Fundamental and Realized Niche

Hutchinson recognizes a species' 'fundamental niche', a multidimensional 'cloud' offavorable conditions determined by all environmental (abiotic and biotic) variables where the species can reproduce and survive, and the 'realized niche', which is a subset of the abstract fundamental niche, where the species can persist given the presence of other species competing for the same resources. Realized niche thus always has a narrower extent along respective dimensions; a species which could potentially live in a broad range of humidity conditions, for instance, may occupy a much narrower range of these conditions in an environment with competing species, since its population growth rate decreases to negative values in some conditions. To a good approximation, if we ignore stochastic sampling from a heterogeneous species' population, species does fill its realized niche.

According to Hutchinson's formalization, niches of different species can be separated along any of these dimensions or by a combination of them (i.e., their pH

Figure 1 A hypothetical example of species niches and their interaction. Two niche axes are shown, pH and temperature. The two species of hypothetical protists have different optima, and one (left) is adapted to wider range of conditions and has overall lower growth rate (measured at a given low density). When the niches of the two species overlap, the growth rates are expected to decrease, potentially to the point where the population cannot be sustained. If one of the species is fully dominant in its niche, the other species can sustain its population only on a part of its fundamental niche, and its growth rate decreases at the overlapping areas. The growth rate isoclines are shown, with the dashed lines depicting the growth rate isoclines of the fundamental niche (i.e., had the other species been absent).

Temperature

Figure 1 A hypothetical example of species niches and their interaction. Two niche axes are shown, pH and temperature. The two species of hypothetical protists have different optima, and one (left) is adapted to wider range of conditions and has overall lower growth rate (measured at a given low density). When the niches of the two species overlap, the growth rates are expected to decrease, potentially to the point where the population cannot be sustained. If one of the species is fully dominant in its niche, the other species can sustain its population only on a part of its fundamental niche, and its growth rate decreases at the overlapping areas. The growth rate isoclines are shown, with the dashed lines depicting the growth rate isoclines of the fundamental niche (i.e., had the other species been absent).

interaction) (Figure 1). Although this formal model of the niche has quite straightforward theoretical consequences, in practice it can be quite difficult to describe properly the ecological niches of real species, because the number of niche dimensions is potentially infinite, and the significant niche axes (and appropriate measurements) may be rather hard to find: a niche overlap among species may mean we did not succeed in determining the crucial niche axes of separation. However, often a few variables are sufficient to separate species' realized niches, and they or their correlates can be inferred assuming we understand the species' biology reasonably well. For example, five species of warblers, analyzed by Robert MacArthur, showed significant (though not complete) separation along only three niche axes (feeding behavior, feeding height, and nesting time).

The difficulties in determining appropriate niche axes, however, still considerably limit the usefulness of the concept in empirical research. Even if we know the important resources, it is still problematic to decide which characteristics to measure. A further problem, albeit rather technical, is posed by including discrete categories: the width of the cloud in the respective dimension would be reduced to zero, and its position can be arbitrary. More importantly, although species can often potentially live in a much broader range of environmental conditions than where they do actually live, the distinction between the 'fundamental' and 'realized' niche is slightly arbitrary, driven purely by the interest in coexistence of species sharing resources. As the dimensions of the fundamental niche are both abiotic and biotic, there is no a priori reason to exclude presence of competitors from the dimensions characterizing fundamental niche. The distinction between the fundamental and realized niche may also be blurred, as species' interactions need not fit to our discrete categories - for example, competitors may act also as predators.

Due to the difficulties with the concept, and for tract-ability, a considerable part of the theory actually dealing with species coexistence works with a one-dimensional approximation of the 'trophic niche', a 'resource utilization function' - given by the frequency distribution of an important characteristic of utilized resource (e.g., a prey size).

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