Responses of interspecific associations in mixed-species bird flocks to selective logging.
Abstract
Non-trophic interactions (or, inter-species associations) play a prominent role in determining community structure and function. Mixed-species bird flocks are networks of non-trophic associations that confer foraging and anti-predator benefits to participant species. Yet, the responses of these interspecific associations to anthropogenic environmental changes are poorly understood. Selective logging is pervasive in the tropics, and can affect associations in mixed-species bird flocks by altering resource availability and/or predation risk, or through the altered abundances of species participating in mixed flocks. Across a gradient of logging intensity, we examined how the number and strength of associations in two different mixed-species flock types responded to logging intensity, while simultaneously controlling statistically for changes in the abundances of species in response to logging. Across the logging gradient, we used network analyses to: (1) quantify the proportion of potentially realizable associations, and (2) measure the strengths of these realized associations. For both these analyses, we used null models to investigate whether changes in the network properties of mixed flocks were simply abundance-driven, or congruent with expectations of how flock properties might be modified by selective logging. In understorey flocks, after controlling statistically for changing abundances of participant species, the proportion of realized associations decreased with increasing logging intensity, whereas mean association strength did not show any relation with logging intensity. In midstorey flocks, both the proportion and mean strength of realized associations increased with increasing logging intensity. Synthesis and applications. By statistically separating abundance and behaviour-mediated effects, we show that interspecific associations in mixed-species bird flocks respond to potential resource and/or predation changes from logging, but that their functional roles persist in logged forest. We show that logged forests can conserve not only community richness, but also function. Thus, these logged forests must be prevented from ongoing conversion into non-forest habitats such as agriculture and plantation.