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COPYRIGHT 2005 Wilson Ornithological Society
ABSTRACT.--Understanding the influences of habitat fragmentation on vertebrate populations is essential for the protection and ecological restoration of strategic sites for native species. We examined the effects of prairie fragmentation on avian reproductive success using artificial and natural nests on 26 randomly selected, privately owned patches of shortgrass prairie ranging in size from 7 to 454 ha within a cropland matrix in Washington County, Colorado, summer 2000. Survival trends of artificial and natural nests differed. Daily survival of artificial nests increased with patch size up to about 65 ha and differed little at larger patch sizes, whereas daily survival of Lark Bunting (Calamospiza melanocorys) and Horned Lark (Eremophila alpestris) nests decreased with increasing size of the grassland patch. We hypothesize that our unexpected findings of lower survival of natural nests with increasing patch sizes and different trends between artificial and natural nests are due to the particular structure of predator communities in our study area and the ways in which individual predators respond to artificial and natural nests. We recommend that the value of small habitat patches in highly fragmented landscapes not be overlooked. Received 1 April 2004, accepted 3 November 2004.
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Understanding the influences of habitat structure and habitat fragmentation on the viability of grassland species is essential to conservation planning, especially for protection and ecological restoration of strategic sites for native species. Many grassland bird species, including those of the shortgrass prairie, have experienced population declines in the past 3 decades (Knopf 1994, Murphy 2003, Sauer et al. 2003). Between 1966 and 2002, populations of Lark Buntings (Calamospiza melanocorys) and Horned Larks (Eremophila alpestris) declined 2.0 and 1.6% per year, respectively, in the High Plains physiographic region (Sauer et al. 2003). Although mechanisms for these declines have not been identified, factors influencing reproductive success are among the possibilities. Shortgrass is the least disturbed of the three prairie types in North America, with as much as 40% remaining unplowed (Samson and Knopf 1996). Even though the extent of habitat loss is considerably less than in the tallgrass prairie (82-99%; Samson and Knopf 1996), habitat loss and fragmentation of breeding areas may contribute to population declines of shortgrass prairie birds.
Broad generalizations regarding the negative effects of habitat fragmentation on density and reproductive success of avian species are common in the scientific literature of the past 2 decades (Ambuel and Temple 1983, Herkert 1994, Donovan et al. 1995, Freemark et al. 1995). Studies on the effects of habitat fragmentation, specifically patch size and isolation, initially were stimulated by island biogeography theory (MacArthur and Wilson 1967, Diamond and May 1981) and subsequently by emerging landscape perspectives (Fahrig and Merriam 1994, Wiens 1995). When detected, patch size effects typically show that smaller habitat patches have lower habitat quality, more edge habitat, fewer species, fewer or no individuals of area-sensitive species, and/or lower reproductive output--due to increased predation and brood parasitism or decreased food abundance (Brittingham and Temple 1983, Herkert 1994, Burke and Nol 1998, Robinson 1998). These generalities are now being incorporated as assumptions in quantitative models of the effects of habitat fragmentation and edge effects on the demography of birds (Donovan and Lamberson 2001, Bollinger and Switzer 2002).
Despite broad support for these generalities, inconsistencies have been documented in well-studied systems. Although larger forest patches in forested landscapes are thought to provide better habitat (Donovan et al. 1995, Robinson et al. 1995, Thompson et al. 2002), not all studies support that pattern (Marzluff and Restani 1999). For example, in western forests, predation rates in fragments are often lower than in unfragmented sites (Tewksbury et al. 1998, Cavitt and Martin 2002). Current paradigms are rapidly evolving with the increased scrutiny of inconsistencies in the forest fragmentation literature (Donovan et al. 1997, Walters 1998, Marzluff and Restani 1999, Heske et al. 2001, Thompson et al. 2002). A major challenge to our understanding of the effects of habitat fragmentation on birds is the variability in their responses to fragmentation, or "differential sensitivity" (Walters 1998)--across regions, landscapes, habitats, species, and populations.
The effects of prairie fragmentation on reproductive success of grassland birds have been documented primarily in tallgrass prairie and in artificial nest studies. The effects are equivocal. Several bird species have experienced lower nest success in smaller grassland patches or near woody edges in field-forest ecotones (Herkert et al. 2003), but such effects do not universally apply to all grassland habitats and species (Gates and Gysel 1978, Johnson and Temple 1990, Winter and Faaborg 1999, Winter et al. 2000). Of five studies that employed artificial nest techniques in grasslands, only one (Burger et al. 1994) reported increased mortality of artificial nests with decreasing grassland patch size and distance from edges (in this case forest edges). Four of these studies reported no differences in mortality of artificial nests relative to grassland patch size or distance to edge, including forested and agricultural edges (Mankin and Warner 1992, Clawson and Rotella 1998, Pasitschniak-Arts et al. 1998, Howard et al. 2001). The lack of a patch size effect in these studies may result, in part, from the range in patch sizes being above or below a threshold at which an effect could be detected.
The search for generalities is often a search for clear and consistent trends reported by several studies. "Similar conclusions obtained from studies of the same phenomenon conducted under widely differing conditions will give us greater confidence in the generality of those findings than would any single study" (Johnson 2002). To contribute to our knowledge of potential effects of prairie fragmentation on birds, we conducted a study in a highly fragmented shortgrass prairie land scape (<15% grassland). The primary objective of our study was to determine the effects of patch size on reproductive success of prairie birds. We selected our study sites randomly so that we could make inferences to our entire target population (see Site selection) rather than just to the individual grassland patches.
METHODS
Study area.--The shortgrass prairie landscape is dominated by xeric grasses, such as buffalograss (Buchloe dactyloides) and blue grama (Bouteloua gracilis). Common breeding birds are Horned Larks, Western Meadowlarks (Sturnella neglecta), Lark Buntings, Chestnut-collared Longspurs (Calcarius ornatus), and Grasshopper Sparrows (Ammodramus
savannarum). Potential mammalian predators of ground-nesting birds include thirteen-lined ground squirrels (Spermophilus tride-cemlineatus), coyotes (Canis latrans), swift foxes (Vulpes velox), long-tailed weasels (Mustela frenata), badgers (Taxidea taxus), and striped skunks (Mephitis mephitis). Common snake species that opportunistically prey on birds include bullsnakes (Pituophis melanoleucus), western hognose snakes (Heterodon nasicus), and prairie rattlesnakes (Crotalus v. viridis).
Our study was conducted during the summer of 2000 in a 4,842-[km.sup.2] agricultural region of Washington County in northeastern Colorado (39[degrees] 34' N to 40[degrees] 27' N; 102[degrees] 48' W to 103[degrees] 28' W). Land-use cover types in the study area include dryland wheat (non-irrigated wheat production in a 2-year rotation system; 73.9%), shortgrass prairie rangeland (14.3%), Conservation Reserve Program (CRP) fields (6.1%), and irrigated crops (3.7%). This agricultural area was adjacent to three large grasslands, totaling 1,689 [km.sup.2], that were not considered in this study.
Site selection.--We used satellite imagery (provided...
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