Ecological Knowledge and Environmental Problem-Solving Concepts and Case Studies (1986) / Chapter Skim
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3. Community Ecology
3. Community Ecology
Pages 47-60
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From page 47... ...
First, whether communities are discrete entities with higher-order "emergent" properties (not readily derivable from an analysis of constituent species) or simply groups of species from the available pool is controversial (Krebs, 1985; Simberloff, in press)
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From page 48... ...
Ecological communities have numerous properties that transcend those of their constituent species and that require study themselves trophic structure, rates of energy and nutrient flow, growth form and physical structure, number of species and their numerical distribution, stability characteristics, and ecotones (zones of transition between two habitat types) , to name a few.
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From page 49... ...
In Lake Washington (Chapter 20) , continued addition of sewage led to the appearance of the blue-green alga, Oscillatoria rubescens, now known to be characteristic of severe eutrophication; when the sewage was reduced, Oscillatoria disappeared and later the zooplankton community changed.
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From page 50... ...
· The species present in a community can change substantially without any significant change in diversity indexes. · Some disturbances can increase diversity if they increase habitat heterogeneity, reduce the influence of competitively dominant species, or create opportunities for new species to invade (discussed below)
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From page 51... ...
In contrast, a low rate of disturbance provides few opportunities for pioneering species and might allow competitively dominant species to usurp limiting resources, particularly in space-limited systems, such as rocky intertidal zones or some terrestrial plant communities. Therefore, the number of species in a community is often greater at intermediate rates of disturbance (Cornell, 1978; Huston, 1979)
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From page 52... ...
For example, experimental removal of starfish from a rocky intertidal zone allows competitively dominant mussels to usurp space from other species; when present, starfish open up space for other species by selectively removing mussels (Paine, 1966, 19741. However, although mussels in the absence of starfish predation can take over space and reduce the diversity of macroinvertebrates in the rocky intertidal zone, the mussel beds provide vertical structure that actually increases total diversity when microinvertebrates are also considered; this shows how the effects of spatial heterogeneity and predation can interact (Suchanek, 19791.
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From page 53... ...
Interspecific territoriality is most common among closely related organisms, but does occur among more distantly related ones as well, especially among fish (Ebersole, 1977; Myerberg and Thresher, 19741. Some cases of interspecific territoriality among birds also involve distantly related species (Cody, 1969; Moore, 1978; Orians and Willson, 19641.
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From page 54... ...
Some forest management plans attempt to maximize diversity by creating configurations of habitat patches with much ecotone while retaining patch sizes and proximities that can support species that rely on single community types (Thomas, 1979~. What constitutes the most appropriate configuration of patch size, shape, and spacing depends on the requirements of the species to be maintained.
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From page 55... ...
Plant species richness is often high in the presence of low soil fertility and periodic disturbance, both of which interact to slow down the takeover of a site by competitively dominant species (Huston, 1979; Tilman, 1982; Chapter 181. A similar phenomenon occurs in rocky intertidal habitats, where animals are the dominant competitors for space (Menge and Sutherland, 1976; Paine, 1966~.
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From page 56... ...
For example, the elimination of the sea otter along the Pacific coast of North America contributed to a large increase in the number of sea urchins, a major food item of otters; and the proliferation of urchins resulted in the decline of kelp beds through excessive urchin grazing (Duggins, 1980; Estes et al., 19821. Lobsters can function similarly as keystone species by preying on urchins off the Atlantic coast (Mann and Breen, 19721.
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From page 57... ...
Many of the escaped captive birds that have established feral populations in North America also exploit new food resources, particularly those provided by extensive plantings of ornamental trees and shrubs in southern cities.
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From page 58... ...
Although ecological succession was originally thought of as a community process, examination of particular successions has shown that abrupt, wholesale extinction of the constituent species of one community with concurrent colonization by the species of another is rare (Drury and Nisbet, 19731. The fates of some pairs or groups of species are inextricably intertwined, as are the fates of some mutualists, but these linkages are in a minority.
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From page 59... ...
The high net production of early succession is harvestable for human use, and this is taken advantage of in agriculture and forestry. Human societies usually try to maintain early successional stages precisely because they are more productive, but maintaining them in the face of the natural tendency for change requires large expenditures of energy, effort, and materials.
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From page 60... ...
Herbicide applications to powerline rights of way can be reduced by planting shrubs that impede succession (Niering and Egler, 19551. CONCLUSIONS All human-induced environmental disturbances alter interactions among species in some way that leads to direct and indirect affects on the composition of ecological communities and their dynamics.
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