[R-sig-eco] Using multiple species data for gam

[David Warton]

Hi Rajendra and Greg,
A couple of quick thoughts:

Firstly, Rajendra the method that is applicable to your data really depends on the research question - what is it that you are trying to achieve.  It is always hard to offer help on what analysis method is suited to a question without knowing the original research objective.  The gamm function for example might be useful to you if you are primarily interested in predictive modelling, and also if you think that you have a common nonlinear response to environmental variables with some "noise" around this pattern for different spp (which can be represented as random effects).  You could alternatively use this function to fit a separate smoother for each spp but that would be a pretty complicated model and few would have sufficient data to justify that level of model complexity.  VGAM y Thomas Yee offers and option in between these two.

Secondly, something you need to worry about with this type of data is interspecies correlation - for various reasons (including species interaction), it is widely thought and even better often observed that species are correlated in abundance (or presence/absence, whatever) even after accounting for environmental predictors.  This makes the problem multivariate.  If you care about making joint inferences across species and you don't account for correlation between species you can get things quite wrong.  The gamm function I think could handle residual correlation, but not the way you specified it, and it would have a lot of trouble, unless you have only a handful of species and quite decent abundance data on each.  On the other hand if you are just making predictions separately for each spp then you don't need to worry too much about this.

All the best
David


David Warton
Professor and Australian Research Council Future Fellow
School of Mathematics and Statistics and the Evolution & Ecology Research Centre
The University of New South Wales NSW 2052 AUSTRALIA
phone (61)(2) 9385-7031
fax (61)(2) 9385-7123

http://www.eco-stats.unsw.edu.au/ecostats15.html


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