Chapter 16 Summary
CONCEPT 16.1 Agents of change act on communities across multiple temporal and spatial scales.
- Agents of change include both abiotic (physical and chemical) and biotic factors.
- Abiotic agents of change can act as disturbances (injuring or killing organisms) or stresses (reducing the growth or reproduction of organisms).
- Biotic agents of change include both positive and negative species interactions. Ecosystem engineers and keystone species are common agents of change.
- Agents of change vary in their intensity, frequency, and areal extent.
CONCEPT 16.2 Succession is the change in species composition over time as a result of abiotic and biotic agents of change.
- Theoretically, succession involves a series of stages that include a stable end point, or climax stage.
- Primary succession involves the colonization of habitats that are devoid of life.
- Secondary succession involves the reestablishment of a community in which most, but not all, of the organisms have been destroyed.
- Early ecologists were fascinated with succession, but disagreed about whether it proceeded in deterministic or indeterministic ways.
- Connell and Slatyer proposed three models of succession in 1977, known as the facilitation model, tolerance model, and inhibition model.
CONCEPT 16.3 Experimental work on succession shows its mechanisms to be diverse and context-dependent.
- Multiple studies of succession have shown that no one model fits any one community. Aspects of the facilitation, tolerance, and inhibition models can be seen in almost all systems studied.
- Generally, experiments show that facilitation tends to be important in early stages of succession and competition in later stages of succession.
CONCEPT 16.4 Communities can follow different successional paths and display alternative states.
- Alternative stable states occur when different communities develop in the same area under similar environmental conditions.
- In communities that experience alternative states, succession is typically controlled by strongly interacting species.
- Human activities have caused “regime shifts” in communities that may or may not be reversible.
