Earth & E-nvironment 1: 205-256
University of Leeds Press
Macroinvertebrates as bioindicators of land
degradation in the Kalahari Desert,
Botswana
Eleanor Jew
School of Earth & Environment, University of Leeds, Leeds, W. Yorkshire LS2 9JT; Tel: 0113 3436461
Abstract
The Kalahari Desert of Botswana is continuing to suffer from significant environmental change in the
form of bush encroachment. This is leading to a reduction in the resource potential of the land, resulting
in land degradation. Indicators are needed that are responsive to changes in the vegetation cover, and that
can be easily measured and quantified. Macroinvertebrates are generally sensitive to environmental stresses
and within the Kalahari, Formicidae (ants) and Coleoptera (beetles) are highly noticeable. These two
families were sampled with the use of pitfall and bait traps across four vegetation zones representing sites
of varying degradation. The abundance and diversity of Formicidae generally showed a decline associated
with an increase in land degradation. Three taxa; Neviamymrex taxanus, Crematogaster sp. and Forelius sp. were
present in all sites. Pachycondyla was only present in areas of high degradation and Brachymymrex and
Cerapachys were only found in areas of negligible degradation. Therefore these three genera can potentially
be used as indicators of degradation. Coleoptera showed small decreases in abundance and diversity with
increases in degradation, with the notable appearance of Carabidae (ground beetles) in negligible
degradation sites. Although further research needs to be conducted within this area to substantiate this
study, preliminary results suggest that macroinvertebrates are a suitable indicator of degradation within the
Kalahari Desert.
ISSN 1744-2893 (Online)
© University of Leeds
Jew E (2004) Macroinvertebrates as bioindicators of land degradation in the Kalahari Desert, Botswana
Earth & E-nvironment 1: 205-256
1 Introduction
Land degradation has been recognised as one of the most pressing current environmental
concerns. It affects 2.6 billion people worldwide, and a third of the earth’s surface (FAO, 2004)
in over 110 countries (UNEP, 1997). It has the most significant affect in dryland areas (arid,
semi-arid and sub-humid regions (FAO, 2004)) where the ratio of precipitation to potential
evaporation ranges from 0.05-0.65 (UNEP, 1997). Dryland areas cover 40% of the world and
support 20% of the world’s population, (UNDP, 2004), where agriculture often provides the sole
means of income. Land degradation in these areas can lead to serious poverty and food security
issues (Mabbutt, 1987). Therefore the development of degradation indicators that can be used
both by experts and local land users is paramount so that susceptible areas can be identified.
Management strategies can then be implemented to prevent further deterioration and reverse the
process before the land is considered to be non-viable.
1.1 Aims and Objectives
The aim of this research was to determine the suitability of the use of macroinvertebrates as
bioindicators of land degradation in the Kalahari Desert of Botswana. To do this the following
objectives were met:
 To measure the diversity and abundance of macroinvertebrates across a range of
degraded lands in the Kalahari Desert of Botswana; and
 To specifically investigate the diversity and abundance of the subfamily Formicidae (ants)
as a potential global indicator.
1.2 The Kalahari
Botswana is a landlocked country of 600,370 km2 (CIA, 2003) in Southern Africa, with a
population of 1.7 million (World Bank, 2002). The Kalahari has been defined many times, and its
dimensions vary as to whether it is defined in a physiographical, geological, ecological or political
context (Thomas and Shaw, 1991). The physiographic limitations of the Kalahari give rise to the
largest area, termed the ‘Mega Kalahari’ (Thomas and Shaw, 1991), an area of about 2.5 million
km2 (Thomas, 2002) (Figure 1.21). This is based geologically on the dominant surface sediment
comprising of the Aeolian-origin Kalahari Sand (Thomas and Shaw, 1991). Within this vast area
the smaller semi-arid Kalahari Desert has been broadly identified, covering the area from the
Orange River in the south to the Etosha-Okavango-Zambezi swamp zone in the north, and its
limitations longitudinally are from the transition from hardveld to sandveld (roughly the
Kalahari-Limpopo watershed) in the east to the ascent to higher country and the Great
Escarpment in the west (Thomas and Shaw, 1991) (figure1.21, shaded pink). The Kalahari
covers three quarters of Botswana, eastern Namibia and the Northern Province of South Africa
(Thomas, 2002).
This area still embraces a large range of ecosystems, from the Kalahari Dune Desert in the southwest to the Okavango Delta in the north, but it is characterised by the highly variable and
unreliable rainfall. This, coupled with the high infiltration rates of the sandy soil, has led to very
few areas of surface water and a high susceptibility to drought (Thomas, 2002). Vegetation
communities consist of bush, grass and tree savannas. Over the last 30-40 years a vegetation
shift from grass to bush dominance has been observed, an ecological transition termed bush
encroachment (Dougill et al., 1999). This leads to a reduction in the grazing capacity of the land
(Tobler et al., 2003), and is widely recognised as land degradation (e.g. Moleele and Perkins, 1998;
Thomas et al., 2000; Dougill, 2002; Dougill et al., 1999).
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Jew E (2004) Macroinvertebrates as bioindicators of land degradation in the Kalahari Desert, Botswana
Earth & E-nvironment 1: 205-256
Figure 1.21 The ‘Mega Kalahari’ (grey) and the Kalahari Desert (pink). (Adapted from Thomas & Shaw,
1991)
1.3 Bioindicators
As land degradation has become increasingly important, it has become equally, if not more so,
important to establish assessment and monitoring systems of degradation by using indicators that
are sensitive to disturbance and can be applied consistently across large areas (Nash et al., 2001).
A variety of indicators are rapidly being developed, and so far they have been focused towards
soil and vegetation properties (Nash et al., 2001). It is necessary to use reliable, responsive
indicators that will show the status of, and trends in, living systems (Kimberling et al., 2001) and
can be used to track changes within the ecosystem in response to human pressures, whether they
are positive or negative. Indicators also need to be easy to sample and ubiquitous (Kimberling et
al., 2001) so that they can be applied worldwide for consistent comparison. Terrestrial
invertebrates fit this description, and are found in a variety of roles in natural environments,
including predators, decomposers, parasites, herbivores and pollinators (Kimberling et al., 2001).
Many taxa have also been shown to respond to varying degrees of disturbance, such as
subterranean termites (Nash et al., 1999); butterflies (Blair and Launer, 1997; Spitzer et al., 1997);
spiders (Cameron et al., 2004); beetles (Seymour and Dean, 1999) and ants (e.g. Lobry de Bruyn,
1999; Nash et al., 1998). This indicates that invertebrates have the potential to be successful
indicators of rangeland degradation within the Kalahari.
1.4 Formicidae (ants)
Ants are increasingly being seen as viable indicators of ecosystem change as they have several
attributes that makes them suitable. They have a worldwide distribution; are extremely abundant;
have a high species abundance which covers a range of trophic levels and functional groups; they
are responsive to environmental change; and are relatively easy to sample and identify (Nash et al.,
2001). The majority of work within this area has centred on semi-arid and arid climates in
Australia, America and South Africa. Much work surrounding ants as indicators have been to
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monitor their response to direct human impact, such as mine adits, fire, grazing and logging
regimes (Anderson, 1997). Therefore there is a void in the literature which encompasses not only
the response of ants to environmental change, but also their abundance and diversity within the
Kalahari Desert. This paper aims to redress this issue not only with regard to ants, but to
macroinvertebrates as a whole.
1.5 Structure
This paper begins with an outline of current understandings of land degradation in the Kalahari,
and the use of existing indicators. It then explores the potential of macroinvertebrates, with
particular regard to ants, as possible indicators, before going on to illustrate the methodologies
used in sampling. The results are then displayed, followed by a discussion upon the merits of
each of the indicators collected as indicator species. This is succeeded by a conclusion which
suggests which genera of Formicidae would be best suited to the role of an indicator species
within the Kalahari. Areas where further research and improvements are needed are then
demonstrated.
2 Literature Review
2.1 The Kalahari environment
The Kalahari is not a true desert (Darkoh, 1989) but is covered with vegetation that is typified as
mixed grass and shrub savanna (Thomas and Shaw, 1991). The extreme south west is classified
as arid shrub savanna (Ringrose et al., 2002) and, moving in a north-easterly direction up the
rainfall gradient, there is moderate to dense tree and bush savanna in the southern central
Kalahari, which changes into sparse savanna woodland and shrubland. In the east there is tree
and bush savanna, and in the very north, above the floodplains of the Okavango Delta, there are
mixed moderate to dense savanna woodlands (Ringrose et al., 2002). This pattern of vegetation is
dominated by rainfall. Precipitation is unreliable as it is influenced by the variability of the
Intertropical Convergence Zone (ITCZ) (Thomas and Shaw, 1991). When the ITCZ is displaced
over the Kalahari in the summer (October to April) it produces convectional rainfall. Quantities
generally decrease in a south westerly direction, and therefore Maun, in central northern
Botswana can receive up to twice as much (500 mm) per year than Tshabong in the south west
(300 mm). Further south west in the Northern Cape the average is less than 150 mm /year
(Thomas, 2002). Rainfall is rarely consistent and the wet season can begin or finish early or late,
or not begin at all (Thomas, 2002). It is possible that the rainfall is linked to a quasi-18-year
drought cycle which affects the whole of the Southern African summer rainfall zone (Tyson,
1979, 1986, cited in Thomas, 2002), but it is also susceptible to other climatic variables, such as
El Ninô (Thomas, 2002).
