Spatial Analysis of Ecological Factors Related to Rangeland Grasshopper (Orthoptera: Acrididae) Outbreaks in Wyoming

Abstract

Spatial analysis was used to test the hypotheses that the frequency of grasshopper outbreaks (≥9.6 grasshoppers per square meter) in Wyoming are dependent on 5 ecological variables: vegetation, precipitation, elevation, evapotranspiration, landform, and soil association. Thirty-one survey maps, compiled annually by government agencies, showing 2 levels of grasshopper densities (< or ≥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 suitable vegetation and endemic populations of species capable of reaching high densities. Analyses of distributions of the areas that supported outbreaks 0, 3, 6, 9, and 12 yr out of 31 yr with respect to each ecological 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 ponderosa 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 outbreaks in the study area using slope, soil depth, and soil erodibility was developed (R2 = 0.885).