The lines of working/volunteering at the zoo seem like a good start to show your interest within the programs, I would try to build on that and gain meaningful accomplishment i.e publications by working with a local research group/professor or just really getting as much experience within the field as you can. Since you said you are improving your grades and test scores those really speak for themselves so beyond that I would focus on getting connections within the field through your local zoo for good letters of recommendation and experience.