[R-sig-eco] Plant biomass

[yann clough]

Dear John,

there is no way to provide a definite answer to your question without 
actually seeing the data and knowing where they come from.
Try to give some thought to what generated the data (e.g. you may have a 
mixture of distributions, for example because processes generating the 
zeros may be different than those generating the biomass data, and/or 
due to experimental treatments or environmental variables).
That being said, the Gamma distribution (see dgamma etc. in R) is the 
continous counterpart of the negative binomial , so that might be a hot 
candidate. Consider also the lognormal - or a mixture distribution. I 
recommend Bolker 2008, Ch.4, for a discussion of distributions and their 
uses in modelling ecological data.

Best wishes

Yann

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> Message: 1
> Date: Mon, 05 Apr 2010 15:56:44 -0500
> From: john polo <jpolo at mail.usf.edu>
> To: r-sig-ecology at r-project.org
> Subject: [R-sig-eco] Plant biomass
> Message-ID: <4BBA4E8C.6000900 at mail.usf.edu>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> 
> Hello,
> I have biomass measurements of plant functional groups that range from 
> .002 g to 2000 g, with quite a few zeros in two of the groups as well. 
> While trying to analyze this data, the negative binomial distribution 
> was suggested for describing the data, with zero inflation for the 2 
> groups. However, negative binomial doesn't allow for non-integer values. 
> I have two questions, the first of which is, is negative binomial the 
> appropriate distribution to use for this range of masses with several 
> zeros? Contingent on negative binomial being the appropriate 
> distribution, how do I correct for non-integer values of my masses?
> Best regards,
> John Polo
> 
> 
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> End of R-sig-ecology Digest, Vol 25, Issue 3
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-- 
Dr. Yann Clough

Agroecology
Department of Crop Sciences
Göttingen University
Waldweg 26
D-37073 Göttingen (Germany)
Tel.: (+49) 0551 3922358
Fax.:(+49) 0551 398806