Bonnie, I am not sure you quite understand rarefaction. First, there are individual-based and sample- based versions. I'm guessing you mean individual based, as sample-based version also take inter- sample (i.e., usually spatial) heterogeneity into account.
Individual-based rarefaction simply tells you the number of species you would expect to get if you took a real sample and extracted a smaller subsample at random. It is used in the context of uneven sample sizes, normally as a way to to standardize species RICHNESS estimates to a constant sample size. For example, if you have samples of 100, 150 and 200 individuals from each of three locations AND the difference is due to your sampling effort, rather than to instrisic differences in local abundance, you can't sensibly compare their raw species richness counts, but you can if you rarefy the '150' and '200' samples down to 100 individuals. One might also want to do this before comparing the sites using a DIVERSITY index, assuming that the diversity index takes into account overall species richness (as they mostly all do). So, rarefaction is a kind of 'pre-processing' step, rather than an alternative, to a diversity index. Also, I can't think of a use for it in the context of equal sample sizes, unless you are simply interested the shapes of the rarefaction curves for the different sites. But those shapes are determined by the abundance distributions, so it would be more straightforward to compare the distributions directly. Hope this helps, Gareth Russell NJIT/Rutgers On Tue, 2 Jan 2007 13:55:51 -0500, bonnie clark <[EMAIL PROTECTED]> wrote: >Dear Colleagues, >I'm considering using rarefaction as a measure of species diversity, since >it takes both species richness and species abundance into account. There >are several benefits of rarefaction over other indices like Shannon >diversity. > >It is usually used when sample size is uneven. Would it be inappropriate to >use it when sample sizes are even (equal)? > >Thank you, >Bonnie > >_________________________________________________________________ >Find sales, coupons, and free shipping, all in one place! MSN Shopping >Sales & Deals >http://shopping.msn.com/content/shp/?ctid=198,ptnrid=176,ptnrdata=200639 >=========================================================== ==============
