Dear List,

I am testing how the species richness (SR) of a plant community is affected
by the presence of an invasive species (Alien). To control for
confounding environmental factors my model also contains two covariates
(CV1 and CV2, both continuous). As I have sampled plant communities in 3
different habitats, I would like to fit a mixed-effects model which allows
for different effects of the invasive on species richness depending on the
habitat in question (in other words separate slopes and intercepts for each
of the 3 habitats). Currently my model looks like this:

model <- SR ~ Alien + CV1 + CV2 + (1 + Alien | Habitat)

However, I think this is not correct as the effects of the two covariates
are not included in the random effect (slope and intercept vary among
habitats, but only account for the effect of the invasive species). How can
I formally include the effects of the two covariates in the random effect
terms and is this necessary?

Any suggestions are much appreciated.

Best wishes,

Tommaso
__________________________________________________________________________

*Tommaso Jucker*

*    Ph.D. student*
*    Forest Ecology and Conservation Group*
*    Department of Plant Sciences*
*    University of Cambridge*

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