Ecosystems and Ecology
Ecosystems and Ecology
Author: Prof Koos Bothma
Licensed under a Creative Commons Attribution license.
ECOSYSTEM FUNCTIONING
It has already been shown in Section 7.6 how human actions can interfere with biodiversity and
ecosystem functioning. The concepts of stability and resilience have been explained in Section 7.4.
Irregular events may affect ecosystem stability while maintaining a healthy biodiversity may hold the key
to provide ecosystems with the resilience that is necessary to return to a state of equilibrium after a
disturbance that is caused by an irregular event. When this irregular event is catastrophic, however, it
may push an ecosystem over a critical ecological threshold from which it may be impossible to recover.
The total or selective deforestation of tropical rainforests by huge ancient civilizations such as the Khmer
of Cambodia (Angkor Wat) and the Maya of Central and South America are proof that crossing such a
threshold may have dire consequences for human survival especially in combination with climatic shifts.
The same fate may await the current human civilization unless humans take heed of the ecological
parameters that govern the utilization of the renewable natural resources of the world. These examples all
involved the human destruction of ecosystem functioning.
Terrestrial ecosystems
Terrestrial ecosystems are the main source of sustenance for humans although oceanic ecosystems at
least once were critical in saving humans from extinction. This happened at a time when humans were
still confined to Africa and this continent largely became unable to sustain these early hunter-gatherers
during a period when Africa became cold and dry. The remnants of the then human population survived
for thousands of years along the south coast of Africa by learning to exploit marine resources and by
utilizing the underground plant storage organs of the fynbos of the Cape Floral region.
Although the scope and nature of this module does not allow a lengthy discussion of all the facets of
ecosystem functioning, selected examples will be given next to indicate the role and effect of some key
elements.
The role of mycorrhizal fungi in soil quality
The role of mycorrhizal fungi in plant growth and therefore in ecosystem health is dealt with in
some detail in the references below. Mycorrhizae are fungi that have developed a mutual
relationship with most vascular plants and are vital in nutrient cycling. Vascular plants are plants
with vascular systems that transport sap, water and nutrients. They include the flowering plants,
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conifers, cycads, ferns, horsetails and clubmosses. The only exceptions are the sedges, and
plants of the family Salvadoraceae, to which the mustard plants belong, while in the family
Proteaceae the clumped, fine roots of the plants seem to play a similar role as the mycorrhizae.
Mycorrhizal filaments are able to penetrate cracks in the soil that are even too small for the finest
of roots. They absorb phosphorus from the soil and transfer it to the plant roots. They also
transfer essential minerals such as zinc, manganese and copper from the soil to the plant roots.
In turn, the mycorrhizae obtain carbohydrates from the plants. The large hyphal network and their
mutual relationship with the mycorrhizal fungi give plants access to much larger soil volumes than
what they are able to utilize with their root system alone. This symbiotic relationship has
developed over many millions of years and has been part of the early development of vascular
plants and their colonization of land surfaces because they are related to the blue-green algae
which were the first forms of life in the primordial oceans.
Endomycorrhizae occur in the roots of some 80 per cent of all vascular plants and they penetrate
the cortical cells where they often form minute and highly branched tree-like structures or
arbuscles. Consequently they are known as arbuscular mycorrhizae. They may also form
swellings in the cells and are then known as vesicular mycorrhizae. Most of the nutrient exchange
between plant roots and mycorrhizae occurs through the arbuscles. Mycorrhizae also promote
soil aggregation and promote plant pollination to some extent.
The inoculation of depaupered soil in degraded ecosystems with mycorrhizae is vital in
ecosystem rehabilitation through new plant succession because the mycorrhizae are one key to
ecosystem development (see Section 4). Ploughing, trampling, overgrazing and catastrophic fires
of high intensity will all destroy mycorrhizae and it requires inoculation to retain or regain full
ecosystem functionality. Mycorrhizae are an essential part of natural biodiversity and are vital to
maintain soil fertility. Yet their role is relatively little known. Moreover, they are major drivers of
nutrient cycles, especially in woodlands and savannas, and form a key component of food-webs.
They are also vital to maintain sustained soil function as a basis of ecosystem health. Mycorrhizal
inoculation techniques and soil rehabilitation are explained in detail in Coetzee, Bothma, Van
Rooyen and Breedlove (2010).
The role of small, burrowing animals in soil quality
In plant succession and productivity, and hence also in ecosystem functioning and health, small,
burrowing animals are vital to create a healthy topsoil. The use of pesticides and the
extermination of small, burrowing mammals have robbed the world in many regions of an
important natural way in which to maintain ecosystem health. This is not a new phenomenon yet
the destruction of this vital soil process is still based on total ignorance of ecological processes
and ecosystem functioning. For example, by as early as 1788, Australia had already lost some 33
per cent of its mammals, many of them small, burrowing mammals that had helped to create an
erstwhile fertile landscape. By the 1880s the once verdant and luxuriant landscape with its
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palatable grazing and browse had been replaced by a near desert of unproductive vegetation that
barely survived on a compacted and infertile soil base. These changes occurred directly as a
result of the massive overutilization by livestock and the active eradication of the small, burrowing
mammals.
In recent centuries, healthy populations of small, burrowing mammals have replaced the former
natural geomorphological soil rejuvenation processes in much of the world. Such mammals keep
soils friable and fertile and spread essential mycorrhizae in their fur and faeces. When digging
their burrows and shallow trenches they also turn over plant litter as they forage for food such as
roots, seeds, insects, tubers, invertebrates and fungi. In the process they mix organic matter into
the topsoil. The result is friable and well-structured topsoil which has a high plant production and
water infiltration capacity. The mixing of plant material into the topsoil also reduces the amount of
combustible organic material on the ground surface and this is beneficial to ecosystems because
it reduces the possibility of hot, smouldering wild fires that inter alia destroy the mycorrhizae in the
topsoil.
Friable, moist and healthy topsoil is also an ideal habitat for the host of invertebrate animals
which form 75 per cent of the living biomass of healthy topsoil. Earthworms can pass up to 250
tonnes of soil and organic material through their digestive system per year and are excellent
sources of organic fertilizer. Dung beetles in turn bury as much as 2 tonnes of wet manure on the
prairies of the USA per year in the topsoil. This increases the water infiltration rate by 129 per
cent. Other invertebrates can detoxify waste products and cleanse the soil.
