The taxa and envs lists are expected to be of equal length and that the ith
element of taxa list is associated with the ith element of the envs list.
That is, the taxa and environmental gradient for treatment 1 are both the
first elements of the respective lists, the taxa and environmental gradient
for treatment 2 are the second elements for the respective lists, etc.
The environmental gradient data.frames are expected to have two columns, one
with the station ID and one with the data defining the gradient.
The taxa data.frames are expected to have the station ID column as well.
Important The station ID column name needs to be the same for all the
taxa and environmental gradient data.frames.