PART FOUR

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Interactions among Individuals and Species

INDIVIDUALS AND POPULATIONS interact in various ways, including courtship and reproduction, competition for resources (nesting sites, mates, cover, food), predator-prey relationships, facilitation, and mutualism. Such interactions among individuals of the same or different species are often based on some form of communication. The chapters in this unit explore the mechanisms and roles of communication (Chapter 10); trophic position and interactions during resource acquisition, such as competition among individuals of the same or different species (Chapter 11); predator-prey interactions during resource acquisition, including avoidance of predation (Chapter 12); and facilitation and mutualism as part of resource use (Chapter 13). These chapters cover most of the major ways in which individuals and populations might interact (Part Table 4.1). Some interactions are symmetrical, either benefitting both parties (+, +) or harming both (−, −), whereas some are asymmetrical, where one member benefits at the expense of the other. However, representing interactions by zeros, pluses, or minuses is an oversimplification. Directions of interactions, although symmetrical in terms of sign, may differ in the strength of the interaction, such as when competition affects one member of the interaction more strongly than the other. Also, a negative interaction, such as resource competition, may in the long term positively affect fitness if it contributes to maintaining a stable population size (R. Ryel, pers. comm. 2011).

PART TABLE 4.1 Some Possible Types of Interactions among Individuals and Populations