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16.2: The niche concept

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    25520
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    A species’ niche is the range of environmental factors that allow that species to survive and reproduce. A particular tree species, for example, may be able to live where temperatures do not drop below −40, and where yearly precipitation is at least 750 mm. Perhaps it also needs open sunlight and an appropriate collection of root fungi. Such are the parameters of a niche.

    G. Evelyn Hutchinson, one of the great ecologists of the twentieth century, envisioned the parameters that form a niche as an “n-dimensional hyperspace.” The “fundamental niche” is the set of conditions allowing the species to survive if there are no other species interfering. Physical conditions are chief among those. The “realized niche” is the real life niche—where species are restricted by interactions with other species.

    Consider latitude on the earth’s surface, which is connected to several parameters such as sunlight and temperature. And consider two species that can thrive anywhere between 40 and 60 latitude, and whose density drops slowly with increasing latitude (Figure \(\PageIndex{1}\)).

    species gradient.JPG
    Figure \(\PageIndex{1}\): Species living apart along a spatial gradient (above) and living together along the same gradient (below).

    At the top of the figure are two nearly horizontal lines representing the abundance you might observe of the two species as you travel north. If free of Species 2 (its competitor), Species 1 (blue line) declines slowly in abundance in more northerly climates. Species 2 similarly declines in abundance (red line), but compared with Species 1 fares a little better in the north and a little worse in the south.

    When these two species are together they compete with each other—each suppressing the other. Using competition equations like those presented in this chapter, we see that in the south, where Species 1 fares better, it takes over and dominates. In the north, in contrast, where Species 2 fares better, it dominates instead (Figure \(\PageIndex{1}\), bottom).

    You see that there can be a sharp change in abundance even with only very slight changes in species characteristics. A range of one species can end and that of a new species can begin, even though you may not be able to discover anything from either species alone as to why they switch their dominance. And the switch-over point need not correspond to the exact place in which their dominance switches. Here the actual switch-over point is a few degrees to the north because of the migration simulated in the model. This phenomenon is called “competitive exclusion” and, when it occurs over space like this, “zonation.”

    Zonation.JPG
    Figure \(\PageIndex{2}\): Zonation on a three-year-old fen, Bluebird Prairie, Upper Midwest.

    Any environmental gradient can induce zonation. Figure \(\PageIndex{2}\) illustrates this on a restored prairie in the North American Upper Midwest. Though less than ten meters, the variation in elevation is enough to induce a mild moisture gradient. The entire area had been converted from a corn field to a restored prairie and seeded uniformly with a mixture of prairie grasses and prairie flowers, but distinct boundaries arose only three years after restoration.

    All the upland areas, labeled 1, contained standard restored prairie flora such as Andropogon gerardi (Big Bluestem grass) and rapidly emerging flowers such as Rudbekia hirta (Black-eyed Susan). But in successive zones surrounding mild depressions in the landscape, labeled 2 to 4, there were sharp transitions to moisture-loving genera such as Typha latifolia (cattail) and Stachys palustris (Smartweed).

    Definitions

    Fundamental niche: The conditions under which a species can live, absent interference from other species.

    Realized niche: The conditions to which a species is restricted by interactions with other species.


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