NULL-MODEL TESTS FOR COMPETITIVE DISPLACEMENT - THE FALLACY OF NOT FOCUSING ON THE WHOLE COMMUNITY

Examines the performance of several parameters that have been used to characterize community structure for the purpose of this comparison. These parameters all focus on selected species pairs within the community rather than considering the community as a whole. Fleming and Partridge (1984) (see 86L/5345) proposed a parameter for examining flowering phenologies that focuses on 1 species pair at a time. They compared the observed overlap between members of a particular species pair with the distribution of possible overlap values for this pair from randomly constructed communities. A 2nd parameter, the overlap between a particular species and all other species combined, was used in a similar way. Using these parameters will always result in acceptance of the null hypothesis when in fact a segregated flowering pattern exists, and may lead to acceptance of the competition hypothesis when in fact the null hypothesis is true. A community-level parameter, mean pairwise overlap, makes neither of these errors. Simberloff and Boecklen (1981) (see 82L/7479) used a family of parameters to examine the competition hypothesis that there is a constant difference in log body size (= a constant ratio of body sizes) between adjacent species ranked according to body size. Their parameters focus on the relative magnitude of the differences between 2 pairs of adjacent species, such as the 2 pairs with the smallest and largest log body size differences. These parameters may not always recognize a community with competitive displacement, and may mistake a clumped community for one with displacement. Several community-level parameters are shown to have greater power in discerning competitive displacement. -from Author