Once topsoil has deteriorated and become capped, the simple removal of overgrazing by larger
herbivores will not restore ecosystem functionality. The repopulation of the soil by small burrowing
mammals can also best be done by creating optimal habitat for their natural predators, including
small carnivores and owls. Topsoil degradation takes years to redress and such degradation will
take many generations and sustained active soil rehabilitation to reverse.
Soil erosion and agricultural sustainability
This topic has been reviewed extensively by Montgomery (2007). As it mainly deals with soil
fertility under crop production regimes it will only be reviewed briefly to illustrate the long-term
negative effects of ploughing. Recognition of the detrimental effect of soil erosion on formerly
flourishing civilizations dates back many centuries as is being proved by archaeological studies.
Although many studies pointed to deforestation as the primary culprit causing the collapse of
these ancient civilizations, the production of crops also played a major role. For example, crop
production, deforestation and climate change all played a role in the decline of the ancient Maya
and Khmer civilizations (see Section 8.6).
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When evaluating the long-term ecosystem impact on soil following crop production the most
pronounced soil loss occurs on upland areas where the topsoil gradually becomes depleted,
loses productivity and starts to erode. The probable rate of soil erosion under conventional
agriculture exceeds the probable rate of natural soil production although soil erosion rates vary
vastly on a regional scale. Nevertheless, tilling the soil is a major contributor to soil loss and notilling procedures are being adopted increasingly. Soil loss rates in ploughed fields are
substantially more rapid than the gains through natural soil production. This makes simple
ecosystem rehabilitation after crop production and sustainable crop production problematic.
Water quality and quantity
Alongside food production, a major crisis that is facing the world is to provide enough water of an
acceptable quality for drinking, irrigation and ecosystem functioning. The absence of water and
the provision of poor quality water have a ripple effect that spreads through entire ecosystems.
Water of acceptable quantity and quality is an important requirement for long-term ecosystem
sustainability but is often underrated or even ignored by ecological planners.
Severe summer thunderstorms bring vital rainfall to the Kgalagadi Transfrontier Park of the southwestern Kalahari ecosystem, often leaving a late afternoon rainbow as here at Dankbaar windmill at
sunset in January 1980
According to Meyer and Casey (2010) water may contain pathogens and parasites in addition to
unacceptable inorganic and organic elements. The effect of the concentration of inorganic and
organic elements is often dependent upon that of the other constituents of water because
seemingly innocuous concentrations may have toxic consequences when they are combined with
other ones. Water of poor quality can also increase soil deterioration, for example through
increasing soil salinity, which will be detrimental to primary productivity by vegetation. Some
animals, however, prefer water that is high in specific elements, and some are more susceptible
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than others to the effects of poor water quality. Some form of moisture in their food is required for
proper digestion in many herbivores, even those that are independent of surface water. The
quantity and quality of water that is available in an ecosystem will therefore largely dictate which
animals will be present in the higher trophic levels. Water quality and quantity therefore show
cascading effects that may affect entire ecosystems because they can affect all the trophic levels
although wetland ecosystems usually filter the water source and yield water of an acceptable
quality.
River in the Kruger National Park, South Africa - June 2004
Risk assessments are required that will take all the elements of water quantity, quality and the
water-use patterns of animals into consideration before a particular water resource can be
declared fit for animal and human consumption. It will also indicate whether the primary producers
will suffer from stress that may be induced by water quality and quantity. Such risk assessments
commonly take at least 50 constituents and elements in water resources into consideration which
will give information on both the macro and trace elements. Risk assessment requires knowledge
of geohydrology, toxicology, nutrition and physiology.
Vegetation determinants in rangeland ecosystems.
The conservation risk and dynamics of the sustainable utilization of rangeland ecosystems in
southern Africa with a broader application as set out by Snyman (1998) will be used as a general
guideline first. It will be followed by a review of the restoration of grasslands and grassy
woodlands in Australia to illustrate the need for the integration of function and biodiversity as it
appears in Prober and Thiele (2005).
The conversion of rainfall into plant production at the producer trophic level is critical, especially in
the arid and semi-arid environments that occur in much of Africa. Sustainable management of this
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resource often requires the sacrifice of short-term benefits. The basic functioning of any
ecosystem depends on the continual interaction between organisms and their environment.
Therefore sustainable rangeland management depends on sustainable ecosystem management.
There are five basic sustaining principles that are involved:

natural resource conservation,

decreasing risks,

maintenance of increasing biological productivity,

economic viability, and

social acceptability.
The sustainable productivity of rangeland ecosystems is determined by the abiotic and biotic
components. Moreover, as in any management programme the expected output of animal
products should never exceed the energy input while losses, but especially soil erosion, must be
limited. The fundamental challenge is to balance the antagonistic relationship between solar
energy capture and efficient harvesting processes under varying ecological relationships.
Biodiversity and biological complexity are essential components for sustainable production in
rangeland ecosystems. Moreover, the processes that affect ecosystem stability, resilience and
equilibrium must be understood. Range condition is a vital component of ecosystem health.
Nevertheless, 66 per cent of the rangelands in South Africa were already moderately to seriously
degraded by 1998, while Africa contributed 36 per cent of the degraded rangelands of the world
then.
In rangeland ecosystems the quality and rate of energy flow is limited by important environmental
factors such as water and nitrogen. However, the soil-climate-vegetation matrix is so complex that
it becomes difficult to manage. Sustainable water management requires that special attention be
given to water penetration and storage in the soil to allow rainfall to be utilized optimally. Different
range conditions show differences in basal cover, evapo-transpiration, deep percolation of the
rainfall, percentage run-off, loss of organic carbon, loss of nitrogen, dry matter production, wateruse efficiency, grazing capacity, sediment loss and gross marginal income from grazers.
Approximately 65 per cent of the rangelands in South Africa occur in arid or semi-arid regions and
are ecologically sensitive. Droughts are common and procedures should be in place to handle
these droughts. Remote sensing through satellite images can be used to quantify the effects of
these droughts. Surface run-off varies with the nature of the rainfall, soil type, slope, plant cover
and soil conservation status. Surface water run-off rates usually show an inverse linear
relationship with the range condition and basal cover of an ecosystem. The best way in which to
counteract surface water run-off is by maintaining a healthy vegetation cover. In an arid region
such as the Nama-Karoo biome the mean surface water run-off is 4,6 per cent of the mean
annual precipitation. However, on a bare, uncultivated soil surface it can be as high as 30 per
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cent. Moreover, increases in stocking density with livestock or wildlife will decrease the rate of
water infiltration because of increased soil compaction from trampling.
