There has been a very similar discussion in the Ecology recently between my
good friends, Hanna Tuomisto & co vs. Pierre Legendre & co. However, the
point here and above exactly was that you cannot use dissimilarities on the
RHS (lack of independence), but you must use rectangular data in dbRDA. If
you use distances on the RHS you won't have dbRDA but you get Mantel family
methods (like MRM in ecodist). The problem, of course, is how to map
distances onto Euclidean space (= rectangular data) *and* still study the
effects of the distances instead of the effects of *location*. I don't know
any really good solution here, but all proposed solutions have their
problems. Pierre Legendre, Daniel Borcard and Hanna Tuomisto have all tried
to convince me of their point of view, and while all their conflicting
arguments make sense, they are not yet an optimal solution.
Cheers, Jari Oksanen