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Segregation and Symmetry in Two-Species Populations as Studied by Nearest- Neighbour Relationships

E. C. Pielou
Journal of Ecology
Vol. 49, No. 2 (Jun., 1961), pp. 255-269
DOI: 10.2307/2257260
Stable URL: http://www.jstor.org/stable/2257260
Page Count: 15
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Segregation and Symmetry in Two-Species Populations as Studied by Nearest- Neighbour Relationships
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Abstract

When the spatial patterns of two species in a community are being investigated, besides determining the spatial pattern of each species with respect to the ground, it is also of value to determine the spatial arrangement of the members of one species with respect to the members of the other. Thus two species may be described as `segregated' if an individual of either species is more likely to be found among members of the same species, than among members of the other species. Conversely, the two species may be described as `unsegregated' if neither shows a tendency to occur as one-species clumps. A two-species population may be segregated or unsegregated regardless of whether or not the population as a whole is clumped. Segregation may be detected and measured by a study of the nearest-neighbour relationships of the members of the population. By examining each individual of a two-species population in turn and determining its species and that of its nearest neighbour, one may draw up a 2 × 2 table showing the observed frequencies of the four possible types of nearest-neighbour relationship. These may be compared with the expected frequencies in an unsegregated population by a χ2 test. A coefficient of segregation for a two-species population is defined in terms of nearest-neighbour relationships. `Negative segregation' is also possible; this occurs when an individual of one of the two species is more likely to be found near individuals of the other, than near individuals of its own species. Regardless of its degree of segregation, a two-species population may also be `symmetrical' or `unsymmetrical'. If it is symmetrical, the members of the two species are equally likely to function as nearest neighbours to other population members. In an unsymmetrical population, individuals of one of the species tend to be more isolated than those of the other and thus serve less often as nearest neighbours. Two tests for symmetry are described. Field results from a mixed stand of Douglas fir and ponderosa pine are used to illustrate the concepts developed.

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