In arid and semi-arid rangelands evapo-transpiration is a major component of water loss and it
can be as high as 96 per cent of the infiltrated precipitation even on rangelands that are in a good
condition. Deep percolation of rainfall below the root zone only occurs when precipitation is
extremely high. The quantity of the water which can be stored in the soil varies mainly with the silt
and clay content of the soil. In the arid and semi-arid rangelands of southern Africa, water is
mainly stored 1 - 1,2 m below the soil surface while most grasses withdraw water from 150 to 200
mm below the soil surface. However, climax grasses are able to withdraw water from deeper than
2,5 m below the soil surface during droughts.
The integrity of arid ecosystems is maintained by soil organisms that affect the distribution of
water and nutrients in time and space. Nutrients accumulate in the surface soil under shrubs and
trees in arid and semi-arid regions. The loss of such surface organic matter through the
eradication of the soil organisms creates a poorer soil structure, a decrease in the water
infiltration rate, an increase in surface sealing, a decrease in water retention ability and soil
fertility, and accelerated wind and water erosion. In rangelands in semi-arid to arid regions the
organic matter content of the soils is usually less than 2,5 per cent.
The decomposition of grass litter is rapid in warm areas with a high rainfall where 50 per cent of
this dead grass biomass will decompose within three months of deposition. In arid and semi-arid
regions the decomposition rate largely depends on the type of plant, water availability and
temperature. The release of nutrients through mineralization and their uptake by rangelands
under optimal conditions are usually in balance with soil degradation.
Water-based soil erosion is a serious ecological problem in arid and semi-arid rangelands
because enough rain falls to cause soil erosion but it is not enough to ensure a stable vegetation
cover every year. Because a grass tuft has a larger basal cover and a network of roots directly
below the soil surface than most types of shrub, tufted grasses are more effective in protecting
soil from erosion than most types of shrub with taproots. Moreover, the potential soil loss from
rangelands is approximately 45 per cent less than from cultivated lands. What is required for the
effective management of rangeland ecosystems is to balance soil degradation with the beneficial
effects of healthy range management principles. Where such rangeland ecosystems are in a
good condition, plant production is usually also good, even with seasonal rainfall. Regardless of
the amount of rain that falls, degraded rangelands are usually inefficient to convert this rainfall
into plant production.
Sustainable rangeland management is a challenge for the sustainable development of emerging
animal producers, whether they produce livestock, wildlife or both. This requires a substantial
investment into human capacity building, particularly through relevant education of the youth and
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adults. The rangeland ecosystems of Africa are an asset with sufficient biological potential for
sustainable animal production and the sustainable development of impoverished communities.
Sub-humid temperate grasslands and grassy woodlands currently occur where there once were
more productive ecosystems in south-eastern Australia. This is the result of over 200 years of
human-induced landscape degradation and alteration for livestock production. A movement is
currently under way to restore the more productive grasslands and grassy woodlands that once
occurred there. Moreover, the objective is also to restore and conserve the associated
biodiversity. The restoration process is difficult and involves the following steps:

learning what the former ecosystems were,

learning how and why they have changed,

restoring biodiversity through effective ecological processes, and

applying active adaptive land management approaches.
This process requires co-ordinated objectives within broad ecological contexts and addressing
ecological function through attention to key ecological processes. Nevertheless, some of the
ecological changes that have occurred are irreversible while others are expensive to rectify. The
only option is to drive such restoration for optimal ecological gains within these restraints as the
major objective. Much the same will be true of reversing rangeland degradation in Africa.
Pollinators
Pollination is one process of plant propagation. Ferns, cycads and coniferous trees are some of
the oldest types of plant in the world and they were probably originally pollinated by wind, flies
and beetles. With the development of the flowering plants some 120 000 years ago, a mutualism
with insects developed and in some fynbos (macchia) regions ants fulfil this role. During
pollination, pollen is transported to various plant propagation organs within the same flower or
plant and between flowers and plants. This results in seeds that germinate to propagate the
flowering plants.
Christian Konrad Sprengel was the first to describe plant pollination in the 18th century. Today it is
known that pollination can occur abiotically (98 per cent wind) and biologically (birds, insects and
some other animals). Abiotic agents are responsible for 20 per cent of all the pollination in plants,
but it especially occurs in grasslands, ferns, most of the conifers and most of the deciduous trees.
Two per cent of the abiotic pollination occurs through water.
There are more than 200 000 types of plant pollinating organism. Plants with highly coloured
flower petals or strong odours are mainly pollinated by insects, while those with nectar may be
pollinated by insects and birds. Plants with white flowers are mainly pollinated by bats while those
with reddish flowers are mainly pollinated by birds.
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Honey bees of the genus Apis are responsible for only 19 per cent of all pollination but they are
one of only a few types of pollinator that live in large swarms. Solitary types of bees pollinate
several types of tree and include the bumblebees which are also the most important pollinator for
the wild flowers of the United Kingdom. In the Philippine Islands, stingless bees of the genus
Trigona are important pollinators of plants.
The role of pollinators in the survival of plants and the production of food for an ever-increasing
human population makes the proper management of ecosystem mosaics vital because the
pollinators have widely variable habitat requirements. The blanket use of pesticides is a direct
threat because it removes a vital functional component from ecosystems.
The impact of herbivory on vegetation
Herbivores form the lower trophic level in ecosystems and the large mammal herbivores are an
essential link in the food chain that ends with the apex carnivores. Herbivores utilise the energy
that is contained in the plants. This energy was produced from solar energy by photosynthesis
mainly. As first level energy consumers these herbivores are much more numerous than the
secondary level carnivore consumers although the smaller carnivores also at times are food for
the larger carnivores and carnivores therefore occupy at least two trophic levels.
