440
JAMES E. DEACON AND W. L. MINCKLEY
viewed in perspective of geologic time through which many of the forms
have lived. Their extinction within a few years therefore seems inevitable.
Conditions which are severe for one fish species may of course enhance
another. Tolerant forms have an advantage in severe conditions and become common. Intolerant forms may shift their ranges, compensating for
severe events, and become locally extinct only to return or even to extend
their ranges when suitable habitat is created (Trautman, 1942, 1957;
Minckley and Cross, 1959; Starre«, 1950b; Minckley and Deacon, 1968).
Short-lived species may almost, or in fact, disappear. Intermittency of
tributaries in a large system results in retreat of the fauna to havens in
rivers or springs (Paloumpis, 1956; Deacon, 1961; Deacon and Metcalf,
1961). However, in desert regimes, the lower, larger parts of a basin may
dry first, and major extinction can occur in disjunct areas of the drainage
system.
The longfin dace is the most adaptable and widespread fish native to
the Sonoran Desert. In years of abundant precipitation, large populations
of dace may build, many kilometers from annually reliable, permanent
flow. In 1965, more than 45 km of surface water persisted in the desert
portion of the Sycamore Creek system, Maricopa County, Arizona, below
about 1000 meters elevation, throughout most of the year (Minckley and
Barber, 1971). Young of 1965 Agosia, displaced in part downstream from
higher elevations, matured and reproduced to densely populate the entire
reach. In the years 1963-1965, and again in 1967-1969, only three
30 to 100 meter segments of surface flow were known in that reach of
creek, and each supported remnant populations of dace. Similar situations
of colonization and extirpation are known for a great number of species
in almost all deserts that have been studied. Some of the most spectacular
examples are the cyprinodontids, especially Cyprinodon and Aphanius.
More subtle changes also occur in stream-fish populations, and sometimes in a remarkably short period of years. Barber and Minckley (1966)
found only one individual of Rhinichthys osculus in Aravaipa Creek, Arizona, in 1963-1964; longfin dace were exceedingly common. After substantial precipitation and increasingly persistent flow in 1965-1966,
speckled dace became more abundant and Agosia relatively less so. In
1968, Rhinichthys was again almost absent, and Agosia was swarming
throughout the area. But in 1970 after good flow in 1969, Agosia was
again rare. In Sonoita Creek, southern Arizona, an identical response occurred over the much longer period from 1904-1965. The fauna of that
stream slowly declined (with unknown fluctuations year to year) as stream
flow deteriorated, until in 1959 only Agosia was found by Miller (1961a).
In 1963, it took 4 hours and a detailed search for suitable habitat to obtain
two specimens each of speckled dace and Gila mountain-sucker (Pan-
JAMES E. DEACON AND W. L. MINCKLEY
viewed in perspective of geologic time through which many of the forms
have lived. Their extinction within a few years therefore seems inevitable.
Conditions which are severe for one fish species may of course enhance
another. Tolerant forms have an advantage in severe conditions and become common. Intolerant forms may shift their ranges, compensating for
severe events, and become locally extinct only to return or even to extend
their ranges when suitable habitat is created (Trautman, 1942, 1957;
Minckley and Cross, 1959; Starre«, 1950b; Minckley and Deacon, 1968).
Short-lived species may almost, or in fact, disappear. Intermittency of
tributaries in a large system results in retreat of the fauna to havens in
rivers or springs (Paloumpis, 1956; Deacon, 1961; Deacon and Metcalf,
1961). However, in desert regimes, the lower, larger parts of a basin may
dry first, and major extinction can occur in disjunct areas of the drainage
system.
The longfin dace is the most adaptable and widespread fish native to
the Sonoran Desert. In years of abundant precipitation, large populations
of dace may build, many kilometers from annually reliable, permanent
flow. In 1965, more than 45 km of surface water persisted in the desert
portion of the Sycamore Creek system, Maricopa County, Arizona, below
about 1000 meters elevation, throughout most of the year (Minckley and
Barber, 1971). Young of 1965 Agosia, displaced in part downstream from
higher elevations, matured and reproduced to densely populate the entire
reach. In the years 1963-1965, and again in 1967-1969, only three
30 to 100 meter segments of surface flow were known in that reach of
creek, and each supported remnant populations of dace. Similar situations
of colonization and extirpation are known for a great number of species
in almost all deserts that have been studied. Some of the most spectacular
examples are the cyprinodontids, especially Cyprinodon and Aphanius.
More subtle changes also occur in stream-fish populations, and sometimes in a remarkably short period of years. Barber and Minckley (1966)
found only one individual of Rhinichthys osculus in Aravaipa Creek, Arizona, in 1963-1964; longfin dace were exceedingly common. After substantial precipitation and increasingly persistent flow in 1965-1966,
speckled dace became more abundant and Agosia relatively less so. In
1968, Rhinichthys was again almost absent, and Agosia was swarming
throughout the area. But in 1970 after good flow in 1969, Agosia was
again rare. In Sonoita Creek, southern Arizona, an identical response occurred over the much longer period from 1904-1965. The fauna of that
stream slowly declined (with unknown fluctuations year to year) as stream
flow deteriorated, until in 1959 only Agosia was found by Miller (1961a).
In 1963, it took 4 hours and a detailed search for suitable habitat to obtain
two specimens each of speckled dace and Gila mountain-sucker (Pan-