THE RELATIVE IMPORTANCE OF PATCH AREA AND PERIMETER-AREA RATIO TO GRASSLAND BREEDING BIRDS.(Statistical Data Included)

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 primary 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, Bobolinks, Upland Sandpipers, Western Meadowlarks, Dickcissels, and Red-winged Blackbirds) was significantly inversely correlated with perimeter-area ratio. The probability of occurrence 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 ([is greater than] 50 ha) and shaped so that they provide abundant interior areas, free from the impacts of edges.

Key words: birds, grassland; habitat fragmentation; landscape ecology; Nebraska; patch area; patch shape; perimeter-area ratio; Platte River, Nebraska; population declines; species-area relationship; wet meadows.

INTRODUCTION

Landscape fragmentation has had a profound effect on the distribution and abundance of bird species in many parts of the world (Whitcomb et al. 1981, Howe 1984, Lynch and Whigham 1984, Opdam et al. 1985, Herkert 1994, Vickery et al. 1994, Hinsley et al. 1996). The strongest evidence of the impact of fragmentation has come from studies of woodland birds. Research in forested habitats suggests that forest-interior birds and neotropical migrants are especially sensitive to smaller habitat patches and the increasing patch isolation that accompanies fragmentation; in general, species richness and relative abundance of area-sensitive species significantly decrease as patch area decreases (Whitcomb et al. 1981, Ambuel and Temple 1983, Howe 1984, Lynch and Whigham 1984, Opdam et al. 1985, Robbins et al. 1989).

Although much less work has concentrated on birds in grasslands, studies suggest that grassland birds are experiencing extensive population declines because of the loss of large grassland patches (Samson 1980, Herkert 1994, Vickery et al. 1994). Recent analysis of data from the North American Breeding Bird Survey between 1966 and 1993 shows that grassland bird species are declining faster than any other group of breeding species in the midwestern United States (Herkert 1995). In particular, Grasshopper Sparrows (Ammodramus savannarum), Western Meadowlarks (Sturnella neglecta), and Bobolinks (Dolichonyx oryzivorus) are among the species that show the greatest declines (Herkert 1995). This loss of grassland birds is concomitant with the tremendous loss of tallgrass prairie, which currently exceeds that of any other major ecosystem type in North America (Samson and Knopf 1994).

Most studies of grassland bird habitat selection have focused only on the importance of vegetation structure (Wiens 1969, Rotenberry and Wiens 1980, Kantrud 1981, Cody 1985, Bowen and Kruse 1993). A few studies have investigated the spatial aspects of grassland fragmentation and found that patch area is an important variable affecting habitat occupancy (Samson 1980, Herkert 1994, Vickery et al. 1994).

Patch area may not adequately explain the effects of fragmentation on habitat occupancy by birds, however, because patches of equal area are not necessarily equal in their ability to support a given population. In fragmented landscapes, patches should be viewed in the context of the surrounding matrix because the matrix can determine the degree of patch isolation (see review in Andren [1994]) and the availability of supplementary resources (Dunning et al. 1992, Burke and Nol 1998). In addition, the matrix itself varies in its degree of hostility (Addicott et al. 1987, Franklin 1993).

Patches of equal area may also vary significantly in the amount of their area exposed to edges. Negative impacts of edges on breeding birds have been documented in both forest and grassland habitats. In forest environments, nest predation and brood parasitism rates increase near edges (Gates and Gysel 1978, Wilcove 1985, Andren et al. 1985, Andren and Angelstam 1988, Burkey 1993, Marini et al. 1995). In grassland habitats, Johnson and Temple (1986, 1990) and Burger et al. (1994) found higher predation and parasitism rates on nests close to wooded edges relative to those away from edges. There is also evidence that some grassland bird species avoid nesting near patch edges (Johnson and Temple 1986, Delisle 1995, Heizer 1996).

Because patch shape, along with area, determines the amount of habitat exposed to edges, patch shape may have a significant effect on habitat occupancy by grassland breeding birds. We are not aware of any research on the effects of patch shape on grassland birds, but Temple (1986) found that the presence and abundance of woodland birds was better predicted by the "core area" (defined as areas [is greater than] 100 m from an edge) than by the total area of forest fragments. Patches that had elongated shapes, indented perimeters, or inclusions of open habitat within the fragment had fewer species and individuals than forest stands with compact shapes and unbroken perimeters.

Temple's findings on the effects of patch shape and core area in forest patches may not be directly applicable to grassland patches because the two breeding habitat types differ in terms of predator species, breeding bird behavior, and structural contrast between the habitat and surrounding edges. There is also some question of whether Temple's method of estimating core area can be extrapolated to different habitats and landscapes. The distance that edge effects extend into a patch vary widely (Faaborg et al. 1993) and edge effects vary between geographic regions (Freemark 1986). This variability points to the need for a relative measure such as perimeter-area ratio that accounts for the amount of patch area exposed to edges without requiring a subjective estimation of the distance that edge effects extend into a patch. Patches with elongated shapes or indented perimeters have higher perimeter-area ratios than patches of the same area with compact shapes and unbroken perimeters. In addition, small patches generally have higher perimeter-area ratios than large patches.

The objective of our study was to evaluate the relative importance of patch area and perimeter-area ratio to both grassland bird species richness and the likelihood of occurrence of individual grassland species. We hypothesized that in landscapes where habitat fragmentation has led to a variety of patch shapes and areas, patches with high perimeter-area ratios (containing little or no core area) are avoided by certain grassland bird species and thus have low species richness.

METHODS

Study area

The study area was located in the floodplain of the central Platte River between Grand Island and Wood River, Nebraska. The floodplain is a relatively flat area composed of a mosaic of grassland, cropland, riparian forest, and stream channels. A dynamic hydrological interaction exists among the grasslands, main channels, side channels, and backwaters. …

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