Effects of shoreline hardening on waterbird community use: Influences of geography, scale, and season in the Chesapeake Bay
The nearshore land-water interface is an important ecological zone that is facing increasing anthropogenic pressure under coastal development. Portions of the Chesapeake Bay and its tributaries have undergone a range of shoreline conversions including bulkhead and riprap armoring to protect human property. This study examines the relative use of four main shoreline types by migrating and wintering waterbirds: bulkhead, riprap, natural marsh, and forest. Waterbirds were surveyed within river tributaries (defined as subestuaries) during two seasons: late summer (August to September) for migrating shorebird and wading birds, and late fall (October to December) for migrating and wintering waterfowl. We used an Index of Waterbird Community Integrity (IWCI) to characterize the waterbird community for each subestuary and season based on sensitivity to human disturbance. Characterization of the shoreline was conducted at three scales using GIS: (1) the subestuary shoreline scale (directly at the shoreline, first using on-screen digitizing of satellite imagery followed by field ground-truthing); (2) the 500m scale surrounding the shoreline edge (using National Land Cover Data, NLCD); and (3) the watershed scale (using NLCD). Akaike’s Information Criterion was used to identify top models from a list of predefined predictors of IWCI for each of the three geographic scales and multivariate regressions were used to relate trends and test for significance.
We surveyed 20 subestuaries across the upper, middle, and lower portions of the Chesapeake Bay in 2010 to 2014. More than 60 waterbird species were observed and IWCI scores ranged from 14.1 to 19.8. The top model at the subestuary scale included percent bulkhead, riprap, and natural marsh (regression p=0.002). At the 500m landscape scale, percent wetland was the top predictor (p=0.03), however, at the watershed landscape scale none of the land cover models were significant. These results indicate that the waterbird communities are responding at the local (subestuary shoreline) and 500m shoreline scales but not the larger watershed scale. At the subestuary scale, IWCI scores decreased with percent bulkhead and riprap within an estuary (p=0.015) and increased with percent natural marsh (p=0.004). Our results indicate that shoreline development at the subestuary scale (at the land-water interface) has a greater effect on waterbird community integrity than the overall watershed scale.