Average daily temperatures range from 20-24°C with greater extremes in the wet season (Thomas
and Shaw, 1991). Temperatures are modified due to the altitude: the Kalahari in Botswana is
900-1000 metres above sea level (Thomas et al., 2000). There is greater diurnal variation during
the winter months when temperatures can fall below freezing at night, and reach the mid-20°Cs
during the day (Thomas and Shaw, 1991). During the summer maximum temperatures are often
over 35°C, and, as these temperatures occur during the wet season this can lead to annual
potential evapotranspiration losses of 3,000-4,000mm (Thomas and Shaw, 1991). However, due
to the lack of standing water and high infiltration rates of the sandy soils these rates are
constrained (Thomas and Shaw, 1991).
Soils are developed from the surficial, nutrient deficient Kalahari Sand (Thomas and Shaw, 1991)
resulting in poorly developed soils with little or no profile development (Thomas and Shaw,
1991), which are classified as arensols (Thomas, 2002) and are moderately acidic, (Thomas and
Shaw, 1991). The very low concentrations of organic matter and nutrients (Thomas and Shaw,
1991; Perkins and Thomas, 1993a), potentially make these soils susceptible to land degradation.
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Figure 2.11 Rainfall in Botswana (Source: Botswana Tourism, 2001)
2.2 Land use in the Kalahari
There had been little large scale or commercial farming in the Kalahari prior to the 1930s due to
the lack of surface water. The Kalahari was sparsely occupied by indigenous nomadic
pastoralists. In the 1930s, however, the first boreholes were sunk (Thomas et al., 2000), to access
the estimated 100,000 million cubic metres of ground water that lies beneath the desert, (Darkoh,
1989). This had only a small impact on the Kalahari, as population densities were very low. By
the 1950s there was increasing livestock pressure in the eastern hardveld which led to further
efforts by the colonial governments to reduce the effects of drought, but little was actually
achieved until after Independence in 1966 (Thomas et al., 2000). As the problems increased the
government made a more concerted effort to improve the livestock industry and this led to the
development of the Tribal Grazing Land Policy (TGLP) in 1975. This aimed to remove large
numbers of cattle off communal lands and into fenced ranches that contained boreholes where
the lessee had sole water rights of the borehole, and therefore control over grazing and resources
within the ranch (Thomas, 2002) Since then (and to some extent before) significant
environmental changes have been seen, and now Botswana is one of the most seriously affected
countries in Southern Africa in terms of desertification (Darkoh, 1989). This was officially
recognised by the Government of Botswana following entry into the United Nations Convention
to Combat Desertification ( UNCCD.) In response to the ratification of this treaty the
Government set up a National Action Programme (NAP), with the help of the UNDP, to
identify causes of desertification and implement action plans to attempt to reduce the impacts.
The NAP recognised thirteen major causes of desertification in Botswana (Government of
Botswana, 2002):
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












Overgrazing
Veldt fires
Drought
Soil erosion
Indiscriminate allocation of arable lands in unsuitable areas
Deforestation
Soil erosion
Population growth
Sand river mining
Loss of cultural values
Decline in land productivity
Increased exploitation of veldt products
Lack of alternative livelihoods.
One of the most important issues that were raised in the NAP was the lack of research into
desertification within Botswana (Government of Botswana, 2002). This is slowly being
addressed, leading to a greater understanding of the extent of desertification and its underlying
causes.
2.3 Land degradation and the Kalahari environment
Land degradation is an effectively permanent decline (Abel and Blaikie, 1989) in the biological or
economical productivity of land in arid, semi-arid and dry sub humid areas (UNCCD, 1996).
Land degradation has effects worldwide (section 1.0 and figure 2.31) and has been widely
recognised within Botswana (e.g. Abel and Blaikie, 1989; Dougill, 2002; Dougill et al., 1999;
Moleele and Mainah, 2003; Perkins and Thomas, 1993a, b; Thomas et al., 2000), and most
documented on TGLP ranches (e.g. Thomas et al., 2000; Perkins and Thomas, 1993a,b; Dougill,
2002). One of the most significant changes is the vegetation shift from grass to bush dominance,
a process termed bush encroachment, (Dougill, 2002) as a result of unsustainable land use
practices (Reed and Dougill, 2003). This shift has occurred radially away from boreholes (Figure
2.32) as a consequence of cattle movement and pressure to and from the watering point. This
phenomenon is known as the piosphere approach (Andrew, 1988; Dougill, 2002; Dougill et al.,
1999; Perkins and Thomas, 1993b) and is demonstrated in figure 2.32.
Figure 2.31 Land degradation in drylands worldwide (UNEP, 1997)
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Figure 2.32 The piosphere approach (Source: Dougill, 2002)
The greatest pressure occurs immediately round the watering point as the cattle return to drink
every day. Therefore the area within two hundred metres of this point is seriously disturbed;
leading to unconsolidated soil and the vegetation is either eaten or crushed. This area is termed
the Sacrifice Zone. The next 2km is where the vegetation shift occurs and there are increased
numbers of unpalatable shrubs and bushes, and it is therefore known as the Bush Encroachment
Zone. The area beyond this remains predominantly grass covered, and is known as the Grazing
Reserve (Perkins and Thomas, 1993a) There is an area which represents the leading front of bush
encroachment into the Grazing Reserve, where shrub dominated vegetation is not yet fully
established, but grasses have been largely removed. This is the Intermediate Zone (section 3.43).
This has also been recognised in central Nevada, where there was a transition from grass
dominated vegetation to an increase in forb amount but without the establishment of shrubs
(Weixelman et al., 1997) leading to a lowering of forage production. The piosphere is a major
ecological pattern on TGLP ranches (Dougill, 2002). When the TGLP was initiated in 1975 there
was an ‘8-kilometre rule’ for distances between boreholes, which meant that they were
significantly far apart to maintain large areas of Grazing Reserve with an ecological fodder
diversity of both grass and bush (Dougill,2002) (figure 2.33). However, as pressure for more
cattle to be grazed on the land has increased this rule has been largely ignored. This is leading to
the possibility that areas of bush encroachment will coalesce (Figure 2.34), which will result in
huge areas of unproductive land (Figure 2.35) (Dougill and Cox, 1995).
A.
B
C
D
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Earth & E-nvironment 1: 205-256
Figure 2.33 The illustrated piosphere approach: A. Grazing Reserve B. Bush Encroachment zone C.
Sacrifice zone D. Borehole (adapted from Dougill, 2002)
As the distance between the boreholes decreases, the area of bush encroachment increases and
the Grazing Reserve is lost (Figures 2.34 & 2.35).
Figure 2.34 Encroaching bush cover
Figure 2.35 Total bush encroachment
The Piosphere model is typical of the Kalahari, and has additionally been noted in Australia
(Andrew, 1988) where there is also a limited water supply. Evidence of bush encroachment as a
sign of land degradation has extensively documented elsewhere, often as a result of overgrazing
in semi-arid and arid regions. In the north Chihuahuan Desert of Mexico overgrazing has led to
the replacement of perennial grasses by shrub dominated communities (Nash et al., 1999), and in
semi-arid central Nevada (Weixelman et al., 1997), severe degradation was characterised by an
increase in the number of forbs and shrubs with an associated decrease in grasses.
There has been much debate as to whether these vegetation changes actually represent true
degradation, or whether they are natural shifts in vegetation dynamics that do not affect the
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resilience of the ecosystem (Dougill and Cox, 1995; Dahlberg, 2000). Past assumptions about
semi-arid and arid ecosystems were that they were fragile environments that were sensitive to
human disturbance, but that if they were left undisturbed they would remain in equilibrium
(Dahlberg, 2000). Now, however, this thinking has shifted towards the notion that these
ecosystems are in fact non- equilibrium systems that are variable but resilient, (Dahlberg, 2000)
and that changes in vegetation are a response to extreme events, and that these changes are not
necessarily degradation unless the resilience of the system is lowered. This can become
complicated, however, as changes in vegetation which may be a response to an event that does
not effect the resilience may lead to a reduction of the resource potential of the land, leading to a
reduction in the economic productivity of the land, a process that is described as land
degradation by the UNCCD (1996). In Botswana vegetation change, especially bush
encroachment, is assumed to be symptomatic of loss of resilience (Dougill and Cox, 1995) and
can therefore be used as an indicator of land degradation. Whilst land degradation unarguably
has occurred in some areas, some contradictory descriptions and evaluation of change and
variation may be related to the changing perceptions of land use practices and the environment
rather than change in the environment itself (Dahlberg, 2000).
2.4 Indicators of land degradation
Indicators of degradation have been developed and used on different scales and targeted at
different groups. They are then implemented according to these scales; at local scales they can be
used to change land use management, at regional and country level they are used to develop
National Action Programmes (NAP) and at global levels they are used to provide global
strategies, such as the UNCCD. As indicators are moved up this scale they become less specific
and much less reliable. Indicators that are used by local land users are often visual aids, such as
vegetation changes and changes in the productivity of the land or condition of the cattle (Reed
and Dougill, 2003). Land users can then use this information to implement better sustainable
management options for the land. Some indicators are developed purely on scientific grounds
and can only be used by people with expert knowledge or training and specialist equipment.
