[R-sig-eco] life forms spectrum analysis

[Aitor Gastón]

Hi Ludovico,

You can compare the proportion of each life form using logistc regression, 
an example with simulated data for 20 plots:

set.seed(100)
n<-20
population<-sample(c("A","B"),n,replace=T)
lifeform1count<-round(runif(n,0,100),0)
otherlifeformscount<-round(runif(n,0,100),0)
f<-glm(cbind(lifeform1count,otherlifeformscount)~population,family=binomial)
summary(f)#significant difference between populations p=0.01137
plogis(coef(f)[1])#mean proportion of life form 1 in population A
plogis(coef(f)[1]+coef(f)[2]) #mean proportion of life form 1 in population 
B

You can run one model for each life form, but maybe multinomial regression 
is more correct (never tried)

Aitor

-----Mensaje original----- 
From: Ludovico Frate
Sent: Monday, March 16, 2015 6:07 PM
To: r-sig-ecology at r-project.org
Subject: [R-sig-eco] life forms spectrum analysis

Dear List,
I would like to analyse the differences  in life form (Raunki�r) composition 
between two populations. The two populations (plant communities) were 
sampled randomly and the presence of plant species were recorded. So for 
each plot I have the frequency of each life form. First, I  thought to a chi 
square or binomial test. However, I am worried about the applicability 
because both tests require that the frequency of life forms observed vs 
expected is grater than 0% (i.e., when a given life form is present in all 
the plot of both populations - observed = 100% expected = 0%) and less than 
100% (better between 20 and 80%).
Pop1, Pop2
present, 70, 56
absent, 0, 0

I have tried also a simple Mann whitney test but it doesn't work well for 
count data. In addition for uncommon life forms I have a lot of zero counts 
in my data!


Do you have any ideas about analysing differences in life forms?
Best,
Ludovico




Dott. For. Ludovico
Frate, Ph.D.
University of Molise - Italy
Environmetrics Lab
http://www.distat.unimol.it/STAT/environmetrica/organico/collaboratori/ludovico-frate-1
Department of Biosciences and Territory - DiBT
Universit� del Molise.
Contrada Fonte
Lappone,
86090 -  Pesche (IS)
ITALIA.
Cel: ++39
3333767557
Fax: ++39 (0874) 404123
E-mail ludovico.frate at unimol.it
ludovicofrate at hotmail.it
https://www.researchgate.net/profile/Ludovico_Frate

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