438
JAMES E. DEACON AND W. L MINCKLEY
only their body shapes, but also the presence of thick, leathery skins, with
scales reduced, absent, or deeply embedded (see Section ΙΙΙ,Α,Ι). This
is especially pronounced on surfaces that first contact onrushing water,
such as the leading edges of fins, dorsum of head, nape, and anterodorsal surfaces of the sides. All the species have relatively small eyes, set
in small, bony orbits, but none is as extreme as a chub from the highly
turbid Missouri River (Hybopsis meeki) which often has an overgrowth
of leathery skin almost covering the eye surface (Moore, 1950; Davis and
Miller, 1967).
2. Fishes in Gradients of Springs
Gradients of dispersion created by gradual changes in habitat downflow
from a spring are somewhat more fixed, and considerably more evident,
than those in streams fed by surface runoff. One, or perhaps two or three,
factors present overriding forces in such a gradient. Thus the Moapa dace
appears as a thermal endemic (Hubbs and Miller, 1948b), as do Crenichthys spp. (Hubbs, 1932). Cyprinodon bifasciatus (Miller, 1968) and others
discussed before are capable of completing their life cycles in thermal and
chemical constancy, but are incapable of doing so downstream, either
because of physiological constraints or because of competitive interactions
with other fish species.
Springheads are notoriously depauperate in species, and this holds, generally, for most animal groups (Minckley, 1963). However, in large, complex limnocrenes, such as in the Cuatro Ciénegas basin, heterogeneity may
allow a substantial fauna to co-inhabit.
When the springs are arranged by size (Fig. 7), faunal diversity may,
however, be demonstrated to increase with decreasing constancy. At the
terminus of the spectrum, the sumps of springs in desiccating lakes and
pools, variability is so great that it acts as an overriding force, again limiting the fauna to one species, or a few species, capable of resisting, or persisting under, the prevailing regime.
The patterns of dispersion in a spring-fed environment therefore may
theoretically result from the same basic factors producing longitudinal succession of fishes in streams of classical physiographic gradient (Fig. 6).
Small faunas of headwater creeks correspond to those in sumps of desert
systems—only colonizers capable of resisting great extremes can live in
either place. Springheads correspond to the relatively greater constancy of
large, lowland rivers of temperate zones. When the springs are large enough
they may support a comparably large and diversified fauna. Intermediate
habitats, by nature both diversified and relatively permanent, support the
greater variety of fishes.
JAMES E. DEACON AND W. L MINCKLEY
only their body shapes, but also the presence of thick, leathery skins, with
scales reduced, absent, or deeply embedded (see Section ΙΙΙ,Α,Ι). This
is especially pronounced on surfaces that first contact onrushing water,
such as the leading edges of fins, dorsum of head, nape, and anterodorsal surfaces of the sides. All the species have relatively small eyes, set
in small, bony orbits, but none is as extreme as a chub from the highly
turbid Missouri River (Hybopsis meeki) which often has an overgrowth
of leathery skin almost covering the eye surface (Moore, 1950; Davis and
Miller, 1967).
2. Fishes in Gradients of Springs
Gradients of dispersion created by gradual changes in habitat downflow
from a spring are somewhat more fixed, and considerably more evident,
than those in streams fed by surface runoff. One, or perhaps two or three,
factors present overriding forces in such a gradient. Thus the Moapa dace
appears as a thermal endemic (Hubbs and Miller, 1948b), as do Crenichthys spp. (Hubbs, 1932). Cyprinodon bifasciatus (Miller, 1968) and others
discussed before are capable of completing their life cycles in thermal and
chemical constancy, but are incapable of doing so downstream, either
because of physiological constraints or because of competitive interactions
with other fish species.
Springheads are notoriously depauperate in species, and this holds, generally, for most animal groups (Minckley, 1963). However, in large, complex limnocrenes, such as in the Cuatro Ciénegas basin, heterogeneity may
allow a substantial fauna to co-inhabit.
When the springs are arranged by size (Fig. 7), faunal diversity may,
however, be demonstrated to increase with decreasing constancy. At the
terminus of the spectrum, the sumps of springs in desiccating lakes and
pools, variability is so great that it acts as an overriding force, again limiting the fauna to one species, or a few species, capable of resisting, or persisting under, the prevailing regime.
The patterns of dispersion in a spring-fed environment therefore may
theoretically result from the same basic factors producing longitudinal succession of fishes in streams of classical physiographic gradient (Fig. 6).
Small faunas of headwater creeks correspond to those in sumps of desert
systems—only colonizers capable of resisting great extremes can live in
either place. Springheads correspond to the relatively greater constancy of
large, lowland rivers of temperate zones. When the springs are large enough
they may support a comparably large and diversified fauna. Intermediate
habitats, by nature both diversified and relatively permanent, support the
greater variety of fishes.