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I'm using the compana function in Adehabitat package to determine if used habitat is selected for from available habitat.

One of the habitat classes I'm using in my available matrix has several very low values (ie. 0.000015000) or zero values. Therefore, when I replace any zero values in the R code with a larger value (0.001), it affects my results. When I replace that larger value with other values (ie. 0.0001 and 0.00000001), to confirm the consistency of the results, my results are not consistent.

Is there an appropriate small value number I should be using?

I've tried using multiple different small values to replace the zero values in the matrix. Smaller numbers than 0.001 do not the top results (working with 9 different habitat classes), but continuously increasing the smaller values changes values of other habitat classes.

bars<-compana(used, avail, test = c("randomisation"),
    rnv = 0.01, nrep = 1000, alpha = 0.1)

Changing the rnv value from 0.01 --> 0.0001 alters results, and continues to alter with smaller numbers

I expect to see consistent results with most or all rnv small number values.

Paul Lemarchand
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Chloe B
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1 Answers1

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I recommend reading the paper by Aebischer et. al. (Compositional Analysis of Habitat Use From Animal Radio-Tracking Data, in Ecology, Vol. 74, No. 5 (Jul., 1993), pp. 1313-1325) which the compana function is built on. It is rather easy to read and explains many aspects running compositional analyses for habitat use.

On pages 1320-1321 the authors say the following:

[...] 0% represents a use too low to be recorded, so should be replaced by a value distinctly less than the smallest nonzero value: an order of magnitude less is probably appropriate to most situations.

I would therefore check for the smallest nonzero value in your dataset and choose a value of one magnitude less to exchange your zeros.

Pelle Lundkvist
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