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Life Sciences


Fragmentation of breeding habitat may cause declines in many bird populations. Our perception of the demographic effects of habitat fragmentation comes primarily from studies in the midwestern and eastern United States and Scandinavia. We know very little about the demographic effects of anthropogenically caused habitat fragmentation in habitats prone to natural disturbance, as is typical of most forest types in the western United States. We located and monitored 1916 nests on eight sites located in mostly forested landscapes and eight sites located in primarily agricultural landscapes to study the effects of landscape-level fragmentation on nest predation and brood parasitism in riparian areas in western Montana. Patterns of nest predation were opposite those documented from more eastern locales; predation rates were higher in forested landscapes than in fragmented landscapes dominated by agriculture. This pattern probably reflects the importance of forest predators in these landscapes: red squirrels (Tamiasciurus hudsonicus) were much more abundant in forested landscapes and declined quickly with decreasing forest cover, whereas predators that typically increase in fragmented landscapes in the Midwest (such as corvids) increased only at very high levels of fragmentation. Patch size and distance to habitat edge did not influence predation rates. Brood parasitism by Brown-headed Cowbirds (Molothrus ater) decreased with increasing forest cover, but the strongest predictors of parasitism were the abundance of human development (farms and houses) on the landscape and the density of cowbird host species, not forest cover. The combined effects of predation and parasitism resulted in low nesting productivity in both forested and agricultural landscapes for heavily parasitized species, while the species not affected by cowbird parasitism had greater nesting productivity in fragmented agricultural landscapes. Our results suggest that the effects of fragmentation are dependent on the habitat structure, the landscape context, the predator community, and the impact of parasitism. All of these factors may differ substantially in western ecosystems when compared to previously studied forests, making generalizations about the effect of fragmentation difficult.




© 1998 by the Ecological Society of America. Joshua J. Tewksbury, Sallie J. Hejl, and Thomas E. Martin 1998. BREEDING PRODUCTIVITY DOES NOT DECLINE WITH INCREASING FRAGMENTATION IN A WESTERN LANDSCAPE. Ecology 79:2890–2903.[2890:BPDNDW]2.0.CO;2.

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