Tuesday, August 7, 2007

PS 38-197: Seed predation, dispersal, and chemical camouflage: Rethinking the benefits of vertebrate seed dispersal

Melissa J. Simon1, Karen M. Reagan1, David C. Haak1, Joshua J. Tewksbury1, and Doug Levey2. (1) University of Washington, (2) University of Florida

Seed dispersal by animals is commonly thought to reduce seed predation by allowing seeds to escape an area of high mortality near the parent plant. Most work has focused on location-related benefits to dispersal. Less often recognized is that seed consumption and defecation can dramatically change context-related factors, which we define as the nutritional, chemical, and structural properties of dispersed seeds. Here we compare location-related benefits of dispersal to context-related benefits in Bolivian wild chilies (Capsicum chacoense).  In two populations, individual chili plants produced 5 to 500 fruits per season, distanced 1 to 10m between plants, and had Elaenia parvirostris (small-billed flycatcher) as their primary fruit consumer.  In both populations, we compared seed predation rates of seeds passed through the gut of elaenias to predation rates of seeds taken directly from fruit. We conducted this experiment under the canopies of the chili plants, and 5m from the nearest chili canopy. We found no evidence for location effects, as seed predation rates were equal under and away from fruiting chilies.  We did find strong evidence for context-related impacts: seed predation was 20-43% lower in seeds that had passed through elaenias than it was for seeds that had not. Using seed choice experiments with ants, the primary seed predators, we determined that removal of lipids from the seed coat during gut passage is the mechanism by which consumption by elaenias reduces the risk of seed predation. We suggest that vertebrate-dispersed seeds may be less conspicuous to seed predators because their chemical identity is “camouflaged” by gut treatment.