Wednesday, August 8, 2007: 11:10 AM
B3&4, San Jose McEnery Convention Center
The role of insectivorous birds in regulating arthropod populations and reducing consequent herbivore damage has received attention in both natural and managed ecosystems. While it is widely believed that bats have similar impacts on herbivore populations, little quantitative data exist demonstrating their effects on arthropods. We therefore studied the impacts of avian and chiropteran insectivory on the standing arthropod crops in Finca Irlanda, an organic shade coffee plantation in southwestern Mexico supporting a diverse population of foliage-gleaning insectivorous birds and bats. In January 2007, we erected 22 blocks of exclosures, each block consisting of 4 treatments: birds excluded, bats excluded, birds+bats excluded, and control. Exclosures consisted of agricultural netting (mesh size 20x20mm) erected around frames enclosing individual coffee plants. Bird exclosures were erected only during the day, and bat exclosures during the night. Arthropods on experimental plants were censused biweekly; after 50 days arthropods were collected for identification, and herbivore damage to leaves assessed. Preliminary results indicate an increase in arthropod populations after 4 weeks of exclosure treatments, with significant increases in large (≥5 mm) arthropods, orthopterans, blattarids, and non-sternorrhynchan homopterans. Combined bird+bat exclosures had the greatest effect, followed by the bird-only and then bat-only treatments. In some cases the effects of bird+bat exclosures were non-additive, exceeding the combined effects of bird- and bat-only exclosures. Our preliminary results suggest a synergistic interaction between bird and bat predation, demonstrating how maintenance of diverse predator communities can result in increased impacts on lower trophic levels. We discuss our results in the contexts of ecosystem services in managed areas and top-down effects of vertebrate insectivory in structuring tropical communities.
