Divergent forecasts for two salt marsh specialists in response to sea level rise

Abstract

Ecosystem specialists are predicted to be more vulnerable to global change than generalists, but whether specialists within an ecosystem will respond similarly to those changes is often largely unknown. Will specialists track changes in their habitats as a group, or are their distributions governed by landscape gradients that will make some species more sensitive to habitat changes? In this study, we forecasted the effects of sea level rise (SLR) on two salt marsh specialist bird species: clapper rails Rallus crepitans and seaside sparrows Ammodramus maritimus. We sampled the abundance of these two species in salt marshes throughout the Georgia, USA, coast in 2013–2014, and analyzed count data using a Bayesian N-mixture model. Model predictions were applied to an SLR land cover model to determine distribution shifts over 100 years. Both species distributions were most sensitive to the relative elevation gradient, with clapper rails using lower elevation marshes and seaside sparrows using higher elevation marshes. These disparities in habitat use, along with other differences according to marsh salinity and distance to forested areas, led to divergent responses to SLR. Clapper rail habitat is predicted to increase with SLR by 52%, but seaside sparrow habitat will contract by 81% by the year 2100. Seaside sparrow habitat is not predicted to decline until sometime between 2025 and 2050, at which point the decline will rapidly accelerate, indicating the importance of careful monitoring in future decades. Diverging responses to a global perturbation create a conservation planning dilemma: if specialists have opposing responses to SLR, it may be difficult to manage conservation areas that accommodate many species.