Dung beetle assemblage structure across the aridity and trophic resource gradient of the Botswana Kalahari: patterns and drivers at regional and local scales

Abstract

Understanding pattern and process at both regional and local scales is important for conservation planning although such knowledge of insects is frequently lacking. To assess patterns along a regional gradient of increasing aridity and diminishing food resources in the Botswana Kalahari, Scarabaeine dung beetles were sampled quantitatively using four dung types at three local sites in six regional areas. At regional scale, factor analysis of species abundance extracted a maximum of six factors, each dominated by a single area. Therefore, the statistical significance of regional spatial variation far outweighed that of dung type association. At local scale, six factor analyses of species abundance extracted from four to six factors. The importance of local dung type associations was relatively high but diminished with increasing local spatial heterogeneity. At regional scale, hierarchical analysis of oblique factors divided assemblages into unique local and shared regional components. Primary extended factors accounted for 40-50% of unique local faunal composition in five out of six areas. Two secondary extended factors showed either high shared proportional contribution to regional assemblage structure in the northeast with a steep decline to the southwest, or an opposite trend. Their point of intersection was consistent with a boundary zone between mesic northeast and arid southwest faunal components in the central Kalahari. Despite some inconsistencies in rank position between regression methods, rainfall, temperature, and mammal density / diversity were the strongest influences on regional patterns defined by secondary factors. Patterns are discussed according to conservation and changes in land usage around reserves.

Species richness – Energy relationships and dung beetle diversity across an aridity and trophic resource gradient

“…2), two were represented across the gradsect, aeolian sands at Chobe in the northeast and at Khutse, Mabuasehube, and Transfrontier in the southwest, with fossil lacustrine deposits in the intervening Mababe (Savuti) and Makgadikgadi Depressions (North CKGR) in the northeast. However, analysis of soil texture at each of 18 study sites showed no significant differences between landscape types as each comprised predominantly sand (adjusted figures: 97.1% -99.6% sand) (Tshikae et al 2012a). …”

“…The two regional components are correlated most strongly to annual rainfall, temperature during sampling, and an index derived from the density of 25 mammal species across the gradient (Tshikae et al 2012a). One southwest-centred regional component declined in proportional contribution to the northeast whereas the other component was centred in the northeast and declined to the southwest.…”

“…Across the three conserved regions studied in the Botswana Kalahari, northeast to southwest declines in the density (4.61 to 1.32 to 1.93 individuals / km 2 ) and combined biomass (5881.3 to 304.6 to 455.9 kg/km 2 ) of mammal species with a body size >4 kg (Tshikae et al 2012a) are consistent with the strong correlation shown between declines in mammal biomass with declines in rainfall and primary production in African savannas (Coe et al 1976). …”

“…The regional gradient in declining annual rainfall (>600 to <200 mm p/a) results in an opposing rainy season gradient of increasing average daily temperatures to the southwest that stems from reduced frequency of local rainfall events during summer when dung beetles were sampled (Tshikae et al 2012a). Substantial rainfall is an initiator of high activity in dung beetles (Davis 1995(Davis , 2002 and with greater frequency of events might increase both the temporal diversity of activity patterns and opportunities for breeding, leading to greater abundance and increased species richness in the northeast.…”

“…Substantial rainfall is an initiator of high activity in dung beetles (Davis 1995(Davis , 2002 and with greater frequency of events might increase both the temporal diversity of activity patterns and opportunities for breeding, leading to greater abundance and increased species richness in the northeast. A parallel regional gradient in declining mammal species richness (>100 to <70) (Andrews and O'Brien 2000), density, and biomass (Tshikae et al 2012a) results in reductions in energy availability through declines in dung type diversity, density of droppings, and amounts per dropping in arid southwestern localities (see Material and methods and Discussion). As a certain amount of dung type specialization has been recorded in dung beetles both in Botswana (Tshikae et al 2008) and elsewhere (Fincher et al 1970, Cambefort 1982, this might result in reduced opportunities for dung type specialization and loss of certain specialist species with increasing aridity (Tshikae et al 2012b).…”