Effects of marsh size on incidence rates and community organization within the lower Chesapeake Bay

B. D. Watts, The Center for Conservation Biology


Bird abundance and species richness was assessed in 50 salt marshes located in the lower Chesapeake Bay. Ten spatial replicates of 1, 2, 3, 4, and 5 ha marshes with similar vegetational composition were included. Over 4,800 observations were made of 100 species. Marsh area was a good indicator of overall species richness and bird abundance. The slopes of species-area relationships were consistent with those derived in an earlier study involving a broader range of marsh sizes. Species loss rates were highest for those species using the marsh directly. Of all primary users, obligate species showed the most dramatic declines with decreasing marsh size. Differences between obligate and facultative species in loss rates resulted in a directional shift in community composition with increasing marsh size. The form of incidence functions varied between species and species groups. Increases in incidence rates ranged from 40 to 70% for obligate breeders and 20 to 40% for facultative breeders. Species that used marshes as primary or alternate foraging areas exhibited variable incidence patterns. Patterns in both species-area relationships and marsh availability were used to formulate size-specific management recommendations. Marshes greater than 50 ha. In size generally support the best examples of unique marsh-bird communities. Because these marshes are rare, they should be the focus of acquisition and management programs. Marshes between 5 and 50 ha. Are also relatively rare and likely support the bulk of all obligate marsh users. Marshes of this size should be formally recognized and given special attention when considering requests for marsh alteration. Marshes between 1 and 5 ha are relatively common and have variable value depending on actual size and particular species of interest. Marshes of this size should be considered on an individual basis taking into account specific species of concern at the locat context of the marsh.