Boat traffic structures seascape genetic patterns in the Florida Manatee

Madison Hall, University of Central Florida
Rodney Dyer, Virginia Commonwealth University
Eric Hoffman, University of Central Florida
Graham Worthy, University of Central Florida
Topic Area
Emerging Issues and New Directions in Transportation Ecology

This intersection of population genetics and landscape ecology is known as landscape genetics when performed in terrestrial systems and seascape genetics when performed in marine systems. Important drivers of seascape genetics patterns can be difficult to ascertain, as marine systems are diffusive and ephemeral and populations of marine organisms are often large and highly mobile. The FL manatee (Trichechus manatus latirostris) presents an especially difficult case, as a previous study revealed that the statewide population possesses low levels of genetic diversity. However, landscape/seascape genetics has proved useful in populations with weak differentiation, low diversity, or chaotic structure, and may be able to explain more of the FL manatee genetic structure than previous methods. There are many ways to conceptualize the movement of genes across a landscape/seascape including Isolation by distance (IBD), Least cost path analysis (LCP), and Isolation by resistance, also known as circuit theory (CT). CT describes genetic differentiation when simultaneously considering all possible pathways. The goal of this study was to elucidate if any CT hypothesis based on abiotic (bathymetry, temperature, salinity), biotic (chlorophyll-a concentration, seagrass cover), anthropogenic (boat activity, coastal habitat disturbance), or a combination of seascape factors could explain genetic distance (GDis) in manatees better than IBD or LCP. I approached this question by generating a GDis matrix for all pairs of 293 individual manatees in the dataset using 20 microsatellite loci and the proportion of shared alleles (Dps). Each seascape variable was then objectively parameterized using eight transformations. I used mantel tests to select informative ecological distance hypotheses, and used maximum-likelihood population-effects mixed modelling (MLPE) to select the model that best explained GDis based on Akaikes corrected Information Criterion (AICc). Results from the MLPE mixed modelling bootstrap (10,000 iterations) revealed that CT distance based on boat activity was selected as the top model most often (86.52%) followed by CT distance using coastal habitat disturbance (13.42%). The results of this study 1) add to a body of literature on CT, and 2) indicate that anthropogenic boat activity and coastal disturbance have created spatial genetic structuring by disrupting gene flow. Previous studies demonstrated that further disruption in gene flow could have drastic effects for FL manatees, suggesting cause for concern. Yet identifying these barriers also present opportunities for management; understanding how landscape/seascape variables affect population connectivity is crucial in targeting conservation and restoration action. This information can be used to implement conservation and recovery plans as anthropogenic pressure on and other environmental threats to FL manatee populations increase.

Abstract Keywords
Seascape genetics Boat traffic Coastal disturbance Circuit theory Manatee conservation