Pressures associated with anthropogenic global change can alter the diversity structure of communities by favoring some species while inhibiting others. Though all ecological communities exhibit some stochastic change over time, for birds, the broad and growing body of literature points to several systematic drivers of community shifts. A better understanding of how recently—and how quickly—communities are changing may help disentangle unusual change from change associated with the community’s historic range of variability. Our objectives are to: (1) determine which ecoregions have experienced the most (and the least) bird community change in North America from 1966-2018; and (2) determine whether ecoregion, latitude, climate, or time best predicts the magnitude of community change. We use principal components analysis to quantify change in bird communities by determining the location of each route-year in “species space” of the entire Breeding Bird Survey dataset count data and measuring the Euclidian distance between points. These distances (community dissimilarities) were used as the response variable in a series of generalized linear mixed models, in which we fitted with a single, explanatory variable (ecoregion, latitude, decade, mean summer temperature, mean winter temperature or average precipitation) and ranked with AIC (models with dAIC <2.0 were considered competitive).
Results/Conclusions
Throughout the 52-year study period, 552 bird species were detected by 9,383 unique observers along 5,178 BBS routes. Unsurprisingly, we found substantial variation between bird community changes across the 25 diverse ecoregions we investigated. North American warm deserts experienced the most community change in recent decades (2000-2018). Our top model (dAIC <2.0) indicated that ecoregion is the best predictor of bird community change at the sub-continent scale. This suggests that ecoregions are not responding uniformly to global change pressures. Because resources are limited and the cost to conserve biodiversity is high conservation triage—the process of prioritizing some conservation actions over others—is likely to be a necessary approach to conservation under global change. Such tactics will require a broad-scale understanding of ecosystem changes over time in order to best inform resource allocation. In other words, some community changes may be more urgent than others (e.g. the rapid and recent bird community change in the warm deserts of North America), so studies that examine the drivers of both rapid and incremental community change may inform future management under global change, especially when conservation resources are limited.