Why are there so few squirrels, possums, skunks, chipmunks and species like turtles and even birds around here? I moved from the Midwest recently and there are less trees, less outdoor area that could house these animals yet there are more there.
Washington County Oregon wildlife
Well, this may be a matter of opinion. In my neighborhood we have a lot of red oak trees and a few walnut trees so I think we have a lot of squirrels. Opossums are not native in this region and there are fewer than when I was growing up so that may be related to more predators for them. These are a couple of examples but is by no means exhaustive.
I'd encourage you to visit some of the nature centers we have in Washington and Multnomah counties. I found this link for Oregon as we have quite a few around the state https://en.wikipedia.org/wiki/List_of_nature_centers_in_Oregon
I did find this paper from 1998 that discusses diversity https://www.jstor.org/stable/2846273?seq=1#page_scan_tab_contents
Certainly what is native here is different as is the climate, terrain and population. I believe there is much diversity in floral and fauna, it's just different than what you have in the Midwest.