~

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.

1

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.

2

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

3

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

4

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

5

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

6

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:

7

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

8

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

9

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

10

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

11

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.

12

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.

13

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

14

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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