Heightened nest loss in tropical forest fragments despite higher predator load in core forest

Abstract

Tropical understory birds have declined due largely to habitat loss and fragmentation. Here, we revisited a study conducted three decades ago and used artificial nests to examine depredation rates in a Costa Rican biological corridor. Using camera trap data, we compared potential nest predator detection rates at experimental tinamou ground nests in La Selva Biological Station and at sites in five local forest fragments. Nest predator detections were positively associated with landscape-scale core forest and distance away from forest edge, as well as with local-scale human trails, and negatively associated with primary forest compared to secondary growth. Twenty-two of 52 artificial nests were depredated, which was similar to previous research in the area. Mammalian and avian predators were common nest predators, but unknown predators (presumably snakes) were responsible for half of nests lost. Nests within La Selva core forest had lower probability of nest loss compared to fragments despite exhibiting higher predator detection rates. Yet other fragmentation covariates such as distance from forest edge, nest occurrence on human trails, or forest age were not associated with nest loss. We suggest that concentrated foraging is the underlying mechanism behind the community interactions that we observed. Community members exist in concentrated use areas within forest fragments, which results in heightened predator foraging rates and thus stronger interactions in fragments despite more predators encountering the nests in core forest. Fragmentation is a global phenomenon and we suspect that concentrated community use of limited resources is driving species to interact more strongly than in natural ecosystems.