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CONSERVATION VALUE OF ROAD VERGESIN SEMI-ARID ECOSYSTEMS:
ANTS (HYMENOPTERA: FORMICIDAE) AS BIO-INDICATORS
BY
THIDINALEI ENNIE TSHIGUVHO
Submitted in partial fulfilment of the requirements for the degree of Master of Science(Conservation Biology)
Percy FitzPatrick Institute of African OrnithologyDepartment of Zoology
University of Cape TownPrivate BagRondebosch
7701
SUPERVISORS: Dr. W.R.J. DeanDr. H.G. Robertson
FebruaryI .1997
The copyright of this thesis rests with the University of Cape Town. No
quotation from it or information derived from it is to be published
without full acknowledgement of the source. The thesis is to be used
for private study or non-commercial research purposes only.
Univers
ity of
Cap
e Tow
n
ABSTRACT
Ground-foraging ants (Hymenoptera: Formicidae) are used to assess the
conservation value of road verges in a semi-arid region around Prince Albert,
Western Cape Province, South Africa. Ant communities were sampled using pitfall
traps at 50 sites along two roads. Four transects were sampled at each site, two in the
road verge and two in the adjacent rangeland. A total of 43298 individuals from 34
species were caught. There were 31 and 32 species observed in the road verges and
the adjacent rangeland respectively. On average, road verges were more species-rich
than the adjacent rangelands. Road verges also contained rare species, namely, those
species that occurred in less than 10% of traps. Ten species preferred road verges to
the adjacent rangeland. No alien ant species were observed in the study area. The
pattern ofspecies richness was similar at bothfenced and unfenced sites.
Ant species richness appears to be influenced by food availability in the area. Ant
species in the rangelands seem to be negatively impacted by higher grazing
intensities. Species in the road verges on the other hand may benefit directly from
road kills; and indirectly from low grazing intensity, surface run-off, and possibly
increased soil surface temperatures.
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No significant correlation was observed between road verge width and species
richness in the road verges. Resultsjrom this study suggest that ant species use road
verges more frequently as feeding sites, rejugia, or nest sites than adjacent
rangeland. Road verges play a valuable role as conservation areas for ant fauna in
this semi-arid region and should be protectedfrom disturbance.
Key words: Ants, Formicidae, road verges, karoo, semi-arid, South Africa.
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INTRODUCTION
Effective wildlife conservation requires that biodiversity be protected at multiple levels
of organisation (Noss 1990). Protection of ecosystems, communities, and biomes
ensures that organisms are conserved together with the interactions that are necessary
for their long term survival. Conservation of biomes in South Africa is not
representative; for example, less than 1% of the Karoo Biome is formally protected
(Hilton-Taylor & Le Raux 1989). The Succulent-Karoo biome in the western Cape has
the highest species richness and highest levels of endemism for plants recorded for
semi-arid regions in southern Africa (Cowling et al. 1989).
Although semi-arid, the region is suitable for grazing by domestic livestock and crop
farming is possible where there is adequate water for irrigation. It is unlikely that land
in areas such as this will be available for conservation because of the current. demands
for land by small farmers. It is therefore important that the conservation status of parts
of the area such as road verges, that are unlikely to be transformed either by
agriculture, settlements, or grazing by livestock, be assessed. The term road verge is
used in this paper to refer to the zone next to the road that is separated from the
adjacent rangeland by a wire fence to exclude domestic livestock.
Very few road verges in semi-arid areas are protected from disturbance. Although road
verges cannotsubstitute for properlymanaged conservation areas, they may have some
value as protected areas for certain groups of plants and animals. Since they traverse
whole landscapes, road verges could potentially protect a large diversity of biota. The
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value of road verges as breeding and foraging habitats, as well as movement corridors
for vertebrates has been reported by many authors (Oetting & Cassel 1971; Key 1978;
Voorhees & Cassel 1980; Laursen 1981, Kitchener, Dell, Muir & Palmer 1982;
Dhindsa, Sandhu, Sandhu & Toor 1988; and Bennett 1990). Invertebrates that occur in
the road verges have been poorly documented (Free et ai. 1975; Keals & Majer 1991).
Road verges are also among the first parts of the landscape that tourists and the South
African public encounter. Because road verges are continually in the public eye, they
can be used to generate public awareness and interest in natural diversity and hence the
need for their protection.
