BEETLE SPECIES RESPONSES TO TROPICAL FOREST FRAGMENTATION

The effects of forest fragmentation on beetle species composition were investigated in an experimentally fragmented tropical forest landscap e in Central Amazonia. Leaf-litter beetles were sampled at seven dista nces from the forest edge (0-420 m) along forest edge-to-interior tran sects in two 100-ha forest fragments and two continuous forest edges, and at an identical series of distances along two deep continuous fore st transects. Additional samples were taken at the centers of two 10-h a forest fragments and two 1-ha fragments. This sampling regime allowe d discrimination between edge and fragment area effects. Beetle specie s composition changed significantly and independently with both decrea sing distance from forest edge and decreasing fragment area. Edge effe cts on species composition were mediated by six important environmenta l variables: air temperature, canopy height, percent ground cover of t wigs, litter biomass, litter moisture content, and an air temperature X distance from edge interaction effect, due to the different temperat ure profiles of edges with differing edge vegetation density. Populati on densities of 15 of the 32 most abundant beetle species tested (47%) were significantly affected by forest fragmentation. Species response s were classified empirically into four major categories: (A) edge sen sitive, area insensitive; (B) area sensitive, edge insensitive; (C) ed ge and area sensitive; and (D) edge and area insensitive. Within these categories, trends in density were either positive (deep-forest speci es), or negative (disturbed-area species), with species showing the fu ll spectrum of responses to fragmentation. The vast majority of specie s were adversely affected. Estimated species loss rates from forest fr agments were: 49.8% of common species from 1-ha fragments, 29.8% from 10-ha fragments, and 13.8% from 100-ha fragments. Declining density wa s a significant precursor of species loss from forest fragments, but o ther species that did not show significant population density response s to fragmentation were also absent from some fragments, presumably by chance. The probability of species loss from forest fragments was not correlated with body size or trophic group for the 32 common species, although for the entire beetle assemblage (993 species) proportions o f species in different trophic groups changed significantly with fragm entation. Rarity and population variability (in undisturbed forest) we re significant predictors of susceptibility to fragmentation. Surprisi ngly, though, common species were significantly more likely to become locally extinct in small fragments than rarer species. This lends empi rical support to models of multispecies coexistence under disturbance that suggest competitively dominant but poorly dispersing species are the first to become extinct due to habitat destruction. Thus, rarer sp ecies are predicted to be better dispersers and better at persisting.