Anthropogenic climate change will indirectly affect species coexistence through shifting distributions which may create novel interactions and alter existing interactions. While there have been many theoretical advances in understanding these changes, empirical data showing such dynamics in wild populations is lacking. To address this gap, we analyzed 42 years of breeding colony census data for two sympatric seabird species along the Atlantic Coast in New Jersey, the Least Tern (Sterna antillarum) and Black Skimmer (Rynchops niger). Bi-weekly perimeter counts of all known colonies were conducted annually by biologists and contractors of the NJ Department of Environmental Protection after these species were listed as state endangered in 1976. Based on anecdotal observations, we hypothesized the frequency of coexistence between Black Skimmers and Least Terns in breeding colonies had increased over four decades. Further, we hypothesized increased breeding season temperature as an indirect driver by promoting greater overlap in nesting habitat preferences. To control for the confounding effect of time we detrended the time series data for both the frequency of coexistence and the mean maximum breeding season temperature. Simple linear models were used to analyze the resulting anomalies.
Results/Conclusions
The frequency of coexistence between Black Skimmers and Least Terns significantly increased from 1976 to 2018, concurrent to greater overlap in nesting habitat. Historically, most Black Skimmer colonies were distributed in salt marshes. However, there was a significant decline in the number of these salt marsh colonies, while the number on sandy beaches, where Black Skimmers can nest with Least Terns, remained stable. Analysis of the detrended coexistence and breeding season temperature time series showed a significant positive relationship where years with temperatures above what would be predicted by the time trend corresponding to those with frequencies of coexistence above what would be predicted by the time trend, and vice versa. Using available literature, we made qualitative comparisons of the heat balance of nests on salt marshes versus sandy beaches. Nests on sandy beaches likely have lower sensible heat, which suggests temperature may be acting as an indirect driver. Survival of avian embryos requires proper thermoregulation, which may directly favor sandy beaches as nesting sites and indirectly favor coexistence under conditions of increasing temperatures. This further concentration of two endangered species on beaches, which receive high levels of human disturbance, may require novel conservation strategies for successful recovery.