Get complete information on Niche Overlap and Competition (environmental variables)

Niche overlap occurs when two organismic units use the same resources or other environmental variables. In Hutchinson’s terminology, each n-dimensional hyper volume includes part of the other, or some points in the two sets that constitute their realized niches are identical.

Overlap is complete when two organismic units have identical niches; there is no overlap if two niches are completely disparate. Usually niches overlap only partially, with some resources being shared an others being used exclusively by each organismic unit.

Hutchinson (1957a) treats niche overlap in a simplistic way, assuming that the environment is fully saturated and that niche overlap cannot be tolerated for any period of time; hence competitive exclusion must occur in the overlapping parts of any two niches.

Competition is assumed to be intense and to result in survival of only a single species in contested niche space. While this simplified approach has its shortcomings, it is useful to examine each of the logically possible cases (Fig.) before considering niche overlap and competition in a more realistic way.

First, two fundamental niches could be identical, corresponding exactly to one another, although such ecological identity is infinitely unlikely. In this most improbable event, the competitively superior organismic unit excludes the other.

Second, one fundamental niche might be completely included within another; given this situation, the outcome of competition depends on the relative competitive abilities of the two organismic units. If the one with the included niche is competitively inferior, it is exterminated and the other occupies the entire niche space; if the former organismic unit is competitively superior, it eliminates the latter from the contested niche space.

The two organism units then coexist with the competitively superior one occupying a niche included within the niche of the other. Third, two fundamental niches may overlap only partially, with some niche space being shared and some used exclusively by each organism unit.

In this case each organismic unit has a “refuge” of uncontested niche space and coexistence is inevitable, with the superior competitor occupying contested (overlapping) niche space. Fourth, fundamental niches might about against one another although no direct competition can occur, such a niche relationship may reflect the avoidance of competition.

Finally, if two fundamental niches are entirely disjunct (no overlap), there can be no competition and both organismic units occupy their entire fundamental niche. Figure illustrates the distinction between the fundamental and the realized niche for an organismic unit with six competitors.

A major shortcoming of the foregoing discussion is that, in nature, niches often do overlap yet competitive exclusion does not take place. Niche overlap in itself obviously need not necessitate competition.

Overlap inhabitants used may simply indicate that competitors have diversified in other ways. Should resources not be in short supply, two organismic units can share them without detriment to one another.

In fact extensive niche overlap may often be correlated with reduced competition, Just as disjunct niches may frequently indicate avoidance of competition in situations where it could potentially be severe (such as in cases of interspecific territoriality).

For these reasons the ratio of demand to supply, or the degree of saturation, is of vital concern in the relationship between ecological overlap and competition.

Indeed, much current research is designed to clarify, both theoretically and empirically, the relationship between competition and niche overlap; modem ecologists are asking questions such as, “How much niche overlap can coexisting species tolerate?” and “How does this maximal tolerable niche overlap vary with the degree of saturation?”

Competition is the conceptual backbone of much current ecological thought. Nonetheless competition remains surprisingly elusive to study in the field and hence is still poorly understood (probably because avoidance of competition is always advantageous when possible).

Precise mechanisms by which available resources are divided among members of a community must be known before determinants of species diversity and community structure can be understood fully. Resource partitioning among coexisting species or niche segregation has therefore attracted considerable recent interest.

The basic raw data for analysis of niche overlap is the resource matrix, which is simply an m by n matrix indicating the amount (or rate of consumption) of each of m resource states utilized by each of n different species.

From this matrix one can generate an n by n matrix of overlap between all pairs of species with ones on the diagonal and values less than unity as off-diagonal elements. Overlap is sometimes equated with competition coefficients (alphas) because overlap is much easier to measure.

Again, the caveat: overlap need not result in competition unless resources are in short supply. Extensive overlap may be possible when there is a surplus of resources (low demand/supply), whereas maximal tolerable overlap may be much less in more saturated environments.

Because the principle of equal opportunity dictates that the ratio of demand over supply be constant along any particular resource gradient, intensity of competition should be directly proportional to the actual overlap observed along any given resource spectrum. Patterns of niche overlap along different resource axes or between different communities must be compared with caution.

In a one-dimensional niche space any given niche can be bounded only on two sides, whereas there can be many more neighbours in a two-dimensional niche space, and still more in three or more dimensions. As the effecitive number of niche dimensions rises, the potential number of neighbours in niche space increases more or less geometrically.

As dimensionality increases, overlap matrices contain fewer off-diagonal elements of zero and the variance in observed overlap usually falls, both within rows and over the entire matrix. Hence niche dimensionality strongly affects the potential for “diffuse” competition arising from the total competitive effect of all interspecific competitors.

