The difference would therefore be that an MA in Biology doesn't exist in the way a typical MS in Biology does. If you see a program advertised as an MA in Biology, it's likely either:
* A mislabeling: The program is actually an MS, but the name is incorrectly used.
* A highly specialized program: It might be a Master's program with a significant emphasis on the humanities aspects of biology (like history of biology or bioethics), but even then, it would be unusual to see it called an MA rather than an MS or some other designation.
In short, if you're looking for a Master's degree in Biology, you should look for an MS, not an MA.