These can range from monitoring nutrients in the soil, e.g. phosphorus, nitrogen and ammonia
(Dougill et al., 1999); using remote sensing (Bowyer et al., 2000); and modelling (Oxley et al., in
press). It is often the results of this scientific research that is then extrapolated and used by policy
makers on a global scale, such as the production of the World Atlas of Desertification first published
in 1992 (UNEP, 1997), which was based on the expert opinions of soil scientists worldwide. The
degree of desertification that was suggested (figure 2.31) has since come under much criticism as
a result of the indicators that were used to assess it (e.g. Warren, 2002; Thomas, 1997).
A traditional approach to monitoring rangeland condition has to been to compare the species
composition of the area in question to the ‘climax’ vegetation that would have been there had the
land not been used, usually through grazing (UCT, 1995). This approach, however, has been
much criticised. Vegetation change can result due to many different reasons, such as fire,
weather events, climate change, and invasion of non-native species (UCT, 1995). Also it is likely
that if the grazing pressure were to be removed the land would not continue to develop to its
climatic climax, but would achieve a plagioclimax. In cases where climax vegetation had not been
achieved the land itself may not be degraded, as exotic species that do not represent the areas’
climatic vegetation may still prevent soil erosion occurring and also would add nutrients to the
soil (UCT, 1995). This has led to the need for further indicators that are not based solely on
vegetation and soils, and the development of criteria that indicators should possess in order to be
of value (table 2.41).
Indicators of degradation are the antithesis of indicators of sustainability. Therefore the two can
be interlinked; if an indicator is demonstrating degradation it must also be indicating poor
sustainability and vice versa (Reed and Dougill, 2003). This is extremely important with regard to
the current global initiatives that are aimed at sustainable development, especially following the
2002 World Summit on Sustainable Development and the introduction of World Development
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Indicators (World Bank, 2004). Integration of these indicators is vital to move forward to reduce
land degradation and develop sustainable livelihood practices.
Table 2.41 A framework of suitable criteria for indicators (Source: Reed and Dougill, 2003)
Easy to measure
Have social appeal and resonance
Rapid to measure
Be policy relevant
Sensitive to spatio-temporal change
Make use of available data
Assess trends over time and provide early Be locally relevant
warning of detrimental change
Cost effective
Accurate
Easy to understand and interpret
Free from bias
Reliable and robust
Derived by the user
Representative of system variability and Simplify complex phenomena
applicable over different regions
Be timely
Quantify information so that its significance is
readily apparent
Scientifically credible
Facilitate communication, particularly between
data collectors and users
Consistency over time
2.4.1 Bioindicators
The use of invertebrates as indicators of land degradation is becoming increasingly more popular
(Kimberling et al., 2000; Hilty and Merenlender, 2000; Nash et al., 2001). Invertebrates often
respond rapidly to habitat changes (Samways, 1994, in Seymour and Dean, 1999) and can
therefore be used to indicate the early signs of degradation, unlike large scale vegetation changes
such as bush encroachment which occur over long periods of time. Bioindicators are required to
monitor changes in soil quality (Lobry de Bruyn, 1999), vegetation (Read and Anderson, 2000)
and responses to general environmental stressors (Whitford et al., 1999). Biological indicators
need to fulfil a range of criteria (table 2.4.11) that is more specified than the criteria mentioned in
table 2.41.
Table 2.4.11 Criteria for successful bioindicators (Source: Hilty and Merenlender, 2000)
Baseline Information
Clear taxonomy
Biology and life history studied
Tolerance levels known
Correlation to ecosystem changes established
Locational Information
Cosmopolitan distribution
Limited mobility
Niche and life history characteristics
Early warning and functional over range of
stress
Trends detectable
Low variability
Specialist
Easy to find and measure
Other
Taxonomic status clear
>30 primary literature articles
Tolerance levels studied
Correlation to ecosystem
Global distribution; not migratory
Home range size small
Reproductive rate high; small body size
Reproductive rate high; small body size; low or
medium trophic level
Small body size; low population fluctuations
Food/ habitat specialist
Easy to find
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Taxa representing multiple agendas
Multiple indicators used
Species at risk; economically valuable
Multiple indicators suggested
Macrofauna (with a body width of 2-20 mm (Lobry de Bruyn, 1999)) often fill many of these
criteria. Indicator taxa can be at species level where specialisation of populations within narrow
habitats will make them more sensitive to habitat change, or at higher taxonomic levels which
occur over broad geographical ranges and habitats so that results can be broadly applicable
(Rodriguez et al., 1998). Indicator taxa can be used as proxy measures of ecosystem conditions,
based on their parameters such as density, presence or absence, or infant survivorship (Hilty and
Merenlender, 2000).
2.4.2 Degradation indicators in the Kalahari
Basing estimations of land degradation on vegetation alone is unwise, as they are often a poor
indicator of damage to resilience (Dougill and Cox, 1995; Thomas, 1993). However, soils in
drylands have a low resilience to degradation (Thomas, 1993) and therefore monitoring changes
in soil quality can be a useful indicator as soil degradation is thought to be the most serious
manifestation of a decline in rangeland condition (Wilson and Tupper, 1982; cited in Weixelman
et al., 1997). Soil degradation in the Kalahari is not necessarily displayed as visible erosion, as the
soils are not particularly susceptible to major soil erosion such as wind and water erosion. The
lack of relief, rapid hydraulic capacity and high infiltration rates means that water erosion is
extremely unlikely (Dougill and Cox, 1995) and although the potential for wind erosion is higher,
erosion is only severe in localised stormy conditions, and it would not be increased by a change in
vegetation cover because a sufficient ground cover is maintained (Perkins and Thomas, 1993b).
However, wind erosion may feature within Sacrifice Zones where vegetation cover is largely
removed. Kalahari soils have low sodium concentrations and low clay values, making them less
susceptible to salinisation, sodicity or physical slumping, slaking and hardsetting (Dougill and
Cox, 1995). Chemical dispersion is also unlikely due to poor cation exchange and low
concentrations of exchangeable sodium (Cox, 1994, in Dougill and Cox, 1995). Therefore soil
degradation is most likely to occur through nutrient loss (chemical degradation), or organic
matter loss (biological degradation) (Dougill and Cox, 1995). The mineralization rate of nitrogen
and phosphorus can determine the ecosystem productivity and structure, and this, in areas like
the Kalahari where the concentrations of these two minerals are low, could be influenced by
external factors such as grazing, as they may interfere with the nutrient cycling processes.
Therefore possible indicators include soil water, pH levels, phosphates, nitrates, and ammonia.
The use of soil formations such as nebkha dunes have also been tentatively proposed as
degradation indicators (Dougill and Thomas, 2001). These formations tend to accumulate
through saltation and creep, processes that increase with reduced vegetation cover and an
increase in unconsolidated soils (Dougill and Thomas, 2001).
These are scientific indicators and cannot be used efficiently by local land users without training
and specialist equipment (Reed and Dougill, 2002). Therefore indicators are needed that can be
used by local people and consequently extensive research has been conducted into land
degradation indicators within the Kalahari. This has been targeted at land user level and has been
accomplished through participatory approaches including focus groups and interviews (Reed,
2004; Reed and Dougill, 2002, 2003). This has resulted in the development of indicators (table
2.4.21) that are scientifically sound and also applicable at pastoralist level (Reed and Dougill,
2003).
Of particular note during the focus groups and interviews was the identification of as large ant
with a grey abdomen and a bad odour, known locally as ‘Malelekatou’, which was said to be an
indicator of poor rangeland health (Reed, 2004). Therefore identification of this species within
this research would be particularly useful.
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Table 2.4.21 Indicators of rangeland condition in the Kalahari (Source: Reed, 2004)
Vegetation
Decreased grass cover
Decreased abundance of wild fruits
Decreased abundance of trees
Decreased abundance of Grewia flava (Berry Bush)
Decreased abundance of thatching grass
Decreased vegetation cover/ increased bare ground
Decreased rain use efficiency
Trees and bushes increasingly stunted
Decreased abundance of wild vegetables
Decreased abundance of Bocia albitrunca (Shepard’s Tree)
Soil
Increased incidence and severity of dust storms
Increased soil looseness
Livestock
Declining livestock condition/ weight loss
Livestock walk further from water/ spend longer between drinking
Increased livestock mortality/ declining herd size
Wild animal and insect
Decreased abundance of game and predators
Decreased abundance of grasshoppers
Socio-Economic
Increased polarisation of rich and poor.
2.5 The Formicidae (ants)
2.5.1 Background to the Formicidae
‘Ants are the most dominant of insects, their species are the most widely distributed, they
outnumber in individuals all other terrestrial animals and they range over the whole world
between the extreme Arctic and Antarctic regions’
(Clarke and Donisthorpe, 1927, p52).
Although this statement was made nearly a century ago, ants are still thought to contribute to
over 10% of total animal biomass, despite representing only 1.5% of the known global insect
fauna (Alonso and Agosti, 2000). They display high diversity, with numerical and biomass
dominance in their occupation of almost every habitat in the world, (Alonso and Agosti, 2000).
These factors alone make them potential indicators of environmental change (section 2.4).
