Constraints on avian seed dispersal reduce potential for resilience in degraded tropical forests

Abstract

1. Seed dispersal - one of the many services supplied by biodiversity - is a critical process underpinning the resilience of tropical forests. Forest loss or degradation typically leads to defaunation, altering seed transfer dynamics and impairing the ability of forested habitats to regenerate or recover from perturbation. However, the extent of defaunation, and its likely impacts on the seed dispersers needed to restore highly degraded or clear-felled areas, remains poorly understood, particularly in human-modified tropical forest landscapes. 2. To quantify defaunation of seed-dispersing birds, we used field survey data from more than 400 transects in three regions of Brazil, first comparing the recorded assemblages with those predicted by geographic range maps, and then assessing frugivore habitat associations across gradients of land cover modification at local scales. 3. We found that current bird assemblages have lower functional trait diversity than predicted by species range maps in Amazonia (4-6%), with a greater reduction (28%) for the Atlantic Forest region, which has been more heavily deforested for a longer period. These reductions are probably caused by local extinctions of forest-dependent bird species following land-use change. 4. Direct measures of seed dispersal are difficult to obtain, so we instead focused on the potential for seed transfer inferred from shared species occurrence between land cover types. Of 83 predominantly frugivorous bird species recorded in relatively intact forests, we show that 10% were absent from degraded forest, and 57% absent from the surrounding matrix of agricultural land covers, including many of the large-beaked species. Of 112 frugivorous species using degraded forest, 47% were absent from matrix habitats. 5. Our findings suggest that degraded forest can supply seed dispersal services to adjacent cleared lands, and that direct transfer of seeds from intact forest to cleared areas may be limited, particularly for large-seeded trees. We conclude that resilience of tropical forest landscapes is best achieved by protecting a mosaic of forest types, including sufficient core areas of intact forest surrounded by buffer zones of degraded forest.