Effects of predation on community assembly and spatial dispersion of breeding forest birds

In a landscape context, nesting avian predators cause variation in predatio n risk with respect to the distance to their nests, which may have both dir ect (predation) and indirect (predation risk) effects on the prey community . We studied the community assembly and spatial distribution of a breeding forest bird community in a structurally complex forest landscape surroundin g forest-hunting Sparrowhawk (Accipirer nisus) nests. Birds were censused o n transect arrays within 1-ha study plots divided into four subplots. We de termined whether the distribution of species within study plots was clumped , random, or uniform. Because predation risk is higher nearer a hawk's nest than farther away, we predicted that species would show a more clumped dis tribution within study plots near the Sparrowhawks' nests than farther away . We also expected densities and community patterns to vary with respect to predation risk. The results suggest that predation and/or its risk has a c omplex and local effect on the prey community. The predation risk negativel y affected the number of species and densities of birds only in the first y ear (1996) of study. The relationship between the density of large birds (g reater than or equal to 20 g, preferred prey) and predation risk was also n egative in 1996, but not in 1997, whereas the density of small birds (<20 g ) was not affected by predation risk in either year. There was no relations hip between predation risk and clumping for all species together. When we a nalyzed separately plots that contained large and small birds or large bird s alone, however, increasing predation risk positively affected the index o f dispersion, but only in thicket (one of the forest types), which is proba bly the preferred hunting habitat for Sparrowhawk. These results are well i n line with studies made on predator-prey interactions in farmlands. In tho se studies, however, the effects of predators on prey communities were much stronger and extended farther in the landscape from the nest of predators than in this forest study. A possible reason for this difference is higher habitat structural complexity in forests, which may mitigate the effect of predators.