Assembly rules are rare in SE Australian bird communities, but sometimes apply in fragmented agricultural landscapes

Abstract

Assembly rules have been difficult to observe partly because species interactions vary with resource availability, the amount of disturbance, and species richness. Using bird survey data from two years in five regions of SE Australia (463 sites), we tested six predictions from assembly rules theory. We developed generalised linear models for all pair-wise species comparisons within a region, after first taking vegetation differences among sites into account. We found: 1) that disturbed and fragmented agricultural landscapes had more negative relationships because aggressive honeyeaters monopolised patches; 2) differences between wet and dry years (high and low resources) were attributed to emigration, immigration, and reduced intensity of negative interactions in the dry year, and not to more intense competition; 3) although negative correlations were expected among ecologically-similar species, we observed the opposite pattern with significant positive associations in fragmented landscapes; 4) relationships involving aggressive honeyeaters led to more negative relationships as predicted; however, this was only observed in fragmented landscapes and was not significant in every year; 5) in two agricultural landscapes, there were fewer negative relationships in low species-richness sites, probably because one species from each pair had low abundance. This enabled us to suggest dominance relationships among species pairs; 6) there was evidence of more negative relationships in low-disturbance sites in agricultural landscapes as predicted. However, instead of the mechanism predicted by theory (reduced disruption allowing assembly processes to approach equilibrium), we suggest that sites with low disturbance represent resources worthy of defence by dominant species. We conclude that bird communities in SE Australia are rarely limited by negative species interactions, although aggressive honeyeaters are a feature of fragmented landscapes. However, diverse mechanisms underlie the temporal and spatial variation in species interactions when they do occur, meaning that simple statements of assembly rules may not be useful in these systems.