Warning this is more of a code dump instead of a blogpost but I'm still putting it out there hoping it's still useful for someone else as it has been sitting in my draft folder for far too long. Anyways the paper by Robert Hijmans is really worth reading if you're interested in the evaluation of species distribution models. If you have questions, don't hesitate to contact me.
Geographic null model from paper by Robert Hijsmans (link to paper).
package dismo (maintained by Robert J. Hijmans). and the package BIOMOD2.
The null_model_evaluation function takes as input a BIOMOD2 model.