Change in Communities

Chapter 17 Change in Communities CONCEPT 17.2 Succession is the process of change in species composition over time as a result of abiotic and biotic agents of change. CONCEPT 17.3 Experimental work on succession shows its mechanisms to be diverse and context-dependent. CONCEPT 17.4 Communities can follow different successional paths and display alternative states.

Concept 17.1 Agents of Change Succession is change in species composition in communities over time. It results from both biotic and abiotic factors.

Concept 17.1 Agents of Change Abiotic agents of change fall into two categories: Disturbance: Events that injure or kill some individuals and create opportunities for other individuals. Stress: Abiotic factors that reduce growth, reproduction, or survival of individuals.

CONCEPT 17.2 Succession is the process of change in species composition over time as a result of abiotic and biotic agents of change.

Concept 17.2 The Basics of Succession Theoretically, succession progresses through various stages that include a climax stage—a stable end point that changes little. But there is some argument about whether succession can ever lead to a stable end point.

Figure 17.5 The Trajectory of Succession Ecology3e-Fig-17-05-0R.jpg

Concept 17.2 The Basics of Succession Two types of succession: Primary succession: Colonization of habitats devoid of life (e.g., volcanic rock). Secondary succession: Reestablishment of a community in which some, but not all, organisms have been destroyed.

Concept 17.2 The Basics of Succession When you use a gradient such as a sand dune to a forest to study succession or change in communities over time. This technique is called “space for time substitution.”

Figure 17.8 Three Models of Succession Ecology3e-Fig-17-08-0.jpg

CONCEPT 17.3 Experimental work on succession shows its mechanisms to be diverse and context-dependent.

Concept 17.3 Mechanisms of Succession Cooper observed increasing plant species richness and change in composition with time and distance from the melting ice front. The pioneer stage is dominated by lichens, mosses, horsetails, willows, and cottonwoods.

Figure 17.10 Successional Communities at Glacier Bay, Alaska Ecology3e-Fig-17-10-0R.jpg

Concept 17.3 Mechanisms of Succession In field experiments, spruce seeds were added to each successional stage. Germination, growth, and survival were monitored over time. Neighboring plants had both facilitative and inhibitory effects on the spruce seedlings, but the direction and strength of those effects varied with successional stage.

Figure 17.12 Both Positive and Negative Effects Influence Succession Ecology3e-Fig-17-12-0R.jpg

Concept 17.3 Mechanisms of Succession Glacier Bay illustrates all three mechanisms in Connell and Slatyer’s models: Early stages show aspects of facilitation—plants modify the habitat in positive ways for other plants and animals. Later, species such as alders have negative effects on later successional species.

Concept 17.3 Mechanisms of Succession In the spruce stage, where dominance is an artifact of slow growth and long life, succession is driven by life history characteristics, as in the tolerance model.

Concept 17.3 Mechanisms of Succession Many experimental studies show that succession is driven by many mechanisms. No one model fits any one community. Facilitative interactions are often important drivers of early succession, especially when physical conditions are stressful.

Concept 17.3 Mechanisms of Succession As succession progresses, larger, slow- growing and long-lived species begin to dominate. Competition probably plays a more dominant role later in succession. In mid- to late-successional stages, an array of both positive and negative interactions are operating.

CONCEPT 17.4 Communities can follow different successional paths and display alternative states.

Concept 17.4 Alternative Stable States Sometimes different communities develop in the same area under similar environmental conditions—alternative stable states. 21

Concept 17.4 Alternative Stable States A community is thought to be stable when it returns to its original state after perturbation. Stability partly depends on the scale of observation, both spatial and temporal. Ecologists have done much research on alternative stable states.

Concept 17.4 Alternative Stable States Lewontin and Sutherland thought that multiple stable states existed and could be driven by addition or exclusion of strong interactors. If the strong interactor was missing, a community could follow alternative successional trajectories.

Concept 17.4 Alternative Stable States The theory of alternative stable states can be depicted as a topographic surface. Valleys represent different community states, a ball represents a community. The ball can move from one valley to another, depending on presence or absence of strongly interacting species.

Figure 17.18 A Model of Alternative Stable States Ecology3e-Fig-17-18-0R.jpg

Concept 17.4 Alternative Stable States A change in one or more dominant species might force the ball into a new valley (stable state). The ball might not be able to move back into the first valley. Hysteresis is an inability to shift back to the original community type, even when original conditions are restored.

Concept 17.4 Alternative Stable States These regime shifts are caused by removal or addition of strong interactors that maintain a community type. It is unclear whether the results can be reversed. For example, will reintroduction of sea otters rejuvenate kelp forests?