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<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 />
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