[R-sig-eco] species richness, GLM and negative values

[Ludovico Frate]

Dear all,I'am trying to fit a very simple linear model. I am analyzing the differences in the number of species (DS) found in several permanent plots in two year of observations.
 Firstly, I have calculated the differences per plot (i.e. number of species in Plot 1 in Time A - number of species in Plot 1 in Time B and so on for all the plots).Secondly, those differences were tested for deviation from zero by means of a linear model
M2<-lm(DC~1, data = gransasso)summary(M2)E2<-residuals(M2)qqnorm(E2, pch = 19, col = "blue"); qqline(E2, col = "red")
The qqnorm has shown that residuals were not normally distributed, thus I need to use a GLM. However GLM (poisson family) does not work with negative values (DS has negative values).I've tried to add a constant value to these differences (i.e. +100) but the result is misleading  since I am testing for deviation from zero.
Do you have any suggestions?
Regards,Ludovico

                                                                                                                             
Ludovico
Frate

PhD student (University of Molise - Italy)
Environmetrics Lab
http://www.distat.unimol.it/STAT/environmetrica/organico/collaboratori/ludovico-frate-1
Department of Biosciences and Territory - DiBT
Università del Molise.
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E-mail ludovico.frate at unimol.it
ludovicofrate at hotmail.it
 		 	   		  
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