Macroecological signals of species interactions in the Danish avifauna

Abstract

The role of intraspecific and interspecific interactions in structuring biotic communities at fine spatial scales is well documented, but the signature of species interactions at coarser spatial scales is unclear. We present evidence that species interactions may be a significant factor in mediating the regional assembly of the Danish avifauna. Because >95% of breeding species (n = 197) are migratory, we hypothesized that dispersal limitation would not be important and that breeding distributions would largely reflect resource availability and autecological habitat preferences. Instead, we detected a striking pattern of spatial segregation between ecologically similar species at two spatial scales with a suite of null models that factored in the spatial distribution of habitats in Denmark as well as population size and biomass of each species. Habitat utilization analyses indicated that community-wide patterns of spatial segregation could not be attributed to the patchy distribution of habitat or to gross differences in habitat utilization among ecologically similar species. We hypothesize that, when habitat patch size is limited, conspecific attraction in concert with interspecific territoriality may result in spatially segregated distributions of ecologically similar species at larger spatial scales. In the Danish avifauna, the effects of species interactions on community assembly appear pervasive and can be discerned at grain sizes up to four orders of magnitude larger than those of individual territories. These results suggest that species interactions should be incorporated into species distribution modeling algorithms designed to predict species occupancy patterns based on environmental variables.

Fig. 1.

Species richness of Danish breeding birds (Left) and spatial variation in habitat diversity (HD) (Right) of grid cells at a grain size of 5 km × 5 km (25 km²). The HD score is the product of relative grid cell area and the probability that two points randomly chosen within a grid cell represent different habitat types (54). The HD score was used to parameterize null models of random species colonization independently. Species richness ranged from 1 to 109 species per cell (16, 60). The best-fitting power function was S = 27.93681(HD)^0.1916, r² = 0.1171. See Fig. S1 for comparable figures at the 10-km × 10-km (100-km²) grain size.

Fig. 2.

Summary of null-model analyses of species co-occurrence in ecological guilds of Danish birds (Tables S4 and S5).

Fig. 3.

Summary of null-model analyses of niche overlap in habitat utilization and electivity in ecological guilds of Danish birds (Tables S6 and S7).