Laserfiche WebLink
<br /><, <br />.~ <br /> <br />. <br /> <br />" <br /> <br />. <br /> <br />" <br />,: . <br />, " <br />.' <br /> <br />." <br /> <br />,. <br /> <br />~ <br /> <br />':"S",:: . <br /> <br />, ." <br />. <br /> <br />, <br /> <br />. <br /> <br />- '.. <br /> <br />... <br /> <br />. <br /> <br />J, <br /> <br />. ;:- . . <br /> <br />~ <br /> <br />. .' <br /> <br />, <br /> <br />4 <br /> <br />'~~ <br /> <br />-...., <br />,-' <br /> <br />" <br /> <br />! <br /> <br />'. <br /> <br />.. <br /> <br />" <br /> <br />'. <br /> <br />I' <br /> <br />. <br />. <br /> <br />',' <br /> <br />~,j " <br /> <br />,. ", <br />~ _.:Jot .':"_.~~_" ! <br /> <br />,t <br />.-:.~- <br /> <br />-.........., <br /> <br />.~,'~ <br /> <br />440 <br /> <br />JAMES E. DEACON AND W. L MINCKLEY <br /> <br />viewed in perspective of geologic time through which many of the forms <br />have lived. Their extinction within a few years therefore seems inevitable. <br />Conditions which are severe for one fish species may of course enhance <br />another. Tolerant forms have an advantage in severe conditions and be- <br />come common. Intolerant forms may shift their ranges, compensating for <br />severe events, and become locally extinct only to return or even to extend <br />their ranges when suitable habitat is created (Trautman, 1942, 1957; <br />Minckley and Cross, 1959; Starrett, 1950b; Minckley and Deacon, 1968). <br />Short-lived species may almost, or in fact, disappear. Intermittency of <br />tributaries in a large system results in retreat of the fauna to havens in <br />rivers or springs (Paloumpis, 1956; Deacon, 1961; Deacon and Metcalf, <br />1961). However, in desert regimes, the lower, larger parts of a basin may <br />dry first, and major extinction can occur in disjunct areas of the drainage <br />system. <br />The longfin dace is the most adaptable and widespread fish native to <br />the Sonoran Desert. In years of abundant precipitation, large populations <br />of dace may build, many kilometers from annually reliable, permanent <br />. flow. In 1965, more than 45 km of surface water persisted in the desert <br />portion of the Sycamore Creek system, Maricopa County, Arizona, below <br />about 1000 meters elevation, throughout most of the year (Minckley and <br />Barber, 1971) . Young of 1965 Agosia, displaced in part downstream from <br />higher elevations, matured and reproduced to densely populate the entire <br />reach. In the years 1963-1965, and again in 1967-1969, only three <br />30 to 100 meter segments of surface flow were known in that reach of <br />creek, and each supported remnant populations of dace. Similar situations <br />of colonization and extirpation are known for a great number of species <br />in almost all deserts that have been studied. Some of the most spectacular <br />examples are the cyprinodontids, especially Cyprinodon and Aphanius. <br />More subtle changes also occur in stream-fish populations, and some- <br />times in a remarkably short period of years. Barber and Minckley (1966) <br />found only one individual of Rhinichlhys oscu/us in Aravaipa Creek, Ari- <br />zona, in 1963-1964; longfin dace were exceedingly common. After sub- <br />stantial precipitation and increasingly persistent flow in 1965-1966, <br />speckled dace became more abundant and Agosia relatively less so. In <br />1968, Rhinichthys was again almost absent, and Agosia was swarming <br />- throughout the area. But in 1970 after good flow in 1969, Agosia was <br />again rare. In Sonoita Creek, southern Arizona, an identical response oc- <br />curred over the much longer period from 1904-1965. The fauna of that <br />stream slowly declined (with unknown fluctuations year to year) as stream <br />flow deteriorated, until in 1959 only Agosia was found by Miller (1961a). <br />In 1963, it took 4 hours and a detailed search for suitable habitat to obtain <br />two specimens each of specklcd dace and Gila mount~in-sucker (.Pan- <br /> <br />.....,........ - ~, ~ ,......7"" <br /> <br />. <br /> <br />... <br /> <br />, <br />