Concept 44.1 Interactions between Species May Be Positive, Negative, or Neutral
- Interspecific interactions have consequences that affect the fitness of individuals and thus the growth rates and densities of populations and the distributions of species.
- Interspecific interactions can be divided into five broad categories depending on whether their effects on the interacting species are positive, negative, or neutral. Review Figure 44.1 and WEB ACTIVITY 44.1
- Interspecific competition refers to interactions in which members of two or more species use the same resource. There generally is one limiting resource that is in the shortest supply relative to demand.
- In consumer–resource interactions, the consumer gains nutrition and the resource species is killed or harmed. Consumer–resource interactions include predation, herbivory, and parasitism.
- A mutualism is an interspecific interaction that benefits both participants.
- Commensalism, in which one participant benefits while the other is unaffected, and amensalism, in which one participant is harmed while the other is unaffected, are not reciprocal and thus do not show the same ecological or evolutionary dynamics as other types of interspecific interactions.
- Many interspecific interactions have both beneficial and harmful aspects, and their effects may be contingent on environmental conditions. Review Figure 44.2
Concept 44.2 Interspecific Interactions Affect Population Dynamics and Species Distributions
- The equation for density-dependent population growth, which describes intraspecific competition, can be extended to include the effects of interspecific competitors on per capita growth rates. Consumer–resource interactions and mutualisms can be described in a similar way by equations.
- The per capita growth rate and the average population size of each interacting species are modified by the presence of the other in a way that depends on the type of interaction.
- In interactions with negative effects on one species (consumer–resource interactions, competition, and amensalism), extinction of one or both of the interacting species is possible. Review Figure 44.3
- The presence or absence of other species can make an environment favorable or unfavorable for a species and so affect its distribution. Review Figure 44.4
- Coexistence of species competing for resources, and of prey species with their predators, is possible if the populations of such species can increase even at low densities. Differences between competing species in their use of resources, known as resource partitioning, often are great enough to generate such a rarity advantage. Review Figure 44.4
Concept 44.3 Interactions Affect Individual Fitness and Can Result in Evolution
- If intraspecific competition results in directional selection for traits that allow individuals to use new resources, or to use resources more efficiently, it can increase the carrying capacity of the population.
- If interspecific competition results in directional selection for traits that allow individuals of one species to use resources different from those a competitor species uses, resource partitioning and coexistence may result. Review Figures 44.6 and 44.7
- The opposing interests of consumer species and resource species can lead to an “evolutionary arms race” in which resource species continually evolve better defenses and consumer species continually evolve better offenses.
- Mutualisms involve mutual exploitation and the exchange of goods and services. The fitness effects of mutualisms may vary with environmental conditions that determine the value and cost of the goods and services that are exchanged. Mutualisms often involve cheating. See ANIMATED TUTORIAL 44.1
Concept 44.4 Introduced Species Alter Interspecific Interactions
- Species introduced into a new region by humans, whether accidentally or intentionally, can alter interactions among the native species of that region.
- Species introduced into a region where their natural enemies are absent may become invasive, reproducing rapidly and spreading widely.
- Invasive species are likely to have negative effects on native species that lack adaptations to compete with or defend themselves against the invaders. Invasive species may even drive native species extinct.