TROPHIC STRUCTURE STABILITY AND EXTINCTION DYNAMICS OF BEETLES (COLEOPTERA) IN TROPICAL FOREST FRAGMENTS

A first analysis of the stability of trophic structure following tropi cal forest fragmentation was performed in an experimentally fragmented tropical forest landscape in Central Amazonia. A taxonomically and tr ophically diverse assemblage of 993 species of beetles was sampled fro m 920 m(2) of leaf litter at 46 sites, varying in distance from forest edge and fragment area. Beetle density increased significantly toward s the forest: edge and showed non-linear changes with fragment area, d ue to the influx of numerous disturbed-area species into 10 ha and 1 h a fragments. There was a marked change in species composition with bot h decreasing distance from forest edge and decreasing fragment area, b ut surprisingly this change in composition was not accompanied by a ch ange in species richness. Rarefied species richness did not vary signi ficantly across any of the sites, indicating that local extinctions of deep forest species were balanced by equivalent colonization rates of disturbed-area species. The change in species composition with fragme ntation was non-random across trophic groups. Proportions of predator species and xylophage species changed significantly with distance from forest edge, but no area-dependent changes in proportions of species in trophic groups were observed. Trophic structure was also analysed w ith respect to proportions of abundance in six trophic groups. Proport ions of abundance of all trophic groups except xylomycetophages change d markedly with respect to both distance from forest edge and fragment area. Local extinction probabilities calculated far individual beetle species supported theoretical predictions of the differential suscept ibility of higher trophic levels to extinction, and of changes in trop hic structure following forest fragmentation. To reduce random effects due to sampling error, only abundant species (n greater than or equal to 46) were analysed for extinction probabilities, as defined by abse nce from samples. Of these common species, 27% had significantly highe r probabilities of local extinction following fragmentation. The major ity of these species were predators; 42% of all abundant predator spec ies were significantly more likely to be absent from samples in forest fragments than in undisturbed forest. These figures are regarded as m inimum estimates for the entire beetle assemblage because rarer specie s drill inevitably have higher extinction probabilities. Absolute loss of biodiversity will affect ecosystem process rates, but the differen tial loss of species from trophic groups will have an even greater des tabilizing effect on food web structure and ecosystem function.