[R] Nested model and variance partitioning

[poulet at cict.fr]

Dear R users,

I face to a nested pattern and despite the numerous examples in the help I am
still confused.
  
I sampled bugs in different habitats within sites which were within rivers
themselves within different regions.
   
The habitat correspond to different substrata (not systematically present in all
sites). For rivers and sites, I have environemental variables (e.g. altitude
and slope of the site, drainage area and geology of the river) and I have only
2 regions. Note that sometimes I have only one site per river.

I would like to know the part of each spatial scale in the species richness
variance. 
 
I looked into the nlme package but I did not found how to proceed...

Thanks for your help

Nicolas Poulet

Laboratoire d'Ecologie des Hydrosystèmes
LEH - UMR UPS-CNRS 5177
Université Paul Sabatier, bât 4R3
118, route de Narbonne
31062 Toulouse Cedex 9