02 ®3 04 05 Temperature

02 ®3 04 05 Temperature

Figure 8.12 Distribution of species 1 and 2.

Consider an extreme case: can two populations occupying the same resource niche coexist in the same environment? (

If two populations occupy same resource niche, then by definition, they utilize all the same resources and in the same manner. Common sense tells us there are three possible outcomes to this situation: (1) share resources more or less equally (neither population changes niche); (2) one or both populations alters niche to reduce overlap (niche partitioning), and (3) one population loses out completely (competitive exclusion). Which outcome will occur? Answer from niche theory = 2 or 3, but not 1. This somewhat counterintuitive finding given the formal name of competitive exclusion principle (CEP) (Gause, 1934) states that no two species can permanently occupy the same niche: either the niches will differ, or one will be excluded by the other (note: "excluded" here means replaced by differential population growth, not necessarily by fighting or territoriality), and has become a central tenet of modern niche theory. Of course, 100% niche overlap is unlikely if not impossible; but such an extreme case is not necessary for competitive exclusion or other forms of niche change.

Much theory and research in ecology has focused on predicting what actually happens when there is niche overlap and competition: when does exclusion result, when coexistence? How much overlap is possible (a question treated by the "theory of limiting similarity")? How do environmental fluctuations affect this? Why are some species gen-eralists, others specialists? Both possible responses to niche competition (competitive exclusion, and coexistence via reduction in niche overlap) are commonly observed, and their determinants and features have been studied by three means: lab experiments, field observations, and mathematical models or simulations. Competitive exclusion is commonly observed when a species colonizes a habitat and out-competes indigenous species— probably due to absence of parasites and predators adapted to exploit the colonizer (e.g. introduced placentals vs. indigenous marsupials in Australia). Coexistence through niche partitioning is rarely observed directly, but can often be inferred from traces left by "the ghost of competition". The typical means of doing so is to examine two populations that overlap spatially, but only partially, and then to compare the niche of each population in the area of overlap versus the area of non-overlap. In such a case, we often observe that in areas where competitors coexist, one or both have narrower niche range (e.g. diet breadth) than in areas where competitor is absent; this is because competition "forces" each competing population to specialize in those resources—or other niche dimensions—in which it has a competitive advantage, and conversely to "give up" on those in which the other population out competes it. Thus, in absence of competitors a given species will often utilize a broader array of resources, closer to its fundamental niche, than it will in competitor's presence; this phenomenon is termed "competitive release".

When niche shift involves an evolutionary change in attributes ("characteristics") of competing populations, it is termed character displacement. This is some of the strongest evidence for the role of competition in shaping niches because it is unlikely to have alternative explanation. A classic example of character displacement is change in length or shape of beaks in ecologically similar bird species that overlap geographically.

Example 1 of the competitive exclusion principle or Gause s principle: two spp. of Paramecium

Gause (1934) placed two species of Paramecium into flasks containing a bacterial culture that served as food. Thus, in this artificial laboratory system both species of paramecium were forced to have the same niche. Gause counted the number of Paramecium each day and found that after a few days (Figure 8.13) one species always became extinct because it apparently was unable to compete with the other species for the single food resource.

However, extinction is not the only possible result of two species having the same niche. If two competing species can co-exist for a long period of time, then the possibilities exists that they will evolve differences to minimize competition; that is, they can evolve different niches.

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