The park is home to a decent amount of desert wildlife, though much of it is shy. Perhaps the most obvious creatures in the park are the lizards - several varieties exist, and they like to sun themselves on the park rocks. Snakes, including rattlesnakes, also inhabit the park.
Soaring through the skies and echoing their rough call off the canyon walls, ravens are one of the more visible birds in the park. More social (read: obnoxious) at the picnic areas are the scrub jays. In the winter, rosy finches and juncos flock through the park (if you want to see all the varieties of dark-eyed junco, this is the place...). In other seasons, the canyon wren supposedly sings here. Golden eagles also soar above the area (they even rate warning signs on the highways). Condors sometimes wander up from Arizona, though they are still exceedingly rare.
Less obvious are the desert bighorn sheep, cougars, coyotes, foxes, and other mammals. The only mammals I see regularly in the park are chipmunks and ground squirrels. There were lots of sheep tracks along the Devil's Garden primitive trail when I was last there, and I saw some Mule Deer at Delicate Arch.
In addition to the rattlesnake, it is worth noting that the area is home to black widow spiders, though I have yet to see either. Insects abound in Spring, pollinating the flowers, but are rare in other seasons.
Vegetation is typical for this area of the desert. Saltbush, sagebrush, rabbitbrush, pinon pine, and Utah juniper make up most of the "larger" plant life. Grasses, an assortment of desert wildflowers, and prickly pear cactus make up the undergrowth. A variety of lichens make interesting patterns across some of the rocks throughout the park.
Perhaps the most common bit of nature in the park is the cryptobiotic crust. Cyanobacteria, fungus, lichen and mosses (the composition varies) form a symbiotic relationship on the top of the harsh desert soils. Together they hold the soil crust together, allowing grasses and larger plants to gain a foothold on what would otherwise be a great sand dune field. In places, the crust is almost invisible - a darkened, solidified patch of sand. In other areas, the crust has matured, and forms peaked masses. The crust is the life of the Colorado Plateau desert, and takes decades to reform - please be aware of this delicate, vital life!