Ecological Niches

Niche is a term that describes the relationship between an organism and its environment. This relationship is thought to be crucial in explaining why certain species exist where they do, and which other species they can coexist with. The constraints on coexistence that come from such niche relations are important in the development of theories about the regulation and the consequences of biodiversity. The niche concept has also been used to understand the functional dynamics (either evolutionary ones that involve genetic adjustments, or short-term adjustment via behavior or physiology) of species to one another. The niche has also been used as a conceptual way of describing how species contribute to ecosystem processes such as productivity and the cycling of materials.

As in all relationships, there are two possible components: the set of responses of the organism to the multiple factors in the environment, and the set of impacts of the organism on these factors. Mathematical theory indicates that in closed local communities, two aspects of niche relations are important in allowing for the stable coexistence of two species: (1) each species must be more sensitive to a different environmental factor (related to the response component), and (2) each species must have a greater net impact on the factor that it is most sensitive to (related to the impact component). The presence of both of these components is essential for theories of stable coexistence, because the absence of either component means that there is no feedback in the system, thus altering many aspects of ecological systems.

History

The niche concept has an origin that harkens back to the origins of the field, but one that has been characterized by some confusion—in part because the concept is perhaps too easily applied in the form of metaphor rather than in strictly logical terms. The confusion has centered on two issues: whether the focus was on biological aspects related to the response of organisms to the environment or to their impacts; and whether the focus was on descriptions of the habitats of the environment or on more detailed (within-habitat) factors.

Joseph Grinnell is the person most frequently cited as the originator of the term. His 1917 paper, "The Niche Relationships of the California Thrasher," is one of the first uses of the term. This paper focuses on the conditions that characterize the habitats used by a single species. Grinnell related these conditions (for example, the presence of chaparral for cover, certain temperature and humidity ranges, and the like) to the requirements of—or to factors that affect the fitness of—the California thrasher, thus emphasizing requirements and habitat perspectives. Intriguingly, Grinnell concluded with the statement that "it is, of course, axiomatic that no two species regularly established in a single fauna have precisely the same niche relationships." Since that was one of the first uses of the term niche and one of the first statements of this principle, one wonders how much theoretical work was being developed outside of print by Grinnell and his collaborators.

Charles Elton used the term niche ten years later in his influential Animal Ecology text in a substantially different way. His definition was vaguely stated as the "role of the species" and emphasized the functional role of species in food webs in relation to their impacts on other organisms and on the environment (that is, reducing food levels, supporting predators, modifying soil structure and so forth). It does not seem that Elton was familiar with Grin-nell's use of the term, and this seems to be an independent derivation of the term. In contrast with Grinnell, Elton emphasized impacts and did not emphasize habitat descriptions.

Alfred Lotka and Vito Volterra developed basic mathematical models of species interactions (interspecific competition, and predator-prey interactions) that confirmed Grin-nell's conclusions. In particular, the simple models they derived showed that Grinnell's "axiom" could be mathematically confirmed and made more precise by restating it as "interspecific competitors can only coexist if the average intraspecific effects are greater than the average interspecific effect." This theoretical work received strong empirical support in the early 1930s by the work of G. C. Gausse, who showed that laboratory populations of pro-tists matched the quantitative predictions of these models remarkably well (the axiom is actually often called Gausse's Axiom). This theory incorporated both responses and impacts to make the predictions, although the links to the niche concept were not made explicit.

George Hutchinson (1959) was the first to provide a rigorous definition of the niche of a species as the n-dimensional hypervolume in which every point corresponds to a set of environmental conditions allowing the species to exist indefinitely. His definition is very strictly focused on the conditions allowing the population of a species to have a stable per capita growth rate of zero or more. Retrospectively, it is clear that this definition does not deal with the impact component of the relationship.

Robert MacArthur and, later, David Tilman developed models of resource competition using Hutchinson's definition to establish how consumers respond to resources. This component of the interaction in their models is defined by the mapping of so-called zero net growth isoclines (or ZNGIs), whereby the response of the species as determined by its per capita growth rate is equal to zero. They also showed that local stable coexistence required that such ZNGIs cross (allowing for a possible equilibrium point), implying the presence of a trade-off in the responses of different species to different resources, so that each species has a greater relative impact on the resource that is most limiting to it (see Leibold, 1995).

Robert MacArthur and his students also developed an alternative approach to niche relations using "resource use functions," one that focuses on using Gaussian distributions to model overlap in consumption of arrays of resources. These models were used to evaluate questions about similarity (as measured by overlap in resource use distributions) and coexistence, and about the evolutionary dynamics of niche adjustments. This body of theory was tremendously influential in the 1970s but has since come under criticism, and many (but not necessarily all) researchers feel that the conclusions are of heuristic use more than anything else.

