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The relationship of grazing to orthopteran diversity in the intermontane grasslands of the South Okanagan, British Columbia Griesdale, Peggy Liu

Abstract

The antelope-brush shrub-steppe of the South Okanagan is small in size yet home to many of the unique and endangered flora and fauna of British Columbia and Canada. More insect species are found in this ecosystem than other grassland ecosystems. Antelope-brush ecosystems are dominated by bunchgrasses, antelope-brush, and a well-developed cryptogam crust, owing to the hot and dry summers of the South Okanagan. Urban and vineyard development are the most immediate threat to this fragile ecosystem, followed by unmanaged livestock grazing. Livestock grazing exposes soil, stunts plant growth, and fragments the cryptogam crust. Less than 9% of the antelope-brush ecosystem is relatively undisturbed and only two small ecological reserves exist. Orthopterans are the most important invertebrate herbivore in North American grasslands and are one of the main biotic influences on grasslands. While Orthopterans assist with biomass turnover and nutrient cycling processes of ecosystem functioning, they may add to the effects of livestock overgrazing. Numerous studies have shown contradictory results of the relationship between grasshopper abundances and grazing pressures. As part of a larger study of the biodiversity and impact of grazing on this threatened ecosystem, this study was conducted to determine how livestock grazing in the intermontane grasslands of the South Okanagan of British Columbia influenced the abundance and species assemblage of Orthopterans. Orthopterans were collected with pitfall traps in ten locations in the antelope-brush ecosystem of the South Okanagan over two years. The study sites were of three different grazing levels: 1) non-grazed; 2) moderately grazed; and 3) heavily grazed. Vegetation data were collected with Daubenmire plots at each site. Twenty-four orthopteran species were captured (seventeen grasshopper species and seven cricket species). All seventeen grasshopper species were previously known to occur in British Columbia, but the taxonomies of four of the cricket species are currently being revised. Grazing did not affect orthopteran species abundance or diversity. Regression analyses showed that the number of orthopteran species and Shannon-Wiener Index values increased with increasing bare soil. The effects of grazing on the vegetation community and structure, and its corresponding effects on the orthopteran species assemblage, are discussed.

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