The study of niche form and niche relationships has become one of the most explosive areas in community ecology in the past few years. More and more studies are devoted to analysis of niche width, separation or overlap within communities in one or many dimensions.
It is in large part this proliferation of studies which has heightened our appreciation of the extent to which niche overlap may occur in real communities. A variety of measures have been proposed so that niche widths, and overlaps may be quantified. While it is not our intention to review all such indices here (an able critique is offered by Abrams, 1980 or Slobodkichoff and Schultz, 1980), it may be valuable to present here, briefly, those measures in commonest use.
The concept of niche width or niche breadth refers essentially to the diversity’ of resource use shown by any one organism or group of organisms.
Those whose resources use is restricted to a small portion of the available resource spectrum are considered to have narrow niches, those which exploit a relatively diverse set of resources within the resource continuum are defined with broad niches.
They note that ‘there are two situations in which one might wish to compare niche metrics: among species within communities, and between species of different communities. In both cases the paramount difficulty is standardization of the procedure so that measurements are comparable for different species and different communities]’ [our addition], Colwell and Futuyma develop further measures of niche breadth, which are reconsidered and refined by Hurlbert (1978), but these become rather complex for general use. Other measures are suggested by Feinsinger et. al., (1981) or Thorman (1982).
In the same was that measures of niche breadth are all primarily derived from various indices of diversity, the various measures of niche overlap are usually based on some analysis of resource partitioning.
Each of these indices measures overlap along one resource dimension only. As noted above, multidimensional overlap may be derived as the arithmetic sum of separate overlaps if resource dimensions are interdependent, the product, if they are independent. Frequently, of course, it is very difficult to determine whether or not the separate dimensions are or are not dependent-or to what extent. Slobodkichoff and Schulz (1980) offer a valuable test for dependence and independence of resource dimensions used.
The different indices obviously each have difference strengths and weaknesses. May (1975a) points out for example, that the overlap matrix produced for Levins’ index is not symmetrical: overlap of upon does not equal overlap of j upon.
Although this has several disadvantages, it has the single advantage that it is sensitive to the number of individuals involved in each species Slobodkichoff and Schulz further note that the different measures embrace to different degrees, competitive events.
They stress a clear distinction between the concepts of resource use overlap and competitive pressure resulting from inche overlap, and note that the same distinction should be recognised in the different indices. They suggest that Pianka’s (1973) overlap index represents resource use overlap (true niche overlap) while Levins’ (1968) index (with its sensitivity to numbers of individuals) is more an index related to competitive pressure resulting from niche overlap.