| Unit 2 Objectives
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Part 2:
Population Dynamics
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| Unit 2 Objectives
|
Part 2:
Population Dynamics
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A niche is an organism's role in the ecosystem, along with all conditions pertinent to that role. Niches develop as each species adapts and evolves along with its environment. A specie's niche includes whether it's a producer, consumer or decomposer, whether a predator or prey, when, where, and how it feeds, its typical habitat as well as many other characteristics. Species can become generalists or specialists. Generalists are adapted to take advantage of a variety of food sources or other resources. A specialist is adapted to one or a few special conditions, food sources etc. Generalists are usually much more abundant than specialists. For instance, in the video "Rift Valley II" we saw that the generalist lions are much more successful than the specialist cheetahs. Cheetahs are adapted for speed and able to catch the fast Thompson's gazelles, but unable to catch larger prey and to defend their prey against usurpation by lions and other predators.
A good example of a North American specialist is the Everglades kite (snail kite).
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(Click on images for large view) The snail kite has a beak which is specialized to open the apple snail (pomatia). Apple snails have declined in the swamps around the Everglades due to pollution, mostly phosphates from fertilizer used by sugar cane plantations. As these snails decline, so do the snail kites, and kites are now found only in a few areas near Lake Okeechobee. The apple snail cannot change its food supply, it is stuck with its specialized beak. In fact most species cannot change what they do or how they do it. Their physical and behavioral characteristics, and their relationships to other species, have evolved over long periods of time and are usually unable to adapt to sudden changes. A few species are opportunists, able to take advantage of new conditions. Africa's fish eagle (seen in the video) is such as species. So is the coyote, and the result can be seen in its rapid expansion as its chief competitor, the wolf, has been eliminated.
The nature of niche evolution can be easily seen when it occurs in isolation. Convergent evolution describes the results when unrelated species evolve in similar habitats. It makes sense that they develop similar physical and behavioral characteristics in response to similar environments. A coral reef and deep lake, for instance, will have fish, eels, etc. which look and behave like one another, although the particular species are not derived from common ancestors. In adaptive radiation many niches are filled by evolution from a common ancestor. This was seen by Charles Darwin in finches in the Galapagos Islands, which had also evolved in isolation, with members of this family filling virtually every possible niche. It is also exemplified by the island continent of Australia where marsupials evolved without competition and predation to fill niches which would otherwise be occupied by other types of mammals.
Limiting Factors - these are factors which determine the distribution and abundance of a species. The fall into two categories:
abiotic factors - these are non-living factors such as moisture, temperature, nutrients, sunlight, physical factors etc. They can be most easily imagined as affecting plants, but such factors influence animals as well.
biological factors - these involve interactions between organisms such as competition, predation, etc.
One can describe a niche as it relates to resources or other abiotic factors. Consider a single resource, for example moisture. A niche can be thought of as a growth curve along the resource continuum. (Figure 1).
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Each organism can survive in a range of conditions: it would have a minimum requirement (A), an optimum level or level of maximum growth (C), and a maximum tolerance (B). For example, dry-adapted desert plants such as the creosote bush survive at very low rainfall levels. A little more rainfall results in more luxuriant and denser stands. But too much rainfall renders the creosote bush unable to compete with species that do better at higher moisture levels, for example desert grasses. Growth curves could be drawn for each and every characteristic or resource important to a species, such as light, heat, altitude, nutrients, salinity, etc. These curves would graphically represent the species' relationship to these environmental characteristics. |
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In Figures 2 and 3 two species are shown which have different requirements
for a particular resource. This means the range of conditions in which they
exist is slightly different. Putting them together on the same axis shows that
they do overlap and therefore coexist in part of their range. 
According to the competitive exclusion principle no two organisms coexisting
in the same
community can have exactly the same niche. If they did, one species would exclude the other
due to interspecific competition.
When two similar species do coexist in the same community, competition between them may push one or both into a slightly different realized niche (def) than the fundamental niche (def) determined by biology. In Figures 2 and 3 you can see how species S1 and S2 exist when apart from one another. But when competing (Figure 4) S1 is reduced to a small fraction of its original habitat while S2 is only slightly affected. Apparently S2 is dominant, except in conditions outside is adapted range. Competition, along with many other factors contributes to the niche differentiation by which similar species with similar niches can coexist.
The following example will be described in class:
Barnacles are mollusks which attach to rocks and other substrates. They reach out and comb the water with feathery "cirri" (combs) to pull in tiny organisms for food. When the tide recedes and they are exposed to the air they close plates to prevent drying out. (Click on images for large view)
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Along the coast of Scotland two species inhabit slightly different niches on the rocks. In order to determine if competition was determining the distribution of these barnacles, an experiment was performed in which each one of the species was removed from a given area and the change of distribution of the other was observed.
The result, shown above, indicate that the larger Balanus is the dominant species in a competition which produces a significantly restricted realized niche for the smaller Chthamalus (pronounced thamalus). Apparently the Balanus cannot survive the desiccation in the uppermost life zone. But the smaller Chthamalus can survive there. And without competition the Chthamalus spreads into all three zones, its fundamental niche.
