There is the method of Brown-Blanquet (1927, English translation available under http://krishikosh.egranth.ac.in/bitstream/1/22218/1/BPT7871.pdf), which simply assigns the following values:

• r: very rare
• +: few individuals, little coverage
• 1: coverage < 5%
• 2: coverage < 25%
• 3: coverage < 50%
• 4: coverage < 75%
• 5: coverage > 75%

Note that such statements make only sense for homogeneous areas. For example, if you have a dry slope running into a valey with a creek or even a swamp, you need to evaluate them separately. Your case of a rare plant concentrated in a few areas would indicate non-homogeneity.

Hi Daniela,

we are using Rabinowitz concept. We published a few papers with it, recently a book and it was accepted in Biological Conservation an complete analysis of Santa Catarina species. Maybe it can helps you.

Hi Daniela - the Rabinowitz concept is very useful. I also recommend you read Kevin Gaston's book 'Rarity' (Gaston, K. 1994: Rarity. London, Chapman & Hall). In New Zealand we have long pondered your problem because of course there are degree's of 'rarity' and here regional, narrow-range, naturally uncommon endemic plants are the usual situation.

If it helps consider looking at these:

Article Revisiting rarity: A botanical perspective on the meanings o...

Book The New Zealand Threat Classification System Manual

Which show the thought processes and conclusions reached in New Zealand over how to classify the different kinds of 'rarity' one see's in the wild.

Currently New Zealand is revising the 'New Zealand Threat Classification System Manual' - but the system is working very well for a nation where narrow range endemics are widespread.

Hope this is useful.