A new method to measure spatial association for ecological count data

A new method is introduced to assess the spatial association between two sets of count data. This features a measure of local association for counts, defined for each sample unit. The new measure is based on a comparison of the spatial SADIE clustering index of the two sets at each sample unit; the mean of the measure is represented by the simple correlation coefficient between the clustering indices of the two sets. The randomization method allows the construction of a test and critical values. For the first time, spatial association may be mapped for count data; clusters of units with positive association or negative dissociation may be identified. The method is exemplified by analysis of spatial pattern and spatial association of counts of male and female tupelo trees from three plots in a South Carolina swamp forest. In addition, methods are presented to distinguish larger-scale apparent association between the sexes, caused by indirect effects, from direct smaller-scale association. No tendency was found for the sexes to occur together at the small-scale, only an apparent affinity caused through their co-location in particular subareas of each plot. The conversion from mapped to count data requires a choice of unit size; the conclusions of these analyses were not affected greatly by changes in unit size.