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Shrubsteppe Bird Response to Habitat and Landscape Variables in Eastern Washington PDF Format - [1.2MB] 
Acknowledgements: We thank those who assisted in the field work for this study: G. Adams, D. Bowers, K. Brady, S.
Cushman, D. Harvey, C. Perry, K. Riener, and Q. Sheehan. D. Swanson supervised data collection in
1993. J. Skalski provided advise on statistical analyses. J. Jacobson conducted Landsat image analyses,
developed the landcover classes, and prepared the GIS figures. The manuscript benefitted from reviews
by S. Knick, J. Skalski, and C. Thompson. We are grateful to the landowners and land managers who
provided access to their lands. This research was funded by the Washington Department of Fish and
Wildlife, Wildlife Diversity Program. |
Shrubsteppe Bird Response to Habitat ABSTRACT: The landscape of the intermountain west has changed dramatically in the last 150 years, particularly in the state of Washington where over half the native shrubsteppe ecosystem has been converted to agricultural lands resulting in a fragmented landscape with few extensive tracts of shrubsteppe. We examined the historical and current distribution of shrubsteppe on different soil types in eastern Washington and we surveyed bird communities at 78 sites in shrubsteppe from 1991 to 1993. We compared abundance of species among soil types and range conditions and developed models of species occurrence using site-specific vegetation and landscape variables. The pattern of shrubsteppe conversion has resulted in a disproportionate loss of deep soil communities. Eight species showed strong relationships with soil type and three with range condition. These associations likely resulted from the influence of soil type and range history on the vegetation of these communities. Brewer's Sparrows (Spizella breweri) and Sage Sparrows (Amphispiza belli) reached their highest abundances in deep, loamy soils, whereas Loggerhead Shrikes (Lanius ludovicianus) were most abundant in deep, sandy soils. Sage Sparrows occurred more frequently in landscapes dominated by shrubsteppe, indicating a negative relationship with fragmentation. Our results suggest that fragmentation of shrubsteppe and the pattern of agricultural conversion among soil types have had detrimental effects on numerous shrubsteppe species. The landscape for species with an affinity for deep, loamy soil communities has changed considerably more than the overall loss of shrubsteppe would indicate. Conservation practices that emphasize retention of shrubsteppe communities on deep soils and that reduce further fragmentation will be critical to the maintenance of avian biological diversity in this system. INTRODUCTION The landscape of the intermountain west has changed dramatically in the last 150 years, particularly within the semi-arid shrubsteppe ecosystems (Quigley & Arbelbide 1997). Anthropogenic changes in these shrub and grass dominated communities has been especially severe in the state of Washington where over half the native shrubsteppe has been converted to agricultural lands (Dobler et al. 1996). Unlike forest communities that can regenerate after clearcutting, shrubsteppe that has been ploughed and is growing agricultural crops is unlikely to return to its former, native community. Furthermore, a long history of grazing, fire, and invasion by exotic vegetation has altered the composition of the plant community within much of the extant shrubsteppe in this region (Quigley & Arbelbide 1997; Knick 1999). Conversion of shrubsteppe communities to agricultural fields in Washington has resulted in a fragmented landscape with few extensive tracts of shrubsteppe (Dobler et al. 1996). Fragmentation of previously extensive landscapes can influence the distribution and abundance of birds through redistribution of habitat types and through the pattern of habitat fragmentation, including characteristics such as decreased patch area and increased habitat edge (Ambuel & Temple 1983; Wilcove et al. 1986; Robbins et al. 1989; Bolger et al. 1991, 1997). Fragmentation also can reduce avian productivity through increased rates of nest predation (Gates & Gysel 1978; Wilcove 1985), increased nest parasitism (Brittingham & Temple 1983; Robinson et al. 1995), and reduced pairing success of males (Gibbs & Faaborg 1990; Villard et al. 1993; Hagan et al. 1996). Much of our knowledge of fragmentation effects on birds comes from studies in eastern and central regions of North America, with relatively little research from western landscapes (Bolger et al. 1991, 1997; Knick & Rotenberry 1995; Tewksbury et al. 1998). We know little about the influence of fragmentation on shrubsteppe bird communities (Knick & Rotenberry 1995). Disturbance plays an important role in determining successional pathways in shrubsteppe communities (Daubenmire 1970; Smith et al. 1995). Excessive grazing by livestock can reduce abundance of some native plants while increasing others and can allow exotic species to enter and in some cases to dominate communities (Branson 1985). Wildfire can promote spread of annual grasses to the detriment of native plants (Whisenant 1990). Effects of livestock grazing on shrubsteppe vegetation can influence use of sites by birds, although the direction of influence (positive or negative) can vary (Saab et al. 1995). Invasion by exotic plants changes floristics and vegetation structure and can have adverse effects on site use by some birds (Knick and Rotenberry 1995). The loss of once extensive shrubsteppe communities has reduced substantially the habitat available to a wide range of shrubsteppe-associated wildlife including several birds that are found only in this community type (Quigley & Arbelbide 1997; Saab & Rich 1997). Sage Sparrows, Brewer's Sparrows, Sage Thrashers and Sage Grouse are considered shrubsteppe-obligates and numerous other species are associated primarily with shrubsteppe at a regional scale. In a recent analysis of birds at risk within the interior Columbia Basin, the majority of species identified as of high management concern were shrubsteppe species. Moreover, over half of these species have experienced long-term population declines according to the Breeding Bird Survey (Saab & Rich 1997). Soils are a conspicuous component of shrubsteppe ecosystems and influence the composition of the vegetation community. The composition, texture, and depth of soils affect drainage, nutrient availability, and rooting depth and result in a variety of edaphic climax communities (Daubenmire 1970). Much of the interior Columbia Basin in eastern Washington is underlain by basaltic flows, and the soils vary from deep accumulations of loess-derived loams to shallow lithosols in areas where glacial floods scoured the loess from underlying basalt. Sandy soils cover extensive areas in the west-central and southern parts of the Basin, the result of glacial outwash and alluvial and wind-blown deposition (Daubenmire 1970; Wildung & Garland 1988). Results of a previous survey of shrubsteppe birds in eastern Washington suggested that the abundance of some species might vary with soil type of the vegetation community (Dobler et al. 1996). If true, this relationship might prove to be a valuable asset to management, because soils are a mapable component of the landscape and could be incorporated into spatially explicit models of resource use and availability. We examined the response of birds to local and landscape variables in shrubsteppe communities in eastern Washington. Our primary objective was to determine if bird abundance varied with soil type and range condition and to compare present and historic distribution of shrubsteppe among soil types to assess the potential effects of this pattern on shrubsteppe bird communities. Our second objective was to develop models of relationships between site-specific characteristics, including a simple index to habitat fragmentation, and the probability of specific species occurring at a site. By building such models and testing them against independent data from the study area we aimed to identify variables to help describe the distribution of shrubsteppe birds on the landscape. Related links:
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