Status of Washington's Shrub-Steppe Ecosystem: Extent, ownership, and wildlife/vegetation relationships

Changes in land use over the past century have resulted in the loss of over half of Washington's shrub-steppe habitat. Dramatic increases in dry-land agriculture and use of irrigation to expand farming and orchards has reduced the once expansive native grasslands and shrub-steppe to a fragmented landscape with very few large areas of native vegetation. The few remaining large areas of shrub-steppe are primarily on federal holdings (Yakima Training Center, Hanford Nuclear Site, and the Yakama Indian Nation) and may represent the only sites suitable for species requiring extensive areas of continuous shrub-steppe. Combined, the Washington Department of Fish and Wildlife (WDFW) and Washington Department of Natural Resources manage 12% of Washington's remaining shrub-steppe. The majority of extant shrub-steppe in Washington, however, exists on land owned by private individuals or corporations.

Washington's shrub-steppe communities support a wide diversity of wildlife. Ninety-four species of birds were recorded on 55 transects surveyed over 3 years. The diversity of species encountered resulted both from the placement of transects across of range of shrub-steppe range sites, and the interspersion of other land-use types (e.g., agricultural fields, Conservation Reserve Program lands) among the remaining areas of shrub-steppe. The two most common species on the survey, western meadowlark and horned lark, are grassland birds that have adapted well to agriculture and also use shrub-steppe. Studies in more extensive areas of shrub-steppe have found sage sparrows or Brewer's sparrows (both shrub-steppe obligates) to be the most numerous species, perhaps reflecting a lower influence of agriculture in these other regions. Brewer's sparrows and sage sparrows ranked third and eighth in abundance in the Washington surveys.

Although the survey technique used in this study was probably better suited to counting birds than to counting most mammals, 13 species of mammal were recorded ranging from elk to sagebrush voles. Surveys designed specifically for mammals would likely identify additional species.

The occurrence of specific birds on study transects was more closely related to the extent of sagebrush cover and cover of introduced annual grasses than to any other measured vegetation parameter. Sage thrashers, sage sparrows, and Brewer's sparrows were found only where shrubs were the dominant structural feature in the landscape. This was expected, because nesting by these shrub-steppe obligates is dependent upon presence of sagebrush. Both species that occurred more frequently on sites with lower cover of big sagebrush were grassland species (savannah sparrow, long-billed curlew). All species that were negatively associated with percent cover of annual grasses also were positively associated with cover of big sagebrush.

The suitability of Washington's shrub-steppe habitat for wildlife differs from that which occurred a century ago. Conversion of sites with deep, loamy soil to agriculture; invasion by non-native grasses and forbs; and fragmentation of the remaining shrub-steppe habitats have likely lowered the suitability of Washington's shrub-steppe habitat for many native species. WDFW supports a continuing research program on shrub-steppe wildlife to help address these issues.