[R-sig-eco] Differences between total constrained inertia (rda) and variance explained (varpart()) in vegan

[Tim Richter-Heitmann]

Dear list,

when i perform RDA on a species set constrained with a set of 
predictors, which i have tested for their relevance by forward 
selection, i get values for inertia:

Inertia Proportion Rank
Total         0.12626    1.00000
Constrained   0.05493    0.43507   11
Unconstrained 0.07133    0.56493   48

So, the amount of explained inertia is
0.05493/0.1262

or roughly 43%.

I am now grouping the same predictors into three groups (soil, 
vegetation, space) and perform varpart() on them, i
get a total variance (sum of all explained variance) explained of only  
30%. This has been consistent for me in a variety of datasets.
Can somebody explain to me the difference?

Thank you!


-- 
Tim Richter-Heitmann (M.Sc.)
PhD Candidate



International Max-Planck Research School for Marine Microbiology
University of Bremen
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