COS 60-7
Using current salt marsh bird distributions to project vulnerability to sea level rise
Sea level rise will affect salt marsh bird species through changes to landcover distribution and structure. If current species occupancy and abundance relationships with landscape gradients remain constant over time, then those relationships can be projected using sea level rise models to predict future species distributions. We sampled the abundance of two salt marsh obligate bird species (Clapper Rail, Rallus longirostris, and Seaside Sparrow, Ammodramus maritimus) in salt marshes across coastal Georgia, USA. Given Clapper Rails’ apparent ubiquity in Georgia salt marshes, we predicted that the current Clapper Rail distribution would be more weakly related to landscape variables and less uncertain than the Seaside Sparrow distribution. We stratified sampling across seven landscape gradients to maximize the power of detecting landscape relationships. We conducted point counts at 214 sampling points during March-June 2013 with three visits made to each point using a double-observer protocol, which allowed for robust estimation of detection probability, occupancy rates, and abundance using a Bayesian hierarchical model. Making spatial predictions of habitat quality from the probability distributions of effects sizes allowed us to assess the uncertainty in our predictions and how this uncertainty can be propagated through time with sea level rise models.
Results/Conclusions
Seaside Sparrow occupancy was positively related to distance to forested and urban areas, and elevation and salinity were positively related to abundance of the species. Landscape variables explained 85% of the variation in occupancy data and 30% of the abundance variation, indicating that Seaside Sparrow distribution is largely determined by the landscape variables that we used to stratify our sampling. However, uncertainty about the size of these effects is large. Clapper Rails were so widespread in Georgia salt marshes that we were not able to model an occupancy process, but abundance was negatively related to distance to forested areas and salinity. Landscape variables only explained 6% of the variability in Clapper Rail abundance, but with small uncertainty around effect sizes. Comparison of the landscape relationships to the distributions of these two species show that Seaside Sparrows may be more negatively affected by sea level rise as high elevation marshes that are far from forests are likely to have greater loss rates than marsh types used by Clapper Rails. Large uncertainty around the current Seaside Sparrow distribution will make predictions about future distributions with sea level rise more difficult. We recommend structuring monitoring around this uncertainty to improve future predictions.