The Direct and Indirect Effects of Insectivory by Birds in Two Neotropical Forests
Most forest birds include herbivorous arthropods in their diet. Experimental tests of whether bird predation can affect arthropod abundance and plant damage are few, however, and restricted to saplings in relatively low diversity systems. In two lowland tropical forests, I tested whether birds indirectly defend canopy and understory plant foliage from arthropod herbivores. The two forest sites differed in age, tree species diversity and rainfall seasonality. Birds significantly reduced local arthropod densities in the canopy of a seasonally dry forest. Moreover, the taxonomic composition of arthropods changed in the absence of bird predation, with greater densities of chewing arthropods where foliage was inaccessible to birds. Consequentially, leaf damage increased by 86% where foliage was inaccessible to bird foraging. In contrast, effects of bird predation were not observed on understory saplings, where leaf production and turnover rates were lower. Furthermore, I observed little evidence of strong predator effects in either stratum within the more mature, wetter forest. Densities of birds were greater in the seasonally dry forest relative to the wetter site. Attack rates by birds were similar in the two forests, but capture rates were greater in the seasonally dry forest. Overall, cascading effects of bird predation were observed when and where resource availability for arthropods was greatest. These results imply that birds are an important element of some Neotropical communities via their indirect defense of canopy trees. Such conditional results, however, suggest that broad generalizations about the outcome of multitrophic interactions are inappropriate for tropical forests, where environmental heterogeneity is great.
University of Illinois - Urbana-Champaign
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