Runs of One Species with Respect to Another in Transects through Plant Populations

To investigate the relationship between two species of plant growing together in an area it has been suggested [Pielou 1961] that their degree of 'segregation' be determined. Two species are said to be 'unsegregated' if the individuals of each are randomly intermingled with those of the other; if not, they are 'segregated'. The method proposed for detecting and measuring segregation between two species involves the recording, for each individual of a two-species population, both its species and that of its nearest neighbor. The observations are then compared with those to be expected from an unsegregated population of the same numerical composition. A coefficient of segregation was devised based on the difference between observation and expectation. The coefficient thus provides a measure of the amount of non-randomness of the two species with respect to each other. The coefficient of segregation is therefore analogous to the coefficients of non-randomness that have been used to measure degrees of spatial non-randomness in one-species populations. Many such coefficients have been devised by, for example, David and Moore [1954], Hopkins [1954], Morisita [1959] and Pielou [1959]. These coefficients provide a measure of the extent to which the pattern of a population departs from randomness, and nothing more. It is impossible, from a single statistic, to draw any conclusions as to the cause of spatial non-randomness in any given case. In searching for these causes, students of spatial pattern have therefore made more detailed investigations. In particular, by comparing observed frequency distributions obtained from quadrat samples with theoretical distributions based on various conceptual models, they have sought to determine which of these models might account for the patterns of naturally occurring non-random populations, (cf. Bliss and Fisher [1953], Gurland [1958] and Cole [1946]). Analogous investigations of two-species populations have not, so far, been attempted. In studying the spatial pattern of individuals of a single species, one is concerned with their pattern with respect