Master of Science (M.Sc.)
Virginia Institute of Marine Science
The threat to biodiversity posed by increasing rates of anthropogenic habitat fragmentation necessitates an understanding of the consequences of spatial pattern for natural communities. Reduction of patch size, loss of habitat, changes in the quantity and proportion of habitat edge, and reduced connectivity among habitats can all shape ecological processes and faunal behavior. Seagrass habitats provide a natural model system for examining spatial inf1uences on marine fauna, but separating the effects of habitat structure, environmental conditions, and spatial pattern is difficult because shoot density, percent cover, and hydrodynamic regime often co-vary with patch size. This study used experimentally manipulated seagrass patches to measure the response of seagrass-associated fauna to patch size and bed fragmentation on the scale of meters. Replicate plots were created by transplanting eelgrass, Zostera marina, at two sites in the lower Chesapeake Bay, USA. Plots were designed to examine effects of patch size and bed fragmentation on the density of epibenthic decapod and peracarid crustaceans (crabs, shrimps, amphipods, and isopods), gastropods, and demersal fish. Densities of most species examined did not vary significantly among fragmented and unfragmented plots, or among plots with differing amounts of habitat area. Furthermore, seasonal edge effects were observed within both fragmented and unfragmented plots for five amphipod crustaceans (Atnpithoe longimana, A. valida, Caprella penantis, Elasmopus levis, and Microprotopus raneyi), the gastropod Mitrella lunata, and blue crab postlarvae, indicating that the processes regulating small-scale distribution within seagrass plots were insensitive to meter-scale habitat patchiness. In addition, two isopods (Edotea triloha and Erichsonella attenuata), the gastropod Nassarius vibex, and the amphipod Erichthonius brasiliensis exhibited edge effects within individual plot types, with few commonalities to suggest that any particular habitat configuration promotes edge effects. In 24 of 27 instances (species - plot type combinations) where effects of plot edges were detected, faunal densities were higher near plot edges than in their interiors. High densities in plot edges were not accompanied by higher overall densities in plots with greater proportions of habitat edges, suggesting that the processes regulating the distribution of individuals among plots are distinct from within-plot processes. Secondary production of epifauna, estimated from size distribution data, was not significantly affected by fragmentation treatments. Although commonly employed faunal categorizations such as body size, mobility, and trophic position did not completely predict response to fragmentation, all of the species exhibiting edge effects were among the smallest and least mobile of those studied. The results suggest that fragmented patches, which are common features of the Chesapeake Bay's seagrass habitat, support a dense faunal assemblage, and that seagrass habitat edges may be zones of enhanced faunal density. Far from supporting a positive influence of seagrass fragmentation, the results indicate that at the scale studied, there is little compensation for habitat loss via enhanced faunal densities in edges. Since fragmented and unfragmented plots support similar faunal densities, total abundances are lower in fragmented plots due to the loss of seagrass area. The mechanisms by which spatial pattern influences faunal dynamics, and the effects of fragmentation at larger scales remain subjects for future research.
© The Author
Marion, Scott R., "Effects of Habitat Fragmentation on the Utilization of Eelgrass (Zostera marina) by Mobile Epifauna and Macrofauna" (2002). Dissertations, Theses, and Masters Projects. Paper 1539617783.