Are there any plants on Tatooine? If not, what do the animals at the bottom of the food chain eat?
Above, we see various plants at the Lars homestead, where Luke grew up.
Also, according to the official Star Wars databank, Tatooine consists of deserts, canyons, and mesas. Mesas typically have low-lying shrubbery and grasses, due to water flowing close to the surface between soft shale layers.
For reference, here is a mesa in New Mexico:
Finally, here are some wild shrubs spotted on Tatooine:
We also know that fungi grows in Tattooine, mostly on the moisture vaporators. In Attack of the Clones, Shmi was captured by the Tusken Raiders when she was out picking mushrooms that grew on the vaporators.
In addition to the other answers, Wookieepedia cites Star Wars 7: From the Journals of Old Ben Kenobi: "The Last of His Breed" as a canon source for the existence of a plant native to Tatooine which produces a black melon.
The melon contains a milk which Tusken Raiders and humans can drink (and presumably other animals).