In the Kgalagadi Transfrontier Park of the south-western Kalahari ecosystem the springbok Antidorcas
masupialis, as here at Grootkolk in the Nossob riverbed in January 1984, is the only true gazelle of
South Africa. It favours the fossil riverbed with its sweet grasses and three-thorn Rhigozum
trichotomum bushes on the banks when foraging because it is a mixed feeder
According to Pringle, Young, Rubenstein and McCauley (2007) and Larson and Paine (2007),
African savanna ecosystems experience interaction cascades that are initiated by herbivores and
are modulated by productivity. Based on ungulate-exclusion plots, the effects that large ungulates
have on various taxa including plants, arthropods and an insectivorous lizard were studied at the
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Mpala Research Centre in the Laikipia District of central Kenya, an area which receives from 450
to 550 mm of rain per year. The larger mammals included ungulates as large as the African
elephant Loxodonta africana, large carnivores such as lions, leopards, cheetahs, spotted hyaenas
and striped hyaenas Hyaena hyaena. The herbivore-exclusion plot spanned a landscape-scale
gradient in productivity in a typical African savanna.
The direct and indirect effects of ungulate exclusion showed that herbivory had a strong effect on
tree density and a weaker but still significant effect on the herbaceous cover. However, there was
no significant effect on arthropod abundance although the abundance of some of the arthropods
did increase significantly after herbivory by large ungulates was excluded, but this was not a
general trend. Arthropod abundance could be predicted strongly on the basis of herbaceous
cover but tree density was not indicative of arthropod abundance. Most of the variation in the
coleopterans was explained by changes in the herbaceous cover. Lizard density was strongly
predicted by tree abundance and that of their arthropod food source and was a mean of 61 per
cent (range: 24 to 214 per cent) greater where large ungulates were absent. The response of
lizards was independently proportional to the responses of both their arboreal micro-habitat and
their arthropod food source. The densities of large herbivores such as zebras Equus burchellii
and Equus grevi, impala Aepyceros melampus, Grant’s gazelle Gazella granti, eland Taurotragus
oryx, African elephant Loxodonta africana, giraffe Giraffa camelopardalis, hartebeest Alcelaphus
buselaphus, African savanna buffalo Syncerus caffer and cattle Bos indicus consistently
depressed the density and primary productivity of trees, insectivorous lizards and the dominant
order of arthropods (Coleoptera: beetles). It was inferred that herbivory by large ungulates
regulated lizard abundance indirectly by suppressing the density of the lizard micro-habitats
(trees). A similar suppression of beetle density was based on food availability. Therefore large
herbivores not only strongly interact with the ecosystem features but they also do so in increasing
intensity at lower levels of ecosystem productivity. In lizards the absence of large ungulates at
least creates greater prey availability (more beetles). More productive plant communities will
absorb the impact of herbivory better and buffer the effect of herbivory on the remainder of the
plant community. In rangeland ecosystems there generally is a well-established relationship
between primary productivity and herbivory rates. This concept forms the basis of modern
stocking density calculations for wildlife. Moreover, some trees such as the dominant tree Acacia
depranolobium in the highest areas of productivity is defended against herbivory by symbiotic
ants and consequently suffer lower rates of browsing pressure by elephants than, for example,
Acacia brevispica and the black thorn.
Complex interactions in ecosystems through herbivory make predictions of any effects
problematic especially because indirect effects are highly sensitive to environmental variations.
This may require that the effects of the possible removal of large mammals in savanna
ecosystems be examined on at least the landscape scale. Nonetheless, the population decline in
many African ungulate species has cascading effects in ecosystems that are at least comparable
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to those that have been found following the loss of apex carnivores. Consequently the presence
of large-bodied herbivores such as the African elephant, eland and African savanna buffalo may
well be as critical to ecosystem functioning as that of apex carnivores.
It is also believed that ecosystems with a low primary productivity will be more susceptible to
human-induced modifications than more productive ones. One of the anthropogenic problems is
the introduction of exotic herbivores into ecosystems where they never occurred before. This has
often been done, as Castley Boshoff and Taylor (2001) point out, for purely economic reasons
with a total lack of knowledge of the ecological sustainability of such introductions.
Another threat that was reviewed well by Evans (1998) is that of overgrazing the natural
ecosystems. Of the world’s land surface, 80 per cent is arid or semi-arid and in some 35,4 per
cent of the world’s grazing land degradation is attributed to grazing animals. However, even in
more temperate regions there can be marked erosional impacts that are being caused by grazing
animals. The problem here is one of overutilization whether by indigenous, wild grazers or by
domesticated ones. Variable climates can exacerbate this impact. One major problem is that any
excess of rainfall will follow animal footpaths and eventually change them into deep eroded
gullies. Another is the trampling of stream banks which destabilizes them. When animals are
being herded daily, the chances of the creation of bare soil and gullies around homesteads are
great. The recovery may be slow or will not happen at all because natural rangelands have
variable grazing intensity thresholds which will initiate detrimental vegetation changes when they
are exceeded. Some of these changes are irreversible.
Erosion that is being initiated and exacerbated by grazing animals occurs widely in the
rangelands of the world. In African savannas the indefinable and immeasurable concept of
carrying capacity has now been replaced by stocking densities of grazers (and browsers) that are
in balance with the quantity and quality of the vegetation that occurs in each plant community. In
many areas, however, the simple removal of herbivores is no longer a management option and
the complete rehabilitation of even whole ecosystems may be the only option that remains. Costly
mistakes can never be rectified with cheap remedies.
Exotic or invasive species
This review is based mainly on Sanders et al. (2003) and Christian (2001). Invasive species have
been introduced world-wide by humans. Some of them either disappear soon after entering an
ecosystem or they are so successful that they replace indigenous species to the detriment of
ecosystem structure and functioning. This problem is now exacerbated by the flourishing plant
and animal trade but it started centuries ago when travellers and explorers first visited strange
and remote lands. By the 1970s the trade in exotic animals alone numbered in the millions per
year. However, not only are indigenous species being replaced or pathogens introduced to new
areas deliberately or by accident, these exotic species also cause the disassembly of existing
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ecosystems and communities. One such effect is the disruption of important seed dispersal
mechanisms.
Among others, the Indian mynah Acridotheres tristis and house sparrow Passer domesticus have
entered Africa from India and Eurasia, the cattle egret Bubulcus ibis has spread from tropical
Africa and south-eastern Asia throughout Africa into Europe and elsewhere, domesticated pigs
Sus scrofa have escaped and become feral in many regions, and the water buffalo Bubalus
bubalis, red fox Vulpes vulpes and European rabbit Oryctolagus cuniculus have become some of
the greatest pests in Australia. Mice that were introduced by whalers almost destroyed several
types of indigenous bird on Marion Island. Rats of various types such as the Norway or brown rat
Rattus norvegicus have also escaped from ships to colonize many islands of the world. However,
only the impact of the introduced Argentine ant Linepithema humile will be discussed below as an
example.
