Odds of Getting In
As a herpetologist, you can expect to be filling that strange gap between the dearth of people who want to dissect crocodiles before lunch and society's need for people who dissect crocodiles before lunch. In other words, the world needs herpetologists, but there are very few experts who've put in the time to actually, you know, become experts. All in all, this leaves you in a great position to succeed as long as you've spent the prerequisite time in school.
One of the funny things about science is that as we get smarter and smarter as a species, scientists have to become more and more specialized in order to make advances. Long gone are the days of the scientific renaissance man or woman, and upon us are the days of people who know way too much about just one thing. Basically, if you want to be Earth's preeminent amphibian authority, you've got a decent shot of getting there if you focus in it.