Birds are thought to be important biocontrol agents in agricultural systems as their diets often include many invertebrates considered to be agricultural pests. The effectiveness of birds in pest management, however, is often context-dependent as the indirect effects of birds on plants are influenced by several factors (e.g. primary productivity, food web complexity, predation efficiency). Understanding which factors play greater roles in contributing to effective biocontrol by birds is important for predicting the community-wide consequences associated with the decline of bird populations. Thus, we asked the question whether birds control agricultural pests on the island of Rota in the western Pacific. Specifically, we tested the following hypotheses: (1) birds control herbivorous insects, leading to reduced plant damage and increased crop productivity, but that (2) spiders respond to bird presence, and thus may influence plant damage and productivity. We planted long beans and eggplants on three working farms and set up bird exclosures using netting around half of the plants. We surveyed the number of spiders, estimated herbivory damage, and counted the number of fruits at harvest on planted crops.
Results/Conclusions
The influence of birds on plant damage and productivity varied between crops and farms, and was greater when pests were more abundant. Herbivore damage did not differ between treatments, but there were differences in spider abundance and fruit production. Spider abundance was higher in the exclosure treatment than in the open treatment at one farm, but the opposite is true for the other two farms. Lastly, we found that more fruit was collected from long beans in the open treatment than the exclosure, but this difference was only significant in the farm where greater spider abundance was observed in the exclosure treatment. This farm had a larger pest outbreak than the other two, which may be the driving force behind our observed results. Overall, our findings suggest that birds may be effective biocontrol agents in systems with greater pests.