Multinomial logist regression

On Mon, 8 Feb 2010 18:29:06 -0500 Steve Friedman <friedman.steve-

Which package and function is more commonly used for ecological
investigations of this kind?

I don't know, but for multinomial models in general, I have been happy 
with the 'multinom' function in the 'nnet' package.

How can I develop a plot illustrating the probabilities of each of the six
response vegetation types for a given concentration of nutrients (the
independent predictors) ?

The 'effects' package is perfect for these kinds of plots, and has 
recently been extended with support for multinomial models:
http://www.jstatsoft.org/v32/i01

Karl Ove Hufthammer