Worldwide the Argentine ant is decimating the indigenous ant fauna. In the fynbos (macchia) of
the Cape Floral Kingdom of South Africa, one of the world’s hotspots for plant endemism, up to
30 per cent of the plants have seeds which are dispersed by indigenous ants with large jaws
which bury the seeds below ground and safe from rodent predation and fire. Argentine ants do
not disperse plant seeds but they often displace the indigenous ants that do so. While some
fynbos ants do live side by side with the Argentine ant, the presence of the latter reduces the
overall seed dispersal rate. Moreover, the two most abundant seed-dispersing types of
indigenous ant that are being displaced in the fynbos by the Argentine ant are the pugnacious ant
Anoplolepis custodicus and the brown house ant Pheidole megacephala. They are also the most
effective seed-dispersal agents for the fynbos. Fynbos plants with larger seeds are especially
vulnerable to a reduction in their populations because the other seed-dispersing ants are not able
to disperse large fynbos seeds. Large seeds that are not being dispersed by ants are more likely
to suffer predation by rodents. If recruitment by large-seed fynbos plants is limited by seed
survival this will become a serious biodiversity and ecosystem functioning problem. Fynbos was
among others critical in the survival of humans in Africa some 70 000 years ago before they
entered Eurasia (see Marean 2010 below) and support a host of indigenous vertebrates.
Experiments on the effect of the Argentine ant on the fynbos community structure showed that
post-fire recruitment of large-seeded members of the Proteaceae was reduced disproportionately
in fynbos sites that had been invaded by Argentine ants. This was due to the loss of refuges in
which the larger seeds could escape rodent predation and consequently to the absence of the
two major indigenous ant dispersers of large fynbos seeds. The ultimate effect is that a change in
fynbos community composition is occurring in areas that have been invaded by Argentine ants.
The ripple effect will be transmitted to the other organisms that depend on the large fynbos plants
for cover, shelter, food or nesting sites.
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One benefit is that these ants could also deter detrimental insects from attacking the large fynbos
plants. However, the negative effects of the invasion by Argentine ants on the survival of large
types of fynbos are greater than the positive ones. It is presently not known whether larger fynbos
types will be able to develop alternative seed dispersal mechanisms, while plant species that rely
on mutualist partners are at a greater risk of decline than others. Moreover, several other types of
ant that are currently spreading around the world could damage both agricultural and natural
ecosystems. This is already happening in some regions, causing changes in plant community
structure, and is leading to rapid natural community disassembly in many parts of the world. It is
clear that invasive or exotic species that are successful pose serious threats to ecosystem
functioning.
In terms of veterinary aspects the threat of the spread of invasive pathogens is real. The rapid
spread of the rinderpest epidemic within a few years from the northern to the southern tip of Africa
in the late 1890s is an excellent example. This epidemic changed the structure and composition
of many African ecosystems. For example, a period of increased rainfall that followed the mass
extinction of wildlife and livestock from rinderpest in the late 1890s has fairly recently allowed the
establishment of the Serengeti ecosystem. This ecosystem is now being maintained as grassland
by the combined effect of elephant browsing and fire on woody plant seedlings. The illegal global
trade in wildlife and wildlife products is also now causing serious concern because it has been
shown that it could act as a conduit for the spread of numerous pathogens that could impact on
human and animal health.
The larger terrestrial animals
Larger terrestrial animals are the consumers of the higher trophic levels in a multitude of
ecosystems. Some of them, such as some reptiles, have been part of ecosystem functioning for
millions of years. Others, such as the multitude of dinosaurs, have helped to shape earlier
ecosystems but these animals and their ecosystems have become extinct. The larger animals
that impact on ecosystem functioning today are mainly large mammals such as large wild
herbivores, but many of them have also already disappeared. Migrating humans were responsible
for the loss of major assemblages of the megafauna of the world, and in many cases this
triggered severe reactions in the existing ecosystems and created new ecosystems.
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The Cape eland Taurotragus oryx, as shown here in its typical habitat at Gnurrie windmill in the
interior area in the Kgalagadi Transfrontier Park of the south-western Kalahari ecosystem in
September 1994, is the largest antelope in southern Africa. It ranges widely for better grazing
The role of the larger mammals in African ecosystems
The role of the larger mammals in process dynamics in African ecosystems as discussed below
has been examined in some detail by McNaughton, Ruess and Seagle (1988) and this paper
forms the basis of the review below. One major conclusion was that the activities of large African
mammals do affect primary productivity and regulate recycling balances. Large mammals are
now known to have powerful and complex interactions with their habitats and the ecosystems in
which they occur. It now appears that they can also control ecosystem processes such as energy
flow and nutrient cycling. The components of African ecosystems are therefore highly interactive
and large mammals form integral parts of the processes that are involved in their dynamics.
The Serengeti Ecosystem of East Africa has been studied in great detail and its dense large
mammal component is remarkable. Nutritionally there is a complete diversity of herbivores and
the apex carnivores that prey on them. Nevertheless, there is substantial partitioning of food
resources. The smaller herbivores such as Thomson’s gazelle Gazella thomsoni generally feed
on relatively rare high-quality plant material while the larger ones such as wildebeest, giraffe,
African savanna buffalo, Burchell’s zebra, eland and African elephants feed on the more
abundant low-quality plant material. Elephants and fire create a delicate balance between the
woody component and the grass sward. Moreover, the elephants push over large trees for the
decomposers. Browsers, but especially African elephants, prevent the growth of woody plant
saplings which are also killed by the regular fires. The combination of elephants, browsers and
frequent fires can convert closed woodlands into open grasslands within a few decades. Few
people know that the Serengeti Plains were in fact closed woodlands before elephants first
appeared there in 1951. Grazing herbivores now influence the species composition and growth
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form of the grasses and the Serengeti grasslands have become dwarfed and low-growing with the
grasses investing heavily into leaf tissue that grows rapidly.
Rainfall, mineralization and large mammals are the primary drivers for primary productivity.
Rainfall and mineralization have a major effect below the soil surface while large mammals have
one on the primary productivity above the ground. Moreover, herbivores will interact with
vegetation to regulate the primary production rates.
