[R] Null models of species co-occurrence
bbolker at gmail.com
Wed Jan 25 03:27:13 CET 2012
Dallas <tad.dallas <at> drakeresearchlab.com> writes:
> I am currently testing species co-occurrence patterns using null models and
> the oecosimu() function within the vegan() package. My issue is that none of
> the methods appear to be the ones that I want. The methods listed are r0,
> r1, r2, r2dtable, swap, tswap. However, I want to know how to go about
> implementing fixed row algorithms, as suggested in Gotelli 2000 in Ecology.
> Also, the null models created seem to be incredibly dependent on the 1)
> burnin and 2) thin values. These are the 1) Number of null communities
> discarded before proper analysis in sequential methods "swap" and "tswap"
> and 2) Number of discarded null communities between two evaluations of
> nestedness statistic in sequential methods "swap" and "tswap". What are the
> significance of these values?
You will probably have better luck with this question on the r-sig-ecology
<at> r-project.org list.
I haven't looked at this stuff in a little while. It sounds from the
?oecosimu description as though there are some fixed row algorithms (?)
Methods ‘r0’, ‘r1’ and ‘r2’ maintain the site (row)
frequencies. Method ‘r0’ fills presences anywhere on the row with
no respect to species (column) frequencies, ‘r1’ uses column
marginal frequencies as probabilities, and ‘r2’ uses squared
column sums. Methods ‘r1’ and ‘r2’ try to simulate original
species frequencies, but they are not strictly constrained. All
these methods are reviewed by Wright et al. (1998).
It might help in your e-mail to r-sig-ecology if you briefly
describe Gotelli 2000's algorithm, so readers don't necessarily
have to go back to the original source to know (at least approximately)
what you want.
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