Freshwater Islands in a Desert Sand Sea: The Hydrology, Flora, and Phytogeography of the Gran Desierto Oases of Northwestern Mexico

Abstract

The Adair Bay pozos (water holes) are small artesian springs scattered along the saltflats of the Gran Desierto near the coast of the Gulf of California in northwestern Sonora. The pozos provide essential fresh water for the rich bird fauna and some of the mammals, and were also utilized earlier by native people. The Gran Desierto aquifer appears to consist of sand and gravel deposited in ancient river beds which were subsequently overlain by dunes. Toward the coast, the alluvial aquifer becomes confined, or buried, beneath the relatively impermeable clays of the saltflats. These clays act as a barrier which causes artesian pressure to develop within the underlying aquifer. Pozos appear to develop at locations in which the permeability of the clay is increased, possibly by desiccation cracking or by flocculation due to ion exchange. The hypothesized existence of a buried fluvial system may explain the occurrence of clusters of pozos in some saltflats and their absence in many others, i.e., pozos only occur in saltflats with an underlying waterway. Alkali Weed (Nitrophila occidentalis) is the first plant to colonize places where the aquifer has broken through the overlying clays and reaches the surface or near the surface. This plant is a good indicator of fresh water. Coyotes seek fresh water in these places. Such action of coyotes and perhaps other animals seems to be related to the formation of smaller pozos. Saltgrass (Distichlis spicata) is the second plant to colonize a pozo and larger oases are colonized by a more diverse flora. The flora of the pozos is markedly different from that of the rest of the Sonoran Desert, both in life -form spectrum and geographic origin. The pozos support 26 species of vascular plants, many of which show temperate affinities. Several members of this flora are new geographic records: Indian Hemp (Apocynum cannabinum in the Apocynaceae), new for Sonora and the Sonoran Desert; Lythrum californicum in the Lythraceae, new for Sonora; Greasewood (Sarcobatus vermiculatus in the Chenopodiaceae), a new generic record for Mexico. The pozos are island -like relicts of the delta of the Colorado River. With the delta ecosystem now virtually destroyed, the local extinction of any wetland species in the pozo flora will most probably not be followed by new immigrants of the same flora, but by introduced weed species such as Salt Cedar (Tamarix ramosissima). The species -area relationship of the pozo flora is similar in value to that for other island ecosystems, although the exponential parameter (z = 0.263) is significantly higher than Preston's "canonical" value and the scale coefficient is significantly higher (k = 0.75) than those for other small island ecosystems. The species richness of a pozo is nearly four times higher than that of dry terrestrial islands of comparable size. Based on a projection of a biogeographical model fitted to the floristic richness of the pozos, we estimate that the original flora of the Colorado River delta supported 200 to 400 species of wetland vascular plants. Most of these populations have met local extinction with the destruction of the delta ecosystem of the Colorado River earlier in this century.