An efficient way of assessing the value of a habitat to conservation is by using indicator
species, i.e. those species within a habitat that signal the effects of a perturbation on a
number of other species that have similar habitat requirements. Majer (1983) identified ants
as good potential biological indicators, since ant species diversity correlates with the
composition of other components of invertebrate faunas of a particular habitat. Ants are
abundant and diverse in most ecosystems (Roth, Perfecto & Rathcke 1994), and have a
community structure that tends to reflect the nature of the environment in which they occur
(Majer 1983, 1990; Torres 1984; Perfecto 1991). Ground-dwelling ants are also auseful
group as biological indicators because they are easily sampled and the adult stages are
present throughout the year, thus avoiding problems of seasonality.
Ants play important ecological roles particularly in arid areas such as the Karoo (Dean &
Yeaton 1993) and the Namib desert (Marsh 1985). Ants structure communities through
nutrient cycling in the soil, seed dispersal, and through their involvement in symbiotic
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interactions with other species (Carroll & Janzen 1973; Beattie & Culver 1982; Holldobler
& Wilson 1990& Roth et al. 1994). In the Karoo and other arid regions of southern
African, ants are important granivores, and may remove many more seeds than birds and
rodents (Kerley 1991).
The present study was conducted to assess the conservation value of road verges for ants
by comparing the abundance and species diversityof ants in the. road verges with that in the
adjacent rangelands.
STUDY AREA
The study was conducted in the road verges in a semi-arid shrubland around Prince
Albert (33010'S 22°17'E), Western Cape Province, South Africa. Ants were sampled
along two road transects. The first transect was a gravel road running for 54 km in a
South-north direction from Prince Albert to Leeugamka. The second transect was a 54
km stretch of gravel road running in an East-west direction from Prince Albert to
Willowmore. The annual rainfall in the area ranges from 50 - 400 mm with mean of
167 mm (Milton & Dean 1993). The vegetation adjacent to both roads is dominated by
perennial and deciduous succulents and non-succulent shrubs, including species from
genera such as Ruschia, Brownanthus, Drosanthemum, Pteronia, Galenia and
Mesembryanthemum (see O'Farrell 1996 for plant species composition along the two
roads).· The vegetation on theWillowmore road appeared more uniform than that
along the Leeugamka road. The landscape along the Willowmore road was fairly flat
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whereas the landscape along the Leeugamka road ran through both flat areas and hilly
country.
METHODS
Sampling ant communities.
Pitfall traps (henceforth referred to as traps) were set out at 50 sites along the
Leeugamka and the Willowmore roads. Sites were situated opposite each kilometre
marker along/ the road, alternatively on the left and the right road verges. However,
kilometre markers that occurred in watercourses or were adjacent to croplands were
excluded. At each site road verge width was measured as. the perpendicular distance
from the fence to the edge of the road surface; Five of the 35 sites along Leeugamka
road were unfenced. Twelve farms were sampled during the study. Farms that were
mainly used for sheep farming were selected.
The traps were polystyrene cups 9 em deep and 8 em diameter containing 50 mlof
water, to which one drop of detergent per litre had been added to lower the surface
tension. Traps were sunk into the soil so that the mouth of the cup was level with the
soil surface. A grid composed of eight traps, four on each side of the fence, was set up
at each site. The traps were arranged in four lines parallel to the fence. The lines in the
road verge were 1 m from the edge of the road surface and 2 m from the fence; and in
the rangeland were lOmfrom thefence and another 10m further into the rangeland.
Transects were sequentially numbered from 1 to 4 starting with the transect closest to
the road surface to those further into the rangeland. The lines in unfenced sites were at
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1 m, 6 m, 16 m, and 26 m from the edge of the road. The two traps along each line
were spaced 8 m apart. All the trap~s were deployed for six days, from 9 to 14 October
1996.
Sorting and identification
The contents of each cup were washed through a fine sieve and all the invertebrates
removed, counted and preserved in 70% ethanol. Most ants were identified to species
level, but in a few cases it was only possible to identify to the genus because of the
poor state of taxonomy of certain groups. In these cases the species was designated
with the generic name and a number corresponding to a reference collection. Reference
specimens have been deposited in the South African Museum, Cape Town.
Data analysis
Several diversity indices can be used to compare biotic communities, each with its own
strengths and weaknesses. No single index accompases all of the characteristics of an
ideal index including high discriminant ability, low sensibility to sample size, and ease
in calculation (Magurran 1988). The species richness index S (total number of species
in an area) was chosen because it is straightforward, easily conceptualised, more
informative and is comparable across habitats. Sites at 36 km and 37 km along the
Willowmore road were excluded in most analyses except where indicated.
