APPENDIX 6:
Modelling the dispersal of alien invading
plant species in river floodplains
Conservation Planning Framework for the GAENP – Final Report
Introduction
River floodplains and riparian zones are very dynamic environments. The main
driving force is the flow of water, which varies continually on time scales from diurnal
to inter-annual, with floods bringing large-scale changes in both the geomorphology
of the river bed and the associated vegetation (Rowntree 1991, Barrat-Segretain
1996). River floodplains and watercourses are also heavily disturbed by human
activities, particularly the bulldozing of river courses to ‘straighten’ them and land
cultivation right to the edge of the riverbank. This turnover creates ideal environments
for colonisation by alien plant species (Pysek and Prach 1993, Stohlgren et al. 1998,
Manchester and Bullock 2000).
South African river systems are no exceptions to this rule and there are several
species which characteristically invade these environments: Acacia mearnsii (Black
wattle), Acacia longifolia (Long-leaf wattle), Sesbania punicea (Red sesbania) and, in
some areas, Eucalyptus species (Bruwer 1983, Hoffman and Moran 1998, Versfeld
et al. 1998, Henderson 2001). Unfortunately, there is very little information on the
rates of spread of plant invaders in riparian zones worldwide and only occasional
observations and anecdotal information for South Africa. This information is
summarised in the next section, which is used as a basis for the proposed
methodology.
Literature review
Observations in South Africa show that, except for a few species such as Prosopis
glandulosa (Mesquite), Tamarix ramosissima ((Tamarisk) and Nicotiniana glauca
(Wild tobacco), the formation of medium to dense riparian invasions is generally
confined to perennial rivers (Versfeld et al. 1998). The dominant dispersal direction is
downstream but these species can also disperse upstream if they are wind or
vertebrate dispersed (see for example Slingsby 1978).
Long-range dispersal in flooding rivers is probably the primary dispersal mode of
Sesbania (Hoffman and Moran 1988). A survey of 532 km of the major rivers
(Breërivier to Olifants River) in the Western Cape in (Bruwer 1983) found that 31 % of
the river courses had dense, 25 % medium, 8 % light invasions and the remainder
had isolated plants or was uninvaded. The high proportion of dense invasions
illustrates the invasive potential of Sesbania in this short period. Phillips (1928) found
that Acacia melanoxylon dispersed 32 km along a stream within 13 years of its
establishment in a plantation upstream in the southern Cape. Notes on a map of the
Witels River (Slingsby 1978) show that the Acacia (longifolia) spread up the Witels
River at about 420 m per year. An analysis of the maps of the distribution of Prosopis
invasions on a karoo farm (Harding and Bate 1991) found that the rate of expansion
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Conservation Planning Framework for the GAENP – Final Report
in area (r) was about 0.180. This estimate included both dispersal of pods by water
and of the seeds on land by cattle, but still gives an indication of the order of
magnitude of the annual rate of spread.
Fraxinus ornus was spread down river, mainly by via autumn floods, up to 61 km
from the source, much faster than it was spread by wind upstream, about 2 km in the
63 years since it had been introduced (Thebaud and Debussche 1991). Range
expansion of the annual plant Impatiens glandulifera in Great Britain occurred at
rates of about 1.9 to 5 km per year from 1920 to 1960 (Usher 1986) and in some
situations, up to 13-38 km per year (Perrins et al. 1993). These rates were increased
by the establishment of multiple colonies and very long range expansions, probably
due to human-aided dispersal, and so are indicative only. Lonsdale (1993) reports
values for the exponent of the rate of areal expansion (r) of 0.64-0.68 for Mimosa
pigra in riparian and wetland situations in Northern Australia. Dispersal rates are
linked to river flow rates and are particularly rapid during high flows and floods
(Pysek and Prach 1993, Barrat-Segretain 1996) which may result in long range
dispersal (Wadsworth et al. 2000).
Wadsworth et al. (2000) modelled dispersal of two herbaceous species using the
MIGRATE model. They used the parameters below (Table 1) which were based on
data from previous studies of these species and the values needed to match spread
rates observed in those studies.
Table 1. Parameter values used for modelling the spread of two herbaceous
weed species in the United Kingdom (Wadsworth et al. 2000). Both invade
riparian habitats as well as adjacent areas. The half distance is calculated from
the negative exponential function used to estimate dispersal.
Parameter
Impatiens
glandulifera
Half-distance of dispersal along 3
river (km)
Furthest dispersal along river (km) 20
Probability of long dispersal
0.0007
Heracleum
mantegezzanium
1
10
0.0007
Modelled dispersal rates were particularly sensitive to the value for the half distance
on long dispersal and less sensitive to the probability of long-range dispersal which
only involves a small fraction of the seeds.
