Effects of woody plant species richness on mammal species richness in southern Africa


*Hong Qian, Research and Collections Center, Illinois State Museum, 1011 East Ash Street, Springfield, IL 62703, USA. E-mail: hqian@museum.state.il.us


Aim  To determine the relationship between the species richness of woody plants and that of mammals after accounting for the effect of environmental variables.

Location  Southern Africa, including Namibia, South Africa, Lesotho, Swaziland, Botswana, Zimbabwe, and part of Mozambique.

Methods  We used a comprehensive dataset including the species richness of mammals and of woody plants and environmental variables for 118 quadrats (each of 25,000 km2) across southern Africa, and used structural equation models (SEMs) and spatial regressions to examine the relationship between the species richness of woody plants and of mammal trophic guilds (herbivores, insectivores, carni/omnivores) and habitat guilds (aquatic/fossorial, ground-living, climbers, aerial), after controlling for environment. We compared the results of SEMs with those of single-predictor regressions (without controlling for environment) and of spatial regressions (controlling for both environment and residual spatial autocorrelation).

Results  The geographical variation of mammal species richness in southern Africa was strongly and positively related to that of woody plant species richness, and this relationship held for most mammal guilds even when the influence of environment and spatial autocorrelation had been accounted for. However, the effect of woody plant species richness on the richness of aquatic/fossorial species almost disappeared after controlling for environment, suggesting that the congruence in species richness patterns between these two groups results from similar responses to the same environmental variables. For many mammal guilds, the relative role of environmental predictors as measured by standardized partial regression coefficients changed depending on whether non-spatial single-predictor regressions, non-spatial SEMs, or spatial regressions were used.

Main conclusions  Woody plants are important determinants of the species richness of most mammal guilds in southern Africa, even when controlling for environment and residual spatial autocorrelation. Environmental correlates with animal species richness as measured by simple correlations or single-predictor regressions might not always reflect direct effects; they might, at least to some degree, result from indirect effects via woody plants. Interpretations of the strength of the effect of environmental variables on mammal species richness in southern Africa depend largely on whether spatial or non-spatial models are used. We therefore stress the need for caution when interpreting environmental ‘effects’ on broad-scale patterns of species richness if spatial and non-spatial methods yield contrasting results.