11_Community_Ecology (2)

11_Community_Ecology (2) - BI SC 002 LECTURE 10COMMUNITY...

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BI SC 002 LECTURE 10—COMMUNITY ECOLOGY Draft: December 2, 2010 Community ecology deals with how different populations of organisms interact with each other. The possible types of interaction include competition , in which two or organisms are trying to utilize the same resources, symbiosis , in which organisms lives are linked to each other, and predation , in which one organism consumes another organism. These interactions affect the population biology of a particular species because they all influence dN/dt (the rate of growth of a population). Two hypotheses exist about how community interactions take place. H.A. Gleason developed the individualistic concept of communities . This idea states that the community is nothing more than an aggregation of species that happen to coexist in one place. This is opposed by F.E. Clements’ holistic concept of communities , in which communities are integrated units. Clements saw the community as a “super-organism” whose constituent species have coevolved to specific functions (like various organs in a body). Most ecologists tend to prefer Gleason’s individual concept because in reality, organisms tend to respond independently of each other to changing environments. Before we can discuss competition, we must first define the term niche . A niche is the sum total of all of the ways an organism utilizes the resources of its environment. Basically, the niche is the “role” an organism plays in the environment. The niche can include space utilization, food consumption, temperature range, mating conditions, moisture, and other environmental components. Competition is an important factor in defining the niche of an organism. Interspecific competition is competition between individuals of different species, while intraspecific competition is competition between individuals of the same species. An organism may be capable of using many, many resources in the environment. What an organism is capable of using is called the organism’s fundamental niche . However, organisms may compete for some of those resources and another organism may be a better competitor, thus making the original’s niche smaller. The actual niche an organism occupies in the environment is called the organism’s realized niche . The classic example of realized and fundamental niches is J.H. Connell’s work with barnacles. On the seaside cliffs of England grow two types of barnacles— Chthalamus , which is found above the low tide line, and Semibalanus , which is found below the low tide line. If you remove Semibalanus , Chthalamus is capable of growing in its place below the low tide line. Thus, Chthalamus ’s fundamental niche is the entire cliff. However, Semibalanus is a better competitor below the low tide line, so Chthalamus ’s realized niche is just that area that is sometimes exposed to air. If you remove Chthalamus , Semibalanus will not expand its range above the tide line (it cannot survive the exposure to air). Thus, for this organism, the realized niche and fundamental niche are the same.
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When new species enter a habitat, they can usually occupy their full fundamental niche. However, as new species move in, competition (and predation as well) limit the range of the
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11_Community_Ecology (2) - BI SC 002 LECTURE 10COMMUNITY...

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