We are interested in to use a simple rarity index from our community data, using the occurrence of species in the sample units, but we do not find literature that revisited the general concepts about the more precise way to cut-off the groups of species (the non-rare and the rare species). Somebody knows if this topic has been revised and/or some standardized and validated mothod exist?
Hello Alvaro:
Rarity has more tan one meaning. In the context of species abundance models, rare species are those that fall into the first few abundance clases. The proportion of rare species will decline through the sequence of models from geometric series to broken stick. To the conservationist however, rare species can range from one that is endangered to a vagrant which strays far from its natural hábitats in which it is very abundant. Rabinowitz et al. (1986) has devised a scheme which clarifies the concept of rarity by partitioning species distribution and abundance on three scales: 1) species which occurr over large áreas from those which are endemic to a restricted área. 2) species according to their hábitat specificity. 3) local population size. The index of relative rarity by Leroy et. al. 2012 defined as the threshold of occurrence below which species are considered rare or common assign a rarity weight to each species as a function of their occurrence, which is obtained from a biodiversity database. For formulas and other indexs plese check this link: http://borisleroy.com/en/research/rarity-indices/
Regards,
Luis Miguel
In have used a relation between occurrence and abundance to categorize the species as rare, common, predominant and occasional. in the page 223 of the paper in the link you can find how i did it, in case that it can be helpful.
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Alvaro, bear in mind that rarity is a rather complex and subjective matter and it depends largely, among other things as pointed out above, on how your community data has been collected and how biased the sampling method is (regardless how you collect, all are biased one way or the other). A species could be poorly represented in your sample, and you may regard it as rare, but it may be possible that the method you use fails to collect it and the species may as well be very common. Using several methods may help but is not always possible and will depend ultimately on your research question. I guess to be able to assess true rarity you need to know well the biology and natural history of the species in the community, something that is not always feasible or even known. Therefore you need to talk about relative rarity and use ways to assess it as suggested above.
As you can see from the above responses the quick answer to your question is no. Besides some of the ideas above Kevin Gaston has pondered this in his book Rarity. Rob Dillon has several blog posts about the problem that I found to be very worthwhile: http://fwgna.blogspot.com/2013/12/what-is-rarity.html
http://fwgna.blogspot.com/2013/12/what-is-rarity.html
Thank you very much to everyone for your valuable comments and discussion,
Clearly, there are no unique conception of rarity, at least from the point of view of the communities ecology. The work developed by Leroy in this way is very valuable, but few works can to have the different-level data that are needed. At least the mine did not has.
I think that the approximation of a relative rarity, as Jaime suggest, is an interesting approximation, and ways of working at local level as the André's work is a good possibility for to use.
Thank you again to everyone
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