In African savannas, animal distributions are also related to broad ecological gradients that are
determined by the availability of nutrients. The soils in regions receiving more than 700 mm of
rain per year support dystrophic savannas that are low in oxygen because they contain high
levels of low-quality organic material. Moreover, primary production on those soils is more often
limited by nutrients than water and forage quality decreases with increasing rainfall. Such regions
are dominated by large-bodied herbivores such as elephants, and smaller but highly selective
browsers such as steenbok Raphicerus campestris, various duikers, dik-diks Madoqua spp, and
klipspringers Oreotragus oreotragus In contrast, the drier savannas contain eutrophic soils which
are rich in nutrients and support a dense plant cover. The rainfall has minimized the effect of
weathering and leaching and these soils tend to support the grazers of medium size that are
typical of much of Africa.
Decomposition rates vary accordingly and there is a negative relationship between grazing and
the turnover time of litter and standing dead plants above the ground. Where the major ungulate
herds concentrate during the wet season the turnover time of litter in the grasslands is much less
than a year. However, taller grasslands that are only grazed lightly during the wet season contain
enough litter to subject them to regular fires and have turnover times of up to four years.
Consequently, there are three types of nutrient recycling in African savannas: fast cycles, slow
cycles and pulsed cycles.
Fast recycling of nutrients occurs in landscapes where herbivory is intense throughout the
growing season and where the plant tissues are largely prevented form deteriorating in quality
with age. Trampling by the hooves of herbivores prevents the accumulation of litter and nutritional
elements cycle rapidly between small, highly dynamic pools of soil, plant material and animal
wastes.
Slow recycling of nutrients occurs in landscapes where herbivory and fire are prevalent. The plant
tissues accumulate above the ground in resistant materials such as stems and also below the
ground as nutrient molecules. These nutrient pools are considerably larger than what is found
with the rapid recycling of nutrients.
Pulsed recycling of nutrients occurs in landscapes which are periodically subjected to fire or
intense herbivory but largely not during the growing season. The nutritional elements accumulate
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in plant tissue during the growing season and are trampled into the soil as ash or animal wastes.
High-quality forage is recycled through animal wastes, and poor-quality forage as trampled ash.
The nutrients accumulate alternatively in plant and soil pools.
Soil conditions, spatial variability, rainfall variation and the potential impacts of large mammals
and fires on the cycling of nutrients form a complex system of interacting processes with explicit
effects on the dynamics of ecosystems. In a simulation model of the process of nutrient recycling,
grazing stimulated the nitrogen uptake considerably while microbial turnover during the growing
season was also greater following grazing. The latter implies that microbes respond immediately
to dung and urine deposits in the soil. The accumulation of litter and standing dead plant material
retarded the recycling of nutrients and in the absence of large mammals or fire they strongly
affect microbial decomposition and other nutrient recycling processes.
Large mammals and plants in grazing ecosystems are linked by limitations of food quantity
through limited primary productivity and on food quality by nutritional deficiencies. Mammals are
distributed in response to the distribution of food quantity in space and time and to the nutritional
quality of grasslands. However, the major larger grazers in the Serengeti also affect ecosystem
processes by influencing the recycling rates and pathways of nutrients. The interaction between
large grazers and vegetation processes and structures is therefore active and not passive. Fire
competes to some extent with the large mammals. The absence of large mammals would create
decidedly different ecosystems largely through their effect on nutrient cycling.
The role of larger herbivores in African ecosystems
While the role of some larger herbivores in the dynamics and processes of African ecosystems
have been discussed to some degree in sections 8.1.7 and 8.1.9.1, their specific foraging and
ecological hierarchies will be further reviewed here briefly largely based on Senft, Coughenour,
Bailey, Rittenhouse, Sola and Swift (1987).
While large carnivores seek scattered prey of high nutritional quality, large herbivores commonly
forage on widely dispersed food of lower quality. The food resources of large generalist
herbivores such as African savanna buffalo and African elephant occur with a patchy distribution
and different foraging responses are displayed at different landscape scales. The feeding
preferences of large generalist herbivores are non-linearly related to forage quality and quantity.
Plant palatability may be modified by the physical characteristics of plants, the presence of
secondary chemical compounds and by prior feeding experiences. The selection of feeding areas
may further be modified by factors such as topography, the proximity of water or mineral licks and
possibly escape from insect harassment.
On a regional scale, foraging tactics include migration, nomadism and the use of a limited or
specific range. In the Serengeti ecosystem, nomadic large-herbivore herds such as those of the
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wildebeest and Burchell’s zebra follow spatially distributed rainfall production pulses
opportunistically. The ranges of most large herbivores are usually centred on the best foraging
habitat and water resources.
Foraging mechanisms involve two problems at the plant community scale: which plants or plant
parts to select and which location to select for foraging. Diet selection must ensure that food of
maximal quality and adequate quantity be selected within the restraints of a particular digestive
system.
Herbivory by large ungulates in the Laikipia District of central Kenya also initiates specific
cascades. According to Pringle, Young, Rubenstein and McCauley (2007) the large ungulates
have a strong effect on tree density and a weaker one on herbaceous cover and the density of
the dominant arthropods. Insectivorous lizard density seems to be greater in the absence of
herbivores. It is postulated that large herbivory indirectly regulates the abundance of insectivorous
lizards by suppressing tree and consequently beetle density.
These studies have confirmed the importance of the interaction of large herbivores with savanna
ecosystems. Such complex interactions make it difficult to predict the effect of ecological
disturbances in savanna ecosystems. Moreover, cascades that were initiated by large ungulates
may have been important in the development of savanna ecosystems. Large herbivores also
maintain cooperation within a widespread symbiosis of organisms.
The role of carnivores
Carbone, Teucher and Rowecliffe (2007) have proposed that the mammalian carnivores fall into
two broad dietary groups with those weighing less that 20 kg usually feeding on small
invertebrates and vertebrates, while those weighing 20 kg or more usually prey on larger
vertebrates, including the smaller carnivores, and form the trophically apex animals. These diets
are governed by the energy gains and energy expenditure of each type of carnivore and prey.
This balance sets an upper limit of 1100 kg to carnivore size which is equivalent to the largest
extinct species of carnivore, the short-faced bear Arctodus simus which is estimated to have
weighed from 800 to 1000 kg, and the extent polar bear Ursus maritimes in which the largest
known individual weighed 1002 kg.
