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Spatial Distribution of the Montane Unicorn
Stuart H. Hurlbert
Vol. 58, No. 3 (Aug., 1990), pp. 257-271
Stable URL: http://www.jstor.org/stable/3545216
Page Count: 15
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Analysis of the spatial distribution patterns of five populations of the recently discovered montane unicorn (Monoceros montanus) yield several surprising results. Principal among these is the fact that when censused using a grid of 1 km2 quadrats all populations yielded a variance:mean ratio of 1.0, though each population showed a different pattern of aggregation and none correponded to a Poisson distribution. Distributions with these mathematical properties are termed unicornian. It is demonstrated that the variance:mean ratio is useless as a measure of departure from randomness, though it is widely recommended as such. The ratio is also uninterpretable as a measure of aggregation except for its coincidental relationship to indices such as the Morisita index of aggregation ( I M). I M can be defined in terms of the probability of two randomly selected individuals being found in the same quadrat or sampling unit. It can be generalized to yield a family of indices, I Mr, relating to the probability of r randomly selected individuals being found in the same sampling unit. Presentation of a plot of I Mr versus r may often be preferable to condensation of all information on aggregation into a single value, such as I M. Comment is also offered on the need to consider a range of spatial scales, on the corresponding definitional and scale aspects of plotless sampling, and on the fruitlessness of attempting to view spatial patterns themselves as consisting of only two distinct aspects, such as intensity and grain.
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