The similarity in species composition between the road verge and the adjacent
rangeland was calculatedusing Sorenson's coefficient of similarity C (Magurran 1988)
using the formula:
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c = 2j I (a + b), where
a andb are the number of species in the road verge and the adjacent rangeland
respectively, and
j is the number of species common between the road verges and the adjacent
rangeland.
The similarity index ranges from 0 to 1, with 0 representing no species in common and
1 representing 1000/0 similarity.
The number of observed species in an area is usually an underestimate of the true
species richness because of rare and/or localised species that may be missed during
sampling. Species-accumulation curves were calculated to assess the thoroughness of
the census technique used. A bootstrap technique presented by Smith and van Belle
(1984) was used to estimate the true species richness in the road verge and the
adjacent rangeland. The formula used is given by:
sBOOT = S + L (1 - pj)"n, where
j=l
BOOT = predicted species richness
S = the number of species observed at n sites in a particular land-use
p = the proportion ofsites containing speciesj on a particular land-use
n = total number of sites sampled
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Wilcoxon paired-sample tests (Zar 1984) were used to assess the differences between
transects in the road verges and the adjacent rangeland in terms of species richness and
total number 'of ants. Species interactions were assessed by correlating number of
individuals for each pair of species. The relationship between road verge width and the
total number of species observed in the road verge per site was tested by simple linear
regression.
RESULTS
Total observed species richness
A total of 43 298 ants (14 386 from the Leeugamka road and 28 912 from the
Willowmore road) belonging to 34 species were collected during the study. Of the 28
912 ants collected along the Willowmore road most (22 771 ants) came from only two
sites (at 36 km and 37 km) which were dominated by one species (Anoplolepis
steingroeverij. No other ant species were observed at these two sites. The total
number of ants along the Willowmore road excluding sites at 36 km and 37 km was
6141. Site at 36 km and 37 km along the Willowmore road were excluded from
species richness analyses unless if indicated. The similarity coefficient between the
Leeugamka and the Willowmore road was 0.85 with 29 species shared between the
two roads. The data from the two roads were lumped for species richness analyses.
The 34 species observed inthe area belong to 15 genera. Of these genera, Camponotus
was the most species-rich (8 species), followed by Tetramorium (6 species) and
Monomorium (5 species) (Table 1). Nine of the 15 genera were each represented by a
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single species. Pheidole capensis was the most abundant species in the area followed
by Monomorium willowmorense,~ Tetramorium signatum, Ocymyrmex barbiger,
Anoplolepis steingroeveri, Anoplolepis trimenii. and Messor capensis (Table 1). No
alien ant species were observed in the area.
Table 1. The percentage number of traps occupied by species and the percentagenumber of individuals in the road verges and the adjacent rangelands. Tot1 and Tot2indicate the total number of traps and the percentage total number of ants at fencedsites. Guild symbols are as follows: P = obligate predator; PIS =predator/scavenger;S =.scavenger; G = granivore; GIN = generalist; and U = unknown. V = road vergeand R = rangeland.
