Document: DAV-3-48-35

Grasshopper population dynamics and vegetation patterns: An experimental analysis.

BRANSON, D.H.*

USDA-Agricultural Research Service, Sidney, MT 59270, USA ¹

Abstract:
I experimentally examined the effect of grasshopper densities, climate, vegetation and natural enemies on population dynamics and reproduction of experimental grasshopper communities. The experiment was conducted in eastern Montana in mixed grass prairie highly dominated by western wheatgrass (>90%). The objectives of the experiment were to examine a number of factors potentially important for grasshopper population dynamics. Grasshoppers were stocked at a variety of densities in 10 m² cages, with 4 replicate cages per treatment. The experiment was conducted in an area with naturally occurring high grasshopper densities. Grasshopper densities at the site averaged between 25 to 30 nymphs and adult grasshoppers per m². Grass biomass declined over the summer, although high quality regrowth occurred late in the season. Treatments affected both vegetation quantity and vegetation quality. Both high density and increased competition treatments had lower amounts of vegetation remaining at the end of the experiment. In addition, a high percentage of grass blades were damaged by grasshoppers. Treatments with grasshoppers had significantly lower damage than a control treatment with no grasshoppers. Survival of grasshoppers in the field was not strongly affected by resource availability, although some treatments significantly impacted grasshopper survival. There were higher numbers of surviving grasshopper in both high density and increased competition treatments. Elevated spider densities did not have an impact on grasshopper survival or reproduction. However, a microbial grasshopper pathogen reduced grasshopper populations by the end of the experiment. Therefore, although some treatments do not affect survival, they can have large effects on reproduction and thus population dynamics of grasshoppers. Both survival and reproduction must be carefully examined to understand grasshopper population dynamics.

Keywords: grasshoppers, population dynamics

This abstract is being presented at: 3:30 PM in session:
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