Robert Holt, James Grover, David Tilman, Mathew Leibold, and J. Tim Wootton have extended the use of the isocline models of MacArthur and Tilman to interactions involving predators, omnivores, and disturbances. The conclusions again show that local coexistence of species requires that each species be relatively more limited by a different environmental factor (so that the ZNGIs that describe their responses to different environmental factors cross) and that each species has a proportionately greater impact on the environmental factor that it is most sensitive to. These extensions of niche theory to address multiple trophic levels and multiple types of factors are important because they allow the niche concept to be used to evaluate ecological processes and phenomena as well as the coexistence of species that simply share resources.

Richard Levins and more recently Peter Chesson (2000) have also used mechanistic models to address how temporal and spatial variability affects niche relations. They again emphasize that it is not only the relative responses to variability that matter but also the relative impacts on variability that affect conditions for coexistence.

Because of criticisms leveled at the niche theory developed with resource use functions, recent work in ecology tends to avoid use of the term niche, even though the underlying idea that relationships between organisms and environment are key to understanding the distribution of species is still critical. One of the more pressing issues is the extension of the concept at scales larger than the local community level.

Extensions of the Concept

Although the niche concept has a central place in theoretical community ecology, it permeates almost all aspects of ecology and evolutionary biology. Of key importance is how diversity of species originates and is maintained. The fundamental insight to arise from the work on the niche is that sustained coexistence of species at any scale depends on trade-offs among such species in their responses to different environmental factors and (at least at the local level) related trade-offs in their impacts on the environment.

The dynamics associated with evolutionary adjustments of species to one another can be evaluated in terms of such trade-offs. Hutchinson made a distinction between the "fundamental niche" of an organism, describing environmental conditions that allow a species to exist in the absence of competing species, versus the "realized niche," describing environmental conditions that allow a species to coexist with another species. The realized niche is thus a subset of the fundamental niche when interspecific competition between pairs of species is involved and the set of environmental habitat conditions that allows a species to coexist with a competitor is smaller than the set of habitat conditions that would allow such a species to exist in the absence of competitors. Much work has gone into thinking about the evolutionary dynamics that result. Especially intriguing was the idea of "character displacement," which argued that species could evolve in response to the presence of competitors to reduce how similar they are to each other by diverging in their niche relations. However, recent work has shown that convergence is also possible or likely.

Case Studies

One of the biggest problems with the niche concept is that it is very open-ended. Although one can study particular environmental factors, one often can't tell if there isn't some unsus pected dimension of the "n-dimensional" hypervolume that isn't important. In the lab, the environment can be controlled so that these problems are minimal. The classic work by Gausse on protists and a number of subsequent studies using protozoans, bacteria, or algae (Grover, 1997) provide good support for the conclusions of niche theories. They provide good support for the so-called Gausse's axiom; they show that more complicated models such as those using ZNGIs are useful in understanding interactions among multiple species; and they show how environmental conditions such as resource supply, predation intensity, and the effects of stressors (for example, temperature, pH, and harvesting) and variability can alter the outcome of species interactions.

Under field conditions rigorous application of these explicit niche models is much more difficult, and there are very few examples that even begin to match the rigor of the lab experiments described above. Instead, the concept is often used in a much more heuristic fashion to interpret data in the context of the theory (Giller, 1984). Such case studies include the classic work on intertidal barnacles by Connell, who showed that the distribution of two species along a stress gradient is controlled by competition between them, such that each dominates only a part of the gradient. More recent work has illustrated the role of niche relations in evolutionary dynamics, the role of predators in modifying niche relations among species, and the role of species interactions on regulating the relative amount of biomass in different trophic levels.

Two intriguing and important recent applications of the niche concept involve its application to coexistence in larger so-called meta-communities (sets of local communities linked by dispersal), and its application to evaluat ing the role of species in the functioning of ecosystems.

—Mathew Leibold

See also: Ecology; Ecosystems; Food Webs and Food Pyramids; Nutrient/Energy Cycling

Bibliography

Chesson, Peter. 2000. "Mechanisms of Maintenance of Species Diversity." Annual Review of Ecology and Sys-tematics 31: 343-366; Giller, Paul S. 1984. Community Structure and the Niche. London: Chapman and Hall; Grover, James P. 1997. Resource Competition. London: Chapman and Hall; Hutchinson, George E. 1957. "Concluding Remarks." Cold Spring Harbor Symposia on Quantitative Biology 22: 415-427; Leibold, Mathew A. 1995. "The Niche Concept Revisited: Mechanistic Models and Community Context." Ecology 76: 1371-1382.

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