SREL Reprint #1889

Testing spatial segregation using a nearest-neighbor contingency table

Philip Dixon

Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, South Carolina 29802-0005 USA and
Biomathematics Program, Department of Statistics, North Carolina State University, Raleigh, North Carolina 27695 USA

Abstract: 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.

Keywords: nearest neighbor; Pielou test; segregation; spatial pattern

SREL Reprint #1889

Dixon, P. 1994. Testing spatial segregation using a nearest-neighbor contingency table. Ecology 75:1940-1948. 

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).