Getting a masters in WildLife Biology after receiving a PhD in math advice
You almost certainly won't need a BS in biology and you should, if possible, take the undergraduate deficiency courses for a grade.  Now for the more general advice:

I have known many people (in my university) who have been in Ph.D. programs while taking courses in other graduate programs or even entering other graduate programs, so it won't be a problem, except you need to talk with both your committee in mathematics and a representative of the Wildlife graduate program.  From the latter, you should find out if you can enter the program now (while in the Ph.D.) and what undergraduate deficiency courses would allow you to be accepted to their Master's program.  These might only be a fraction of the courses in the undergraduate curriculum.  It is hard for someone outside the their department to tell what they think is a minimum amount of background needed to enter a program.  Some programs will also let you enter their program before all of the deficiency courses have been taken.  You can't guess at this without talking with an advisor in the Wildlife Biology program.  In fact, they might prefer that you take a graduate level broad-based course rather than some of the undergraduate courses.

At the same time, in order to have your tuition covered, you'll probably have to have your Ph.D. committee approve the courses.  If you tell them that you are interested in applications of mathematics to wildlife biology (that's true, I'm guessing) and want to take a couple of applied courses, they'll likely approve as long as it doesn't interfere with your progress in the Ph.D.

In other words, don't guess at what you need to do.  Talk with your advisor and an advisor in the Wildlife Biology program.

(In addition, you may also want to take a couple of statistics courses if you haven't had any statistics in your mathematics program.)