[R-sig-ME] GlmmADMB: random slopes and fixed effects

[Genevieve Perkins]

Hello,

I am a masters student new to the world of GLMMs. I have developed a mixed
model using the glmmADMB package and I have been scouring the literature
and help files, and trying to find an answer to my questions with no
success.

I want to estimate the effect of cats on bird abundance for birds with
particular traits (all traits are binary coded (0,1);
Specifically I am looking at the interaction estimate.

I included species as a random effect, and I wanted the species response to
vary with Vegetation (Veg) and Population (Pop). I also added a random
level observation term.

      Model 1: fitn <- glmmadmb(as.formula(bird.abund ~ Cat + trait + Cat:trait
+ (1 + Veg + Pop + Cat|Species) + (1|ID)), data = bdata,family= "nbinom")


I noticed however that if I include Veg and Pop as fixed effects (model 2)
my model estimate for cats at the fixed effect level and species level also
change.

      Model 2: fitn <- glmmadmb(as.formula(bird.abund ~ Cats + trait +
Cat:trait
+ Veg + Pop + (1 + Veg + Pop + Cats|Species) + (1|ID)), data = bdata,
family= "nbinom")


My questions are:
1)  Is it possible to include varying slope coefficients (ie: Veg and Pop)
in a GLMM model without including them as fixed effects? (I couldn't find
any examples of this format)

2) How are the estimates for the random effects treated without a corresponding
fixed effect in Glmmadmb. I was guessing they may be pooled to a group mean
of zero, but I was not able to find this information in the glmmadmb
literature.

All suggestions greatly appreciated!
Thanks

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