you can be any major you want to be. as long as you complete to pre reqs each dental school want you to do (chem, bio, phys.........).
most people are science majors because they can complete their science degree while completeing their pre reqs for each school. the science degree also allows you to take many upper division science courses.
nevertheless, you can complete your psychology and environmental studies degree while taking the pre reqs and taking some extra upper division science classes.
each school has different requirements. you should check out their websites.
i think you should pick one you are interested more.
however, if i had to pick i would pick psychology. i personally think psychology is more interesting than environmental studies. but i would pick bio over psyc. i just think it would be easier being a bio major than a psyc major for reasons stated above.