Afr. J . Ecol. 1990, Volume 28, pages 103-1 10

Zoogeography of the herpetofauna of Botswana and its management implications

M . P . SIMBOTWE'* a n d L. J . G U I L L E T T E , Jr . ' 'Environmental Sciences Consultancy Company ( P T Y ) Ltd., PO Box 60127. Livingstone, Zambia, 'Department of Zoology, University of Florida, Gainesville. FL 32611, U.S .A .

Summary A taxonomic analysis of data reveals that there are more reptilian than amphibian species in Botswana and that the number of reptilian species generally decreases from the mesic north to the arid southern part of the country. Squamates (lizards and snakes) compose the majority of the reptilian fauna. An analysis ofcommunity reveals that the area of Botswana north of the 22" parallel is the most important in terms of faunal diversity, and thus special consideration in terms of fauna conser- vation and management is warranted. Results show that climatic conditions in Botswana have profound affects on the distribution of herpetofaunal species.

Ke-v words: Zoogeography, reptiles, Botswana amphibians.

RCsumk Une analyse taxonomique des donnkes revele qu'il y a plus d'especes de reptiles que d'amphibiens au Botswana et que le nombre d'espkces de reptiles diminue gCnCrale- ment du nord vers le sud aride du pays. Les squamates (lezards et serpents) compo- sent la majeure partie de la faune reptilienne. Une analyse des communautes revele que la partie du Botswana au nord du 22e parallde est la plus importante en terme de diversit6 faunistique et que ceci justifie une considkration spkciale pour la conser- vation et la gestionde la faune. Les resultatsmontrent que lesconditionsclimatiques au Botswana affectent profondement la distribution des especes de reptiles.

Introduction Botswana occupies 586,000 km2 of the down-warped Kalahari Basin of the great southern Africa plateau. The majority of the country comprises undulating plateaux and is noticeably hilly in the east. Botswana lacks extensive surface drain- age, apart from those areas bordering the Limpopo and Chobe Rivers, the rest of the country's drainage is interior and does not reach the ocean. The perennial Okavango River flows from the Angolan highlands forming the major drainage system for Botswana. From the massive inland Okavango swamps, water flows eastwards seasonally along the Boteti River to Lake Xau and into the Makgadikgadi Salt Pans. Lake Ngami receives a seasonal run-off intake from local monsoonal rain.

In Botswana, aridity increases from the north to the south (Amer, 1976). The south and west, with an average annual rainfall of 130 mm, experience near desert 'To whom correspondence should be addressed.

104 M . P . Simbotwe and L. J . Guillette Jr

conditions whereas there is a gradual increase in precipitation towards the east (380-500 mm) and north (635 mm) (Amer, 1976). This rainfall distribution is reflected in a parallel transition in the natural vegetation. The transition is from sparse thornveld in the Kalahari desert to dry woodland savanna in the north and east. Soils are generally of low fertility, but do improve going eastwards. Temperatures in winter drop to freezing point in the temperate west and east but are warmer in the north.

Poynton (1964) analysed and interpreted the nature of the distribution of the amphibia of southern Africa in terms of the various climatic zones (i.e. tropical vs temperate zones). According to the biotic divisions of southern Africa (Poynton, 1964), Botswana falls primarily into a ‘subtropical moist’ zone to the north and ‘temperate west’ zone from the southernmost tip of the country up to the Tropic of Capricorn. In the area between the ‘subtropical moist’ zone and the ‘temperate west’ zone lies the ‘subtropical arid’ zone and a small area, the ‘temperate east’ zone (Fig. I).

Over the years, data has accumulated (Auerbach, 1987) to the point where a zoogeographical analysis of the herpetofauna of Botswana is feasible. Such analysis is essential for the development of management plans.

Materials and methbds

The taxonomic status of the reptiles and amphibians of Botswana is sufficient to generate a taxonomic profile from which patterns of taxal evolution can be hypoth- esized. The major weakness in this approach is a serious lack of corroborating data based on fossil history of the fauna.

From available data on species distribution, a community analysis has been attempted. To determine levels of associations among taxa inhabiting different climatic zones, an index of species structure in communities was needed. Pair-wise analysis of similarity ( S ) between samples was performed such that

2c s= - A + B

where A equals the number of species in sample A, B equals the number of species in sample B, and C equals the number of species common to both samples (Odum, 197 1). An index of dissimilarity between tropical and temperature populations was computed as 1 - S (Odum, 1971).

Patterns of species geographical distributions also have been analysed (chi- squared analysis) and discussed in relation to climatic features, major developments in the country and the likely impact of man on their populations.

Results Population analysis

The reptiles and amphibians of Botswana are rich in species (Table 1). The reptilian fauna is composed of 158 species and subspecies whereas there are 38 amphibian species.

