Throughout the glacial interludes of the Pleistocene, places of ice free zones or refugia existed in North America. It was within these refuges that both plants and animals persisted and/or migrated to escape the ice covered land. During the last glacial maxima, for example, three large regions served as refugia in North America: midlatitude North America, Berengia, and coastal plains east of the ice sheet. Within these ice free zones, smaller refuges or islands with specific environmental characteristics existed, providing opportunity for evolutionary change. The Cuatro Ciénegas basin, located in northern Mexico can be thought of as such an island refuge.
This paper explores the Cuatro Ciénegas basin as an ancient and modern refugia, within which, populations of plants and animals evolved and persisted. The abundance of surficial waters in the midst of an arid desert has acted as an evolutionary island, distinct and separate from the surrounding areas. Evidence of the basin as an evolutionary island is found in the high degree of endemism. In addition, evidence of environmental stability since at least the mid-Wisconsin has allowed the organisms to evolve and persist.
While the high degree of endemism implies long term isolation, the similarity of relict species with Chihuahuan Desert populations implies invasion of the area in the recent past during pluvial periods (Milstead 1960). Pluvial periods existed during the Pleistocene causing a withdrawal of the eastern borders of the desert. These pluvial periods provided an opportunity for the invasion of more mesic-adapted species.
The persistence of the relict species combined with the high diversity of plants and animals in the basin provides evidence that the region acts as a current refugia. With its abundant surficial water, Cuatro Ciénegas provides an oasis for mesic-species, which lead a precarious existence in the midst of the arid desert.
Just as ice free zones during the Pleistocene served as a refuge for a variety of plants and animals, areas of permanent water in the midst of the Chihuahuan Desert served as refugia in the ancient past as well as the present.