The cactus in the foreground, with it’s sharp and spiny defenses, like so many desert plants seem to have, strikes a sharp (no pun intended) contrast with the smooth lines of the zebra and the bland brown behind it. I think only in New Mexico would you see a prickly pear in front of a zebra pen at the zoo. There is probably a good possibility that only a New Mexico zoo would trust patrons enough to not grab the spiny cactus when it is so easily in reach.
Anyway. Contrast. I think there are several layers of it in this picture, not just in the colors. I’ll leave it to you figure out what they might be. Maybe you’ll see some that I missed.