Species Guild 0/0 number of traps 0/0 number of antsoccupiedV R Tot! V R Tot2
Aenictus rotundatus Mayr P 1.2 0.0 0.6 0.3 0.0 0.2Anochetus levaillanti Emery P 0.0 0.6 0.3 0.0 0.0 0.0Anoplolepis steingroeveri Forel S 36.6 38.4 37.5 4.9 9.8 7.0Anoplolepis trimenii Forel PIS 56.4 42.4 49.4 5.6 6.4 5.9Camponotus cuneiscapus Forel PIS 10.5 8.1 9.3 0.2 0.2 0.2Camponotusfulvopilosus De Geer PIS 34.3 26.7 30.5 1.0 1.1 1.0Camponotus mystaceus Emery PIS 8.7 6.4 7.6 0.5 0.3 0.5Camponotus simulans Forel PIS 0.6 1.7 1.2 0.0 0.1 0.0Camponotus sp.J(emarginatus-group) PIS 1.2 0.6 0.9 0.0 0.0 0.0Camponotus sp.2 (mystaceus-group) PIS 1.2 4.1 2.6 0.0 0.1 0.0Camponotussp.3 (emarginatus-group) PIS 4.1 7.6 5.8 0.1 0.3 0.2Camponotus vestitus Smith PIS 18.6 19.2 18.9 0.8 0.9 0.8Cardiocondyla emeryi Forel GIN 0.0 1.2 0.6 0.0 0.0 0.0Crematogaster melanogaster Emery PIS 0.6 4.1 2.3 0.0 0.4 0.2Lepisiota capensis Mayr PIS 6.4 16.3 11.3 0.4 0.7 0.5Lepisiota sp.2 PIS 4.1 2.3 3.2 0.1 0.1 0.1Leptothorax sp.J GIN 2.3 1.7 2.0 0.0 0.0 0.0Leptothorax sp.2 GIN 3.5 2.3 2.9 0.1 0.1 0.1Leptothorax sp.3 GIN 0.0 1.2 0.6 0.0 0.0 0.0Messor capensis Mayr G 37.2 33.7 35.5 6.5 6.8 6.6Monomorium havilandi Forel G 9.9 4.7 7.3 0.9 0.7 0.8lvfonomorium macrops Arnold G 29.7 33.7 31.7 2.6 3.1 2.8Monomorium monomorium-group U 0.6 0.6 0.6 0.0 0.0 0.0Monomorium ocel/atum Arnold U 30.8 14.0 22.4 6.6 1.9 4.7lvJonomorium willowmorenseBolton U 74.4 75.0 74.7 16.5 20.9 18.3Ocymyrmex barbiger Emery PIS 82.0 65.7 73.8 7.2 6.9 7.0Pachycondyla hottentota Emery P 11.0 6.4 8.7 0.4 0.2 0.3Pheidole capensis Mayr GIN 90.1 70.3 80.2 35.0 23.2 30.1Solenopsis sp.l P 1.7 0.0 0.9 0.0 0.0 0.0Tetramorium bevisi Arnold U 0.0 0.6 0.3 0.0 0.0 0.0Tetramoriumperingueyi Arnold G 11.6 12.2 11.9 1.2 2.2 1.6Tetramorium quadrispinosum Emery PIS 15.7 6.4 11.0 1.0 0.4 0.8Tetramorium signatum Emery G 43.0 39.0 41.0 7.9 11.4 9.3Tetramorium solidum Emery G 4.7 12.2 8.4 0.2 1.6 0.7Tetramorium sp.l (oculatum-complex) U 1.2 0.0 0.6 0.1 0.0 0.0
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Species-accumulation curves
The species-accumulation curves constructed for -the road verges and the rangelands at
sites along the Leeugamka and the Willowmore roads rose fairly steeply and levelled
off relatively quickly indicating that most of the species in the area were sampled
(Figure 1).
35 -r-----------------------------.,30
25
20
15
10 -
5
---road verge I• • • • • rangeland I
, ..... -.
Number of samples
Figure 1. Species-accumulation curves for ant species in the road verges and theadjacent rangeland along the Leeugamka and Willowmore roads.
More than 600/0 of species were sampled at the first four sites in both areas. The
curves, however, did not reach a definite asymptote because of few more species that
might have been missed during sampling. There are species that have been recorded in
the study area before (Milton, :r:>~<l:rl&Kerley 1992; Dean & Milton 1995) that were
not observed during this study. Anoplolepis custodiens, for example, has often been
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observed in households and road verges around Prince Albert but was not caught
during this study.
Species richness in the road verges versus the adj acent rangelands
There were 31 and 32 species in the road verges and the adjacent rangelands
respectively. The estimated values for true species richness calculated by. bootstrap
were 31.8 and 32.4 for the road verges and the adjacent rangeland respectively. The
coefficient of similarity calculated between the road verges and the adjacent rangelands
was 0.82 with 27 species common to both areas. The mean number of species per site
(± SE) were 10.5 ± 0.5 and 9.5 ± 0.5 (N = 43) for the road verge and the adjacent
rangelands respectively. Wilcoxon paired-sample tests showed significant differences in
mean species richness per site between the road verge and the adjacent rangeland (T =
179, P < 0.01, N = 43). Transectl had the highest mean number of species followed
by transect 2, 3, and 4 (Figure 2).
12 ,..- --.- ""'t-"" ..- --.
10
8VIVI~
S"ti 6'CVI~
'u~c.. 4rJ1
2
OL-.--------a.-------------.a-...-------'2
Transects
3 4
Figure 2. Box-and whisker plot ofmean number of ant species in the road verges andthe adjacent rangeland. The point inside each box represents the mean number ofspecies, the box defines the standard error of the mean and the vertical lines give thestandard deviation from of the mean.
