Intensive agriculture erodes beta-diversity at large scales

Background/Question/Methods

Biodiversity is declining from unprecedented land conversions, which replace diverse, low-intensity agriculture with vast land expanses under homogeneous, intensive production. Despite documented losses of species richness, consequences for beta-diversity, the compositional change of communities between sites, are largely unknown. beta-diversity depends critically on maintaining differences in environmental conditions between sites; therefore, replacing previously distinct habitats with homogenous agricultural fields may erode biodiversity beyond current dire predictions. We analyze the consequences of agricultural intensification for tropical bird beta-diversity. The dataset consists of 2,640 censuses, conducted at 44 transects across Costa Rica and surveyed by the same expert observer from 2000 to 2009. We contrast bird communities located in native forest (forest fragments and preserves; n= 12), low-intensity agriculture (mixed crop fields with remnant trees; n= 16), and high-intensity agriculture (vast monocultures and pastures; n= 16).

Results/Conclusions

We show that lower-intensity agricultural practices sustain beta-diversity across large scales on a par with forest. In both forest and low-intensity agriculture, bird communities change as vegetation structure naturally turns over with distance. By contrast, we find that high-intensity agricultural practices homogenize vegetation structure across great distances, causing bird community turnover to decline by ~40% relative to the turnover in forest or lower-intensity agriculture. At large scales, this results in a significant decline in bird beta-diversity. This pattern suggests that biological mechanisms (turnover with distance and vegetation structure) dominate in low-intensity agriculture, while neutral-like mechanisms dominate in high-intensity agriculture. Thus conversion to high-intensity agriculture is a recipe for accelerating diversity loss, especially in a world in which forests will likely be subject to increasing clearing and degradation. These findings point the way towards multi-functional agricultural systems that maintain productivity while simultaneously conserving biodiversity.