Mixed Support for Spatial Heterogeneity in Species Interactions: Hummingbirds in a Tropical Disturbance Mosaic

Abstract

Many natural landscapes experience frequent disturbance on a small scale. Disturbance loosens or disrupts relations between species, or between species and resources, characteristic of intact communities. One result is the release of previously scarce resources, leading to increased productivity and increased intensity of species interactions in disturbed patches as compared with undisturbed patches. Additionally, populations in disturbed sites may exploit resources in a more haphazard and opportunistic fashion than populations in undisturbed sites. The altered ecological conditions of disturbed sites may favor species different from those occupying undisturbed sites, leading to spatial heterogeneity in community composition. Nectar-feeding birds (mainly hummingbirds) inhabiting the natural disturbance mosaic of a Costa Rican cloud forest responded to habitat heterogeneity in complex ways. Whereas most ecological traits of hummingbird assemblages varied among patch types (understory of canopied forest; treefall gaps; large, landslide-like gaps), the direction of variation differed for different traits. Density of hummingbird food (nectar) was highest in treefall gaps, and some characteristics of hummingbirds (e.g., species diversity) reflected this enrichment. Variables that involve collective foraging by the entire hummingbird assemblage (e.g., intensity of interspecific competition) suggest that species interactions in the forest are the least haphazard, those in treefall gaps more haphazard, and those in large gaps the most haphazard. Even the largest gaps examined, however, were rarely invaded by hummingbird "weeds" available in the regional species pool, and interactions in these gaps showed only faint resemblance to those in the tremendously fluctuating competitive environments that characterize nectar-feeding bird assemblages in large anthropogenic old fields nearby or at other tropical sites. Our results, and reconsideration of results from other studies involving natural disturbance mosaics, suggest that responses of consumers to disturbance mosaics may often be subtle and complex. Comparisons between patch types in a natural disturbance mosaic need not resemble comparisons between points in a successional sequence after anthropogenic disturbance.