[R] grouping explanatory variables into "sets" for GLMM

[Maria Kernecker]

Dear all, 

I am trying to run a GLMM following the procedure described by Rhodes et al. (Ch. 21) in the Zuur book Mixed effects models and extensions in R . Like in his example, I have four "sets" of explanatory variables: 
1. Land use - 1 variable, factor (forest or agriculture)
2. Location - 1 variable, factor (riparian or upland)
3. Agricultural management - 3 variables that are binary (0 or 1 for till, manure, annual crop)
4. Vegetation patterns - 4 variables that are continuous (# of plant species in 4 different functional guilds)

How do I create these "sets"?  I would like to build my model with these "sets" only instead of listing every variable. 

Also: is there a way of running all possible models with the different combinations of these sets and/or variables, sort of like running ordistep for ordinations?

Thanks a bunch in advance for your help!
Maria