Grasses, Grazers, Mutualism and Coevolution: A Comment about Handwaving in Ecology

desert ecosystems (Gary Polis, pers. comm.) and may be found in most communities possessing many euryphagous invertebrate predators. If one naively conducted a manipulative experiment on the guild foraging on sand-treaders, erroneous results could be obtained. For example, removal of one scorpion species might leave more sandtreaders available to the other two predaceous scorpions. If one believed that food was limiting, a positive functional or numerical response by these species might be expected due to competitive release. Suppose one of the predators on the removed scorpion (e.g. Onychomys) switched its foraging to the remaining scorpions, the opposite change might be seen regardless of the actual importance of competition. If the removed scorpion species were a major predator on one or both remaining scorpion species, a positive numerical response by these species could occur for a reason entirely unrelated to exploitation competition. If complex food webs are common, the usefulness of manipulative field experiments in ecology may be seriously compromised. It should be possible to overcome this problem by conducting more thorough analyses of the community being studied to delimit the appropriate predictions of an experiment. Specifically the relative magnitudes of links in the food web being studied need to be quantified. This will not be an easy task, but it is essential if we are to gain a real understanding of the processes shaping community interactions.