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The leopard Panthera pardus, as shown here at Gnurrie windmill in the interior area in the Kgalagadi
Transfrontier Park of the south-western Kalahari ecosystem in January 1995, is the second largest
type of cat in Africa and this ecosystem where it has the largest ranges of all leopards in the world
because of a low prey density
Hunting costs and the resulting energy requirements increase with prey size. Carnivores that
weigh more than 45 kg are too large to sustain themselves on small prey only, although most
carnivores are opportunistic hunters that will kill any available prey. Prey size becomes of
especial importance to the canids that usually are more sociable while the felids mostly hunt
alone. The high costs that are involved in transporting a large body moreover may limit the ability
of the largest carnivores to counter prey shortages by increasing their hunting effort. Therefore,
slight environmental perturbations which may create lower prey availability could upset delicate
carnivore-prey balances in ecosystems.
Diseases and parasites
Epidemiology is the branch of human or animal medicine that is concerned with the incidence and
distribution of diseases. In natural ecosystems an outbreak of a disease is often the symptom of
an ecological disturbance. Under normal conditions there is an epidemiological triangle that
consists of a host, a vector and a causal agent. When natural ecological balances are disturbed,
certain diseases may become prevalent.
It is not the scope of this module to discuss the causative factors of animal diseases in depth as
this will be done in separate modules. Diseases that are transmitted between animals or between
animals and humans constrain land-use options in sub-Saharan Africa. For domesticated
livestock, mainly the African savanna buffalo (Syncerus caffer caffer), warthog (Phacochoerus
africanus), bushpig (Potamochoerus larvatus), greater kudu (Tragelaphus strepsiceros),
bushbuck (Tragelaphus scriptus) and various subspecies of the wildebeest (Connecheates
taurinus) are important reservoirs of livestock diseases. These wild herbivores have all been
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linked to serious and deadly livestock diseases such as rinderpest, foot-and-mouth disease,
African swine fever, malignant catarrhal fever, East Coast fever, bovine petechial fever,
trypanosomosis and bovine tuberculosis (See the module on High Impact Diseases).
African ecosystems have developed over millennia as did natural endemic stability between their
wildlife and pathogenic micro-organisms and macro-parasites. Currently the international trade
and elicit movement of invasive organisms are creating new disturbances and epidemics. The
global illegal trade in wildlife products, of which the USA is the current largest importer, has
introduced countless viruses into the world from Africa. These include Simian foamy virus and
herpes viruses such as cytomegalovirus and lymphocryptovirus.
Rinderpest which swept through Africa in the late 1800s is a classic example of the scope of the
social, political and economic impact of an introduced disease. This disease probably first came
to Africa from India when a German military expedition brought infected cattle from Aden and
Bombay into Eritrea. It spread rapidly through Africa, killing countless wild and domesticated
ungulates and causing enormous human hardships. When the tsetse fly Glossina spp. was
eradicated in many regions, it in turn affected ecosystems through biogeographic adaptations in
some hosts. In the post-colonial era, rabies entered Africa with severe impacts on some sociable
carnivores, notably the Ethiopian wolf (Canis simensis) and the African wild dog (Lycaon pictus).
The major current diseases of free-ranging wildlife in Africa include anthrax, bovine tuberculosis,
brucellosis, foot-and-mouth disease, bovine malignant catarrhal fever, African swine fever, African
horse-sickness, heartwater, rabies, botulism, ehrlichiosis, distemper, viral diseases of cats,
corridor disease, cytauxzoönosis, redwater, nagana and besnoitiosis. (See the module on High
Impact Diseases). Other diseases and disease conditions have nutritional causes and include the
excess or deficiency of minerals, toxic plants, mineral toxins and organic ones.
The major ecto- and endoparasites affect livestock, wildlife and humans in various ways. (See the
module on Ticks and Helminths of Wildlife). Arthropod-borne diseases can cause serious
illnesses and death, while flies, midges, mosquitoes, lice and fleas can also cause serious
problems. The major problematic endoparasites are flukes, tapeworms, roundworms and
tongueworms. Animals and humans that are in a poor physical condition, often through
ecosystem degradation, are more susceptible to diseases and disease conditions than those that
are in a good to excellent physical condition.
Marine ecosystems
Although it is accepted that marine ecosystems cover around two-thirds of the surface of the Earth, the
general consensus is that they only yield about a third of the productivity of the Earth. In part this is due to
the occurrence of deep sea trenches where solar energy seldom penetrates and there is a de facto
marine desert. Another problem is the technical ability to measure productivity in marine ecosystems. As
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it is equally difficult to measure the primary productivity of underground organic material, this discrepancy
may even be greater than what has been thought to date. A general estimate is that the net primary
productivity of terrestrial ecosystems is in the order of 110 to 120 x 10 9 tonnes of dry weight organic
material per year as opposed to 50 to 60 x 10 9 tonnes in the marine ones. It is believed that the
underground primary productivity of terrestrial ecosystems may equal or exceed that above the ground. In
marine ecosystems primary productivity is often limited by nutrient deficiencies with high levels of
productivity occurring where there is upwelling of nutrient-rich waters from oceanic currents, even at high
latitudes, because solar energy is not an on-site factor. Water is of course not a problem in marine
ecosystems.
Territorial dispute, seal bulls, Kleinzee, Namaqualand, South Africa - November 1983
In marine ecosystems, the organic matter is produced through photosynthesis in large plants and
attached algae in shallow waters and by microscopic plankton in open water. However, dead organic
material forms a substantial energetic resource. In the open ocean the input of organic material from
terrestrial communities is negligible, while deep oceans preclude photosynthesis from solar energy. The
greatest limitation to primary productivity in marine ecosystems is the availability of solar energy, nutrients
and the effect of “grazing”.
Marine ecosystems may locally have high levels of nutrient inputs from two sources. The first source is a
constant and continual input of nutrients into coastal shelf regions from estuaries. The second is the local
upwelling of high nutrient concentrations along continental shelves where the wind blows consistently
parallel to or nearly so to the coast. This wind action moves water offshore to be replaced by cooler water
that originates from the ocean bottom and is rich in nutrients. Nutrient-rich water sets off a phytoplankton
bloom and a chain of heterotrophic organisms take advantage of the food abundance. Consequently, the
best marine fish resources are found in water that is rich in nutrients. In some marine ecosystems there
may be two peaks in biomass production per year but the reason is still uncertain. Trophic levels in
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marine ecosystems follow much the same pattern as in terrestrial ones, with herbivores forming the bulk
of the producers and primary and secondary carnivores preying on them.
