Chapter 18 Summary

CONCEPT 18.1 Species richness differs among communities due to variation in regional species pools, abiotic conditions, and species interactions.

• The regional species pool and the dispersal abilities of species play important roles in supplying species to communities.
• Humans have greatly expanded the regional species pools of communities by serving as vectors for the dispersal of non-native species.
• Physiological constraints imposed by abiotic conditions act as a strong “filter” for community membership.
• When species depend on other species for their growth, reproduction, and survival, those other species must be present if they are to gain membership in a community.
• Species may be excluded from communities by competition, predation, parasitism, or disease.

CONCEPT 18.2 Resource partitioning among the species in a community reduces competition and increases species richness.

• Resource partitioning is an equilibrium theory predicting that species must use slightly different resources along a resource spectrum if they are to avoid competitive exclusion.
• One model of resource partitioning states that the less overlap between the species in a community in their resource use, the more species can coexist in the community.
• The resource ratio hypothesis posits that species that use the same set of resources are able to partition them by using them in different proportions.

CONCEPT 18.3 Nonequilbrium processes such as disturbance, stress, and predation can mediate resource availability, thus affecting species interactions and coexistence.

• Nonequilibrium theories state that disturbance, stress, or predation can cause species’ populations to fluctuate in space and time, making resources available and increasing species coexistence.
• The intermediate disturbance hypothesis states that intermediate levels of disturbance promote species diversity by reducing competitive exclusion. Species diversity is low at low levels of disturbance due to competitive exclusion and at high levels of disturbance due to high mortality.
• The dynamic equilibrium model predicts that species diversity will be highest when the level of disturbance and the rate of competitive displacement are roughly equivalent.
• Positive interactions can promote species diversity, particularly at intermediate to high levels of disturbance, stress, or predation.
• Lottery models assume that resources made available by disturbance, stress, or predation are captured at random by recruits from a larger pool of colonists, all of which have an equal chance to do so.

CONCEPT 18.4 Experiments show that species diversity is positively related to community function.

• Species can control numerous functions of communities, including productivity, soil fertility, water quality and availability, atmospheric chemistry, and responses to disturbance.
• Many manipulative experiments in different communities have shown that as species diversity increases, so does community function.
• Hypotheses proposed to explain the positive relationship between species diversity and community function fall into four general categories that include varying assumptions about the degree of overlap in the ecological functions of species and about variation in the strengths of the effects of the ecological functions of species.