Chapter 15 Summary
CONCEPT 15.1 Communities are groups of interacting species that occur together at the same place and time.
- Communities can be delineated by the characteristics of their physical environment or by biological characteristics such as the presence of common species.
- Ecologists use subsets of species to define and study communities because it is impractical to count all the species within a community, especially if they are small or undescribed species.
- Subsets of species used to study communities include taxonomic groups, guilds, functional groups, and food and interaction webs.
CONCEPT 15.2 Species diversity and species composition are important descriptors of community structure.
- Species diversity, the most commonly used measure of community structure, is a combination of the number of species (species richness) and the abundances of those species relative to one another (species evenness).
- Communities differ in the commonness or rarity of their species, such that even in communities with the same numbers of species, one community might have a few very abundant species and many rare species, while another might have species of equal abundance.
- Species richness estimates for communities improve with increased sampling effort up to a certain point, at which additional samples reveal few or no new species.
- Species composition—the identity of the species present in a community—is an obvious but important characteristic of community structure that is not revealed in measures of species diversity.
CONCEPT 15.3 Communities can be characterized by complex networks of direct and indirect interactions that vary in strength and direction.
- Indirect species interactions, in which the relationship between two species is mediated by a third (or more) species, can have large effects on the outcomes of species interactions.
- Some species have a strong negative or positive effect on their communities, but others probably have little or no effect.
- Species that have strong effects by virtue of their large abundance and biomass are known as dominant species while those that have strong effects due to their interactions, rather than size are known as keystone species.
- Ecosystem engineers such as trees create, modify, or maintain physical habitat for themselves and other species.
- The environmental context of species interactions can modify them enough to change their strength or direction.
