Global drivers of change, such as climate and land use, may significantly impact organisms, yet ecologists know very little about how these stressors interact across scales to affect avian species. Our objective was to assess whether cross-scale interactions involving regional climate and the local proportion of land cover in agriculture influenced the persistence of three bird species of conservation concern: wood thrush (Hylocichla mustelina), prairie warbler (Setophaga discolor), and Kentucky warbler (Geothlypis formosa). We used data collected along North American Breeding Bird Survey (BBS) routes (each 39.4 km long with 50 survey stops) for four Bird Conservation Regions (BCRs) in the eastern United States. A species’ persistence for a route was calculated as the 3-year (2010 to 2012) mean proportion of the 10-stop route segments in which the species was detected. We measured the regional-scale variable (30-year mean daily temperature during the breeding season) within BCRs, and we measured the local-scale variable (proportion of land cover in agriculture) for landscapes within the species’ mean natal dispersal distance (1.1 km) along the BBS routes. Linear mixed-effect models and spatial eigenvector analyses were used to assess relationships between persistence and the main and interaction effects of climate and agricultural land cover.
Results/Conclusions
Persistence of each species was lower in areas with higher proportions of agricultural land cover. The decline in persistence of prairie warbler and Kentucky warbler in response to land cover was not affected by cross-scale interactions with regional climate, but the decline in persistence of wood thrush was. The decrease in persistence of wood thrush in response to increased agricultural land cover was stronger with higher regional temperatures. These results demonstrate that one environmental stressor acting at a broader spatial scale may exacerbate the effect of another stressor acting at a finer spatial scale. Examining cross-scale interactions, therefore, may uncover information vital for conserving species that are declining due to multiple stressors operating at different spatial scales. Cross-scale interaction effects of global drivers of change deserve much more research attention than they have received to date, and it will be important to incorporate any evidence of such effects into conservation planning and management for declining bird species.