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Abstract
There has been an ongoing controversy over how to decide whether the distribution
of species is "random" - i.e., whether it is not greatly different from what it would
be if species did not interact. We recently showed (Roberts and Stone (1990)) that
in the case of the Vanuatu (formerly New Hebrides) avifauna, the number of islands
shared by species pairs was incompatible with a "random" null hypothesis. However,
it was difficult to determine the causes or direction of the community's exceptionality.
In this paper, the latter problem is examined further. We use Diamond's (1975) notion
of checkerboard distributions (originally developed as an indicator of competition)
and construct a C-score statistic which quantifies "checkerboardedness". This statistic
is based on the way two species might colonise a pair of islands; whenever each species
colonises a different island this adds 1 to the C-score. Following Connor and Simberloff
(1979) we generate a "control group" of random colonisation patterns (matrices), and
use the C-score to determine their checkerboard characteristics. As an alternative
mode of enquiry, we make slight alterations to the observed data, repeating this process
many times so as to obtain another "control group". In both cases, when we compare
the observed data for the Vanuatu avifauna and the Antillean bat communities with
that given by their respective "control group", we find that these communities have
significantly large checkerboard distributions, making implausible the hypothesis
that their species distributions are a product of random colonisation.