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Ants were also significantly more abundant in the road verges than the adjacent
rangelands (Figure 3)[Wilcoxon test: T = 174,P < 0.01, N =43]. The mean number of
ants in the road verges and the adjacent rangelands were 254.1 ± 24.1 and 178.8 ±
20.5 respectively..Again transect 1 had the largest number of ants followed by transect
2, 3, and 4 (Figure 3).
240
220
200
180
160en~ 140=...
1200loo~.c 1002::
Z 80
60
40
20
02
T'runsects
3 4
Figure 3 Box-and whisker plot of mean number of ants in the road verges and theadjacent rangeland. Symbols are used as explained in the previous graph.
Three species were unique to the road verges, namely, Aenictus rotundatus, Solenopis
sp. 1, and Tetramorium sp.1. Four species unique to the rangelands were Anochetus
levaillanti, Cardiocondyla emery, Leptothorax sp. 3, and Tetramorium bevisi. Three
of these rangeland species were subterranean. All species that were unique to either
land-use were rare. The two land-uses were dominated by predator/scavenger species.
Road verges contained one more predator/scavenger species, one more generalist and
two more granivores than the adjacent rangeland.
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Ant-habitat associations
Four species, namely Monomorium ocellatum, Ocymyrmex barbiger, Pheidole
capensis and Tetramorium quadrispinosum, occupied more traps and also occurred in
large numbers in the road verges than in the adjacent rangelands (Table 2). Anop/o/epis
trimenii was also more common in the road verges although its abundance in the two
land-uses did not differ significantly. Other species that tend to occur more frequently
in traps along the road verges, although the results were not significant, were
Camponotus julvopilosus, Messor capensis, Monomorium havilandi, and
Pachycondyla hottentota. Lepisiota capensis occurred significantly more frequently
and in large numbers in traps in the rangeland than those in the road verges. Other
abundant species did not show any habitat preference.
Table 2. Frequency of occurrence of species in traps and their abundance along theroad verge and in the adjacent rangeland. V = road verge; R = adjacent rangeland;N = number of sites at which the species occur; T = Critical value for Wilcoxon test:and P = significance level. Species which were more common and more abundant intraps in the road verges are highlighted.
Mean (± SE) Wilcoxon statistic Mean (± SE) number Wilcoxonnumber of traps of ants per site statisticoccupied per site
Species V R T P N V R T PA. trimenii 2.6±O.2 1.9±O.2 172 0.05 38 16.0±2.8 13.0±3.0 259 0.10C. fulvopilosus 1.8±O.2 1.4±O.2 183 0.20 33 3.3±O.6 2.6±O.5 211 0.47L. capensis 0.6±O.2 1.5±O.2 24 0.02 19 2.4±1.3 3.0±O.5 50 0.12M. capensis 2.0±O.2 1.9±O.2 138 0.51 31 22.9±7.2 16.9±4.7 192 0.571.\1. havilandi 1.5±OA 0.7±O.3 10 0.07 11 8.6±3.2 4.6±3.0 13 < 0.01lvf.ocellatum 1.7±O.2 0.8±O.2 93 < 0.01 31 23±8.6 5.0±2.2 114 < 0.010. barbiger 3.4±O.2 2.7±O.2 53 < 0.01 42 18.6±2.9 12.6±3.1 147 < 0.01P. hottentota 1.3±O.3 0.7±O.2 26 0.17 15 2.8±1.1 0.9±O.3 16 0.04P. capensis 3.8±O.1 3.0±O.2 40 < 0.01 41 93±13. 43.5±9.6 151 < 0.01T. quadrispinosum 1.8±O.3 0.7±O.2 10 0.04 15 7.5±2.4 1.9±O.6 18 0.05
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Species richness at unfenced sites
Twenty three percent of all ants .(from 19 species) occurred at unfenced sites. All
species observed in these sites were also found at fenced sites. None of the rare species
(species that occupied less than 100/0 traps) were observed at unfenced sites. The mean
number of species on the transects closer to the edge of the road was significantly
higher than on transects further into the rangeland (Figure 4) [Wilcoxon test: T = 0.1,
P =0.04 , N = 5].
Figure 4. Box-and whisker plot of mean number of species on transects at unfencedsites. Symbols are used as in the previous graphs.
