Besides isolation, stability is a key factor for evolutionary change. A study conducted by Edward R. Meyer (1973) provided evidence that aquatic and terrestrial habitats found in the basin today are ecologically equivalent to those in existence since at least mid-Wisconsin time. Meyer arrived at this conclusion after analyzing the pollen spectra of the basin.
Cores from two spring sites were examined for fossil pollen evidence. Five different radiocarbon dates were obtained from the cores -- the oldest dated at 31,400 years BP. Meyer compared the modern pollen rain with the fossil pollen record to determine changes in valley flora.
Meyer categorized the pollen as arboreal pollen (AP) including pine, oak, fir, spruce, and cypress. Grasses (Gramineae), cheno-ams (Chenopodiaceae & Amaranthaceae) and aquatic plants (sedge and cattail) composed the GCA category. The Compositae category contained pollen from the sunflower family.
The modern samples from montane locations surrounding the valley were characterized by AP and Compositae pollen, exceeding 50 percent collectively, with GCA proportions less than 35 percent. The pollen rain from the basin floor, however, was dominated by anemophilous plants, characteristic of the vegetation growing there now, and GCA pollen comprising at least 58 percent of each sample. Total arboreal pollen from the floor of the basin ranged from 5 to 20 percent. Compositae was consistently present from the valley but did not exceed 22 percent in any sample.
Water lilies in one of the many pools in the basin. (Photo from National Geographic, October 1995) |
Fossil records were composed primarily of pollen from plants growing in the basin now. The GCA portion of the record provides evidence that the type of vegetation growing at the core sites changed repeatedly during the late-Quaternary, probably in response to changes in soil moisture and soil salinity. For example, core samples dominated by grass pollen and cheno-ams represent periods of less active springs. On the other hand, core samples containing more than 10 percent aquatic pollen indicate times of active springs. Meyer states that the high values for grass and cheno-ams in the fossil records indicates that the springs have been surrounded by grassland and xeric shrub throughout the late-Quaternary. The fossil pollen record at both core sites was dominated by GCA pollen, specifically grass and cheno-ams. However, aquatic pollen types were present throughout the spectra. Meyer points out that sedge and cattail grains do not travel far or in great quantities from points of origin. This led him and others to conclude that appearance of these pollen types in Quaternary fossil pollen records from arid regions in the western North America, is evidence of ancient aquatic environments. Therefore, fossil aquatic pollen in Cuatro Ciénegas provides evidence of an ancient aquatic environment. Although Meyer's study suggests that the valley floor has been in a stable state for at least 30,000 years, that is not the case of the surrounding mountains. The mountain pine forests and woodlands were much more extensive in the past than at present. However, the unique environment of the valley floor prohibited the expansion of the woodlands. Meyer's study concludes that the stream and pond habitats of the Cuatro Ciénegas basin have existed since the Tertiary suggesting that the floor of the valley remained a stable environment throughout the Quaternary - regardless of changes in climate. This stable, isolated environment provided a refuge and an opportunity for evolutionary change as exhibited by the diversity of organisms in the basin.
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