Niche

Species have limited abilities to tolerate extremes of environmental conditions. In other words, for all environmental factors there are upper and lower bounds of intensity that organisms can endure. For example, a particular species of plant or animal might be able to survive over an extended period of time within a zone of temperature bounded by certain high and low extremes. Extended exposures to hotter or colder temperatures cannot be tolerated, and the species will not occur in such environments. The boundaries of temperature tolerance can be represented as a component of the niche of the species in a single dimension-that of environmental temperature.

There are similar boundaries of tolerance for other environmental factors including climatic, chemical, and habitat variables, each of which can be similarly represented as a component of the larger niche in a single dimension. Conceptually, the niche of the species can be viewed as a multidimensional composite in which all of the boundaries of tolerance of diverse environmental influences are assembled into a single, multivariate factor. This is known as the fundamental niche, or the multidimensional zone (also known as a hypervolume) of environmental tolerance, in which an individual can potentially survive or in which a species can maintain viable populations.

However, in natural ecosystems species are rarely faced with habitat opportunities that are only defined by the boundaries of their tolerance of diverse environmental factors. The actual utilization of the fundamental niche is also significantly influenced by ecological interactions of various sorts. For example, other species may have similar tolerances of environmental factors. These species will seek to utilize some portion of the environmental opportunities that are available, resulting in the ecological interaction known as competition. Competition exerts a very important influence on the ability of species to optimally exploit their fundamental niche and on the structure of ecological communities. If species have very similar fundamental niches, then competition between them will be intense. In extreme cases this can cause one species to be eliminated from the community through a process known as competitive exclusion. More often, however, species are displaced by competition to particular zones within their fundamental niche.

Exploitation of the fundamental niche is also constrained by other ecological interactions such as predation, parasitism, and disease. All of these can restrict the opportunities for species to exploit their fundamental niche in an optimized fashion. Ecologists define the realized niche as the multidimensional hypervolume of environmental factors that species actually manage to exploit in nature in view of the powerful influences of competition, predation, parasitism, and disease.

Species must be present in the habitat in order to realize the benefits of some part of the range of their fundamental niche. If a location containing potentially suitable habitat cannot be colonized by a species, then it cannot utilize that part of its fundamental niche.

Realized niches are variable over time because they can respond to changes in the nature of ecological interactions. The introduction of a new, more capable competitor can eliminate an original species from its ecological community through competitive exclusion. A similar effect can be caused by introduced predators, parasites, and diseases. In contrast, the elimination of an important competitor, predator, parasite, or disease can release a species from a previously controlling influence, allowing it to expand the dimensions of its realized niche.

In general, the realized niche of species does not represent the environment conditions to which they are optimally adapted. The combined influences of diverse ecological interactions commonly relegate species to sub-optimal portions of their fundamental niche.

Ecological communities can be viewed as populations of various species that co-occur in space and time. Each species in the community maintains its populations by utilizing the opportunities available in its realized niche within the larger habitat. The number of species that can be maintained in the community and their relative abundance are determined by the diversity of niche opportunities, their stability over time, and the intensity of ecological interactions. The influences of these factors are optimized in old-growth tropical rain forests which maintain a greater diversity of species than any other terrestrial ecosystem. Among oceanic ecosystems, species diversity is greatest in coral reefs.