Ants were also significantly more abundant on transects 1 and 2 than transects 3 and 4
(Figure 5) [Wilcoxon test: T = 0.2, P = 0.04, N = 5].
15
200
160
<II4.l.~ 1204.ls:::.. •<II
e-Q
¢-4.l.Q
80§Z ¢40
02 3 4
Transects
Figure 5. Box-and whisker plot of mean number of ants on transects at unfenced sites.Symbols are used as in the previous graphs.
Some of the species that showed preference to ,road verges than the adjacent rangeland
at fenced sites (A. trimenii, M. capensis, and P. capensis) again occurred significantly
more frequently on transects closer to the road verges than those further away (Table
3). Monomorium havilandi, Monomorium ocellatum and Ocymyrmex barbiger also
showed preference towards road verges although the results were insignificant.
Table 3. Frequency of occurrence of species and their abundance on transects atunfenced sites. Closer = transects closer to the road (i.e. transects 1 and 2); Further =
transects further into the rangeland (transect 3 and 4); N= number of sites at which thespecies occur; T = Critical value for Wilcoxon test; and P = significance level. Specieswhich were more common and more abundant in traps on transects closer to the roadverges are highlighted.
Mean (± SE) Wilcoxon statistic Mean (± SE) number Wilcoxonnumber of traps per of ants per site statisticsite
Species Closer Further T P N Closer Further T P
A. trimenii 2.1±O.3 0.2±O.1 1.2 0.05 3 12.3±4.9 2.9±1.5 5 0.04!vI. capensis 2.0±O.2 0.9±O.2 0.1 0.04 3 3.1±1.7 0.5±O.2 0.2 0.01
.M. havi Landi 0.3±O.4 0.2±O.1 0.4 0.09 2 5.1±2.8 3.1±1.3 1.2 0.06
Miocellatum 2.0±O.2 1.9±O.1 4 0.25 4 6.9±4.3 0.6±O.5 12 0.07
0. barbiger 2.8±O.1 2.5±O.1 0.1 0.06 3 8.5±1.1 6.1±1.2 0.1 0.05
P. capensis 3.4±O.1 1.9±O.1 2 <0.01 5 35.3±8.4 12.4±4.2 8 0.02
16
The relationship between road verge width and species richness in the road verge
The mean road verge width in the ~area was 11.0 ± 0.9,. ranging from 3.7 m to 31.7 m.
There was no' significant correlation betweenverge width and number of ant species
(r = - 0.06, P = 0.67, N = 43). Road verge width did not correlate significantly with
total number of ants either (r = - 0.2, P = 0.58, N = 43). Narrow road verges did not
have less species than the adjacent rangeland.
Species interactions
Species belonging to similar guilds co-occurred at most sites except for Tetramorium
peringueyi and Tetramorium solidum. The two species did not co-occur except at one
site. Tetramorium peringueyi occurred along the first 21 km along the Leeugamka
road, whereafter T. solidum was more abundant (Figure 6). T solidum was also more
abundant than T peringueyi at sites along the Willowmore road. Other species did not
show any significant interactions.
120
C"J 100 I,..... .,e ,~ I .~ 80 I ,0 ,-..~..c 60e "::: "c: ,
"; 40.....0~ 20 ,
", \,
I', \
I " I\
, \ .v ,
0
--T.solidum
••.•. T. peringueyi
..... (") (0 co 0..... N ~ (0 co 0 N
..... ..... ..... ..... N N(0 co 0N N ("1
Kilometers from Prince AIbert
Figure 6. The occurrence of Tetramorium peringueyi and Tetramorium solidum atsites. Sites marked with asterisks occurred along the Willowmore road whereas othersites were along the Leeugamka road.
17
DISCUSSION
Road verges in the study area were as species-rich as the adjacent rangelands. More
than 80% of the species were common to both the road verges and the adjacent
rangelands. The species diversity of an area may be influenced by the surrounding land
use system. Road verges and/or any remnant vegetation adjacent to cultivated or
afforestated lands, for example, tend to. contain more. plant and animal species than the
adjacent land (Adams & Geis 1983~ Adams 1984), possibly because of high degree of
vegetation change that takes place in the adjacent lands. The use of fertilisers and other
farm chemicals in cultivated lands may also impact very negatively on the biodiversity
of the adjacent natural remnant. The degree of environmental impact on the rangeland,
on the other hand, is usually lower (although not negligible) than in arable land, and the
vegetation remains semi-natural. The vegetation in the rangeland in the study area was
superficially not very different from that in the road verges. The road verges were
adjacent to rangelands and the two. were separated by a fence that does not act as a
barrier to the movement of invertebrate species. Keals & Majer (1991) also found road
verges in semi-natural lands to contain as many insect species as adjacent blocks of
native vegetation.
