That's actually more like a 1/4 or 1/5 of the entire exhibit (including the portion that isn't for public viewing, even though you can kind of look into that area if you happen to be standing in the right place). If you go to Google Maps, Google Earth, or a similar location to see an overhead view of the zoo, you'll see two large open spaces north of the entrance. The one to the northwest is the main Asian Elephant yard; the one to the southeast is the pronghorn/bison yard.
I recall from my visit many years ago that the North America section was quite nice and the best part of the zoo. (This was well before recent additions like polar bear and African savanna).
