TESTING SPATIAL SEGREGATION USING A NEAREST-NEIGHBOR CONTINGENCY TABLE

Segregation of species occurs when a species tends to be found near conspecifics. This is frequently investigated using a contingency table, classifying each point by its species and the species of its nearest neighbor. Pielou proposed using a 1-df chi-square test of independence as a test of segregation. This test is inappropriate if all locations within a study area are mapped. For completely mapped data, I derive the expectations and variances of the cell counts in the nearest-neighbor contingency table under the null hypothesis that species labels are randomly assigned to points. The properties of the cell counts suggest a new 2-df chi-square test of spatial segregation, a pair of species-specific tests, and a pair of species-specific measures of segregation. In small samples, the proposed tests have the appropriate size, unlike the Pielou test. The new test is illustrated with three examples: Pielou's Douglas-fir/ponderosa pine data, a realization of a mother-daughter process, and the locations of male and female water tupelo trees.