On Thu, 2010-12-02 at 21:02 -0200, Alexandre F. Souza wrote: > Dear friends, > > It has been recently suggested by Legendre and co-authors that we > include quadratic and product terms of our environmental variables in > constrained ordinations. For instance, to include not only latitude but > also latitude^2 and latitude*longitude, the same with physical and > biotic variables aimed at being correlated with ordination axis in some > way. > > I am following McCune and Grace's book (2002) suggestion of > correlating environmental variables with NMDS axes. However common > correlations of multiple variables x each nmds axis are not the best > options because at each correlation they do not control for the other > variables in the dataset. Those authors also state that multiple > regression use partial correlation coefficients and thus would be a > better methos for correlating several environmental variables with > ordination axes.
I think correlating anything with individual "axes" of an nMDS is a silly thing to. There are no "axes" in nMDS, the configuration is king! Fit a response surface to the nMDS solution instead. See ?ordisurf in the R package 'vegan' for examples of what I mean. > However, multiple regression suffers from multicolinearity, which is > greatly enhanced when we use product or quadratic terms of the > environmental variables. Quadratic terms and interactions are used all the time in statistical modelling - the point of them is to model a non-linear (in the English language sense of the term) relationship between response and the particular covariate (latitude in your case). Interactions adjust the effect of one covariate on the response by the effect of another covariate. What they are suggesting be done is to not simply consider the spatial component as a linear function of laitude. HTH G > What do you think about that? > > Best whishes, > > Alexandre > > Dr. Alexandre F. Souza > Programa de Pós-Graduação em Biologia: Diversidade e Manejo da Vida > Silvestre > Universidade do Vale do Rio dos Sinos (UNISINOS) > Av. UNISINOS 950 - C.P. 275, São Leopoldo 93022-000, RS - Brasil > Telefone: (051)3590-8477 ramal 1263 > Skype: alexfadigas > [email protected] > http://www.unisinos.br/laboratorios/lecopop -- %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~% Dr. Gavin Simpson [t] +44 (0)20 7679 0522 ECRC, UCL Geography, [f] +44 (0)20 7679 0565 Pearson Building, [e] gavin.simpsonATNOSPAMucl.ac.uk Gower Street, London [w] http://www.ucl.ac.uk/~ucfagls/ UK. WC1E 6BT. [w] http://www.freshwaters.org.uk %~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%~%