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Conservation Planning Framework for the GAENP – Final Report
The findings of this brief review have been summarised in Table 2. The data are
rather sparse but they do show that the estimates of the annual increase (r) range
from about 0.07 to 0.7 if the very high values recorded for the herbaceous Impatiens
are excluded.
Table 2. Estimates of rates of spread of different species in riparian habitats or
for species where water is an important dispersal agent. Values in italics were
estimated using the other data in the table.
Species
Distance Time
(km)
(yrs)
Acacia
longifolia
Prosopis
species
Acacia
melanoxylo
n
Fraxinus
ornus
Impatiens
glandulifera
Impatiens
glandulifera
1.26
3
Rate (r) Juvenile Source
period
(yrs)
0.125
3
Slingsby 1978 (upstream)
4.4
10
0.18
3
Harding and Bate 1991
32
13
0.338
3-5
Phillips 1928
61
63
0.0695
10?
1.9 - 5
1
±0.9
annual
Thebaud and Debussche
1991
Usher 1986
13 - 38
1
>1.3
annual
Perrins et al. 1993
Data on the rates of increase in density are lacking. Obervations of Impatiens
species found that, in large rivers (minimum flow rates >5m 3/second), it took the
equivalent of 75 years to go from initial colonisation to 100 % occupation (Pysek and
Prach 1993). In terrestrial environments, the transition from first invasion to dense
stands takes from 40 to 160 years over a wide range of situations and species
(Versfeld et al. 1998). This is equivalent to a rate of increase in percentage cover of
4.5 to 20 % per year or, using an exponential equation, a coefficient (r) of 0.0450.194. Anecdotal observations suggest that Acacia mearnsii can take as little as 30
years to invade and form a dense stand after being planted upstream (Dirk Versfeld
pers. comm. 1998).
Proposed approach
This approach is based on a scenario where no control operations are carried out
and the species spread unhindered.
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Conservation Planning Framework for the GAENP – Final Report
Rates of spread
The rates of spread of invading organisms in terrestrial environments typically show a
sigmoid curve with the rates slowing down as the available area (habitat) becomes
fully invaded (Versfeld et al. 1998). This would not apply to downstream dispersal in
riparian environments where invadable habitat is available for as far downstream as
the sea or until the riparian habitat becomes unsuitable for invasion. Therefore, we
have used a standard exponential function to model the increase in area. The data
are clearly very sparse, but a rate of increase (r) of between 0.15 and 0.25 would be
reasonable for a species with a short juvenile period. Species with purely vegetative
reproduction (poplars, willows) probably would be more episodic dispersers as major
floods are required to uproot trees and break off branches that can root themselves
again. A rate (r) of 0.20 has been used to generate the results below.
Invadable area
Modelling of expansion was restricted to the perennial systems as the main invading
species are confined to perennial rivers.
Predictions
The predictions have been summarised in Table 3. The importance of the rapid
increase in the later years is evident from the differences between the situations at
20 and at 30 years. The approach is based on spread from a single colonisation point
in year one. As can be seen from this summary, the spread from the existing
invasions could be considerably more rapid because the large seed source they
provide means that the initially slow expansion phase has already been completed.
Table 3. Predicted downstream spread and the mean density in each of the
invaded sections.
Years
5
10
15
20
25
30
Spread
downstream
(km)
2.1
5.2
12.8
32.0
80.0
198.0
Mean density at Mean density
20
years at 30 years (%
(mean % cover) cover)
7.9
26.2
4.3
14.4
2.4
8.0
1.3
4.3
2.4
1.3
Density increases in existing stands
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Conservation Planning Framework for the GAENP – Final Report
The data on density increases were even sparser but a rate of increase in the
percentage cover of 0.128, or about 40 years from colonisation to dense stands
would be reasonable for riparian invasions. Within the next 20 years all stands which
are currently in the scattered class will have become closed stands (>75 % canopy
cover) with smaller changes in the lower density classes (Table 4).
Table 4. Changes in density predicted for different times in the future given the
initial starting density. Density classes were based on the standards developed
for the Working for Water Programme (WfW 1999)
Year Starting density (% cover)
Rare
Occasion Very
(0.01 %) al (<1 %) scattered
(1-5 %)
5
0.0
0.8
4.9
10
0.0
1.5
8.9
15
0.1
2.7
16.2
20
0.1
4.9
29.6
25
0.2
9.0
54.0
30
0.3
16.4
98.6
Scattered Medium Dense
(5-25 %) (25-50 %) (50-75 %)
24.3
44.3
81.0
100.0
100.0
100.0
60.7
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
References
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Conservation Planning Framework for the GAENP – Final Report
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