Time trumps habitat in the dynamics of an avian community

Predictions of change in avian biodiversity often fail to account for the natural dynamism of species assemblages. Information from historical datasets can be useful indicators of how avian communities may change in the future. However, simple comparisons of diversity and abundances over time may overlook additional changes in bird-habitat associations. In 2013, we revisited sites from a unique, highly detailed avian survey from the Willamette Valley, Oregon, conducted in 1952. Our objectives were to quantify and describe the extent of avian community change; relate observed species assemblages to changes in vegetation type and land-use class; and identify species with specific associations to either survey era. Using nonparametric ordination, we assessed whether the distribution of the avian community within the environmental space varied temporally. We used blocked indicator species analysis to identify species with specific association to historical or modern survey eras. Nonparametric permutation procedures identified both plot, each representing a different major habitat type, and year as significant factors defining communities. Year was more strongly related to the second ordination axis than the other environmental variables examined. Ordination of survey sites in species space confirmed modern avian communities were significantly different from their historical counterparts. Bird species richness increased and community composition changed by at least 50% despite the physiognomic characteristics of each plot's habitat and the surrounding landscape remaining comparatively stable. Because of this, we conclude time trumped, or was more strongly associated with community change, than alterations in physiognomic habitat. These results suggest avian communities are naturally dynamic even in areas with relatively stable habitat conditions. Predictions that only address changes in climate, land use, or vegetation cover type may fail to predict changes in avian community composition. Historical datasets are a valuable means of understanding real-world changes in avian communities. We suggest additional factors, such as vegetation structure and microhabitat, may predict fine-scale shifts in avian species assemblages. Nevertheless, major changes in community composition occurred with apparently minor shifts in physiognomic habitat characteristics in our study area.