Academic journal article The American Midland Naturalist

Habitat Relationships among Grasshoppers (Orthoptera: Acrididae) at the Western Limit of the Great Plains in Colorado

Academic journal article The American Midland Naturalist

Habitat Relationships among Grasshoppers (Orthoptera: Acrididae) at the Western Limit of the Great Plains in Colorado

Article excerpt

ABSTRACT.-We measured grasshopper densities on 66 shortgrass, mixed grass, tallgrass and hayfield plots on Boulder, Colorado, open space in 1995-1996. Grasshoppers as a group, and most species individually, were more abundant on relatively sparse short and mixed grass plots than on lusher hayfields and tallgrass plots-a result consistent with the hypothesis that most species require warm and sunny open ground for survival and reproduction. Bandwinged grasshoppers (Oedipodinae) were particularly associated with open ground in shortgrass vegetation, whereas spur-throated grasshoppers (Melanoplinae) were most abundant in mixed grass plots with a high percentage of forb vs. graminoid cover. Slant-faced grasshoppers (Gomphocerinae) were the most uniformly distributed among habitats, but were generally associated with plots including relatively high proportions of grass vs. forb cover. While 20 of the 25 most common grasshopper species on Boulder open space occurred in all four grassland types, each habitat was dominated by a distinctive set of species. However, numerically dominant grasshoppers on short, mixed and tallgrass plots only loosely resembled groups of dominant species in the same habitats elsewhere on the Great Plains. While most Great Plains grasshoppers are widely distributed, they are apparently numerically responsive to combinations of environmental conditions expressed at local scales. Common species of grasshoppers on Boulder open space in 1995-1996 were the same as those collected in the region in the 1950s. Our results suggest these protected grasslands, although invaded by alien vegetation and fragmented by suburbanization, are still effectively conserving this particular insect group.

INTRODUCTION

Insects in general, and grasshoppers in particular, are dominant above ground herbivores in most grassland ecosystems but much remains to be learned about the structure and function of their communities (Otte, 1981; Curry, 1994; Tscharntke and Greiler, 1995). Most grasshoppers of the North American Great Plains are widely distributed, with ranges that encompass eastern tallgrass, western shortgrass and northern mixed grass prairies that comprise the natural vegetation across broad expanses of this region (Daubenmire, 1978; Otte, 1981, 1984; Sims, 1988; Pfadt, 1994). Local and regional studies suggest that many Great Plains grasshoppers have distinct habitat requirements, despite their large ranges (Joern, 1982; Capinera and Thompson, 1987; Evans, 1988; Kemp et al., 1990; Quinn and Walgenbach, 1990; Welch et al., 1991; Kemp, 1992). However, we are not aware of any investigation attempting a quantitative comparison of grasshopper densities among short, mixed and tallgrass habitats. One difficulty is the notorious and apparently climate-related volatility of many grasshopper populations (Capinera and Horton, 1989; Joern and Gaines, 1990; Kemp, 1992). These fluctuations can make it difficult to distinguish fundamental habitat differences from temporal variations in widely separated sites sampled in different years (but see Joern and Pruess, 1986).

The arid plains of eastern Colorado are, or were, dominated by relatively uniform shortgrass prairie. However, fine-scale mosaics of short, mixed and tallgrass habitats occur where the plains meet the eastern edge of the Colorado Rocky Mountains, apparently the result of increased precipitation, topographic heterogeneity and differences in soils and land-use history (Livingston, 1952; Branson et al., 1965; Moir, 1969; Bennett, 1997). This circumstance permitted a comparison of grasshoppers occupying the three fundamentally different sorts of Great Plains grasslands at a local scale where their regional populations would be experiencing similar patterns of temperature and precipitation, and presumably where individuals could move among the available habitats.

Grasslands at the base of the Rocky Mountains in Colorado (the so-called Front Range Corridor) are rapidly being fragmented and lost to development (Mutel and Emerick, 1992; Long, 1997). …

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