Slowly eroding diversity: A century of climate change and land-use intensification on avian metacommunity dynamics in California

Background/Question/Methods

Forecasts of global change on biodiversity project recent range relationships with climate variables to the future, often with alarming conclusions. Yet, 20^th century environmental change has been pervasive, and few studies have documented local extinctions and population declines associated with recent climate change. Historical resurveys – where biodiversity surveys from the past are resampled – provide important opportunities to understand the influence of past environmental change on biodiversity and establish new benchmarks to understand future change. We conducted resurveys in 2009-10 of 100 species of songbirds at 70 sites originally surveyed by Joseph Grinnell and colleagues from 1911-40 in the California’s Coastal and Klamath Ranges, spanning 1000 km north to south from the Oregon border to northern San Luis Obispo County. We used occupancy models to provide unbiased estimates of detectability, occupancy, colonization, and extinction between the survey periods for each species, amalgamating results at the site level to measure effects on metacommunity turnover dynamics

Results/Conclusions

Model-averaged detection probabilities were two times higher for modern compared to historic surveys. The probability of site occupancy (ψ) changed little (-0.1 < Δψ < 0.1) for 57 of the 100 bird species resurveyed; for the other half, however, about twice as many species (n = 32) decreased in occupancy by > 0.1 as those that increased in occupancy (n = 15). At the species level, occupancy increases were primarily related to extinction rate negatively associated with climate change or land use intensification, whereas occupancy declines were associated with varied causes. Almost half the species that declined were not related to any covariate tested. At the site level, metacommunity rates of extinction (E) were positively related to change in mean annual temperature and to latitude, while colonization (C) was unrelated to climate or land-use change.  When rates were combined to model metacommunity dynamics, turnover (E + C) was positively related to changes in mean annual temperature and latitude, and negatively related to elevation. Based on 20^th Century patterns, future climate change is projected to lead to a slow erosion of avian species richness in this region.