Processes driving temporal dynamics in the nested pattern of waterbird communities

Nestedness is a common pattern of bird communities in habitat patches, and it describes the situation where smaller communities form proper subsets of larger communities. Several studies have examined the processes causing nestedness and the implications for conservation, but few have considered the temporal changes in these processes. We used data from 6 years and two seasons (wintering and breeding) to explore the temporal changes in the causes of the nested pattern of a waterbird community in man-made irrigation ponds. Nestedness was significant in both seasons and in all years, and thus temporally stable. Despite the nestedness of waterbird communities, the proportion of idiosyncratic species (species that do not follow the nested pattern) was higher than in other studies. Furthermore, the idiosyncratic species often had endangered status. Selective colonisation and, mainly, selective extinction were the most important factors producing the nested pattern. In addition, the nested structure of the microhabitats at the ponds also caused the pattern. The causes of the pattern changed temporally even in the absence of big disturbance events. In general, breeding communities were more stable than wintering communities, and the seasonal differences in the causes of the nestedness were larger than the inter-annual differences. Consequently, studies of community nestedness from only one snapshot in time should be considered with caution.