Dear Todd,
I don't really understand how you can obtain a negative Simpson's Index
value. Simpson's index formula is :
H = 1-sum(pi²) where pi is the relative abundance of the ith species. So,
H is always greater or equal to 0.
You can always use Simpson's index as far as you have a set of relative
abundances (ie. sum to one). The kind of abundance measure has no
importance for the index. Of course it change the type of diversity you
want to estimate.
You must know that Simspson's Index give a very high importance to
abundant species and don't care about rare one. So if one species have a
very high weight in comparaison to the other ones, your index will be low
even if you have many species. Consequently, you must ask yourself if it
make sense for you.
Regards
Nicolas
Le Thu, 09 Aug 2012 01:38:51 +0200, Shuskey, Todd
<[email protected]> a écrit:
I am studying how the biodiversity within the weedbeds of a freshwater
lake
has changed and one parameter I would like to investigate is species
diversity using Simpson's Index. The data I have available is the wet
weights and dry weights for each species from quadrat sampling. I do not
have available the actual number of plants within each quadrat. For
another
comparison, I am using the Proportional Index of Community Similarity.
Can either of these data sets be used in the Simpson's Index of
Diversity?
My problem is that many of the weights are less than one, which gives me
a
negative value when trying to calculate Simpson's Index. Is there a
proper
transformation that I can do with the data?
Thank you in advance,
Todd Shuskey
--
Nicolas PERU, PhD
33-(0)4 72 43 28 94
06-88-15-23-10
CNRS, UMR 5023 - LEHNA
Université Claude Bernard - Lyon 1
43 Bld du 11 novembre 1918
Rdc Bât Forel
69622 VILLEURBANNE cedex FRANCE
