Chapter 17 Changes in communities: Ecological succession & disturbance

______can initiate community change by causing ______ or _____ _______of a species.

Diseases death slow growth

Inhibition model

assumes early species modify conditions in negative ways that hinder later successional species.

Alternative stable states

occur when different communities develop, or community states, at the same place under similar conditions

Elton believed that______ and the______ interact to shape succession. In English pine forests, succession depended on________ conditions. Wet areas developed into sphagnum bogs, Drier areas developed into grass and sedge marshes. The only way to_____the _____ of_____ was to______ the biological and________ ______ ______ ________ ____ _________

organisms, environment,moisture predict, trajectory,succession understand, environmental context in which it occurred.

Biotic interactions may result in the __________of one species with another.

replacement

A community is thought to be_____ when it returns to its original state after disturbance. Scale dependent.

stable

Frederick Clements believed plant communities are like "__________" groups of species working together toward some ______ ___________

superorganisms deterministic end.

Henry Cowles (1899), studied succession on sand dunes along Lake Michigan. He assumed that..... plant assemblages _____from the lake's edge were the ______ _________ ________ the ones ______ the lake were the ______ ________ _________ Predicted how communities would change over time without waiting decades to centuries. "Space for time substitution"

the sand dunes presented a time series of successional stages: farthest oldest seral stage; nearest youngest seral stage.

Clements felt that each community reaches a stable end point called the "_______ _______."

"climax community."

Disturbance:

A change in a structural aspect of an ecosystem that injure or kill some individuals, but it may also create opportunities for other individuals.

Stress:

Any factor that reduces the growth or reproduction of individuals.

Abiotic factors (agents of change): Waves, currents Wind Water supply Chemical composition Temperature Volcanic Activity

Biotic Factors Negative Interactions: competition, predation, herbivory, disease, parasitism, trampling, digging, boring

Primary Succession and Mutualism: The role of plant microbial symbioses All the examples of primary succession have involved plants with nitrogen-fixing bacteria. Some of these bacteria form nodules in the roots of their plant hosts, where they convert N2 gas from the atmosphere into a form usable by plants (NH4). The bacteria receive carbohydrates from the plant. Dryas and alders in Glacier Bay, and lupines on Mount St. Helens all have associations with nitrogen-fixers.

Figure 17.21 Dwarf Lupines and Nitrogen-Fixing Bacteria (A) Dwarf lupine (Lupinus lepidus), a legume with symbiotic nitrogen-fixing bacteria, was the first plant to colonize Mount St. Helens. (B) Root nodule development is the result of a strong interaction between the plant and the bacteria.

Disturbance and succession

High disturbance, low frequency: primary succession Moderate disturbance, moderate freq: Secondary succession Low disturbance, High frequency: Little succession

Type of Succession

Primary succession Secondary succession We call the different stages seral stages.

Tolerance model

also assumes the earliest species modify the environment, but in neutral ways that neither benefit nor inhibit later species.

Henry Gleason (1882-1975) Henry Gleason thought communities are the random product of fluctuating environmental conditions acting on individual species. Communities are not the predictable and repeatable result of coordinated interactions among species; each community is unique. Succession not as predictable as Clements suggested

Charles Elton (1900-1991) Elton believed that organisms and the environment interact to shape succession. In English pine forests, succession depended on moisture conditions. Wet areas developed into sphagnum bogs, Drier areas developed into grass and sedge marshes. The only way to predict the trajectory of succession was to understand the biological and environmental context in which it occurred.

Agents of change fall into two categories:

Disturbance Stress

Facilitation model,

Early species modify the environment in ways that benefit later species. The sequence of species facilitations leads to a climax community.

Primary succession can be very slow. Initial conditions are very inhospitable. The first colonizers (pioneer or early successional species) tend to be stress-tolerant. They transform the habitat.

Secondary succession occurs after fires, storms, logging, etc. The legacy of the preexisting species and their interactions with colonizing species play large roles

Figure 17.19 Rapid Amphibian Colonization Frog and salamander species rapidly colonized a wetland complex in the Pumice Plain on Mount St. Helens. (After Crisafulli et al. 2005.)

Small mammals and amphibians had the best chance for survival Figure 17.20 Pocket Gophers to the Rescue The burrowing activity of northern pocket gophers, some of which survived the eruption underground, brought organic matter, seeds, and fungal spores to the soil surface, creating microhabitats, like this one in the Pumice Plain, where plants could grow. (Left from C. Crisafulli.)

Agents of____act on______ across all_______ and_______ scales causing disturbance

change communities temporal spatial

Communities can follow________ ______ paths and display______ ________

different successional alternative states

Climax community

dominant species that persist over many years and provide stability that can be maintained indefinitely.

Ecosystem_________ or _______ _______ can influence community change.

engineer, keystone species

Multiple mechanisms were responsible for primary succession: Dwarf lupines on the Pumice Plain_____ the recolonization of other species by trapping seeds and detritus, and its association with nitrogen-fixing bacteria that increases soil nitrogen. Lupines were_______by insect herbivores, which controlled the pace of succession. Douglas fir and herbaceous species lived together in some habitats, evidence of the species tolerance

facilitated inhibited

Connell and Slatyer (1977) Reviewed the literature on succession and proposed three models:

facilitation model, Tolerance model Inhibition model

Secondary succession

involves reestablishment of a community in which many, but not all, organisms have been destroyed.

Primary succession

involves the colonization of habitats devoid of life (e.g., volcanic rock).

Succession

is the gradual and linear process of change in species composition over time as a result of agents of change causing disturbance or stress

William Cooper, a student of Cowles, began studies of Glacier Bay in 1915, seeing it as a "space for time" substitution opportunity. He established permanent plots that are still being used today. Cooper___.... The____ _____ is dominated by lichens, mosses, horsetails, willows, and cottonwoods.

observed increasing plant species richness and change in composition with time and distance from the melting ice front. pioneer stage