A taxonomic analysis of amphibians reveals that at the higher taxonomic cate- gories, genera are the most differentiated. Taxa are divided into 7 families, 17

Zoogeography of Botswana herpetofauna 105

2 2c

24

FRO

PLATANNA

TOA

2O0S

Fig. 1. The relative distribution of amphibians in the four ecological zones of Botswana. Numbers given in the pie charts represent percentage occurrence compared to total number of species present.

genera and 38 species and subspecies. The ratio of amphibian species to genera is 2:2 whereas that of genera to families is 2:4. Reptiles are the most diversified of the herpetofauna. There are records of 17 families, 75 genera and 158 species and subspecies. The ratio of genera to families is 2: 1 whereas that of species to genera is 4:4.

Table 1 shows the current taxonomic profile of the reptilian communities in Botswana. Lizards and snakes are by far themost diverse and abundant, accounting for 66 (41.7%) and 65 (41.2%) species, respectively. A total of 18 (11.4%) species are amphisbaenians, 8 (5.1 YO) are testudines and 1 (0.6%) crocodile (Table 1).

106 M . P. Simbotwe and L. J . Guillette Jr.

Table 1. Taxa of extant reptiles in Botswana Families Genera Species and subspecies

Testudines 2 5 Squamata

Lacertilia 7 28

Serpentes 4 36 Amphisbaenia 3 5

Crocodylia 1 1

8

66 18 65

I

Totals 17 I 5 158

Of the Anuran taxa, 3 frogs are monogeneric whereas 4 are monospecific. Amongst reptiles, the following are monogeneric: 2 lizards, 2 snakes, 1 amphisbae- nid, and 1 crocodile. Monospecific reptilian taxa include 1 turtle, 13 lizards, 14 snakes and 1 crocodile. No amphisbaenids are monospecific.

Community analysis The distribution of reptilian and amphibian species throughout Botswana in relation to the various climatic zones is shown in Figs 1 and 2. The percentage value given is the percentage of the total species present and includes overlapping ubiquitous species. The trend that clearly emerges shows that amphibian species decrease dramatically from the ‘subtropical moist’ zone of northern Botswana through the ‘subtropical arid’ zone and the ‘temperate east’ zone to the ‘temperate west’ zone (Fig. 1). This is supported by a chi-squared analysis which indicates that amphibian species are not distributed homogeneously among the four ecological zones (x2= 140.3; P<O.OOl; Table 2). The trend among reptiles was similar (x2 = 148.3; P<O.OOI; Table 3). It is important to note that the ‘subtropical moist’ zone has 109 herpetofaunal species. The ‘subtropical arid’ zone has 108, ‘temperate east’ zone 78 and ‘temperate west’ zone 66 species.

A pair-wise comparison of samples of amphibian species between the ‘subtropi- cal moist’ and ‘subtropical arid’ zones produced a similarity index value of 0.53 whereas between the ‘temperate east’ and ‘temperate west’ zones, the similarity value was 0.18. A dissimilarity value of 0.55 existed between ‘subtropical’ and ‘temperate’ zone samples. A similar analysis on the reptilian samples indicated a species similarity value of 0.54 between the ‘subtropical moist’ and ‘subtropical arid’ zones whereas between the ’temperate east’ and ‘temperate west’ zones the value of similarity of taxa was 0-42. Subtropical fauna showed a dissimilarity of 0.61 when compared to the temperate reptilian fauna.

The general distribution of species indicates that only 16 amphibian species occur ubiquitously while 22 are localized. Among reptiles, 57 species are widespread whereas 100 are localized or confined. One species (Agama hispida makarikarica) is endemic only to the Sowa Pan and Makgadikgadi Salt Pans. The record of Bufo lemairii (yellow swamp toad) in Shakawe and Xugana Lediba is important because it is the only record for this species in Southern Africa, south of the Cunene and Zambezi line. This record together with that of Hyperolius benguellensis (Bocage’s sedge frog) represent southward extensions of these species. Limnophis bicolor bangweolicus (eastern striped swamp snake) and Ptychadena subpunctata (tropical

Zoogeography of Botswana herpetofauna 107

SERPENTE MPHlBAENlA

(present)

Fig. 2. The relative distribution of reptiles in the four ecological zones of Botswana See Fig. 1 for further information.

grass frog) are both Angolan and Zambian species. Routes of dispersal being the Chobe-Zambezi and Cubango-Okavango Rivers.

