Chapter Summary
55.1 Species Interactions Vary in Direction and Strength across a Continuum
Species vary in the effects they have on each other (positive, negative, neutral). Species interactions may involve feeding (trophic) effects and/or living in close association (symbiosis).
The four main categories of species interactions are predation (predator kills and/or consumes its prey, or part of its prey), competition (at least two species use some of the same limiting resources and have negative effects on one another), positive interactions, or facilitations (at least one species benefits from the interaction and none are harmed), and amensalism (one species is harmed but the other is not affected). Review Focus: Key Figure 55.1, Activity 55.1
Species interactions are not always clear-
cut but instead fall along a continuum in which species vary in how strongly they affect one other. Review Figures 55.2, 55.3 Some species interactions result in reciprocal evolutionary change over time, also known as coevolution. Review Activity 55.2
55.2 Predation Is a Trophic Interaction in which Predators Benefit and Prey Are Harmed
Predator–
prey interactions result in the evolution of a range of capture and avoidance mechanisms. Review Figures 55.4, 55.5, 55.6, Activity 55.3 Herbivory (animals preying on plants) and parasitism (parasites feeding and living on or in hosts) are widespread but specialized interactions that usually do not result in the death of the prey. Review Figures 55.7, 55.8
Population sizes of predators and their prey tend to cycle as a result of predator–
prey interactions over time. Review Figure 55.9 Some predators, called keystone species, can have dramatic effects on communities by preying on competitively dominant species. Review Figure 55.10
55.3 Competition Is a Negative Interaction in which Species Overlap in the Use of Some Limiting Resource
Interference competition occurs when one species directly interferes with or prevents another species’ access to a limiting resource. Exploitation competition occurs when all competitors have access to the limiting resource but the outcome depends on their relative efficiency at using the resource.
Competitive exclusion occurs when a species prevents another species from using essential resources, which may cause the inferior competitor to become locally extinct. Review Figure 55.11D
Most competing species show competitive coexistence through the processes of resource partitioning, disturbance, stress, and/or predation. Review Figures 55.11E, 55.12, 55.13
55.4 Positive Interactions Occur When at Least One Species Benefits and None Are Harmed
The two types of positive interactions are mutualisms, in which both species benefit from the interaction, and commensalisms, in which one species benefits and the other is unaffected. Some positive interactions are symbiotic relationships. Some positive interactions are obligate (necessary), while others are facultative (optional). Review Table 55.1, Animation 55.1
Positive interactions are more common in stressful environments, where physical conditions can limit the abundance and distribution of some species. Review Figure 55.14
Positive interactions can have dramatic effects on populations and communities, especially by providing habitat for other species.
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