Predator-prey cycles can persist in the field
Natural populations of predators and prey can coexist and show dynamics similar to those of Huffaker's mites. Clumps of mussels off the coast of California, for example, can be driven to local extinction by predatory sea stars.
However, mussel larvae float in ocean currents and hence disperse more rapidly than the sea stars. As a result, the mussels continually establish new clumps that flourish until they are discovered by sea stars. Thus, like the six-spotted mites in Huffaker's experiments, the mussels persist because portions of their population escape detection by predators for a time.Field studies have also shown that predators influence population cycles in species such as southern pine beetles, voles, collared lemmings, snowshoe hares, and moose (Gilg et al. 2003; Turchin 2003). But predation is not the only factor that causes population cycles in these species. The supply of food plants for the herbivorous prey can also play an important role, and in some cases, social interactions are important as well. Thus, reality is not as simple as implied by the results of predator-prey models (in which cycles are maintained purely by predator-prey interactions). In the field, some population cycles may be caused by three-way feeding relationships—by the effects of predators and prey on each other, coupled with the effects of prey and their food plants on each other.
Whether their populations cycle or not, a variety of factors can prevent predators from driving prey to extinction. Such factors include habitat complexity and limited predator dispersal (as in Huffaker's mites), prey switching in predators (see Figure 12.6), spatial refuges (i.e., areas in which predators cannot hunt effectively), and evolutionary changes in the prey population.
In this section, we have seen how predation can alter the population size of predator and prey, resulting in population cycles. We turn next to how predators can have major effects on ecological communities.