ecological niches

IN CONTEMPORARY ecology, the term ecological niches is referred to as the position occupied by representatives of any life form (usually alive) in biocenosis, and, at the same time, the smallest unit of a habitat that is occupied by an organism. The notion reflects an organism's or species's place in the community, taking into account not only its tolerance to natural (physical) agents, but its interaction with other organisms as well. The ecological niche concept originated as an attempt to describe the general role of species in the community and to differentiate on a theoretical level population, community, and ecological systems. The term had been introduced in 1917 by J. Grinnell, who interpreted it in a spatial sense as the ultimate distributional unit of a species. Later C. Elton (1927) concentrated mainly on niche functional aspects when describing an organism's place in its biotic environment in connection with its nutrition and animals.

The middle of 20th century marked a multidimensional approach to the niche concept in the works of L. Ramensky (1924), E. Odum (1968), and others. The multidimensional approach to niche comprehension allows scientists to conceive how species are connected with each other and, in such a way, enhances the understanding of a community's internal organization.

Further development of the niche concept is connected with the name of G. Hutchinson. In 1957, he interpreted niche as hyperspace, in frameworks of which conditions of the ambient allow unrestrictedly durable existence of specimen or species. According to Hutchinson, niche is defined by a general set of environment variables, to which species must be adapted (physically, chemically, biotically) and in which a species population lives and replaces itself indefinitely.

Such niches could be described mathematically with the help of quantitative characteristics of every environment's variable spatial coordinates. Hutchinson distinguished fundamental (now often called potential) niche (the whole set of optimal conditions that could be occupied by a species in its enemies' absence) and realized niche (a real set of conditions in which a species usually exists).

Today, a niche is often regarded as a way by which a species population fits into a certain community and in such a context concentrates on a species's connections in one community. So the niche concept also is concerned with the functional role of an organism in community (for example, its trophic status) and its position in relation to external factors of its habitat (such as temperature, humidity, etc.). It is especially useful for understanding the current distribution of sources among all niche inhabitants.

The latest developments in the framework of ecological niches are connected with a discussion about their attribution as a basic property of community or species. Some researchers believe that any niche is generated by its inhabitants, and its features are defined by the role that its creator is playing in its environment. Others regard niche as a community attribute and deny its significance in any other context. They reveal the phenomenon of ecological equivalence (or the principle of convergent evolution): A niche is created by biotic and abiotic ecosystem components and exists a priori. It becomes filled in the course of a species's adaptations taking place during evolutionary changes; such species' adaptations should be similar even if these species are not interrelated.

NICHE CHARACTERISTICS

Theoretical and empirical development of the ecological niche concept has put forward a series of niche characteristics, a major part of which is defined by function of utilization (or species activity distribution) along the resources' gradient. Most important among niche characteristics are the principle of competitive exclusion, niche width, niche axes, niche dynamics, niche interlocation, and so on.

The principle of competitive exclusion (or Gause principle) was proposed in the 1920s as the result of experimental investigations by G. Gause and J. Grinnel. Accordingly, two ecologically identical species could not survive simultaneously at the same place, or a niche is occupied by only one species. It was mathematically confirmed by the equation of Lotka-Volterra and has become one of the basic principles of ecology. In connection with the niche concept, attention should be paid to its corollary: If two species co-exist, they should differ ecologically, so every species has its own niche.

Niche width (or its size) notion is inherent to a multidimensional approach to niche interpretation. It its framework, it is defined as a general amount of all diversity of different features used by the population. Most often, it is identified through species activity distribution (resources utilization spectrum). Niche axes is a relatively new notion with a minimally independent
set of limits in relation to species factors. It describes a series of important variables, such as form of population response to the axis challenge, relative width of species distribution along certain axes, grade of species overlapping, species's relative position along the axis, and other factors.

The niche dynamics concept envisions investigation of niche changes in at least three contexts. In an evolutionary framework, short-term, life cycle, and generational changes could be distinguished. The width of a realized niche also changes in response to competition and in relation with resource level. Community space changes in accordance with daily or seasonal rhythms. The niche interlocation concept describes niche differences, first of all, with time and place of activity and with ways of other species' communication. In such a context, scientists have distinguished occasional (nonoverlapping), discrete, and overlapping niches.