Print PageTop predators can stabilize communities and influence the quality of ecosystem services by causing indirect effects within food webs known as trophic cascades. The strength of trophic cascades can depend on ecosystem productivity, species traits, and interactions among predator species. Because mangrove forests provide critical ecological and economic services and are threatened by anthropogenic factors, we decided to evaluate the potential for top predators (birds) in these systems to impact animal communities and indirectly alter plant damage and primary productivity through trophic cascades. To quantify the impact of birds on mangrove trees and associated arthropods, we excluded birds from Rhizophora mangle (red mangrove) canopies on two islands off the coast of Belize. We constructed 1m3 PVC frames around entire small, slow-growing trees (situated in nutrient-poor areas) and around individual branches of large, fast-growing trees (situated near nutrient-rich areas). Half of these frames (N=20) were covered with propylene netting to exclude birds, while the other half were left as controls. We searched for arthropods, recorded levels of herbivory, and measured branches on each tree 0, 10, 20, 33, and 54 weeks after treatments were applied.
Results/Conclusions
In the absence of birds, arthropod densities and plant damage increased. Arthropod predators such as ants and spiders were more likely to be present within bird exclosures in both nutrient rich and poor conditions. Herbivores were not commonly found in surveys, except for leaf miners, which were more abundant when birds were absent, regardless of nutrient levels. Plant damage was more strongly affected by bird removal in nutrient poor conditions. Small slow growing plants from which birds were excluded experienced 157% greater leaf herbivory and 50% greater bud herbivory, while fast growing branches of large plants experiences 13% greater folivory and 12% more bud herbivory. Plant biomass did not differ in the presence or absence of birds. From these results, we conclude that avian top predators on mangrove islands exert effects on their arthropod prey that cascade down the food chain, altering plant damage but not plant productivity. An experimental simulation of severe folivory and bud damage indicated that leaf damage has little effect of plant growth, but bud damage increases branching on fast growing trees and decreases branching on slow growing trees. We speculate that the loss of birds on mangrove islands could have important consequences for these ecosystems.
