2013/4/18 Pierre THIRIET <pierre.d.thiriet@gmail.com>

> Bray-curtis is usually the most appropriate, on raw
> abundance/biomass/cover data, or square root/log transformed. So why do you
> Hellinger transform before? This transformation is dedicated to be used
> with euclidean distance, and resulted ordinations (PCA or RDA) have a
> distinct meaning than PCoA or CAP/db-RDA (with bray-curtis) because joint
> abscence are included in first cases and excluded in the latter.
>

Dear Pierre and Aurélie,

Just another point: after Hellinger standardization (site profiles), joint
absences (double zeros) are ignored in PCA and RDA since Hellinger distance
is an asymmetric measure of resemblance (see Legendre and Gallagher 2001,
Oecologia).

All the best,

François

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