Multi-scale determinants of dung beetle assemblage structure across abiotic gradients of the Kalahari–Nama Karoo ecotone, South Africa

Abstract

AIM: Changing conditions across spatial gradients are primary determinants of biotic regions, local habitats, and distributional edges. We investigate how a climatic gradient and edaphic mosaic interact as multi-scale drivers of spatial patterns in scarabaeine dung beetles. The patterns are tested for congruency with ecoregion and floral boundaries over the same gradient, as responses to physical factors often differ among higher taxa. LOCATION: Southern Africa and the Nama Karoo–Kalahari ecotone, Northern Cape, South Africa. METHODS: Data consisted of the climatic distributions of 104 species and their abundances at 223 sites in two ecoregions/floral biomes, four bioregions, and 13 vegetation units. Factor analyses determined the biogeographical composition of the species, and regional- to local-scale patterns in species abundance structure. Hierarchical analysis of oblique factors determined the proportional contribution of spatial variance to patterns. One-way anova was used to test for significant separation of patterns along factor axes. Stepwise multiple regression was used to determine correlations of five physical attributes with species richness, Shannon-Wiener diversity, and factor loadings for the study sites. RESULTS: Four biogeographical influences overlap in the study region, although rank contribution declines from south-west arid through north-east savanna to widespread and south-east highland taxa. Species abundance structure comprises five subregional patterns, two centred to the north-east (Kalahari, Isolated Kalahari Dune) dominated by Kalahari influence, and three to the south-west (Nama Karoo subdivisions: Bushmanland, 'Upper', 'Stony Prieska') dominated by south-west arid influence. Kalahari deep sands are characterized especially by a warmer, moister climate, whereas the Nama Karoo mosaic of deep or stony soils is characterized especially by north-west aridity (Bushmanland), south-east cooler temperatures ('Upper'), or excessively stony soils ('Stony Prieska'). Four of the subregional patterns each comprised three localized patterns related primarily to relative stoniness, edge effects from geographical position, or incidence of rainfall. Species richness and diversity declined with decreasing rainfall and increasing stoniness. MAIN CONCLUSION: Climatic and edaphic factors are important multi-scale determinants of spatial patterns in dung beetle assemblage structure, with edaphic factors becoming more important at local spatial scales. The patterns are roughly congruent with the Kalahari Savanna–Nama Karoo ecotone at the floral biome or ecoregion scale, but show limited coincidence with finer-scale floral classification.