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` ~b}' detnition, involve relative changes in abundance. Some species mac be elim- inated, others may be reduced in num- bers, and some may be benefited. Animals i^ category 4 are the easiest to define, simply because of the general- ly small, unique habitats in which the}• live. Such habitats often lack biological diversity. Minor changes may therefore effect major fluctuations in species abundance. Because of its isolation, an island is an especially likely site for such a catastrophe; indigenous organisms are few, and the system is ecologically un- saturated. Aggressive exotic species that travel with man have, when successfully established, a profound and usually detrimental influence. This was exempli- fied by the rapid destruction of a major part of the isolated avifauna on the Hawaiian Archipelago in the late 1800's and early 1900's (S). In most respects, desert springs are similarly isolated, in- sofar as many aquatic animals are con- cerned-often even more so than oceanic islands (6). Series of springs in desert regions form aquatic archipelagos that differ from their oceanic analogs in that they often contain organisms that are relicts of past ages, rather than orga- nisms resulting from chance invasion and subsequent differentiation. The re- stricted and ecologically .simplified na- ture of these habitats leaves them especially susceptible to faunal destruc- lion, especially when the springs are located in areas of rapid population growth, where the demand for water exceeds the supply. Status of Selected Fishes Faunal depiction in aquatic habitats of the American Southwest is the simple rule. Much surface water is directly re- n~oved for use by man. Most of the re- n;aining natural waters are highly modi- fied, physicochemically or biologically. Because of these factors, big-river fishes (category 3) present a special, pressing problem in the region. Table 1 illustrates the gross changes that have occurred in the fish fafinpa of a mtijor stream in Arizona, the Salt River, near its down- stream end at Tempe (Fig. 1). Extirpa- tion of a major part of the fauna between 1890 and 1926 is evident, cor- responding to early modifications of the stream by Caucasian man and impound- ment of Roosevelt Lake on the river in 1910. A chain of impoundments was then progressively created on the Salt River. between Tempe and Roosevelt. The Verde River, a major confluent of Z9 MARCH 1968 the Salt, maintained some .++ater in the channel at Tempe for a ~,bile. $artlett Reservoir on the ~'crde ++as closed in 1939, ho++ever, and this, in combination with construction of another dam, re- sulted in almost total desiccation of the channel of the Salt River b}~ the late 1950's (7). Only subsurface percolation of water, mostly from underfloor of municipal waste waters, maintained iso- lated fish habitats along the nearly dry stream. Such habitats persist today. In- troduced fishes became increasingly established after 1926, and extirpation ,.~ -, i •' _~>~i ;,. ,. . -ter k 'P ~,:.. ~.~~.~ -- j` - NEVaDA ~ f ~ 1 ~ ~-~ r uTAlt r L_~ l 1 PLUVIAL 2C ~ W RITE R, GREEN R. ~ / 1 a~ ' \4 1, ~ ~: J ~ r J ASH 1 MEADOWS 1 ~ ~ VIRGIN R. ~ 26 ~ { y ~ '~ MOAPA ~ j- _- R~ f '" - ~- PAHRUMP ~ f ~t~ VALLEYS SIT v4--13 \, COLORADO R. CALIFORNIA X15 3 VERDE R. 27 2y 28 ^~I 6 21~~ 1 - _. y __ T2s ~ ~~ GIIA R. +x-11 \~~~~ "~~ ~'~,\ ARIZONA ~, I~ . GULF OF \`^ ~~ CALIFORNIA WYOMING ~~ ` F ~~ _~ ;,~ f;~: I YAMPA R. t~ ~ ~ •-^ .~. - 1 rr~~ ^ `-~ i ' ~ , COLORADO R. f~ 1 f LITTLE .COLORADO R. l ,1 I `1(„_ i 1 / ~ 1 j of additional native fishes quickly fol- lo+ved. All the species that occupied the Salt Ricer at Tempe in 1590 exist today somewhere in the Colorado River basin. The variation in their success in main- taining populations is, however, great; some species remain abundant, others are reduced in number, and a few are on the verge of extinction. This varia- tion illustrates some of the problems involved in the study and definition of "endangered species." Two large species especially relevant r \ IGILA ) ~2a , )rSAN PEDRO R. 11 r7 I COLORApO SAN JUAN R, .~..~ d N NEW ME%ICO Fig. 1. Sketch map of the Colorado River basin, southwestern United States, showing rivers and localities mentioned in the text, (1) Arivaca Creek; (2) Bartlett Dam; (3) Camp Verde, Arizona; (4) Coolidge Dam; (5) Dinosaur National Monument; (6) Dome, Arizona; (7) Fairbank and Tombstone, Arizona; (S) Flaming Gorge Dam; (9) Ft. Thomas, Arizona; (10) Frisco Hot Spring; (11) Gila City (= Gila Bend), Arizona; (12) Glen-Canyon Dam and Lee's Ferry, Arizona; (13) Grand Canyon; (14) Grand Falls; (15) Lake Havasu; (16) Lake Mead; (17) Lake Mojave; (18) Martinez Lake; (19) Navajo Dam; (20) Ouray, Utah; (2t) Phoenix, Arizona; (22) Roosevelt Lake and Roosevelt, Arizona; (23) Safford, Arizona; (24) Salt River Canyon; (25) Saguaro Lake; (26) St. George, Utah; (27) Tempe, Arizona; (26} Yuma, Arizona. 1425 t. F~, 1. l~• ,: }; 'r', Ci, 1';~ +. ~~. ,y ~(_ .,^