Changing study fields After reading countless posts about this topic, I still can't find my answer. I'm a little lost on the future study plan...

I am currently studying Advertising and PR but do not have an amazing interest in it. I will finish my undergrad next year and looking for postgrad opportunities. However, I was wondering how realistic it is to be looking at postgrad courses in animal science, biology, marine biology and so on...

I know it is 'easy' to switch from science to social but is it possible to do the switch the other way around? 

Thanks!
It is very unlikely that you can make such a radical change without having to take a number of additional make-up courses at the undergraduate level, and your current training does not appear to have much overlap with your future plans.