THE RELATIVE IMPORTANCE OF PATCH AREA AND PERIMETER–AREA RATIO TO GRASSLAND BREEDING BIRDS

Abstract

Habitat fragmentation has been implicated as a major cause of population decline in grassland birds. We tested the hypothesis that a combination of area and shape determines the use of grassland patches by breeding birds. We compared both species richness and individual species presence in 45 wet meadow grasslands in the floodplain of the central Platte River, Nebraska. Bird data were collected through the use of belt transects and supplemented by walking and listening outside transects. Our data supported our pri- mary hypothesis that perimeter-area ratio, which reflects both the area and shape of a patch, is the strongest predictor of both individual species presence and overall species richness. The probability of occurrence for all six common species (Grasshopper Sparrows, Bobo- links, Upland Sandpipers, Western Meadowlarks, Dickcissels, and Red-winged Blackbirds) was significantly inversely correlated with perimeter-area ratio. The probability of occur- rence of Grasshopper Sparrows, Bobolinks, Upland Sandpipers, and Western Meadowlarks was also correlated with area. We conclude that species richness is maximized when patches are large ( .50 ha) and shaped so that they provide abundant interior areas, free from the impacts of edges.