[R-sig-eco] Fitting a GLMM to a percent cover data with glmer or glmmTMB

[Vasco Silva]

Hi,

I am trying to fit a GLMM on percent cover for each species using glmer:

>str(cover)
'data.frame': 102 obs. of  114 variables:
$ Plot : Factor w/ 10 levels "P1","P10","P2",..: 1 1 1 1 1 3 3 ...
$ Sub.plot: Factor w/ 5 levels "S1","S2","S3",..: 1 2 3 4 5 1 2 ...
$ Grazing : Factor w/ 2 levels "Fenced","Unfenced": 1 1 1 1 1 1 1  ...
$ sp1 : int  0 0 0 1 0 0 1 ...
$ sp2 : int  0 0 0 0 0 3 3 ...
$ sp3 : int  0 1 0 0 1 3 3 ...
$ sp4 : int  1 3 13 3 3 3 0 ...
$ sp6 : int  0 0 0 0 0 0 0 ...
 ...
$ tot  : int  93 65 120 80 138 113 ...

sp1.glmm <- glmer (cbind (sp1, tot- sp1) ~ Grazing + (1|Plot), data=cover,
family=binomial (link ="logit"))

However, I wonder if binomial distribution can be used (proportion of
species cover from a total cover) or if I should  fitted the GLMM with
glmmTMB (tweedie distribution)?

I would greatly appreciate it if someone could help me.

Cheers.

Vasco Silva

	[[alternative HTML version deleted]]