Landscape constraints on functional diversity in tropical agroecosystems

Understanding functional diversity in the tropics needs a perspective from human-dominated landscapes, because most species experience their surroundings at spatial scales beyond the plot level, and spillover across natural and managed ecosystems is common. Using  new data analyses of tropical insects and tropical birds, we found that agricultural bird species have larger populations and experience greater habitat and diet breadth than forest species. Birds in tropical agroecosystems comprise more frugivorous and less insectivorous species, affecting relative importance of seed dispersal and biological control. Similarly, insect predators of plant-feeding arthropods were more diverse in Ecuadorian agroforest and forest compared with rice and pasture, and also bee diversity in Indonesia profited from forested habitats. Hence, diversity of bird and insect predators as well as bee pollinators suffered from agricultural habitat transformation, thereby contrasting with the pattern in avian seed dispersers. Similarity of communities is higher in agricultural than natural systems, as shown for both birds and insects in our analyses, so the higher beta diversity of natural communities needs to be taken into account. Bird diversity was also higher in small than large farmer landscapes, presumably due to higher management heterogeneity and related community dissimilarity. Such patterns are little known, but important predictors of landscape resilience, i.e. the capacity of communities to reorganize after disturbance and to sustain ecological functioning. Proximity of land-use systems to forests is an important aspect of landscape configuration and known to influence functional diversity, but there is generally little evidence for data-based management, making recommendations to enhance high functional biodiversity and sustainability in tropical human-dominated landscapes difficult.