Effects of terrestrial-aquatic connectivity on an estuarine turtle

Abstract

ABSTRACT Aim Estuaries world-wide have been modified or fragmented due to human stressors in their terrestrial and aquatic components. Estuary fragmentation often results in reductions in species richness, diversity and connectivity. Effects of human modification on estuaries have been well studied, but less is known about how land use alters connectivity of the terrestrial-aquatic ecotone. We studied the relationship between terrestrial-aquatic connectivity and the distribution of an estuarine turtle, diamondback terrapin (Malaclemys terrapin). Location Chesapeake Bay, Virginia, USA. Methods We conducted diamondback terrapin surveys at 165 sites from late spring to mid-summer in 2012 and 2013. We evaluated associations between terrapin occurrence, land use, salt marsh, shoreline armouring and crabbing intensity in concentric-circular neighbourhoods ranging from 0.27 to 2 km to cover daily and annual terrapin movements. We used occupancy modelling and model averaging to identify key terrestrial and aquatic variables explaining heterogeneity in terrapin occupancy. We evaluated the final model with an independent data set and identified occurrence thresholds for key variables. Results Diamondback terrapin occupy areas with ≥ 10% of marsh within a 750-m neighbourhood, ≤ 17% armoured shoreline within a 1-km neighbourhood, ≤ 20% of agriculture within a 500-m neighbourhood, ≤ 33% low-density housing within a 270-m neighbourhood and ≤ 9 active crab pots within a 270-m neighbourhood. Our model performed well when evaluated with an independent data set. Main conclusions We are the first to identify thresholds and quantify negative associations between the distribution of diamondback terrapin and alterations to terrestrial-aquatic connectivity from land development, shoreline armouring, and fishing activity. Because diamondback terrapin responses are reflective of changes in coastal habitats, especially marshes, terrapin occurrence can be used to direct wetlands conservation and restoration efforts