Effects of bush encroachment on an assemblage of diurnal lizard species in central Namibia

Abstract

Bush encroachment is a serious environmental and economic problem in Namibia, but little is known about impacts on native reptile diversity. Area-confined visual surveys were used to examine a diurnal lizard assemblage in central Namibian commercial ranchlands. Surveys were conducted in plots of open savanna habitat and proximal bush-encroached habitat. The following four species comprised 97.5% of all lizard observations: Pedioplanis undata, Mabuya varia, M. striata, and Lygodactylus bradfieldi. Pedioplanis undata was terrestrial, and the remaining three species were largely arboreal in our study plots. Mabuya varia was found in all savanna plots but was absent from all bush-encroached plots. Two species (P. undata and L. bradfieldi) were less abundant in bush-encroached plots. One species (M. striata) was more abundant in bush-encroached plots than in open savanna plots. Arboreal lizards demonstrated an avoidance to invasive woody plant species. Decreased diversity of habitat structure in bush-encroached habitats appears to influence native savanna lizard assemblages. Our results are consistent with accumulating evidence suggesting that bush encroachment and its associated ecological impacts are reorganizing savanna ecosystems throughout southern Africa.

Introduction

Although humans have influenced the natural environment in Namibia for thousands of years, changes in land use practices over the last century have severely altered natural ecological processes (Louw & Seely, 1982, Prins & van der Jaeugd, 1993, Hoffman, 1997, Pallett, 1997). Recent human-mediated impacts include overgrazing, fire suppression, fragmentation of historical migration routes, establishment of permanent water sources and extirpation of the largest herbivores and top carnivores (for a review, see Barnard, 1998). A consequence of these ecological changes is the conversion of savannas to dense, acacia-dominated thickets with little grass cover, a process known as “bush encroachment.” This problem is especially prevalent on commercial ranchlands in central Namibia, to the extent that bush encroachment has measurably reduced the economic productivity of the Namibian livestock industry (Quan et al., 1994).

Livestock grazing has been the predominant land use practice in Namibia for several centuries; consequently, much of the native wildlife in Namibia (and throughout much of sub-Saharan Africa) must coexist with livestock production (Kinahan, 1991, Mabutt, 1984, Archer, 1995). Nevertheless, commercial ranchlands in Namibia still harbor a diverse assemblage of native species and have substantial potential as refuges for Namibia's biota. For example, the majority of Namibia's populations of native large ungulates (over 70%) and the largest remaining population of cheetahs in the world are found on commercial ranchland, outside of the formal protected area network (Joubert & Morsbach, 1982, Marker-Kraus et al., 1996). Considerable conservation efforts and government wildlife policies are focused on these large mammalian species (reviewed by Griffin, 1998, Richardson, 1998). While these efforts complement biodiversity conservation objectives in most cases (e.g. the cheetah, see Marker-Kraus et al., 1996), some less economically measurable and/or popular components of biodiversity may not be compatible with a system designed to promote livestock production and large, huntable game (Richardson, 1998). Most of the native species that do not have direct economic value in Namibia are found outside of the formal protected area network (Barnard et al., 1998). A better understanding of interactions between native species and human impacts is an important step towards native species conservation on commercial ranchland.

Here we present results of systematic surveys of an assemblage of lizard species in open savanna and bush-encroached habitats in central Namibia. Our purpose was to examine lizard species composition, relative abundance, and microhabitat use in open savanna and bush-encroached habitats. In a review of resource partitioning among species of amphibians and reptiles, Toft (1985) found habitat to be more important than temporal or dietary dimensions in interspecific separation. Patterns in the spatial use across habitat gradients by lizard assemblages have been well documented, yet few studies have examined the impacts of ecological processes on lizard assemblage stability at the landscape scale (Case, 1994, Vitt, 1994; but see Pianka, 1996). Although lizards have little direct economic value, they are important components of Namibian savannas and may be impacted by bush encroachment. We compared lizard assemblages associated with open savanna and bush-encroached habitat and interpreted our results with respect to accumulating evidence that bush encroachment and its associated ecological impacts are reorganizing savanna ecosystems throughout southern Africa.