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

The spatial heterogeneity of natural landscapes structures the distributions of species and relationships between species (MacArthur and Pianka 1966; Wiens 1976, 1986; Pickett and White 1985). Patchiness of habitats can affect the dynamics of populations (Reddingius and den Boer 1970; Strong 1983; Strong and Antolin, in press) and the dynamics of interactions between populations, such as exploitation (J-Iuffaker 1958; Gurney and Nisbet 1978), competition (Levin 1974; Chesson and Case 1986), and mutualism (Thompson 1982; Barton 1986). Even the nature of coevolution among species may vary among patch types (Thompson 1982). Most frequently, environmental patchiness is caused by disturbance. Many natural landscapes experience frequent small-scale disturbances, resulting in mosaics of recently disturbed patches and patches undisturbed for some time (Whittaker and Levin 1977; White 1979; Sousa 1984b; Pickett and White 1985). Biological effects of disturbance mosaics range from slight to profound, depending on the scale, frequency, and intensity of disturbances (Connell and Slatyer 1977; Connell 1978). The overall effect is to loosen or disrupt the species interactions, or interactions between species and abiotic resources, that characterize undisturbed habitat. One result is the release of critical resources, for example nutrients and sunlight, previously in short supply. If organisms or propagules that can exploit these resources are available, disturbance may increase the density of organisms, leading to regenerating patches that are pockets of high productivity and intense interactions (Connell and Slatyer 1977; Canham and Marks 1985; Sousa 1985; Blake and Hoppes 1986). The "loosening" process may also have other effects on resource use: responses of individuals, populations, or species assemblages to resources in disturbed sites may be more flexible and less precise than those in