Spatial synchrony is a widespread phenomenon in which nearby dynamic systems exhibit synchronous fluctuations that become less synchronous at greater distances. Synchrony in populations can be driven by dispersal, regional trophic interactions, or regional environmental variation (the Moran Effect). Recent work suggests that examining the ‘geography of spatial synchrony’ as well as temporal variation in this phenomenon, can yield valuable insights into regional population drivers. Grassland birds are a declining guild dependent on agricultural lands in many parts of North America. Farm management can affect survival and reproduction and vary annually due to regional economic and climatic forces. These effects, along with direct effects of climate, have the potential to leave differing geographic and temporal signatures of spatial synchrony. We used North American Breeding Bird Survey data to evaluate the degree of spatial synchrony in nineteen grassland bird species across North America over time (1966-1990 vs. 1991-2017).
Results/Conclusions
Numerous species exhibited pronounced spatial synchrony, and there was an overall increase in magnitude between the two time periods. Fifteen of 19 species became more synchronous, while only four became less synchronous. Changes were not uniform geographically or among species, but there were some common patterns. We found 'hot spots' of increasing and decreasing synchrony across all species in the Midwest and Northeast, respectively. Six passerine species contributed most to the increasing hot spot and, furthermore, showed high and increasing levels of cross-synchrony, or coincident population fluctuations, in this region. Mechanisms are currently being investigated and could relate to agricultural intensification, grassland conversion, or extreme weather fluctuations. In general, patterns of spatial synchrony in guilds of conservation concern have the potential to take us beyond the geography of population trends to illuminate regional population drivers as well as metapopulation vulnerability.