As discussed in Chapter 6, competition and predation have been recognized as two primary mechanisms, along with resource quality and quantity, for limiting population growth of a given species (e.g., May 1983). Any particular species usually interacts with at least 2-5 other species as prey (see Chapter 9) and with additional species as a competitor. Life table analysis often is used to identify key factors, especially predators or other interactions, that contribute most to population change, but the combination of interactions provides for "redundant" control of population growth. If the major regulating species should disappear, other predators, parasites, or competitors usually compensate.
As noted earlier in the chapter, mutualistic interactions may reduce the probability that either species will decline to extinction. Mutualistic species often are closely associated, especially in obligate relationships, and enhance each other's resource acquisition, energy and nutrient balance, or reproduction. Although mutualism is likely to become unstable at low population densities of either partner, depending on the degree of obligation (May 1983), mutualism could help to maintain the two populations above extinction thresholds (Dean 1983).
The combination of various interactions involving a particular species should maintain its population levels within a narrower range than would occur in the absence of these various interactions. Croft and Slone (1997) found that three predaceous mite species maintained populations of the European red mite, Panonychus ulmi, at lower equilibrium levels than did fewer predator species. However, few studies have documented the importance of species diversity or food web structure to the stability of population levels.
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