Discussion The present analysis reveals trends in the distribution of the herpetofaunal taxa that are worthy of comment when considered from a management policy perspective. It is clear from the taxonomic profile that in terms of faunal diversity, the area of Botswana north of the 22"s parallel is very important. This is the area herein

108 M . P. Sirnbotwe and L. J . Guillette Jr.

Table 2. Patterns of distribution of amphibian taxa in different ecological zones in Botswana

Table 3. Patterns of distribution of reptilian taxa in different ecological zones in Botswana

Subtropical Subtropical Temperate Temperate moist arid east west

Anura Platanna 2 I 1 0 Toads 5 4 4 0 Frogs 23 14 12 5

Totals 30 19 17 5

“Number of individual species and subspecies

Subtropical Subtropical Temperate Temperate moist arid east west

Testudines I* 4 5 3 Squamata

Lacertilia 25 39 25 30

Serpentes 39 40 26 23 Amphibaenia 7 5 5 5

Crocodylia 1 1 0 0

Totals 19 89 61 61

‘Number of individual species and subspecies.

referred to as the ‘subtropical moist’ and ‘subtropical arid’ zones. An important feature of this area is that its most northern part receives high annual precipitation (> 500 mm) and its vegetation structure is complex. Major national development projects in this area include: (1) irrigated farming, (2) timber harvesting, (3) fisher- ies and wildlife and (4) tourism. Likely impacts on the herpetofauna would be destruction of habitats and modification of water composition to which aquatic and semiaquatic species are adapted. Simbotwe (1988) mentions several micro- habitats important to reptiles in the Okavango Delta that are worth managing. These include: deep water channels, perennial off-channels choked with papyrus and leading to a sandy bank or areas often overgrown with phragmites scattered sandy covered islands, sandy banks along the main channels or rivers and anthill areas.

In the subtropical arid zone, there is an evident absence of natural perennial surface water. Rainfall is lower (300-500 mm per year) and vegetation is composed principally of tree and bush savanna. This area of the country records the largest varieties of reptiles. Lizard and snake species are plentiful. The reasons for this distribution pattern are many. We hypothesize that the presence of low but adequate rainfall maintains abundant food supplies (rodents, seeds, insect). Varied

Zoogeography of Botswana herpetofauna 109

microhabitats (arboreal, terrestrial, ripiculous, fossorial) allow species of the same genera to occur sympatrically supporting a larger diversity of amphibians and reptiles. Additionally, this area is less inhabited, and the least developed in the country. Future studies should test these hypotheses. Major development pro- grammes are planned for this area including mining and mineral prospecting (e.g. Sowa Pan Project) as well as continued expansion of livestock grazing areas. Likely impacts include impoundments and trampling, resulting in habitat degradation and consequent depletion of faunal diversity.

A surprising number of species occur in the temperate east zone of the country, which is also the major industrial area. Annual rainfall is greater than 500 mm and the vegetation composition includes Limpopo riparian woodland, tree and bush savanna and semi-sweet mixed bushveld. As suggested above, this complexity may help explain the high species diversity in this relatively small area. This is especially important to note, as the temperate east zone has an area approximately a fifth of that of the subtropical arid zone. Due to high population densities, likely impacts include road kills, poaching, habitat destruction through land clearing and pollution.

The temperate west zone has desert-like conditions (< 300 mm rainfall per year). The major environmental features include thorn veld and scattered scrub bushes, deep Kalahari sand and extreme temperature variations (2-34°C). Only burrowing species of frogs Breviceps, Pyxicephalus, Cacosternum and Tomopterna have colonized this region. A quarter of the total known species of reptiles inhabit this biotope. The most successful groups include geckos, tortoises and turtles, which seem well adapted to Kalahari desert conditions. However, major national development projects include livestock ranching and mineral prospecting, and habitat degradation is the likely impact.

A high frequency of genera with single species (monospecific taxa) was recorded among lizards and snakes. Large numbers of these represent highly specialized groups (e.g. burrowing snakes, aquatic reptiles and desert geckos). Monogeneric amphibian taxa are composed of groups whose phylogenetic related- ness to local taxa is poorly understood. There are few other species of frog, for example, that resemble Xenopus in the family Pipidae. No other frogs appear to be closely related to the monospecific representatives of the class Hemisus and Chirornantis. They indeed belong to their own families Hemisotidae and Rhacophoridae, respectively. Phylogenetic studies have not yet begun and are required so that relationships among taxa can be clarified.

Reptiles and amphibians are protected under the Fauna Conservation Laws of Botswana. Pythons are conserved and so cannot be exploited while amphibians are protected through the regulation of imports and exports (Simbotwe, 1987). Crocodiles are currently exploited under ranching schemes based on a quota system administered by the Department of Wildlife and National Parks (Anon, 1989) that reduces the chance of over-exploitation. Habitat loss is prevented through the Environmental Impact Assessments that accompany every major development project in Botswana. That environmental degradation is a real threat to the herpetofauna of Africa, however, is documented by the work of Jansen (1976) in east Africa. However, data from our very successful country-wide project on the herpetofauna has enabled us to plan for sustainable conservation and exploitation and better management.

110 M . P . Simbotwe and L. J . Guillette Jr .

Acknowledgments

We would like to thank Miss P. Modiko who typed the final draft, Ms D. Harrison for drawing the artwork and the Director of Wildlife and National Parks who gave us permission to publish the paper.

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(Manuscript accepted 30 March 1990)