How can I identify research opportunities I am qualified for, and position myself as a competitive candidate, in a different field from my degree?
Frankly, most undergraduate students (and new graduates) are under-qualified to do research in any field, including their own. For various altruistic and non-altruistic reasons, we're often willing to take them on anyways. 

When I hire an undergraduate or M.S. student to work in my research group, I look for qualities that tell me they'll be train-able, and also a good investment for me. Things like:


motivation (as demonstrated by showing that they actually know what we are currently working on, have looked at our publications before asking for a job, etc.)
interest in pursuing graduate studies related to my field of research 
soft skills (oral and written communication, organization, note-taking, etc.)
ability to learn new concepts and skills quickly and somewhat independently
basic quantitative and technical ability


I don't expect anybody I hire at this level to have relevant experience or experimental skills specific to my lab, whether they have a degree in the field or not. 

So, keep pursuing opportunities like these, be honest about your lack of a biology degree, but highlight your eagerness to learn and your "transferable" abilities.

You might also look for research opportunities in cross-disciplinary labs that combine your new interest (biology) with your previous degree (environmental science). For these opportunities, your degree might actually give you a competitive advantage over biology students.