In 1976, a cooperative team began a comparative study of wildlife populations in developed and undeveloped areas at The Geysers Geothermal Field. This portion of the study analyzes habitat variation and derives habitat relationship models for small mammals. Developed and undeveloped plots were compared in five habitat types. (Developed plots were subject to disturbance from geothermal development, while undeveloped plots were free from this disturbance). Natural habitat variation was described by measurements of vegetative structure and composition, ground cover type, and topography and solar radiation. Because geothermal development produces high noise levels from steam wells and construction operations, measurements of noise level were used to assess the geothermal influence on each plot. All habitat and small mammal measurements were replicated on 10 sampling units in each study plot. Noise levels were consistently higher in developed than in undeveloped plots. Some developed plots showed alterations in shrub cover and ground cover that were attributed to geothermal development, but in general the developed and undeveloped plots of the same habitat type were similar in vegetative structure. Plant species composition depended on location of the plot. Of the eight small mammal species captured, the deer mouse (Peromyscus maniculatus) and the pinyon mouse (P. truei) comprised 80 to 95 percent of the catch in each plot. However, the dominance relationships of these two species seemed to be affected by geothermal development, with the pinyon mouse dominating in undeveloped plots and the deer mouse dominating in developed plots. Discriminant analysis was used to develop models of small mammal species relationships as influenced by vegetation and noise level.
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