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Terms in this set (70)

Exploitation competition is competition in which the competitors interact only indirectly, through their shared resources. Direct interference is in which competitors compete directly to prevent organisms from using the limited resource by using that resource for themself. An example of direct interference is of the Dolly Varden charr versus the white-spotted charr, which are two morphologically similar and closely related species of salmonid fish found in many streams in Hokkaido Island, Japan. The Dolly Varden charr is located further upstream than the white-spotted charr. The water temperature has profound consequences for fish ecology, in which it increases downstream. When maintained together, white-spotted charr did consistently better than Dolly Varden charr at higher, upstream, temperatures. The lower altitudinal boundary of Dolly Varden charr in the Japanese streams was due to temperature-mediated competition favoring the white-spotted charr, which were more aggressive, foraged more effectively, and grew far faster. An example of exploitation (indirect interference) is the competition amongst phytoplankton for phosphorous. There were 5 different single-freshwater phytoplankton species that compete in pairs, all of which require phosphorus as an essential resource for their growth. When any of the species was grown alone, it established a steady population density, reducing the phosphorous to a constant low concentration. However, when any 2 species was grown together, there was only one survivor, and that survivor was whichever species had previously reduced phosphorous to the lower level.
Niche complementarity is niche differentiation in a community of species involved in several niche dimensions, with an example of fish species occupying a similar position along one dimension tending to differ along another dimension, be it species of anemone, zone on the shore, or food particle size. The fish can also build a guild, a group of species that exploit the same class of environmental resource in a similar way. Interspecific competition plays a role in structuring communities; it tends to do so, as here, not by affecting some random sample of the members of that community, or by affecting every member, but by acting within guilds. The fish example illustrates the properties of niche complementarity. An area in Papua, New Guinea, has the highest reported species richness of both anemone fish (nine) and of sea anemones with which they are associated (ten). Anemones seem to be a limiting resource for the fish in that almost all anemones are occupied, and if some are transplanted to new sites, they are quickly colonized. However, each individual anemone tends to be occupied by individual of just one species of anemone fish, because the residents aggressively exclude intruders, through aggressive interactions are less frequently observed between anemone fish of very different sizes. In the survey, the four zones should that each species of anemone fish was primarily associated with a particular species of anemone. Each also showed a characteristic preference for a particular zone. Crucially, though, anemone fish that lived with the same anemone were typically associated with different zones.
The patterns that are apparent in these examples have also been uncovered in many others, and have been elevated to the status of a principle, named the Competitive Exclusion Principle (also known as the Gause's principle). It can be stated with two principles: if two competing species coexist in a stable environment, then they do so as a result of niche differentiation and if, however, there is no such differentiation, or if it is precluded by the habitat, then one competing species will eliminate or exclude the other. This principle has emerged here from examining patterns evident in real sets of data. There is no question that there is some truth in the principle that competitor species can coexist as a result of niche differentiation, and that one competitor species may exclude another by denying it a realized niche. But it is crucial also to be aware of what the Competition Exclusion Principle does not say. It does not say that whenever we see coexisting species with different niches, it is reasonable to jump to the conclusion that this is the principle in operation. All species, on close inspection, have their own unique niches. Niche differentiation does not prove that there are coexisting competitors. The species may not be competing at all and may never have done so in their evolutionary history. We require proof of interspecific competition. Most cases for competitors coexisting as a result of niche differentiation have not been subjected to experimental proof. Part of the problem is that although species may not be competing now, their ancestors may have competed in the past, so that the mark of interspecific competition is left imprinted by evolution on either their niches, or their behavior, or their morphology. This principle investigates the habitats' conditions and supply of resources that remain more or less constant and if species compete, then that competition runs its course, either until one of the species is eliminated, or until the species settle into a pattern of coexistence within their realized niches. However, most of the time, the environments are not stable for long periods of time.
Neutral, or null, models, asks the question, "Does the observed pattern, even if it appears to implicate competition, differ significantly from the sort of pattern that could arise in the community even in the absence of any interactions between species?" These questions seek to compare real communities with these models. The idea is that the data are rearranged into a form representing what the data would look like in the absence of interspecific competition. Then, if the actual data show a significant statistical difference from the null model, the action of interspecific competition is strongly inferred. So, the null model analyses are attempts to follow a much more general approach to scientific investigation, namely, the construction and testing of 'null hypotheses.' This approach can be applied to three predictions of what communities structure should look like by potential competitors that coexist in a community should exhibit niche differentiation, this niche differentiation will often manifest itself as morphological differentiation, and within a community, potential competitors with little or no niche differentiation should not coexist, so each should tend to occur only where the other is absent. This application to community structure has not satisfied all ecologists. The problems with this model is that the patterns that are observed, sometimes have a confirmation that a role for competition is occurring, while other times it is not, which is the general conclusion for the neutral model. This concentrates the minds of investigators by stopping them from jumping quickly to conclusions. However, this approach can never take the place of a detailed understanding of the field ecology of the species in question, or of manipulative experiments designed to reveal competition by increasing or reducing species abundances.