Driving westward across the Great Plains of North America, one notices that the woodlands thin out, the greenery diminishes and the air loses its humidity. While there is a gradual transition from eastern to western ecosystems, with no definitive border, geographers and botanists often use the 100th Meridian as the dividing line. West of this longitude, which runs through or near Abilene, Texas, Dodge City, Kansas and Pierre, South Dakota, increasing distance from Gulf of Mexico moisture, rising surface elevation and the "rain shadow" of the Rocky Mountains all combine to produce a drier landscape.
In reality, the leading edge of "the west" is angled a bit from southwest to northeast. After all, the 100th Meridian approaches the Gulf of Mexico in south Texas and the eastern Dakotas, though east of that longitude, are far removed from Gulf moisture. A line from the Big Bend of Texas to the high plains escarpment of eastern North Dakota might be more accurate; this margin would cut through west-central Oklahoma and central Kansas, both of which have a relatively dry, western landscape.
On these western High Plains, trees are limited to stream beds (except where artificially watered), crop fields require significant irrigation and shortgrass prairie replaces the rich, tallgrass province of the eastern Plains. A change in fauna is also observed; Swainson's and ferruginous hawks, prairie falcons, pronghorns, swift fox, jackrabbits, prairie dogs, thirteen-lined ground squirrels, burrowing owls, long-billed curlews, lark buntings and longspurs are among the High Plains residents.
