An Ecosystem Perspective on Grasshopper Control : Possible Advantages to No Treatment

It has been hypothesized that some herbivores may increase the cycling of nutrients in ecosystems, thereby increasing plant production even after herbivore consumption is considered. Such herbivory could maintain the productivity of ecosystems and should not be curtailed. This contrasts with the traditional view whereby herbivory always reduces plant production. Two grasshoppers (Melanoplus sanguinipes and Ageneotettix deorum), considered pest species in Western US rangelands, are experimentally shown to enhance plant production under certain conditions and to diminish it under others. Plant production increased when grasshoppers' consumption increased nitrogen cycling, nitrogen being the limiting resource for plants at the study sites. This happened when grasshoppers consumed plant species with slowly decomposing litter, favoring plants whose litter decomposed rapidly, such that their store of nitrogen was more rapidly released to the soil. However, feeding preferences change between sites and with grasshopper species, creating a spatial mosaic in which grasshoppers can increase and decrease plant production. Because changes in plant species composition are the major reason for changes in nutrient cycling rate, plant communities are created that will persistently exhibit enhanced or diminished production. Therefore, grasshoppers cannot be generally viewed as detrimental to rangeland productivity and grasshopper control may need to be more judiciously applied.