Ants have long been of great interest to man, due to their great abundance and the fascination
that their constant activity promotes. Early attempts to identify ants include Saunders’ (1896)
detailed account of the ant fauna of the British Isles, and continued with works by Wheeler
(1922), Arnold (1915-1927) and Clarke and Donisthorpe (1927). Interest soon began to spread
world wide and it was not long before tentative estimates as to the diversity of the family were
made. In 1952 Morley proposed that there were 15,000 species in eight subfamilies (Morley,
1953). By the 1960’s this figure had dropped to 6000 (Chauvin, 1969; Malyshev, 1966), with
Malyshev going on to suggest that the ants were the smallest family of their order, the
Hymenoptera. Currently 9538 ant species have been described and named out of an estimated
15,000 species (Bolton 1994) with both figures expected to continue to increase. There are
thought to be 296 genera (Bolton, 1994), but the number of subfamilies to which they belong
continues to be a matter for considerable debate, with classifications ranging from ten, (Goulet
and Huber, 1993), to eleven (Hölldobler and Wilson, 1990), to twelve (Gauld and Bolton, 1988)
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and finally to sixteen extant subfamilies, and four extinct subfamilies (Bolton, 1994), a figure
which is becoming more generally adopted as more evidence comes to light.
CLASS Insecta
ORDER
Hymenoptera
SUBORDER
Apocrita
SUPERFAMILY
FAMILY
Coleoptera
Diptera
Lepidoptera
Symphyta
Aculeata
Parasitica
Chrysidoidea
(spheciformes)
Vespoidea Apoidea (apiformes) Apoidea
Formicidae
(+ 9 others)
SUBFAMILIES
Aenictinae
Aenictogitoninae
Aneuretinae
Apomyrminae
Cerapachyinae
Dolichoderinae
Dorylinae
Ecitoninae
Formicinae
Lepanilloidinae
Leptanillinae
Myrmeciinae
Myrmicinae
Nothomyrmeciinae
Ponerinae
Pseudomyrmecinae
EXTINCT
Armaniinae
Formiciinae
Paleosminthurinae
Shecomyrmecinae
Figure 2.51 A cladogram showing the relationship of the Ants (Formicidae) (Sources: Bolton, 1994; Gauld
and Bolton, 1988; Goulet and Huber, 1993)
Ants belong to one of the most diverse classes in the world, the Insecta. There are four orders of
insects, Coleoptera, Lepidoptera, Diptera and Hymnoptera, containing over 100,000 known
species (Goulet and Huber, 1993). The Hymnoptera include the ants, wasps, bees, sawflies, and
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other similar forms (Hölldobler and Wilson, 1990; Gauld and Bolton, 1988; Goulet and Huber,
1993). The Hymenoptera are divided into two sub orders, the Symphyta and the Apocrita
(Gauld and Bolton, 1988). The Symphyta includes sawflies and wood wasps and they are
characterised by the lack of a slender waist and the production of caterpillar-like larvae (Gauld
and Bolton, 1988). The Apocrita are the ‘waisted’, Hymenoptera (Gauld and Bolton, 1988), to
which the ants and wasps belong, and are characterised by the extremely narrow connection
between the abdominal segments 1 and 2 (Goulet and Huber, 1993). The Apocrites are further
divided into the Parasitica and the Aculeata- the ‘stinging’ Hymenoptera, representing the wasps,
bees and ants (Gauld and Bolton, 1988). The Aculeata are then divided into four superfamilies;
(figure 2.511) (Goulet and Huber, 1993; Gauld and Bolton, 1988), of which the Vespoidea
contains the family Formicidae to which the ants belong. The subfamilies (figure 2.511)
represent monophyletic taxa, but they are far from exhaustive, and it is likely that they will be
subjected to future change, as arguments for collapsing and expanding the groups based on
apomorphies and synapomorphies can be made (Bolton, 1994).
2.5.2 Ants as indicators
Ants have several attributes that make them suitable as indicators of ecosystem change: high
abundance, relatively high species richness; there are specialist species and species at higher
trophic levels; they are responsive to changing environmental conditions; and are relatively easily
sampled and identified (Nash et al., 2001), (table 2.4.11).
Species abundance, diversity and
functional groups are used to indicate changes across environmental gradients (Andersen, 1997).
Ants have been used extensively within the mining industry in Australia to demonstrate
restoration success (Andersen, 1997), but use of ants as indicators of land degradation has only
occurred in the last decade. Most of this has focused on Australia (Lobry de Bruyn, 1999; Read
and Andersen, 2000); semi arid regions of the USA (Nash et al., 1998; Nash et al., 2001; Whitford
et al., 1999); the Karoo in South Africa (Seymour and Dean, 1999); and the Chihuahuan Desert in
Mexico (Rojas and Fragoso, 2000). There is little published work on ant communities within the
Kalahari Desert, and their response to land degradation. Therefore this is a region where
research is needed, as ants may provide another indicator of rangeland condition in this region.
2.5.3 Field techniques used to capture ants
Ants are usually anecic (transfer material between the soil and litter habitats) or epigeic – (process
organic matter on or near the soil), and are therefore predominantly subterranean or ground
dwelling, although arboreal species do exist (Lobry de Bruyn, 1999). Methods used to capture
ants should be realistic, rapid, repeatable, quantitative and cost effective (Parr and Chown, 2001).
A number of techniques have been used, e.g. baits, nest mapping, hand sampling, litter and/ or
soil extraction, pitfall traps (Lobry de Bruyn, 1999; Bestelmeyer et al., 2000), fogging, beating and
sweeping (Wang et al., 2001). The most frequently used methods are discussed here; pitfall traps,
bait traps and Winkler litter sampling.
Pitfall trapping is one of the most consistently reliable methods for sampling epigeic
communities (Samways et al., 1996, cited in Seymour and Dean, 1999; Parr and Chown, 2001) and
has been used extensively (e.g. Lindsey and Skinner, 2001; Luff, 1975; Nash et al., 1998; Rojas and
Fragoso, 2000; Tshiguvho et al., 1999). Pitfall traps consist of a cup or vial of plastic, polystyrene
or glass, (Luff, 1975) that is part filled with a preserving agent, such as ethylene glycol (Read and
Andersen, 2000). They are then buried in the soil so that the lip of the cup is flush with the soil
(Bestelmeyer et al., 2000). They are then left for a period of time, from 24 hours (Whitford et al.,
1999) to a week (Rojas and Fragoso, 2000). The traps are then collected and the contents
analysed. There are a number of disadvantages with pitfall traps: - the diameter of the trap may
effect the size of the specimen caught; some ants are easier to trap than others; not a true
representation of all ant fauna is caught; only surface active ants are trapped; it is not an
indication of absolute diversity, and there is uncertainty as to whether the species resides there or
is merely foraging in the area (Lobry de Bruyn, 1999). However, the advantages of this method
outweigh the weaknesses. They are simple to use (Ward et al., 2001; Lobry de Bruyn, 1999; Parr
and Chown, 2001) reliable, (Lindsey and Skinner, 2001,) low cost (Ward et al., 2001; Parr and
Chown, 2001) can be operated day and night (Lobry de Bruyn, 1999; Lindsey and Skinner, 2001)
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and are rapid, repeatable and relatively unbiased (Nash et al., 2001). They can be subject to a
‘digging in effect’ (Digweed et al., 1995) where a larger number of specimens are caught
immediately after the trap is set. However this can be useful in short term studies as it maximises
the number of specimen caught (Ward et al., 2001). When there is a clumped nest distribution or
traps are along foraging trails some results may become distorted, and pitfall traps are more
effective in open habitats as vegetation may compromise the results (Parr and Chown, 2001).
Therefore pitfall traps should be used in conjunction with another method (Lindsey and Skinner,
2001; Lobry de Bruyn, 1999).
Winkler sampling involves removing collecting leaf litter from a quadrat, usually 1m2, and then
sifting the litter through a coarse sieve (1cm2) (Parr and Chown, 2001). The litter is then
suspended in a mesh bag for 48 hours, in which time the ants and other invertebrates work their
way downwards and out of the dry litter and into a cup of preservative which is suspended at the
bottom of the bag (Parr and Chown, 2001; Bestelmeyer et al., 2000). This is time consuming and
labour intensive, (Parr and Chown, 2001) and also needs significant litter, and therefore would be
inappropriate in the Kalahari.
Bait traps use food substances such as tuna, sardine, fruit jelly, cookie crumbs, honey, peanut
butter or sugar solutions (Bestelmeyer et al., 2000) to attract the ants. They can be used to show
the territories and frequencies of ants, behaviour patterns and the composition and richness of
foraging species. Bait traps are influenced by the feeding preferences of the ants and also the
time of day and weather patterns (Wang et al., 2001). They are useful to check that pitfall
trapping has adequately sampled an area and they are easy to use and obtain cleaner samples than
pitfall traps (Wang et al., 2001) as they are specifically targeted at ants.
Other methods include nest mapping, when the nests are very obvious. This has to be done
throughout the year to record seasonal fluctuations and monitor individual nests (Lobry de
Bruyn, 1999). Soil cores and hand picking involve removing cores of soil and sifting through
them, this tends to under sample fast moving ants (Lindsey and Skinner, 2001).