Herbivores
There are large variations in the intensity and scope of herbivory in marine ecosystems but little is
known about it. Controlled herbivory has developed in marine ecosystems which have been
present on the Earth for many millions of years longer than terrestrial ones. Nevertheless,
imbalances may still occur with devastating effects particularly because humans largely still
regard the oceans as pristine sources of energy but are already overutilizing most of them. The
example of the control of chemically rich seaweeds which poison coral reefs will be discussed
briefly next as one example of the interactions in marine ecosystems.
One of the current features of marine ecosystems is the global decline in the health of coral reefs.
Seaweeds commonly replace coral reefs that decline. However, it has not been clear if the
seaweeds cause the coral reefs to decline or merely replace them opportunistically once they
have declined. In a recent study in the tropical Caribbean Sea near Panama it was now found that
up to 70 per cent of the common seaweeds will bleach and kill corals upon contact. Therefore,
where marine herbivores become depleted and no longer control seaweed populations, the
allelopathic interaction between seaweeds and corals will kill the corals through lipid-soluble
metabolites that are transferred from the seaweed to the corals upon direct contact. In marine
reserves, where coral reefs are protected form fishing, the coral reefs are either left intact or are
affected at a reduced rate. Coral reefs with intact food-webs are therefore less prone to damage
by seaweeds than damaged ones.
In Panama, five of seven types of seaweed present caused bleaching in Porites porites corals
and three of eight did so in Porites cylindrica corals. However, not all the types of seaweed that
occur commonly following the removal of herbivorous fishes damaged the coral reefs rapidly.
Moreover, the suppression of a single type of herbivorous fish could elevate the risk of damage.
Chemical interactions between corals and seaweed may limit the recovery of damaged coral
reefs even when the original cause of the decline has been removed, making such ecosystem
recovery difficult. Such information may help in managing coral reef resilience to improve the
health of tropical marine ecosystems.
Carnivores
As in terrestrial ecosystems, apex carnivores play a vital role in maintaining marine ecosystem
health. Chronic overfishing of the apex carnivorous fishes has been shown to have severe
impacts on marine ecosystems because they release mesopredator-prey populations from
predator control. Moreover, it leads to the onset of trophic cascade effects on the indirect trophic
levels. Fishing has already reduced the apex carnivores of many of the world’s marine
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ecosystems considerably and yet there is still wide disagreement about the ecological
consequences. Overfishing of large sharks has intensified in recent years. There now is evidence
for a consequent marine ecosystem transformation with a cascading effect through the lower
trophic levels and a proliferation of these elements. This cascade could potentially extend to the
seagrass habitats and put the coastal benthic ecosystems that are already under severe pressure
under increased pressure. The elimination of functional components such as the apex predatory
shark therefore carries the risk of broader marine ecosystem degradation. This has important
implications for management which aims at producing sustainable and healthy marine
ecosystems.Ecosystem and habitat fragmentation
The impact of land fragmentation on ecosystem management has already been discussed in some detail
in Section 8.3. Here, some additional specific examples are given. It is intuitive to believe that pollution
and fragmentation of ecosystems can threaten biodiversity and the very thread of life on the Earth. Yet,
one aspect that is often overlooked is the impact of highways on the natural flow of genetic material
among populations, communities and ecosystems. This may be so because there are few clearly
documented cases.
In a recent study on genetic diversity in the desert bighorn sheep Ovis canadensis nelsoni in the central
Mojave, southern Mojave and Sonoran Desert regions of the USA, however, there was a direct link
between the rapid reduction of genetic diversity following the building of man-made barriers after only 40
years. Wide interstate highways, irrigation canals and developed suburban areas were found to have
virtually eliminated normal gene flow. This is now posing a severe threat to the persistence of biodiversity
and natural population survival in the desert bighorn sheep.
It has been estimated that nuclear genetic diversity in animal populations that have been completely
isolated by man-made barriers has declined by up to 15 per cent in circa 40 years. The current rate of
loss would lead to the loss of 40 per cent of pre-barrier genetic diversity in the next 60 years. One of the
causative factors was a suppression of migration. Consequently, man-made barriers may greatly reduce
the stability of ecosystems and the maintenance of a matrix of habitats that will allow gene flow between
meta-population members becomes vital. In highways, disconnectivity can be mitigated by building
suitable passages underneath the roads provided that the passages are wide enough to allow the natural
movements of the taxa that are involved. Changes in fencing design may also allow access to migration
passages. This problem requires species-specific solutions, however, as generalizations will be invalid
because the ecological requirements of the organisms differ and the solutions are subject to differences
in scale. For many species, improving the quality of habitat or ecosystem matrix connectivity may lead to
higher conservation returns than the manipulation of the size and configuration of remnant patches of
habitat.
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Transfrontier national parks
Many ecosystems have become dysfunctional due to fragmentation. A general rule is that the more arid
an ecosystem is the larger size it requires to remain fully functional because its primary productivity is
directly related to rainfall. For example, an apex predator such as an adult male leopard can survive in
small exclusive ranges as small as 10 km2 along the edge of the prey-rich tropical forests of the Royal
Chitwan National Park in southern Nepal where primary productivity and prey density are high while in the
arid and prey-poor south-western Kalahari with its low primary productivity an adult male leopard requires
a mean range size of 1322 km2 .The effective minimum population size for various animals vary with
their size and genetic heterogeneity. Taking into consideration that adult male leopards share their ranges
with up to four adult females and that there is a mean overlap of up to 48 per cent between adult males
and 30 per cent between adult females in the south-western Kalahari, the former Kalahari Gemsbok
National Park of South Africa (9591 km2) would only be able to accommodate 35 adult leopards which
would in all probability not have been viable. However, the creation of the larger Kgalagadi Transfrontier
Park in a joint venture with Botswana has created a protected ecosystem of 37 991 km 2 which is able to
support at least 136 adult leopards. Such a population will be more conducive to the maintenance of
ecosystem functioning.
Similar principles will apply to other wildlife but the effective size of any
transfrontier park will depend on the type of wildlife that it contains and its primary productivity as
influenced by the climate. African elephants in particular require large transfrontier parks to survive
without becoming destructive to their own habitat. In some case transfrontier parks are not large
contiguous areas but are large blocks of conservation land that are linked by suitable habitat to allow the
movement of wildlife between them. The concept of transfrontier parks will be discussed in more detail in
a separate module.
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