Gerbils are illegal in California because the state is afraid that if any find their way into the wild, they could cause significant damage to crops and native animals and plants. Hawaii also prohibits the breeding, distribution and ownership of gerbils.
The most common variety of pet gerbil in the United States is the Mongolian gerbil. The climate and natural areas found in California and Hawaii are very similar to the rodent's natural habitat. If these gerbils are either let loose or escape into the wild, they could quickly breed and establish colonies. Because they are burrowing creatures, they would create large series of underground tunnels that could threaten commercial crops and native plants.