2.6 Ant genera expected in Africa
Eight zoogeographical regions are used when discussing ant fauna (Bolton, 1994) and the region
including sub-Saharan Africa is known as the Afrotropical region (Bolton, 1994). Eleven
subfamilies are found in sub-Saharan Africa, (Bolton, 1994) containing 2,500 species (Bolton,
1986, in Hölldobler and Wilson, 1990, p4):











Aenictinae
Aenictogitoninae
Apomyrminae
Cerapachyinae
Dolichoderinae
Dorylinae
Formicinae
Leptanillinae
Myrmicinae
Ponerinae
Pseudomyrmecinae
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3.0 Research Design and Methodology
3.1 Aims and hypothesis


To sample macroinvertebrates from sites of varying grazing intensity (Sacrifice Zone,
Bush Encroachment Zone, Intermediate Zone and Grazing Reserve) to determine
changes in abundance and diversity in relation to variations in degradation.
To map the vegetation within each sample area to detect any relationships between
vegetation cover and macroinvertebrate distribution.
Hypotheses:
 H0- There will be no significant change in the abundance or in the diversity of
macroinvertebrates between the four vegetation types.
 H1- There will be a significant change in the abundance or in the diversity of
macroinvertebrates between the four vegetation types.
3.2 Study Site
The study site was located in south wesetern Botswana (figure 3.21) 8km north of the town of
Tshabong, neighbouring a commercial ranch called Berrybush Farm (Figure 3.23).
Figure 3.21 Botswana, showing study site (Source: Botswana Tourism, 2001)
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Figure 3.23 The study site
Tshabong is the largest town in south west Botswana, in the Kalagadi district. Annual rainfall is
about 300mm (Dougill and Thomas, 2004), and is extremely unreliable, with an interannual
variability of 45% (Bhalotra, 1985a; in Perkins and Thomas, 1993b) (figure 3.24).
140
120
100
Rainfall (mm)
1999
2000
2001
2002
2003
80
60
40
20
0
Feb
Mars
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Month
Figure 3.24 Inter-annual rainfall variability in Tshabong (Source: Meteorological Services, Government of
Botswana)
3.3 Research design- site justification
The piosphere model (Andrew, 1998; Dougill, 2002; Perkins and Thomas, 1993a,b; Dougill et al.,
1999) was adopted as a basis for the identification of each site. The aim was to sample from each
of these recognised Kalahari vegetation zones, the Sacrifice Zone, the Bush Encroachment Zone,
the Intermediate Zone and the Grazing Reserve (see section 2.2). There are two methods that
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can be applied to sample within the piosphere; those based on a transect, where sampling occurs
every 0, 15, 30, 60, 120, 240, 500, 1000, 1500, 3000, and 5000m (Moleele and Perkins, 1998;
Dougill et al., 1999); and sampling within each zone after classification of the vegetation
composition. It was felt that although the piosphere model is a good general classification of the
succession in vegetation types radiating from a borehole, in the field it does not necessarily
produce the concentric rings shown in the model (figures 2.32. and 2.33). Although this pattern
is broadly produced there may be variations in shape due to the influence of roads, settlements,
and, most importantly within this study site, movement of cattle away from the watering point.
This is because cattle will usually move off after each other and in the same direction on a regular
basis, and therefore the grazing pressures in this direction will be greater than in other areas. It
was felt that using a transect approach would be unsuitable for the level of sampling that was to
take place, and therefore each study site was chosen as an area that was most representative of
the vegetation zone that was to be sampled. Within each study area the sampling sites were
chosen as the most homogenous of the vegetation within each site and placed so that the fauna
within each quadrat would not be affected by the sampling occurring in the other quadrat or
from disturbance that could occur.
3.4 Site descriptions
3.4.1 Sacrifice Zone
The sacrifice zone was clearly identifiable by its proximity to the watering point, and the lack of
vegetation in the area. There were kraals that contained cattle and a number of donkeys were in
the vicinity. The soils were sandy and unconsolidated due to disturbance and a high number of
cattle tracks. There were large amounts of cattle dung visible, and this was often focused under
Boscia albitrunca (shepard’s trees), which are used for shade. The only vegetation was that of large
trees and the occasional bush. This is due to smaller plants and seedlings being either eaten or
trampled on before they can get to a size that may lead to their survival. There was a large
amount of dead wood within this zone. This was produced by several means; overgrazing
pressure, lack of water, and there was also evidence of fire damage. The Sacrifice Zone
represents an area of high degradation.
Figure 3.41 The Sacrifice Zone
3.4.2 Bush Encroachment Zone
This area is dominated by bushes and shrubs that often form impenetrable thickets. It is
dominated by Acacia mellifera and Grewia flava bushes, there are a few large trees: Boscia albitrunca
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and Acacia eriloba (camel thorn). The soils are consolidated, except on cattle tracks which
circumnavigate the bushes. There are few grasses as these have been either grazed or outcompeted by the bushes. However, some grasses are found underneath the bushes, where the
cattle cannot reach them. This provides an important seed bank or ‘islands of fertility’
(Schlesinger et al., 1990, in Dougill and Thomas, 2004).
Figure 3.42 The Bush Encroachment Zone
3.4.3 Intermediate Zone
This zone represents the frontier of the bush encroachment. It contained a heterogeneous
mixture of grasses, bushes and trees. The vegetation diversity was higher than that of the Bush
Encroachment Zone, but less than that of the Grazing Reserve. Cattle tracks were present in
lesser quantities than that of the Bush Encroachment Zone, and were not solely around the
bushes. The soils were consolidated and showed fewer signs of disturbance.
Figure 3.43 The Intermediate Zone
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3.4.4 Grazing Reserve
This was essentially the control site. This area is unaffected by overgrazing and has a high
percentage of grass cover. There are a small number of bushes and trees, but grasses are
Figure 3.44 The Grazing Reserve
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dominant e.g. Schmidtia kalihaiensis (Kalahari sour grass); Eragrostis lehmanniana (layman’s love
grass); and Aristida congesta (tassel three awn). There are few signs of disturbance. Signs of cattle
are mainly under larges trees that are used for shade. There are few cattle tracks. This area is
representative of semi-natural Kalahari vegetation cover. This site was located 8 km between
Berrybush Farm and the town of Omaweneno. This was the most representative site that was
found of this vegetation type, illustrating the need to deviate from a strict transect approach to
site location.
3.5 Methodology
3.5.1 Pitfall Traps
At each sample site a 35 x 35 metre grid was marked out. This size was used so that 49 traps
could be laid with inter-trap spacing of 5 metres. This spacing was shown not to cause a
depletion effect and also showed a significantly higher number of species than a 1 metre spacing
(Ward et al., 2001). Each trap consisted of a plastic pot of 58 mm diameter and 72 mm in depth.
Wide trap diameters are more efficient at sampling a higher diversity of invertebrates (Luff, 1975;
Abensperg-Traun and Steven (1995) cited in Lobry de Bruyn 1999). 42 mm diameter traps were
shown to perform as well as traps of 86 and 135 mm
in diameter (Bestelmeyer et al., 2000). Therefore the
diameter of 58 mm was used as convenient and easy
to use size which did not affect the results. Each pot
contained 12 ml of 30% ethylene-glycol a widely
used preservative and killing agent that is nonattracting and non-evaporating (Greenslade and
Greenslade, 1971, cited in Read and Anderson,
2000). It is, however, thought to be toxic to small
mammals (Bestelmeyer et al., 2000). The traps were
then buried into the sand so that the lips were flush
to the ground; as if the lip is only slightly raised it
may deter small and /or wary ant species
(Bestelmeyer et al., 2000). The ground litter was then
replaced by hand. A lid was used to prevent soil
being blown into the trap, reduce evaporation and
deter small mammals (figure 3.51). This was
constructed from tin pie lids punctured with nails
which were pushed into the soil so that the lid was
suspended at 1cm above the ground (Figure 3.31).
The traps were then left for 72 hours (Bestelmeyer et
al., 2000; Parr and Chown, 2001) before being
collected and the contents analysed.
Figure 3.31 A laid pitfall trap
3.3.2 Methodology- Bait traps
At each site a second 35 x 35 m grid was marked out, at least 20 metres away from the pitfall
traps. Inter-trap spacing was kept the same as for the pitfall traps to retain consistency; 49 traps
placed at 5 metre intervals. The bait traps were plastic pots 86
mm in diameter and 120 mm deep (figure 3,52). Four 5 mm
holes (Wang et al., 2001) were drilled into the pot at equal
distances from each other 30 mm from the bottom of the pot.
2ml of honey and 2ml of peanut butter, as in Wang et al.,
(2001), were placed at the bottom of the trap (Figure 3.52).
These substances were used as most ants are attracted to
either carbohydrates or sugar (Bestelmeyer et al., 2000). Cling
film was then placed over the top of the pot and secured with
Dec
Dec
Dec
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sellotape. This was sufficient to prevent ants escaping. The traps were then laid; buried into the
sand to just below the holes. This was to stabilise the pots, whilst allowing the ants’ easy access
to the bait. The traps were left for 24 hours (Wang et al., 2001), then collected and the holes
sellotaped over to prevent escapes during transition. The contents were then analysed and the
pots cleaned before being re-baited and laid at the next site (e.g. figure 3.53).
Figure 3.53 Bait traps laid at the Sacrifice Zone
3.6 Vegetation Mapping
After sampling had taken place the main vegetation at each site was recorded per 5 x 5 m square.
This was so any major vegetation- species associations could be analysed and also demonstrated
the different vegetation compositions at each site. Mapping occurred after sampling was
completed so that minimal disturbance was caused during sampling.
