null models for a single species
On 8/03/11 10:54 AM, "Penner, Johannes" <Johannes.Penner at mfn-berlin.de> wrote:
Dear List members, I would like to test whether an observed occupancy of lakes in a landscape has occurred randomly (by chance) or not. How can I do that? The problem is that it concerns only a single species and I would like to use binary data only. At first I thought of generating null models and test the observed occupancy against the randomly generated one. However, this needs more than one species... Any hints are highly appreciated!
Johanne, Actually many of the null models as defined in vegan would work here: you only provide a one-column matrix. Although they work, they would not make much sense: null models of type "r00" and "c0" would only give you random permutation of your data (and "c0" would give you the data). Naturally, this is one way to go: just permute your observations. For simple permutations you can use sample() function of base R, and for constrained permutation you can download Gavin Simpson's 'permute' package from http://www.r-forge.r-project.org/. However, if you have a structured model and a structured hypothesis you can do much better than have a simple permutation. I have no idea of your hypothesis, though, and I can't help here. Cheers, Jari Oksanen