The Preble's meadow jumping mouse (Zapus hudsonius preblei) is a small mouse, about three inches long, but with a six-inch long tail. They get their name from the fact they can use their large hind legs to jump 18 inches into the air. The mouse is not a distinct species but is on the endangered species list because it is recognized as a distinct subspecies.
For some years the mouse, found in Colorado and Wyoming, has been on the federal endangered species list. Its declining numbers are probably due to habitat loss and much of the remaining mouse habitat is also under threat. Its preferred habitat is a narrow band of land along the eastern edge of the Rocky Mountains that is being keenly eyed by developers.
A subspecies, by definition, differs morphologically from members of other subspecies of the species. Subspecies may, in some cases, be on their way to forming full species. In other cases they may not ever form separate species but will contain important genetic variation. Although the endangered species act preserves both species and 'subspecies' there is no clear definition of how distinct a 'subspecies' has to be before it deserves separate protection. In the current case opinions are sharply divided over whether the Preble's Jumping Mouse is distinct enough from the more common Bear Lodge meadow mouse. The story was summarized by the Jackson Hole Star Tribune when the US Fish and Wildlife Service convened a panel of scientists to review the evidence. A google search on the mouse will show the history of the debate.
This may sound like a small academic debate but it could have important outcomes for how the endangered species act is interpreted in the future. Genetic data is becoming increasingly important in deciding what is a valid subspecies but there are no guidelines for how much genetic difference is required between two populations for them to be considered different subspecies.
There are a lot of issues here, some of which we touched on and some of which we will touch on in the ecology section. It is important to realize that we may not currently have enough information to make these decisions.