Geographic coincidence of abundance, richness, conservation value, and response to climate of U.S. landbirds

Background/Question/Methods

Distributional patterns across the U.S. of five avian community breeding season characteristics – mass, richness, ranking of species vulnerability to extirpation, percentage of species’ global abundance present at sites (Conservation Index, CI), and position along the ecological gradient underlying species composition - were analyzed, their co-variation described, and projected effects of climate change on the characteristics and their co-variation modeled.  Higher values of mass, richness, and CI are generally preferred from a conservation perspective.  However, higher values may not coincide among the characteristics geographically so we ask how regions of the U.S.  differ in their value for conservation depending on which characteristic is chosen for setting conservation priorities.

Results/Conclusions

Over approximately 75% of the U.S., projected changes in June temperature and precipitation from current time to year 2080 conditions were associated with decreased averaged values of richness, mass, and CI, implying decreased conservation value for birds.  Among the five characteristics, highest correlation with longitude was observed for community richness, which declined from east to west across the lower 48 states of the U.S., and the ecological gradient that underlies community composition, as represented by principal curve (PC) scores.  The Combined Score (CS), a measure of a species’ threat of decline or extirpation, exhibited the strongest latitudinal pattern, declining from south to north.  The ecological gradient underlying composition exhibited a demarcation near the 100^th meridian, separating the U.S. grossly into two similar sized avian ecological provinces that diverged from each other with projected changes in June temperatures and precipitation from current to 2080.  Anticipated climate related changes in the five characteristics by 2080 were more weakly correlated with latitude or longitude then the responses themselves, implying less distinct geographic patterns of characteristic change than in the characteristics themselves.  Climate changes projected for 2080 were associated with geographic shifts in mass, CS, and PC values, a moderate overall decline in CI, and general decline in species richness.