|
<br />0006J8
<br />
<br />DELTA HABI
<br />
<br />
<br />ND RESTORATION POTENTIAL
<br />
<br />. the totoaba, now virtually extinct, a steel-blue fish
<br />that grows up to seven feet (2 m) and 300 pounds
<br />(136 kg), and once supported a commercial fishery
<br />that closed in 1975 (Postel et aI., n.d.).
<br />
<br />In addition, Mexico lists five threatened species: the
<br />yellow-footed gull, Heermann's gull, elegant tern, red-
<br />dish egret, and peregrine falcon; three species for special
<br />protection: the brant, house finch, and mockingbird; and
<br />one rare species: the great blue heron.
<br />
<br />Although not extensively studied, the delta's signifi-
<br />cance for migratory birds is indisputable, as it is the
<br />principle freshwater marsh in the region. From 1980 to
<br />1985, some 45,000 ducks and 200 geese wintered in the
<br />delta (Payne et aI., 1992). A 1992 winter survey found
<br />more than 160,000 birds in the delta, of which some 9000
<br />were avocets and 8000 were willets (the remainder be-
<br />ing smaller species such as sandpipers) (Morrison et aI.,
<br />1992 in Mellink et aI., 1997). A series of delta surveys in
<br />1993-1994 documented 21 seabird species, with more
<br />than 16,000 individuals; 6 heron species, and more than
<br />220 individuals; 20 shorebird species, and nearly 150,000
<br />individuals (Mellink, et aI., 1997). The delta also pro-
<br />vides nesting, breeding, and nursing sites for egrets,
<br />sandpipers, avocets, cormorants, ducks, pelicans, gulls,
<br />and terns. (See Appendix A for a selected species lists.)
<br />
<br />In addition to bird counts, there is evidence that delta
<br />habitats are of greater value to birds than riparian habi-
<br />tat upstream (in the United States) on the Colorado River
<br />(see discussion of Delta Wetlands and Riparian Vegetation
<br />below). Tree species composition is known to be critical
<br />in avian habitat selection, particularly in desert, ripar-
<br />ian habitats (Rice et aI., 1984). On the lower Colorado
<br />River, birds prefer gallery forests of cottonwood and
<br />willow, both native species, over screw bean and mes-
<br />
<br />
<br />The still waters were of a deep emerald hue.... A verdant wall of mesquite and willow separated the channel from
<br />the thorny desert beyond. At each bend we saw egrets standing in the pools ahead.... Fleets of cormorants drove
<br />their black prows in quest of skittering mullets/ avocets, rtillets, and yellow-legs dozed one-legged on the bars/
<br />mallards, widgeons, and teal sprang skyward in alarm.... At every shallow ford were tracks of burro deer.
<br />We always examined these deer trails, hoping to find signs of the despot of the Delta, the great jaguar, el tigre....
<br />We sa w neither hide nor hair of him, but his personality pervaded the wildemess....
<br />
<br />-Aldo Leopold, A Sand County Almanac, 1948
<br />
<br />THE DELTA'S ECOLOGICAL SIGNIFICANCE
<br />
<br />Productivity and diversity in the delta have declined
<br />over the last century, but the delta ecosystem remains
<br />an important biological resource nevertheless. It remains
<br />an oasis of life in the midst of the arid Sonoran Desert.
<br />Although reduced flows and the construction of levees
<br />have transformed the delta, floodwaters, agricultural
<br />drainage, municipal wastewater, and seawater in the
<br />tidal zone continue to support large riparian areas and
<br />marshes. The size of these areas tends to vary dramati-
<br />cally from one season to the next: during the period from
<br />1973 to 1993, freshwater and brackish wetlands ranged
<br />from 2300 to 25,500 acres (5800 to 63,000 ha). In 1997,
<br />flood releases reestablished native vegetation along the
<br />delta's Colorado River floodplain and riparian zones as
<br />well as in southeastern delta wetlands (Valdes-Casillas
<br />et aI., 1998c).
<br />
<br />The delta supports a variety of wildlife, including
<br />several threatened and endangered species. Mexico's
<br />Environmental Regulations on Endangered Species lists
<br />the following endangered species found in the terres-
<br />trial and aquatic regions of the delta (Diario Officiel,
<br />1994):
<br />
<br />. the desert pupfish, also listed as an endangered
<br />species in the u.s. - the largest remaining population
<br />anywhere is in La Cienega de Santa Clara;
<br />
<br />. the Yuma clapper rail, also listed as an endangered
<br />species in the Us.;
<br />
<br />. the bobcat;
<br />
<br />. the vaquita porpoise, the world's smallest marine
<br />mammal, listed as a species of special concern by the
<br />U.S. Marine Mammal Commission; and
<br />
|