Road verges were on average species-rich than the adjacent rangelands. There are two
possible explanations for the observed pattern of occurrence; either resource
availability or microclimatic conditions or both. The amount of food available may
determine the distribution of species in an area. Guild representation in this area
18
indicates that seeds, as well as scavenging matter are the most important food items
required by most species.
The results from a study done concurrently with this indicated that road verges have a
higher mean percentage plant cover than the adjacent rangelands (O'Farrell, 1996).
The low mean percentage plant cover in the rangelands is possibly due to the impacts
of grazing. Impacts by livestock on ant species diversity might be direct or indirect (by
impacting on plant species associated with particular ant species). Livestock may
directly affect plants through the removal of foliage and by trampling over plants. In a
study in the Karoo, sheep grazing was found to cause a reduction in canopy size,
flower, and seed production of the preferred forage plants (Milton & Dean 1990;
Milton 1993). Grazing may thus alter the competitive abilities of plant species in the
rangeland. Waste products released during grazing may elevate nutrient levels in the
soil and thus altering soil structure. Different plant species may respond differently to
the effects of a changed soil structure. A change in plant cover in the rangelands may
thus affect ant fauna associated with the affected plant species, particularly if the
association is obligatory (e.g. obligate mutualists).
The direct impacts of grazing on ants might be through trampling which compacts the
soil which in turn may cause a reduction in burrowing activities (Scougall et al. 1993).
The low number of ants in the rangeland may thus indicate a change in soil quality.
19
The high abundance of· seed harvester species such as Messor capensis and
Monomorium havilandi might be a result of increased seed production of their
preferred plant species in the road verges. The distribution of Messor capensis is
determined by the amount of seeds available in an area (Milton & Dean 1993).
Scavenger species in the road verges may benefit from animals killed by the passing
vehicles. The distribution of predator species may also be influenced by the distribution
of their prey.
Species richness at unfenced sites followed the same pattern as at fenced sites, with a
high mean number of species and more ants in the transects closer to the road than
those further away. Unlike at fenced sites, the vegetation on transects closer to the
road is not protected from grazing by livestock. The high species richness on transects
closer to the road therefore suggests that grazing is not the only factor determining
species richness in the area. The influence of abiotic factors such as run-off and soil
surface temperatures on species richness were also considered. The influence of these
factors on ant species might be through their influence on plant species associated with
particular ant species.
During rain road verges receive additional water supply from the road surface run-off
Transects closer to the edge of the road will receive more run-off than those further
away. Soil moisture on transects immediately adjacent to the edge of the road averaged
12 % greater them on transectsawayfrom the road edge (O'Farrell 1996). Raised
levels of moisture content together with other factors such as increased temperature
and CO2 levels might result in an increased plant productivity. Milton &
20
Dean (1988) observed a higher flower and fruit production of Rhigozum obovatum
(Bignoniaceae) on road verges~ompared with the adjacent rangelands. The
productivity was even higher in the road verges situated below the level of the road
than otherwise, suggesting these road verges receive more run-off than those situated
above the road surface. The percentage soil moisture in the. area correlated with the
mean percentage plant cover on transects (Figure 7).
35
30;.,Q,),
25 -0c;....= 20c:Q..
'S. 150
=c: 10Q,)
~5
0
8.9 7.9 7.8 7.5
Mean % soil moiture
Figure 7. The mean percentage soil moisture versus mean percentage plant cover in theroad verges and the adjacent rangeland (Data from O'Farrell 1996).
The lower percentage plant cover in the rangelands may also provides more bare
ground which might be more convenient for movement by ants. Species such as
Ocymyrmex barbiger, Pachycondyla hottentota, and Tetramorium quadrispinosum
prefer more bare ground (Robertson, H.G., personal communication). All these species
were observed to prefer the road verges than the rangelands and that may indicate that,
although more vegetated than the adjacent rangelands, road verges provide adequate
bare ground for ant movement. It may also be that more bare ground exposes ants to
21
desiccation than otherwise. Perfecto & Snelling (1995) found soil moisture to be a
determinant factor of the foraging activity of ants.
