Concept 43.1   Interactions between Species May Increase, Decrease, or Have No Effect on Fitness

• Interspecific interactions have consequences that affect the fitness of individuals and thus the dynamics and densities of populations and the distributions of species.
• Interspecific interactions can be divided into five broad categories depending on whether their effects on fitness are positive, negative, or neutral. Review Figure 43.1 and ACTIVITY 43.1
• Interspecific competition refers to mutually detrimental interactions in which members of two or more species use the same resource. Competition is generally strongest for the 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 a mutually beneficial interspecific interaction.
• Commensalism, in which one participant benefits while the other is unaffected, and amensalism, in which one participant is harmed while the other is unaffected, differ from other types of interspecific interactions in affecting only one of the participants.
• Many interspecific interactions have both beneficial and harmful aspects, and their effects may be contingent on environmental conditions. Review Figure 43.2

Concept 43.2   Interactions within and among Species 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. Review ANIMATED TUTORIAL 43.1
• 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), local extinction of one or both of the populations of the interacting species is possible. Review Figure 43.3
• The presence or absence of other species can make an environment favorable or unfavorable for a species and so affect its distribution. Review Figure 43.4
• Coexistence of species competing for resources, and persistence of prey species and their predators, is possible if each species gains a growth advantage when it is rare. Differences between competing species in their use of resources, known as resource partitioning, are often great enough to generate such a rarity advantage. Review Figure 43.5

Concept 43.3   Species Are Embedded in Complex Interaction Webs

• Most species are involved in webs of interaction with many other species.
• Trophic interactions, which determine the flow of energy and materials through communities, form the core of these interaction webs.
• Energy and materials enter communities through primary producers, which produce organic compounds from inorganic sources.
• Species that feed on primary producers are called primary consumers. Those that consume primary consumers are secondary consumers, and so on. These positions in the feeding hierarchy are called trophic levels. Review Table 43.1 and ACTIVITY 43.2
• Trophic interactions can be diagrammed as food webs. Review Figure 43.6
• Changes in species composition or abundance at higher trophic levels may cause a trophic cascade of changes at lower trophic levels. Review Figure 43.7
• Species introduced by humans to a new region where their natural enemies are absent may become invasive species that reproduce rapidly, spread widely, and alter interactions among the native species of the region.

Concept 43.4   Interactions within and among Species Can Result in Evolution

• Intraspecific competition can increase the carrying capacity by causing selection for traits that allow individuals to use resources more efficiently.
• Interspecific competition can lead to resource partitioning and coexistence. It does so through selection for traits that allow individuals of one or both competing species to decrease use of resources that the competitor species uses. Review Figure 43.8 and Figure 43.9
• 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 affect the value and cost of the goods and services that are exchanged. Review ANIMATED TUTORIAL 43.2