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., et al. 2012. Positive relationships between association strength and phenotypic similarity characterize the assembly of mixed-species bird flocks worldwide. American Naturalist 180: 777-90. AuthorsHari Sridhar, Umesh Srinivasan, Robert A. Askins, Julio Cesar Canales-Delgadillo, Chao-Chieh Chen Submitted February 10, 2012; Accepted August 1, 2012; Electronically published MONTH? xx, 2012 Online enhancement: appendix. Dryad data: http://dx.doi.org/10.5061/dryad.th198.abstract: Competition theory predicts that local communities should consist of species that are more dissimilar than expected by chance. We find a strikingly different pattern in a multicontinent data set (55 presence-absence matrixes from 24 locations) on the composition of mixed-species bird flocks, which are important subunits of local bird communities the world over. By using null models and randomization tests followed by meta-analysis, we find the association strengths of species in flocks to be strongly related to similarity in body size and foraging behavior and higher for congeneric compared with noncongeneric species pairs. Given the local spatial scales of our individual analyses, differences in the habitat preferences of species are unlikely to have caused these association patterns; the patterns observed are most likely the outcome of species interactions. Extending group-living and social-information-use theory to a heterospecific context, we discuss potential behavioral mechanisms that lead to positive interactions among similar species in flocks, as well as ways in which competition costs are reduced. Our findings highlight the need to consider positive interactions along with competition when seeking to explain community assembly.

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Orchid bees (Apidae, Euglossini) are important pollinators in the Amazon forest. In eastern Brazilian Amazon, secondary forest and pastures are being replaced by oil palm plantations. Here, we tested the role of forest reserves and riparian corridors in maintaining orchid bees. We sampled bees in three different soil-type uses, comparing richness, abundance, and assemblage composition. Estimated richness was lowest in palm plantations than in forest reserves and riparian corridors on diversity of orchid bees. Riparian corridors had the highest abundance, followed by reserves, and oil palm plantations. Bee assemblage also varied with land cover, with the reserves having the most distinct composition. We also identified indicator bees for primary forest. Our results demonstrate riparian corridors and forest reserves can maintain orchid bees in oil palm landscapes.

Interactions between species in groups are often ignored in studies of the effects of anthropogenic change on species' persistence, and yet, given their global ubiquity, mixed‐species groups have the potential to be models for community ecology. In this paper we examine the impacts of rural community activities that are pervasive in tropical regions on mixed‐species flocks of birds in a unique coastal tropical dry forest ecosystem, with the aim of advancing both our understanding of mixed‐species associations and how best to conserve them. We examined the effects of small‐scale clearing of trees and livestock grazing on mixed flocks in two vegetation types in the highly threatened Tumbesian region of Ecuador. Because the fitness benefits of flocking come from a reduced risk of predation or enhanced feeding efficiencies, or both, habitat degradation could lead to changes in the benefits of flocking and thus flock characteristics. We predicted that, in more disturbed vegetation, flocks would have fewer species and individuals and that species would show lower flocking propensities and feeding efficiencies. Based on observations of 431 flocks, we found that these predictions were supported in tropical dry forest but not always in arid scrub vegetation. Less disturbed tropical dry forest had greater mean species richness per flock than more disturbed tropical dry forest, but mean species richness per flock did not differ between more and less disturbed arid scrub. Flock species richness and composition differed across the disturbance levels in tropical dry forest, but not in arid scrub. Many species had their highest flocking propensities and greatest foraging efficiencies in less disturbed, compared to more disturbed, vegetation. We show that existing park management, which allows community development and livestock grazing within national park borders, leads to degradation of the vegetation and disrupts species' interactions in tropical dry forest. We provide specific conservation recommendations to mitigate these disruptive influences. Examining intraspecific associations across diverse communities is of the utmost importance to both answering basic theoretical questions in community ecology and conserving species diversity in the face of anthropogenic landscape change.