The overall effect of relatively weak competitive inhibition per species summed over many other species could well be as strong as or even stronger than much more intense competitive inhibition (per species) by fewer competing species. Thus an increased number of niche dimensions, by generating a greater potential for immediate neighbours in niche space, can intensify diffuse competition.

Imagine that height above ground and prey size are two such critical niche dimensions which species use differentially and thereby avoid or reduce interspecific competition.

Analysis of resource utilization and niche separation along more than a single niche dimension should ideally proceed through estimation of proportional simultaneous utilization of all resources along each separate niche dimension.

These define a three- dimensional resource matrix, with each entry representing the probability of capture of a prey item of a given size category at a particular height interval by each species present. Obtaining such multidimensional utilization data is extremely difficult, however, because most animals move and integrate over both space and time.

Accurate estimates of an animal’s true use of a multidimensional niche space could only be obtained by continually monitoring an individual’s use of all resources. (Even then the degree to which prey individuals move between microhabitats will affect competition in obscure but vitally important way!) As such continual observation is often extremely tedious or even impossible; one usually approximates from separate unidimensional utilization distributions.

Just as the three-dimensional shape of a mountain cannot be accurately determined from two of its silhouettes viewed at right angles, these “shadows” do not allow inference of the time multidimensional utilization.

The question of the degree of dependence or independence of dimensions becomes critical. Provided that niche dimensions are truly independent, with prey of any size being equally likely to be captured at any height, overall multidimensional utilization is simply the product of the separate unidimensional utilization functions.

Under perfect independence, the probability of capture of prey item in microhabitat is then equal to the probability of capture of item i times the probability of being in microhabitat. Unidimensional estimates of various niche parameters (including overlap) along component niche dimensions may then simpiy be multiplied to obtain multi-dimensional estimates. However, should niche dimensions be partially interdependent, there is no substitute for knowledge of true multidimensional utilization?

True multidimensional overlap can vary greatly depending on the exact form of this dependence. In the extreme case of complete dependence (if, for example, prey of each size are found only at one height) there is actually only a single niche dimension, and a simple average provides the best estimate of true utilization.

Moreover, the arithmetic average of estimates of unidimensional niche overlap obtained from two or more separate unidimensional patterns of resource use actually constitutes an upper bound on the true multidimensional overlap (May, 1975).

It is difficult (or virtually impossible) to evaluate the degree of interdependence of niche dimensions for many species. However, in relatively sedentary species, the degree to which food eaten are influenced by microhabitat can sometimes be assessed.

One such study of a sedentary legless lizard showed that most species and castes of termites are eaten in fairly similar proportions by lizards taken from different microhabitats, suggesting that these two niche dimensions are largely independent.

In reviewing major factors leading to ecological isolation among birds, Lack (1971) concluded that the most important were differences in geographic range, habitat, and food eaten. Schoener (1974) recently reviewed patterns of resource partitioning in over 80 natural communities ranging from simple organisms such as slime molds through various mollusks, crustaceans, insects, and other arthropods to various members of the five classes of vertebrates, including lizards. He identified and ranked five resource dimensions by degree of importance in niche segregation: macrohabitat, microhabitat, food type, time of day, and seasonality of activity.

Schoener concludes that habitat dimensions are generally more important in separating niches than food type dimensions, which in turn tend to be important more often than temporal dimensions. Tenestrial poikilotherms partition food by being active at different times of day relatively often compared with other animals.

Predator’s partition resources by diurnal differences in time of activity more than do other groups and vertebrates segregate less by seasonal activity differences than do lower animals. Schoener also found that segregation by food type is more important for animals feeding on large foods relative to their own size than it is among animals that feed on relatively small items.

Guild Structure

To what extent are species over dispersed in niche space? Do clusters of functionally similar species exist? Members of such a group of similar species, known as a guild (Root, 1967), interact strongly with one another but weakly with the remainder of their community.

Although techniques of objectively defining a guild are still in their infancy, the concept is clearly of some interest because guilds presumably represent the arenas of most intense interspecific competition.

An operational means of delimiting the members of guild has been developed by L.R. Lawlor (personal communication), who defines a guild as a cluster of species separated from all other such clusters by distance greater than the largest distance between the two most disparate members of the guild concerned.

Under this rather conservative definition, a community of n species whose component species are evenly spread out in niche space contains n one-species guilds (each species is its own guild) and one large n-speciec guild (the entire community) but no guilds of intermediate size. Real communities may usually contain many guilds of intermediate size.