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Correlations Between Incidence and Abundance are Expected by Chance

David Hamilton Wright
Journal of Biogeography
Vol. 18, No. 4 (Jul., 1991), pp. 463-466
Published by: Wiley
DOI: 10.2307/2845487
Stable URL: http://www.jstor.org/stable/2845487
Page Count: 4
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Correlations Between Incidence and Abundance are Expected by Chance
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Abstract

Relationships between the distribution and the abundance of species are current topic of interest among biogeographers. When distribution is measured by the frequency with which a species occurs in a set of samples (`incidence'), however, correlations between incidence and abundance are expected by chance. This is because rarer species are less likely to be found in samples. Plotting incidence against abundance-when-present (omitting samples with zero individuals from the calculation of mean abundance) does not eliminate this problem. Therefore the existence of a correlation between incidence and abundance cannot be used as support for any particular ecological hypothesis, since a correlation is the null expectation. The appropriate null hypothesis is never `no correlation between incidence and abundance.' I present a statistical test that can differentiate observed patterns of incidence from the simplest random pattern: a Poisson model. More sophisticated models suggest that many ecological phenomena can shape patterns of incidence and abundance, including spatial variation in habitat quality, social behaviour, localize reproduction, and stochastic immigration/extinction processes. The relative importance of these different factors cannot be distinguished with the current methods. I suggest that simple correlations between distribution and abundance are of limited utility, and that other, more promising, avenues of analysis of this interesting area of biogeography should be investigated.

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