Do I use MLA or APA citation for my Environmental Science project?

I have to do a project on an endangered species, and I have to cite the sources that I use in my project. My teacher did not tell me which format to use because he wanted "to be surprised". Just so you know I will have all of this information on a poster so its not really like putting citations in a paper at all. I don't have to cite where each individual piece of information came from, I just need a works cited or bibliography section on my poster. Oh, and I can only use 1 side.