Back in 2004, I wrote about the Geography of American Working Terriers:
In the U.S., the bread-and-butter quarry of the working terrier is the groundhog. Raccoons cannot dig their own dens, and neither can possums. If dirt dens are not available, they will seek other alternatives -- hay lofts, brush piles, hollow trees, farm outbuildings, hay stacks, rock crevices, or old squirrel nests.
With the exception of rock dens, these are not locations where a small dog follows quarry to ground and is then dug to. In short, it is not earthwork.
Red fox will dig dens on their own, of course, but in the American west they face real on-the-ground competition in the form of the coyote. A coyote will generally kill a red fox if given half a chance, as they directly compete with red fox for food.
In addition, the red fox is not native to the U.S., and its dispersal in the West is uneven as a consequence. While red fox are common in some areas (such as the prairie pothole region) they are quite rare in other areas (such as western Oklahoma).
Weather and time are another important reason fewer people hunt in the West. Fox will not den in warm weather unless they have kits, and in the U.S. we do leave fox pups alone.
Without hounds to drive fox to ground, the U.S. fox-digging season is very short -- generally only 10 weeks long, and for most people with jobs this presents a very short period of time to get out into the field.
When people do get out into the field, of course, they have to find their fox! This is easier said than done, and is very hard job for a novice hunter with a novice dog. Red fox densities are variable, but settlement is generally much thinner in the West where there is less food than in the East, and where the fox face direct competition with coyotes for both food and den sites.
Raccoons are not native to the West, though they have spread with humans during the last 50 years, helped immeasurably by the creation of denning shelters in the form of barns, out buildings, road culverts, abandoned cars, and brush piles.
For some reason, raccoons are never found above 4,500 feet, however, which means that they are absent from a large portion of the Rockie Mountains. A raccoon can expand a ground den a little bit, but it is not really made for digging. A skunk can and will dig its own hole, which is suitable for possum, but too small for anything but the smallest of adolescent raccoons. An armadillo will dig a hole, and in areas where they are common there is some hope of finding a raccoon to ground in an old armadillo hole.
Marmots and prairie dogs are found in some locations in the West, but the prairie dog is far too small for a dog to work, while the various species of rock marmots tend to gravitate towards areas with large boulders and talus slopes -- areas very hard to dig. Marmots are also absent from larger parts of the West outside of the Rockie Mountains. That said, if found in the right location, marmots are excellent quarry for working terriers in the Mountain States.
Some states -- notably California -- have very diverse geography and wildlife but also have very restrictive game laws which make working terriers difficult.
The American badger is common in some parts of the West, but the population densities are generally very low. Badger are also hard to locate, as they will move every few days or so as they eat out, or chase out, a local rodent population (rats, mice, ground squirrels, and prairie dogs). Once the food is gone, so too is the badger.
Unlike in Europe, the presence of a badger hole in the U.S. does not mean you have actually found a badger. More likely you have found a blank hole or, in some areas, worse -- a skunk, porcupine or snake.
Porcupines, rattlesnakes and skunks are fairly common in many parts of the West, and all three animals are a very serious threat to a dog. A dog sprayed underground by a skunk can be overpowered and die from anemia if not gotten out of the ground in pretty short order. A porcupine's defense system consists of barbed quills which can leave a dog wrecked in short order (and the owner's wallet drained after a visit to the vet). A dog bitten by a rattlesnake rarely lives, as the venom from even a small rattler is more than enough to kill a terrier. The dog commonly dies of asphyxiation as the throat swells and chokes off the wind pipe.
In the South there are tens of millions of nutria, but they do not seem to be worked very often. In part this is due to the fact that in those parts of the Deep South where where nutria are most numerous alligators also tend to be present.