Hi Meg,
Thank you for taking the time to help me.
What you suggest ( (1 + var1 + var2 + var3 | species) + (1 + var4 || species)), however, does not work, as it gives me the following message:
Error: Duplicated group-level effects are not allowed.
Occured for effect ‘Intercept’ of group ‘species’.
Moreover, I don’t think this totally encapsulate what I’d like to do, as my objective here is to discover how to code that (1) the coefficients of var1, var2, var3, var4 and var5 (level 1 covariates) all depend on the group ‘species’ (varying or random slopes), but (2) I only want the model to include the correlation between 1 (the intercept) and var1, between 1, var2, and var3, and between 1, var4 and var5. I have no biological/ecological reason to consider the correlation between var2 and var4, for example, and so on.
I hope I made things clearer. :-)
Thank you very much.