SPATIAL-ANALYSIS OF ECOLOGICAL FACTORS RELATED TO RANGELAND GRASSHOPPER (ORTHOPTERA, ACRIDIDAE) OUTBREAKS IN WYOMING

Spatial analysis was used to test the hypotheses that the frequency of grasshopper outbreaks (greater than or equal to 9.6 grasshoppers per square meter) in Wyoming are dependent on 5 ecological variables: vege tation, precipitation, elevation, evapotranspiration, landform, and so il association. Thirty-one survey maps, compiled annually by governmen t agencies, showing 2 levels of grasshopper densities (< or greater th an or equal to 9.6 grasshoppers per square meter) were digitized (100- ha pixels) and overlaid using a geographic information system. Maps of all the ecological factors were digitized and the areas of geographic overlap with the grasshopper outbreak frequency map were determined. The resulting maps showed that grasshopper outbreaks occur repeatedly in some areas but rarely in others, even though they apparently had su itable vegetation and endemic populations of species capable of reachi ng high densities. Analyses of distributions of the areas that support ed outbreaks 0, 3, 6, 9, and 12 yr out of 31 yr with respect to each e cological factor showed that the distributions of outbreak frequencies were not random. Cross-tabulations showed that frequent outbreaks are associated with particular vegetation types (ponderosa pine, Pinus po nderosa Lawson, savannah, which covers only 5.5% of Wyoming) and soil associations (those on the steep hills of Wyoming's eastern plains). A regression model for describing the susceptibility to grasshopper out breaks in the study area using slope, soil depth, and soil erodibility was developed (R-2 = 0.885).