4 Results and Analysis
4.1 The Formicidae (ants)
The specimens that were retrieved at each site were identified based on their morphological
features and assigned sample numbers. Fifteen different sample sets were recognized. Theses
were subsequently identified at the National History Museum of Botswana. The identifications
are displayed in table 4.1.
Samples were identified to genus, with several identified to species. However, with the exception
of Neivamymrex taxanus the species were only consistently identified to genus. Despite members
of Forelius being identified to two different species levels (samples 6,7,12 and 2), Forelius samples 8
and 3 were not identified to species level. Therefore it is appropriate to use only the genus
identification. Therefore six genera were identified, and the sample numbers for each genus were
amalgamated to produce six sets of data.
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Table 4.1 Identified sample numbers
Sample number
Subfamily
Genus
Species
1
Myrmicinae
Crematogaster
2
Dolichoderinae
Forelius
cooki
3
Dolichoderinae
Forelius
4
Ecitoninae
Neivamymrex
taxanus
5
Myrmicinae
Crematogaster
6
Dolichoderinae
Forelius
pruinosus
7
Dolichoderinae
Forelius
pruinosus
8
Dolichoderinae
Forelius
9
Myrmicinae
Crematogaster
10
Myrmicinae
Crematogaster
11
Ponerinae
Pachycondyla
12
Dolichoderinae
Forelius
pruinosus
13
Formicinae
Brachymymrex
14
Cerapachyinae
Cerapachysaugasta*
15
Myrmicinae
Crematogaster
*The abbreviated form of Cerapachysaugasta -Cerapachys- is used within the literature, and therefore
this genera will henceforth be referred to as Cerapachys.
4.1.1 Results of Ants
Results are displayed for each vegetation zone, from the pitfall traps and bait traps, followed by
the total ants from both data sets.
Table 4.1.21 Ants recorded in pitfall traps
Site
Genus
Sacrifice
Zone
Crematogaster
Forelius
Neivamyrmex
Pachycondyla
Brachymyrmex
Cerapachys
59
49
Bush
Encroachment
Zone
2
752
10
1
Intermediate
Zone
178
5
2
Grazing
Reserve
33
7
5
15
Table 4.1.22 Ants recorded in bait traps
Site
Genus
Crematogaster
Forelius
Neivamyrmex
Pachycondyla
Brachymyrmex
Cerapachys
Sacrifice
Zone
6
175
6
11
Bush
Encroachment
Zone
Intermediate
Zone
19
326
891
39
250
530
Grazing
Reserve
1305
4
537
7
310
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Table 4.1.23 Total ants from both methods of trapping
Site
Genus
Sacrifice
Zone
Crematogaster
Forelius
Neivamyrmex
Pachycondyla
Brachymyrmex
Cerapachys
65
175
55
11
Bush
Encroachment
Zone
21
1078
901
1
Intermediate
Zone
217
257
532
Grazing
Reserve
1338
11
542
7
325
Using two methods to sample the ant communities proved valuable, as they each collected genera
that were not recorded by the other method at the same site, e.g. Brachymymrex and Pachycondyla.
4.1.3 Statistical analysis
The data was analysed to demonstrate relationships between the sites for abundance and
diversity. The chi-squared test shows if there is a significant difference in the abundance of the
genera between each site and the Shannon Wiener diversity index illustrates if there are changes
in diversity at each site.
Chi squared was calculated for the three sets of data:
χ2 = Σ (O-E)2/E
Degrees of freedom = 20
Therefore χ2 tab at 0.05 = 31.4
Pitfall traps χ2 = 1580.48
Bait traps χ2 = 3394.95
Total ants χ2 = 3374.56
In each case χ2 calc> χ2 tab which indicates a significant difference between the abundance of ant
genera between the sites. This is further indicated in figures 4.1.41- 4.1.43.
Performing further analysis on this data after Cohen and Holliday (1996, p234-340) including
‘standardising margins’ and using partitioned contingency tables, did not add to this data.
The Shannon Wiener Index was used to determine changes in diversity at each site.
Shannon Wiener equation:
This can also be used to determine the evenness of the distribution between the genera groups:
The values for the Shannon Wiener and Evenness indices are then multiplied by -1, to give a
positive number between 0 and 1. Values of 1 represent high diversity and high evenness
respectively (Beals et al., 2000).
The combined results of the pitfall and bait traps were used for this analysis in order to use the
full spectra of genera.
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Table 4.1.32 Shannon Wiener analysis for Formicinae
Site
Shannon Wiener Index
Evenness
Sacrifice Zone
0.363
0.603
Bush Encroachment Zone
0.323
0.365
Intermediate Zone
0.154
0.316
Grazing Reserve
0.423
0.606
These results show that the Grazing Reserve had the highest diversity and evenness scores. This
is because 5 genera were present here, and they were relatively evenly distributed. The
Intermediate Zone had the lowest scores, and the Sacrifice Zone and Bush Encroachment Zones
had similar diversity scores, with distribution being a little more even in the Sacrifice Zone.
4.1.4 Graphical Analysis
The abundance and diversity are displayed for each method of trapping and for total values
across each site. This shows the low abundances for the Sacrifice Zone. It also shows that values
in the Bush Encroachment Zone are higher than those in the Intermediate Zones, but that values
of abundance and diversity are both the highest in the Grazing Reserve.
800
700
600
No. of individuals
500
Crematogaster
Forelius
Neivamyrmex
Pachycondyla
Brachymyrmex
Cerapachys
400
300
200
100
0
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Site
Figure 4.1.41 Ant abundance in pitfall traps
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1400
1200
1000
No. of individuals
Crematogaster
800
Forelius
Neivamyrmex
Pachycondyla
Brachymyrmex
600
Cerapachys
400
200
0
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Grazing Reserve
Site
Figure 4 .1.42 Bait trap ant abundance
1600
1400
1200
No. of individuals
1000
Crematogaster
Forelius
Neivamyrmex
800
Pachycondyla
Brachymyrmex
Cerapachys
600
400
200
0
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Site
Figure 4.1.43 Total ant abundance
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4.1.5 Vegetation and Species mapping
This section shows the distribution of vegetation in each sample site, and the distribution and
genera variety of ants captured at each site. This is to demonstrate if there is any relationship
between the vegetation and the distribution of ants.
Vegetation mapping:
Key
Senna italica
Leaf litter
Acacia erioloba
Grewia flava
Dead wood
Small bushmans grasses
Acacia mellifera
Sour grass
Boscia albitrunca
No grass
G Grass cover: tassel three-awn; Laymans love grass; sour grass
Ant genera distribution:
Key
1-5
Crematogaster sp.
6-10
11-15
Neivamymrex taxanus
Forelius sp.
16+
Pachycondyla sp.
Cerapachys sp.
Brachymyrmex sp
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4.1.51 The Sacrifice Zone
The sparsely distributed vegetation demonstrates the severely degraded aspect of this site. There
are no grasses, and those trees and bushes that are present show stunted growth and evidence of
browsing. The cattle visit this site every day for water, and therefore it is heavily disturbed.
Figure 4.1.51a The pitfall trap site
Figure 4.1.51b The bait trap site
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4.1.52 Spatial distribution of ant genera
This shows the species present at the Sacrifice Zone, and the difference in the species captured
by the pitfall traps and bait traps. The distribution is sparse, and the genera diversity is low,
especially in the pitfall traps. The bait traps show a bias, as they actively attract the ants to the
trap.
4.1.52a The pitfall trap distribution
Figure 4.1.52b Bait trap distribution
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4.1.53 Bush Encroachment Zone
This shows the Bush Encroachment Zone, clearly demonstrating the increase in bush and tree
cover, with an increased occurrence of Acacia Mellifera and Grewia flava.
Figure 4.1.53a Pitfall trap site
Figure 4.1.53b The bait trap site
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4.1.54 Spatial distribution of ant genera
This shows a wider distribution of ants in the area, the relationship is not as aggregated. The
difference between the two types of traps is demonstrated by the much higher number of
individuals captured by the bait traps.
304
Figure 4.1.54a The pitfall trap distribution
Figure 4.1.54b The bait trap distribution
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4.1.55 The Intermediate Zone
The intermediate zone vegetation shows that the bush cover is much less dense, however, there
is little grass as this has been overgrazed, but there has not been sufficient time for bush cover to
increase. This site indicates the leading edge of the bush encroachment.
Figure 4.1.55 a The pitfall trap site
Figure 4.1.55b The bait trap site
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4.1.56 Spatial distribution of ant genera
This shows that the distribution of genera in the Intermediate Zone is dominated by the
Crematogaster species. It is highly aggregated with large numbers found in the pitfall traps,
indicating that there was a high population of this genus in this area.
Figure 4.1.56a The pitfall trap distribution
Figure 4.1.56b The bait trap distribution
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4.1.57 The Grazing Reserve
This site is only lightly grazed, and there is ~90% grass cover. There is only light bush cover in
both sample sites.
Figure 41.57a The pitfall site
Figure 4.1.57b The bait trap site
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4.1.58. Spatial distribution of ant genera
The Grazing Reserve shows a greater number of individual ants and a larger diversity in genera
than in previous sites.