Soil surface temperature may also affect the distribution of insects in a habitat
(Levings 1983; Torres 1984). Levings (1983) and Dean (1992) found soil surface
temperature to be an important factor determining the activity patterns of ant species in
the study area, with certain species having wider thermal niches than others. Road
surface tends to absorb more heat than the surrounding area, and may retain it longer.
It is possible that road verges experience higher temperatures than the adjacent
rangeland. Species that prefer foraging at high temperatures such as Ocymyrmex
barbiger and Camponotus fulvopilosus (Dean 1992) were more common in the road
verges than the rangeland. The occurrence of these species more frequently in the road
verges than in the adjacent rangeland may indicate higher soil surface temperatures in
the road verges.
Ant species in similar guilds seem to co-occur at sites. It is difficult to infer any form of
interaction from short-run trapping data. Ants also have a variety of means to promote
coexistence and alleviate competition, including partitioning of activity to different
hours of the day and different temperature regimes, and different foraging strata
(Wilson 1971, Felles 1987). Tetramorium peringueyi occurred in low numbers or was
absence at sites occupied by Tetramorium solidum. It is unlikely that this pattern of
occurrence is due to competitionbetween the two species. It seems that the two
species prefer different habitat or soil types. The area preferred by T. peringueyi, i.e.
22
>20 km from Prince Albert, was more dry.and less flat although the vegetation did not
appear very different superficially.
Road verge width did not correlate significantly with species richness. Studies on
vertebrates have found road verge width to correlate significantly with species richness
(Keals & Majer 1991). A wider road verge may contain more species than a narrow
one, because it may provide habitat requirement for more species, even those with
wide habitat ranges. Narrow road verges on the other hand .may be an entirely edge
habitat that may not be used by rangeland-interior species. The reason why road verge
width did not correlate significantly with species richness might be that insects, in
contrast to vertebrates, require little area to meet their habitat requirements and to
provide adequate food resources. The status of the adjacent rangeland might also
influence the relationship between road verge width and species richness in the road
verge. For example, a wide road verge next to an overgrazed rangeland might have
lower species richness than a narrow verge adjacent to a medium-grazed rangeland.
The influence of road verge width on species richness of other groups of animals in the
road verge needs to be investigated.
CONSERVATION VALUE OF ROAD VERGES
Road verges in the study area provide a greater opportunity for conserving ant
diversity. The road verges are still ina relatively pristine condition and no alien species
have yet entered into the area. Road verges act as buffer zones that allow ant species
to persist in the rangeland, providing ants in the area with alternative foraging habitats.
23
Movement of species between the road verge and the adjacent rangeland may diminish
the chances for species extinctions. Road verges also protects a wide variety of species
because they cover a variety of habitats, for example, the occurrence of Tetramorium
peringueyi and Tetramorium solidum.
Road verges are also attractive settings for recreation and they may support a tourist
industry. The bright flowers in the road verges during summer months (personal
observation) in the area attract motorists. Road verges are also vital as outdoor
teaching laboratories depicting how ecosystems function.
Disturbance in the road verges should be minimised. Road workers should consider the
protection of biodiversity in the road verges when they construct or maintain roads.
The use of off-roads vehicles should be avoided since this may impact negatively on
the species diversity in road verges. Ground-foraging ants may serve as indicators of a
general trend, and the results from this study might indicate similar trends in other
invertebrate populations in the road verges. The value of road verges as conservation
areas to other groups of animals should be investigated. Studies such as this should be
extended to other ecosystems as well.
ACKNOWLEDGEMENTS
I am thankful to the University ofVenda who granted me a leave in order to undertake
the study. The Foundation for Research and Development funded the project. I am
thankful to my supervisors Dr. W.RJ. Dean (University of Cape Town) and Dr. H.G.
24
Robertson (South African Museum). I also thank Dr. H.G. Robertson for the
identification of specimens. I am also grateful for comments on an earlier draft of the
paper from Dr. P.G. Ryan and Dr. SJ. Milton (University of Cape Town), Dr. lE.
Crafford and Mr. M. Ligavha (University of Venda). I appreciate the technical support
offered by Chris Tobler and Lionel Mansfield (VCT). I would also like to express my
gratitude to farm owners who gave permission to work on their farms. Patrick
O'Farrell assisted with field work. I appreciate the support provided by Mr. N. K.
Mavhungu throughout the research.
25
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