Congrats! Personally I do think that it is always best to dress a little more formally than required. It shows you care and know how to be professional. However, I'm not terribly surprised that an ecology interview might be more casual... When I went to the ESA meeting, no one wore a full-on suit (except a few students), and a few people presented in jeans and/or hiking shoes. Not what I was expecting for a national conference!
Perhaps there is a side to ecologists that always wants to be field-ready... ha!