Figure 4.1.58a The pitfall trap distribution
Figure 4.1.58b Bait trap distributions
4.2 Other invertebrates caught
Other invertebrates were captured through pitfall trapping at each site. They were not identified,
but they serve to show that this method is useful for trapping other types of macroinvertebrates.
There are no results for bait traps, as these were targeted at the ants.
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Coleoptera (beetles):
Table 4.2.1 Coleoptera species
Site
Beetle
species
Sacrifice
Zone
1
2
3
4
5
6
7
8
Bush
Encroachment Intermediate Grazing
Zone
Zone
Reserve
3
31
1
1
9
2
12
32
1
1
1
1
10
2
1
3
1
4.2.2 Statistical Analysis-Coleoptera
Chi squared analysis:
Degrees of freedom = 21
Therefore χ2 tab at 0.05 = 32.7
Coleoptera χ2 = 50.11
χ2 calc > χ2 tab
This indicates that there is a significant difference in the abundance of these beetles across the
sites.
4.2.22 The Shannon Wiener Index
There was little change in diversity across the sites. This can be seen from the distribution in
table 4.2.22 and from figure 4.2.23.
Table 4.2.22 Shannon Wiener Analysis of Coleoptera
Site
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Grazing Reserve
Shannon Wiener
0.457
0.103
0.467
0.298
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Evenness
0.759
0.147
0.669
0.142
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35
30
25
Number of individuals
Species 1
Species 2
Species 3
Species 4
Species 5
Species 6
Species 7
Species 8
20
15
10
5
0
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Grazing Reserve
Site
Figure 4.2.23 Coleoptera abundance in pitfall traps
The dominance of species 2 is very apparent at each site.
4.3.1 Araneae (spiders):
Table 4.31 Araneae species
Site
Spider
species
Sacrifice
Zone
1
2
3
4
5
6
7
8
9
10
11
2
1
Bush
Encroachment Intermediate Grazing
Zone
Zone
Reserve
1
4
1
1
4.3.2 Statistical analysis
Chi-squared analysis:
Degrees of freedom = 30
Therefore χ2 tab at 0.05 = 43.8
Araneae χ2 = 41.36
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3
2
1
1
1
1
2
1
5
2
2
1
1
2
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As χ2 calc< χ2 tab the distribution conforms to the expected and therefore there is not a
significant difference in the abundance of Araneae species between the sites.
4.3.22 The Shannon Wiener Index
The Shannon Wiener Index (table 4.3.2) shows a higher diversity in the Intermediate Zone and
Grazing Reserve. This is also illustrated in figure 4.3.23.
Table 4.3.22 Shannon Wiener analysis of Araneae
Site
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Grazing Reserve
Shannon Wiener
0.579
0.217
0.858
0.773
Evenness
0.961
0.722
0.950
0.915
6
5
Species 1
Species 2
4
Species 3
Species 4
Number of individuals
Species 5
3
Species 6
Species 7
Species 8
Species 9
2
Species 10
Species 11
1
0
Sacrifice Zone
Bush Encroachment Zone
Intermediate Zone
Grazing Reserve
Site
Figure 4.3.23 Araneae abundance from pitfall traps
Carabidae (ground beetles):
Table 4.4.1 Carabidae species
Site
Ground
beetles
Sacrifice
Zone
Bush
Encroachment Intermediate
Zone
Zone
1
2
3
4
5
Grazing
Reserve
5
2
6
3
1
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It is evident from table 4.41 that Carabidae (ground beetles were only found in the Grazing
Reserve. Therefore no statistical tests were performed on this data set.
Scorpion species:
Table 4.4.2 Scorpion species
site
scorpion
1
Sacrifice
Zone
Bush
Encroachment
Zone
1
Intermediate
Zone
Grazing
Reserve
1
Only two scorpions were trapped and therefore they can not be regarded as useful in this data
set.
5 Discussion
The majority of invertebrates that were collected were Formicidae, which were the target group
of the research. Other invertebrates were also trapped, most notably Coleoptera and Araneae.
The suitability of these macroinvertebrates as indicators of degradation within the Kalahari is
discussed below.
5.1 Coleoptera (beetles)
Coleoptera were present at each of the four sites and did show a significant difference in
abundance between the sites. However, most species were represented by only one individual,
and therefore conclusions cannot be drawn from diversity distributions. Species 2 appeared to
be the dominant species as it was present in all four sites, showing greater abundances within the
Bush Encroachment Zone and the Intermediate Zone, but without identifications for this species
it is impossible to draw significant conclusions regarding their use as indicators.
However, ground beetles (Carabidae) were not trapped at any of the degraded sites and were only
found in the relatively undisturbed Grazing Reserve, where five different species were found.
Therefore it is possible that the presence of ground beetles could be used as indicators of good
rangeland condition. However, within the lowland tropical rainforest belt of Nigeria
participatory indicator research indicated that an increase in the presence of beetles was
associated with an increase in degradation (Chokor and Odemerho, 1994). And therefore the
presence of beetles as an indicator of good rangeland condition could not be applied across
broad regions. The lack of large beetles in the degraded areas of the Sacrifice Zone and Bush
Encroachment Zone was unexpected, as dung beetles (Coleoptera: Scarabaeidae) are usually
present within the Kalahari, and in areas where there is a presence of dung, such as the Sacrifice
Zone (Davis and Scholtz, 2004). However, this, and the low presence of beetles in general, could
be explained by climatic conditions. Sampling took place during the African winter and dry
season, and ground beetles have been shown to be responsive to climatic variables more than
habitat change (Irmler, 2003) and dung beetles are thought to have higher abundances during the
late summer rainfall (Davis and Scholtz, 2004). Therefore although beetles did show variations in
abundance and diversity throughout the four vegetation sites they could not be recommended as
a suitable bioindicator without further research, which sampled during different climatic
conditions.
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5.2 Araneae (spiders)
Araneae were collected at each site, at relatively low abundances. Again, many species were
represented by a single individual. The differences in abundances between the four sites were not
shown to be significant, but diversity did decrease with an increase in degradation. This pattern
has also been shown in a range of degraded marsh sites; where abundances and diversities were
greatest in the least degraded areas and lowest in the degraded areas (Milakovic and Jefferies,
2003). Although the diversity of spiders throughout these sites did vary, they are not a suitable
indicator as they were not trapped in large numbers or on a regular basis, due to their reclusive
behaviour and natural low abundances.
5.3 Formicidae (ants)
The two methods used to trap ants were successful, and using two different techniques proved to
be beneficial as each method caught species that the other method failed to do in the same site.
Six genera were discovered from six different subfamilies and each of these is discussed below.
5.3.1 Cerapachyinae Cerapachys sp.
Cerapachys is the largest and most widely distributed genera of this subfamily, with species in every
zoogeographical region (Bolton, 1994). This genus was discovered only in the Grazing Reserve,
and therefore may potentially be used as an indicator of good rangeland condition. Species of
Cerapachys are specialist predators of other ants (Shaltuck and Barnett, 2001). Colonies are usually
of less than 100, and hunting occurs during the day. Scouts are sent out to find a suitable nest to
raid, and these return to the home nest to recruit more workers (Shaltuck and Barnett, 2001).
This may explain why very few were caught during the pitfall trapping, whereas more were found
during the bait trapping, possibly attracted to the presence of other ants. They nest in the soil
through single entrance holes, but have also been known to nest in wood or cracks in rocks.
They have been seen to associate with army ant columns of Neivamymrex sp. (Ants of Arizona,
2004).
Figure 5.31 Cerapachys sp. (Source: Ants of Arizona, 2004)
5.32 Dolichoderinae Forelius
Two members of this genus were identified to species level: Forelius cooki and Forelius pruinosus
(figure 5.32). Forelius are small, 1-2 mm in length (Ants of Arizona, 2004). They are attracted to
nectar and sugar sources (Ness, 2003) and produce a fruity odour when crushed (Ants of
Arizona, 2004). They are thermophilic and forage during the hottest parts of the day (Ness,
2003) in open arid habitats (Wild, 2004).
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Figure 5.32 Forelius pruinosus (Source: Ants of Arizona, 2004)
5.33 Ecitoninae Neivamyrmex
The single genus found of this sub family was identified to species level; Neivamyrmex taxanus.
This is an army or legionary ant, a type of ant well known from the Dorylinae and Ecitoninae
subfamilies. However, the subfamily Ecitoninae was not expected to be found in this region
(section 2.5). Indeed, Bolton (1994) states that the five genera of this subfamily (including
Neivamyrmex) are distributed entirely in the New World. Driver ants from the closely related
Dorylus are well known in sub-Saharan Africa as they ‘drive’ all other species of invertebrates and
small vertebrates before them. Little is documented about the species Neivamyrmex taxanus, but
some behavioural traits of the genus (figure 5.33) are known. They generally exist in worker
colonies of 10,000- 140,000 which raid in weak dendricitic columns preying on other ants, beetles
and other small arthropods (Holldobler and Wilson, 1990). Raids occur at night or on cloudy
days. This would explain why no army ant activity was observed; raids occur at night and occur
in weak columns rather than the well documented swarms that are inescapably noticed, such as
those of the Ecitoini butecheni.
Neivamyrmex nests are usually subterranean or under logs, rocks, or in rotting logs. They move
through periods of migration, where the nest site is moved frequently within a period of about 14
days, followed by a stationary phase of 19-21 days. Migratory phases are thought to be initiated
by the emergence of new adults from the pupae. The frequency of the group foraging raids is
determined by the availability of food (Holldobler and Wilson, 1990).
