Strange things happen near the edge of a plant or animal's geographic range. Indeed, some biologists believe that evolutionary change tends to be concentrated around a species' periphery. This is because the physical and biological forces restraining the organism's spread place great stress on the marginal populations. But these forces are not necessarily all-powerful, and populations sometimes evade them, managing to set up isolated colonies beyond the main body of the species.
One such escapee, skipping beyond the northern edge of the Chihuahuan
Desert, is creosotebush, usually considered a desert species. As you move north along
the Rio Grande Valley, creosotebush first disappears from the cooler, northern slopes
of arroyos and prominences. Becoming scantier and scantier, it peters out entirely
around Socorro, New Mexico. Yet, over 60 miles farther north, just south of
Albuquerque, a colony thrives. We have no real idea how it got there, but its reason
for success seems obvious. The plants are happily encamped amidst a black,
heat-gathering and heat-radiating lava flow that brings a bit of the south to this
colony of the north.
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.