The geographic ranges of species vary in size

As we discussed, the geographic range of a species is the entire region over which that species is found. Although there are no species that are found everywhere, there is considerable variation in the sizes of their geographic ranges.

Other species, such as coyotes, live over most of one continent (North America), while still others, such as gray wolves, live on small portions of several continents (North America and Eurasia). Relatively few terrestrial species are found on all or most of the world's continents. Notable exceptions include humans, Norway rats, and the bacterium Escherichia coli, which lives in the intestinal tracts of reptiles, birds, and mammals (including humans) and thus is found wherever its host organisms are found. Some marine species, including invertebrates with planktonic larvae (see Figure 7.11) and whales (see Figure 9.13), have large geographic ranges. But while range sizes vary greatly, the pattern in the oceans is similar to that on land, and for most marine species the geographic range is relatively small (Gaston 2003).

The geographic range of a species includes the areas it occupies during all of its life stages. It is particularly important to keep this fact in mind for species that migrate and for species whose biology is poorly understood. For example, if we wish to protect monarch butterfly populations, we must ensure that conditions are favorable for them in both their summer breeding grounds and their overwintering sites. In some cases, we understand an organism's range poorly because it has life stages that are hard to find or study; this is true of many fungi, plants, and insects. We may know under what conditions the adult organism lives, yet have no idea where or how other life stages live. In fact, that was long the case for the monarch butterfly. Biologists knew that each spring these butterflies arrived in eastern North America from the south, but it took almost 120 years (from 1857 to 1975) before their overwintering sites were discovered in mountains west of Mexico City.