COMPETITION AND HABITAT SELECTION IN NAMIB DESERT TENEBRIONID BEETLES

We tested whether intra- and interspecific competition could affect ha bitat selection in the two most abundant tenebrionid beetles, Physades mia globosa and Onymacris rugatipennis, in a dry riverbed in the Namib desert. The spatial distributions of these beetles at the microhabita t scale were negatively correlated. We performed a removal experiment, progressively removing first 25% and then a further 25% of the popula tion of the most abundant species, P. globosa, under the trees where m ost of the preferred food of both species is concentrated. There was n o response of O. rugatipennis to this removal in the tree habitat. In the open, barely-vegetated habitat where most O. rugatipennis are foun d, the number of this species caught in pitfall traps increased follow ing both removals and decreased following P. globosa replacement under the trees. It appears that intraspecific competition forces some P. g lobosa to occupy the open habitat. Interspecific competition between P . globosa and O. rugatipennis in the open habitat reduces the number o f O. rugatipennis that can co-exist with P. globosa there. Removal of P. globosa under the trees allows conspecifics in the open habitat to move under the trees, releasing O. rugatipennis in the open habitat fr om competition. This then results in an increase in the numbers of O. rugatipennis in the open habitat as a result of immigration from neigh bouring areas. We found that differences in foraging efficiency, measu red as giving-up times in artificial food patches, create a likely mec hanism of co-existence that explains the distinct preferences of these two species for tree and open habitats.