Flightlines AUDIO: Black Hills is unique for many reasons
That the Black Hills of South Dakota is a unique place is not lost on people around here. Most everyone I know has made the trek at one time or another and seen Mt. Rushmore, Badlands National Park, maybe a cave or two along with the much-hyped Reptile Gardens, or perhaps even the Sturgis Motorcycle Rally. It's close enough to get there in a day and makes for a great--albeit tourist-choked--getaway for us in this region. Yet I would venture to guess most visitors do not fully appreciate just how geographically and biologically special these five thousand square miles really are.
While living in Nevada for a few months some years ago I became familiar with the term, "sky island." It means an isolated range of mountains surrounded by environments completely different than it. That's a definition that fits the Black Hills quite well, being surrounded on all sides by High Plains.
It's not altogether different from an oceanic island really. Just envision the Galapagos Islands and their well-studied groups of finches, tortoises, iguanas, etc. and you begin to get the idea.
Woven into many sky islands is a complex web of biogeography where endemic species (those found nowhere else) and remnant populations are the norm. Left on mountains after the retreat of Ice Age glaciers, some species have nowhere to go. Over time, small isolated mountain ranges become a refuge for the stranded. Thus the term, sky island.
A worthy example is the Phoebus Parnassian butterfly. This is an alpine species of the West and Northwest that frequents mountain meadows and clearings. Speciation appears to be taking place right before our eyes. Differences between the butterflies among the various mountain ranges are small but noticeable. So much so that experts can tell you exactly which mountain range a particular specimen came from.
When one takes the time to study range maps in bird field guides as they relate spatially to the area of western South Dakota, a strange and notable trend emerges. Standing apart from the shapeless blobs that depict the breeding ranges of a lot of our birds is a starkly localized point on the map, the Black Hills; a breeding "island" apart from the rest.
Again its geography plays an interesting card and it's all about longitude, the east-westness of the area. Given its placement on the continental map, the Black Hills can be expected to reach to the west and attract Rocky Mountain species to its relatively lofty heights. Consequently, species such as the broad-tailed hummingbird, dusky flycatcher, Clark's nutcracker, and Virginia's warbler use the Black Hills as the farthest northeast outpost of their typical mountain west breeding range.
More surprising, perhaps, is the few species that count the Black Hills as the farthest west extension (or at least near the edge) of their normal "eastern" nesting ranges. After all, in order to place themselves in suitable forested habitat, these are birds that must ignore or overfly mile after grassy mile of treeless plains. Here one could easily argue this is nothing more than species fidelity to breeding grounds left isolated from the last glaciation. Still, birders are surprised to find such species as broad-winged hawk and black-and-white warbler doing just that.
Among bird species, perhaps none more clearly defines the Black Hills as a sky island than the white-winged junco. This is the subspecies that cleanly carves out these forested granite ridges as a truly extraordinary place. This species complex is riddled with disparate races and subspecies that make up the dark-eyed junco species, yet it's only in the Black Hills that the white-winged race breeds.
While motorcycle riders and waterslide-seeking families take to western South Dakota for various reasons, a few of us look to the Black Hills as representing something different. Not only is it a relief from the hot dry plains which surround it, but it's home to a complex and startling mix of eastern and western species found nearly nowhere else. Biogeographically, it is absolutely a continental crossroad, a singularly unique place, and one worth exploring in just about any season.