The extent to which the network of regulatory interactions maintains stable community structure (see Chapters 9, 10, and 15) has been a topic of considerable debate. Although some irruptive species show wide amplitude in population size over time and space, such irruptions often reflect disruption of normal interactions as a result of anthropogenic habitat alteration or introduction into new habitats (see Chapters 6 and 7). The range in population size may be narrower, and the duration of deviations shorter, when regulatory interactions are intact.
The capacity for the network of interactions to stabilize species populations may be enhanced by compensatory interactions and changes in the nature or strength of interaction with changing environmental conditions. For example, the many plant species at a site can, at the same time, compete for resources, share nutrients via mycorrhizae, be growth-limited by herbivores, and limit herbivore populations through the mingling of attractive host odors and repellent (or unattractive) nonhost odors (E. Allen and Allen 1990, A. Hunter and Arssen 1988, Visser 1986). The net result of these negative and positive effects of interaction may be balanced co-existence (W. Carson and Root 2000). Ants maximize energy gain by preying on aphids when the value of honeydew rewards is low (e.g., scattered individuals or individuals dispersing from dense colonies) and by tending aphids when the value of honeydew rewards is high (Bristow 1991, Cushman and Addicott 1991). Competitive interactions could become mutualis-tic if two competitors mutually exclude a third, more competitive, species from the intermediate region of the shared niche (Pianka 1981). Such flexibility in species interactions may facilitate regulation in a variable environment. If the various species in the community respond to changes in each other's population densities in ways that are neutral or beneficial at low densities and increasingly negative at higher densities (see Chapters 12 and 15), then community structure should be relatively stable. Stabilization of community structure has substantial implications for the stability of ecosystem processes (see Chapter 15).
Interactions strongly affect energy or nutrient balances, survival, and reproduction of the associated species and therefore represent major selective factors. Strongly negative interactions should select for adaptive responses that minimize the negative effect (e.g., niche partitioning among competitors, prey defenses, etc.). Therefore, negative interactions should evolve toward more neutral or mutualistic interactions (G. Carroll 1988, Price 1997).
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