Figure 5.33 Neviamyrmex sp. (Source: Wild, 2004 www.myremecos.net)
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5.34 Formicinae Brachymyrmex
The genus Brachymyrmex is also known as the Rover Ant (Florida University, 2004). It belongs to
the tribe Brachymyrmecini and has a worldwide distribution (Bolton, 1994). They are some of the
smallest species in the world (Florida University, 2004) and eat honeydew, which they obtain
from sap sucking insects on the roots of plants (Florida University, 2004). This genus was only
found in the Grazing Reserve. Very little is documented regarding this genus.
Figure 5.34 Brachymyrmex sp. (Source: Florida University, 2004)
5.35 Myrmicinae Crematogaster
Crematogaster sp. have a worldwide distribution. They are often known as acrobat ants as they can
raise their abdomens over their heads when they are disturbed; to release their well developed
chemical defences (Shaltuck and Barnett, 2001; Ants of Arizona, 2004). They are thought to be
generalist predators and also tend to hemiptera and sap-sucking insects. They are attracted to
sugar and proteins, (Florida University, 2004), which explains their attraction to the bait traps.
This genus was found in all the sites, showing a resilience to land degradation.
Figure 5.35 Crematogaster sp. (Source: Addison, 2001; Ants of Arizona, 2004)
5.36 Ponerinae Pachycondyla
Pachycondyla are a wide and diverse group of large bodied Ponerinae which are usually general
predators or scavengers, with some specialist termite predators.
They have a worldwide
distribution (Bolton, 1994), but very little is documented about this genus.
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Figure 5.36 Pachycondyla sp. from South Africa (Source: IZIKO, 2004)
5.4 Formicidae distribution in the Kalahari
The knowledge of ants in Africa is very limited (Lindsey and Skinner, 2001). This is
demonstrated here by the unexpected presence of Neivamymrex taxanus (section 2.6) in Botswana
when it has been documented as only a New World genus (Bolton, 1994). However, before any
conclusions as to the distribution of this genus can be drawn, further sampling and identification
of the genus is required. This would not be difficult, as this genus was present in generally large
abundances throughout the four sites. Very little is known about the morphology, habitat or
behaviour of any of the ants found in Botswana, as demonstrated in section 5.3.
5.4.1 The Sacrifice Zone
Four genera were found in the Sacrifice Zone, of which only two were found in the bait traps.
This was surprising as Forelius sp. had a relatively high abundance in the bait traps and yet was not
found in the pitfall traps. This demonstrates the need to use more than one sampling technique
(Lobry de Bruyn, 1999). Overall abundance here was low. This is due to unconsolidated soils
which makes establishing the typical nests difficult. The lack of vegetation also reduces food
sources. The notable presence within this zone is that of Pachycondyla sp. This is a large ant, and
as it is shown only in this zone and the Bush Encroachment Zone it is likely to be representative
of degradation sites. This could therefore be the ‘Malelekatou’ ant identified as an indicator of
degradation in Botswana by Reed (2004) (section 2.42).
There is no notable link with vegetation in this area, possible due to the scarcity of vegetation
here.
5.4.2 Bush Encroachment Zone
This zone demonstrated a high abundance containing four genera, one of which was a solitary
representative of Pachycondyla sp. The high abundance is likely to be due to the high proportion of
bush cover. Forelius species appeared to be associated with Acacia mellifera bushes. The Acacia
mellifera provide sub-canopy microhabitats, where the soils have enhanced ammonium and
phosphate concentrations, higher levels of organic matter and there is an increase in plant
biomass (Dougill and Thomas, 2004). This can lead to seed banks which are protected from
grazing by the thorny Acacias and have been described as ‘islands of fertility’ (Schlesinger et al.,
1990, in Dougill and Thomas, 2004). They provide suitable habitat and foraging supplies for the
ants, leading to increased abundance.
5.4.3 Intermediate Zone
This zone shows paucity in both abundance and diversity, although it is well represented by the
three genera that are present. The Crematogaster sp., which is present at all the sites, begins to
show an increase in abundance here, possibly indicating a preference for less degraded
conditions. In this area the vegetation is sparse, as the grasses have been removed by overgrazing
and the bush cover is not yet fully established, as the Intermediate Zone represents the leading
edge of the bush encroachment. Therefore there is not the range of habitats present to support
larger diverse populations. Forelius also shows an association with Acacia mellifera in this site.
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5.4.4 Grazing Reserve
Thus area has the highest abundance and diversity of all the sites. This is to be expected, as it
also contains the most heterogeneous mixture of vegetation (section 3.44) therefore providing
good habitat and foraging opportunities. This zone had the first appearance of two new general
Brachymyrmex and Cerapachys. The genera were well distributed throughout the site, and the
decline of Forelius could be related to the reduction in the numbers of Acacia mellifera.
5.5 Ants as indicators
The ants fulfil many of the criteria outlined in section 2.4. They show responses to vegetation
changes as a result of land degradation both in terms of abundance and diversity, as both
parameters showed a decline with an increase in degradation. The results from the Bush
Encroachment Zone do show an increase in abundance, and give weight to the suggestion
outlined in Nash et al., (2001) that ants are only useful as indicators of large-scale changes.
However, it appears that the subtle changes within the data here can be identified and used in
conjunction with other indicators to signify degradation.
It is possible to tentatively suggest that three of the genera recorded within the Kalahari can be
used as indicators of land degradation. Pachycondyla was only present within the degraded areas of
the Sacrifice Zone and the Bush Encroachment Zone. Therefore it can be suggested that this
genera is an indicator of increased degradation. Brachymymrex and Cerapachys were both present
only in the Grazing Reserve. Therefore these can both be suggested as indicators of good
rangeland condition, but due to the low abundance of Brachymymrex this suggestion must be
approached with caution. It could also be suggested that the distribution of Forelius is related to
the presence of Acacia mellifera. This preliminary research outlines a real potential for the use of
ants as indicators of land degradation in the Kalahari.
6 Conclusions






Macroinvertebrates are suitable indicators of land degradation within the Kalahari
Desert.
Both Araneae and Coleoptera showed responses to habitat change associated to land
degradation. However, due to the lack of identification for the species found and the
low numbers of specimen recovered of each species, a single keystone indicator cannot
be identified.
Formicidae showed declines in both abundance and diversity associated with increases in
degradation
It is likely that the genus Pachycondyla is the ant identified as the ‘Malelekatou’ ant by local
land users (section 2.4.2) as in indicator of increased degradation.
The genus Cerapachys is suitable as an indicator of good rangeland condition.
The genus Brachymymrex is a possible indicator of good rangeland condition.
This research has led to the tentative proposal of three key indicator taxa representing both
extremes of degradation. However, further research is needed before they can be accepted as
robust indicators of land degradation.
6.1 Limitations
The two methods used to sample ant communities were rapid repeatable and efficient. However,
there were significant problems with the identification of ants; a microscope is essential, in
addition to a good key and some knowledge of the morphology of the family. Fortunately these
specimens were successfully identified to genus level at the Botswana National Museum, but in
order to identify indicator species identification needs to be to species level, for which an expert
within this field is needed. The lack of identifications of the beetle and spider species meant that
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it was not possible to identify indicator taxa for either of these two groups. Identification was the
major limiting factor within this research.
6.2 Further Research
This research has indicated that there is a variation in the diversity and abundance of ant
communities across a range of degraded habitats in the Kalahari. This concept needs to be
further explored. Research is needed over longer periods of time that would sample from both
the wet and the dry season. This would enable a larger sample size to be collected, and
identification to species level would be required. Research over several years would enable
variations due to climatic anomalies to be identified, as it is possible that the drought of
2002/2003 within this region may have affected the community structures. Further development
of this research would lead to a scientifically robust Formicidae keystone indicator. Further
research into beetle diversity and abundance is also likely to provide indicator species for
degradation. This would also involve long-term studies and identification to species level.
Land degradation within the Kalahari Desert is detrimental not only to the environment but also
to the livelihoods of the people dependent on the land. The development of indicators is crucial
in the battle to identify vulnerable regions and unsustainable management practices so that the
decline of Botswana’s drylands can be halted and the country can move forward into a
sustainable future.
Acknowledgements
I would like to thank Dr. Andy Dougill for his help, patience and support throughout the last year, Mark
Reed for his endless enthusiasm, motivation and assistance in transporting ants across Africa and Dr.
“Tom” Thomas for providing such entertainment and help whilst we were in Africa. I would also like to
thank Dr. Colin Pitts for his help at various times throughout the year. I would like to express my gratitude
to Elisah Namati from the Botswana National Museum who very kindly identified my ant specimens and
battle with temperamental Internet connections. Jill and Keith Thomas were amazing hosts and provided
us with brilliant food and entertainment and showed remarkable resilience to my endless demands for
honey and peanut butter, (not to mention anti-freeze!), without their help our research would not have
been so immensely enjoyable. I would also like to thank my fellow researchers from Manchester
Metropolitan University; Andy for providing us with raindrops, Candy, for just about tolerating our
renditions of raindrops and Alex for those bags of sand! And last, but by no means least, I would like to
thank Angela and Suzie for putting up with me for two months in a tent and for their constant support and
friendship ever since.
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