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Insect herbivory and vertebrate grazing impact food limitation and grasshopper populations during a severe outbreak

David H. Branson, Marshall A. Haferkamp
Ecological entomology 2014 v.39 no.1 pp. 371-381
biomass, cages, experimental design, foods, grasshoppers, grasslands, grazing, interspecific competition, land use, phytophagous insects, population density, population dynamics, sheep
Interspecific competition between distantly related herbivores, as well as between large vertebrate herbivores and phytophagous insects, has received little attention. Livestock grazing is the dominant land use in western North American grasslands, where phytophagous insects can be the dominant herbivore. Large scale studies have examined interactions between livestock grazing and grasshoppers, but manipulations of grazing and grasshopper densities to clearly examine competition are lacking. In a novel design, we manipulated grasshopper population densities and the timing and frequency of sheep grazing inside 10m2 caged mesocosms to examine their impact on grasshopper population dynamics and vegetation. We utilized a replicated, factorial experimental design during a naturally occurring grasshopper outbreak. Field densities of grasshoppers reached 130 per m2 in 2000. Grasshoppers removed much of the available vegetation biomass during the experiment, as green grass biomass was eight times higher in vegetation control cages than in field density cages. As a result of high densities, exploitative competition from grasshopper herbivory was dominant in all treatments. Proportional survival and number of grasshoppers surviving were lower in field relative to reduced density cages. Higher grasshopper population densities and late season and multiple grazing both accelerated the population decline resulting from food limitation. Strong density dependence was evident in reproduction, as fewer grasshoppers hatched in field density than in reduced density treatments in 2001. Few detailed experiments have been conducted during grasshopper outbreaks. The results indicate that phytophagous insect herbivory can have stronger effects than livestock grazing on population dynamics during an outbreak, but livestock grazing and grasshopper herbivory interacted at the lower density. Additional research examining interactions between livestock grazing and grasshoppers is needed at lower grasshopper densities.