Chapter 16 Ecology
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the change in species composition in communities over time.
an event that injures or kills some individuals and creates opportunities for other individuals (e.g., the 2004 tsunami killed or injured many individuals).
- an abiotic factor reduces the growth or reproduction of individuals (e.g., temperature increase).
- Both disturbance and stress are thought to play critical roles in succession.
Biotic interactions can
also can result in the replacement of one species with another.
beach gross took over first
Ecosystem engineers or keystone species can
also influence community change.
R vs K
r is first, high reproduction ect, K is next
Succession was similar to the development of an organism
a stable end point that experiences little change.
Primary succession involves
the colonization of habitats devoid of life (e.g., volcanic rock).
Secondary succession involves
reestablishment of a community in which some, but not all, organisms have been destroyed.
Frederick Clements believed plant communities were like
- “superorganisms,” groups of species working together toward some deterministic end.
- Succession was similar to the development of an organism. Clements felt that each community had a predictable life history and, if left undisturbed, ultimately reached a stable end point called the “climax community”.
The climax community was composed of dominant species that persisted over many years and provided stability that could be maintained indefinitely due to work of early species
Henry Gleason thought that communities
- were the random product of fluctuating environmental conditions acting on individual species.
- Communities were not the predictable and repeatable result of coordinated interactions among speciesElements of both theories are found in the many successional studies carried out since then.
inspired by Clements. Early species modify the environment in ways that benefit later species. The sequence of species facilitations leads to a climax community.
also assumes the earliest species modify the environment, but in neutral ways that neither benefit nor inhibit later species.
assumes early species modify conditions in negative ways that hinder later successional species.
Connell and Slatyer (1977) reviewed the literature on succession and proposed three models:
Facilitation, Tolerance, and Inhibition
In a newly exposed habitat...
- a pioneer stage develops, dominated by lichens, mosses, horsetails, willows, and cottonwoods.
- Pioneer stage—spruce seedlings had low germination rate; higher survival rate.
After about 30 years,
- the Dryas community develops, named for a small shrub.
- Dryas stage—increase in seed predators led to weak germination and survival; but survivors had better growth. Dryas has N-fixing bacteria.
After about 50 years (or 20 km from the ice front),
- alders dominate, forming the alder stage.
- Alder stage—more nitrogen (alders also have N-fixing bacteria) and soil organic matter produced positive effects; shading and seed predators led to overall poor germination and survival rates.
100 years later,
a mature Sitka spruce forest is in place.Spruce stage—effects of large spruce were mostly negative. Growth and survival rates were low due to competition with adult spruce for light, space, and nitrogen.
Two hundred years later, species richness decreases somewhat as Sitka spruce are replaced by Western hemlocks.
In some cases different communities develop in the same area under similar environmental conditions, called
—alternative stable states.
A community is thought to be stable when
it returns to its original state after some perturbation.
The stability of a community partly depends on
the scale of observation, both spatially and temporally.
Connell and Sousa's (1983) requirement that the physical environment not change is problematic because
it excludes as drivers of succession all species that interact with other species by modifying their physical environment—that is, all ecosystem engineers.
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