R help - Oct 2008 - A covariate correlated with other explanatory variables

Dear all,

I have here a difficult problem: We are currently analysing data from a
biodiversity experiment,
where the following models are compared using AICc:

a) y ~ plant.diversity
b) y ~ plant.biomass
c) y ~ plant.diversity + plant.biomass

The problem now is that plant.diversity and plant.biomass are correlated at
about r=0.6.

Plant.diversity is a variable that is maintained experimentally at fixed values,
while plant.biomass
is the result of plant.diversity (i.e., higher plant.diversity leads to higher
productivity).

Is it allowed to compare these models at all? And is it allowed to have these
two correlated terms
in the same model (assuming Type I sums of squares)?

Or should we ignore all plant.biomass effects, because they are caused by
plant.diversity?

I would highly appreciate your help on that problem.

Many thanks and best wishes
Christoph







-- 
Dr. rer.nat. Christoph Scherber
University of Goettingen
DNPW, Agroecology
Waldweg 26
D-37073 Goettingen
Germany

phone +49 (0)551 39 8807
fax   +49 (0)551 39 8806

Homepage http://www.gwdg.de/~cscherb1