Honors Thesis Defense, Logan Timm

Comparison of edge effects across patchy and contiguous seagrass landscapes

Abstract: Coastal marine habitats are subject to fragmentation from a variety of processes including human-induced disturbances such as dredging and natural processes such as high-energy storms. It is widely accepted that habitat edges, made more common by fragmentation, affect biodiversity and faunal survival rates, but there is little research on the differences between edge effects on discrete patches within the same seagrass meadow – some of which occur along the meadow’s outer margin and some of which occur with the meadow interior. My study aims to explore if and how edge effects (occurring within individual patches) vary across landscapes. I classified four distinct seagrass meadows as either contiguous or fragmented based on spatial metrics and examined (a) seagrass composition and (b) consumption rates between edges and interiors of habitat patches in each. In each meadow, I worked across a gradient from patches along the meadow margin toward patches within the meadow core, calculating “edge effects” as the ratio in metrics (i.e., seagrass composition, consumption) measured along the edge and interior of each patch. I found that position within a meadow (landscape scale: meadow core versus margin) had notable consequences on the direction and magnitude of edge effects (at patch scales), and that this was further modulated by the nature of the meadow as either contiguous or patchy. In contiguous meadows, seagrass cover and height were greater in patch edges – relative to patch interiors – within the meadow core, but lower in patch edges within the meadow margin. The opposite was observed in patchy meadows: seagrass cover and height were greater in patch edges – relative to patch interiors – within the meadow margin, but lower in patch edges within the meadow core. Across all meadows, edge effects on consumption rates also varied as a function of position with a meadow. Along meadow margins, consumption rates along patch edges were higher than in corresponding patch interiors (i.e., edge effect >1). Conversely, within meadow interiors, consumption rates in patch edges were lower than in corresponding patch interiors (i.e., edge effect <1). These data highlight that the effect of patch edges is potentially context dependent with respect to position of patches in a broader landscape.