Differential responses and sensitivity to edge of seagrass fish and their primary prey faunal groups with respect to two distinct adjacent habitat edge types bordering Posidonia australis beds on the northern shores of Jervis Bay, NSW, Australia

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Several studies over the past decade have examined “edge effects” on fauna within a focal habitat using a model consisting of two habitat types forming the edge. With regards to seagrass faunal studies this edge effect is more commonly investigated at seagrass-sand interfaces. These observed effects are in many cases largely biased towards the structure of the seagrass itself at the edge and not the type of adjacent with which the seagrass forms the edge. As real landscapes contain multiple edges with the structure of each determined by the type of adjacent habitat with which the focal one is forming the edge, it seems imperative to begin assessing the dynamics of edge effects under these conditions. As such this study investigates the spatial distribution of fish and their prey communities – zooplankton and benthic invertebrates, along with epiphyte load, within Posidonia australis seagrass beds in northern Jervis Bay, NSW, Australia with respect to two adjacent habitat edges i.e. rocky algal reef and bare sediment. Rocky reef habitats are common features in the nearshore environment of the south-east Australian coast. The relationship between the prey and fish distribution relative to both habitat edge types was examined along with the relative sensitivity to edge of the different seagrass fauna. Manipulative aquarium experiments were also performed to assess the role of predatory threat in the spatial distribution of selected seagrass species with respect to these habitat edges. Results illustrated (1) differing responses of seagrass fauna to different habitat edge types, with the rocky reef exhibiting an overall greater effect, (2) site-specific and species-specific and for fish taxa, functional guild-specific responses to the different edge types, (3) a close relationship between edge effects and sensitivity to edge exhibited by fish and their prey, (4) a greater sensitivity to edge displayed by the more sessile benthic invertebrate community compared to the more mobile zooplankton and fish taxa, and (5) that predatory threat influenced habitat position of certain fish taxa. Results of this study highlight the importance of adjacent habitat type in edge studies, illustrating different responses to the differently structured edge types, likely driven by differing microclimatic conditions experienced at each. As such, adjacent habitat type should feature more in the discussion of edge effects and the need exists to conduct more edge studies in landscapes representing real multiple edge conditions to better inform management and planning initiatives.
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