2E. Environmental variation and local adaptation
The habitats of the White Sands pupfish vary considerably in regard to
salinity levels. Salinity is relatively high at Lost River (13.5-60ppt),
moderate at Salt Creek (10.5-21ppt), and relatively low at Mound (1.5-4ppt)
and spatially variable at Malpais Springs (3.5-21.5ppt)(Stockwell and
Mulvey 1998).
This difference in salinity appears to provide the conditions for local
adaptation; indeed an allozyme locus appears to be under selection due
to differences in environmental salinity in these habitats (Stockwell
& Mulvey 1998). Stockwell and Mulvey (1998) reported rapid evolution
at an allozyme locus (Phosphogluconate dehydrogenase; Pgdh) in recently
established populations of White Sands pupfish (C. tularosa). The allele
frequencies in two recently established populations (Lost River and Mound
Spring) varied significantly from allele frequencies in the native parental
population (Salt Creek). This shift and additional evidence suggested
that Pgdh might be under selection by salinity (Stockwell & Mulvey
1998).
Further, salinity has indirect effects on pupfish, because complex life
cycle parasites can be excluded from the sites with high salinity (Rogowski
and Stockwell 2006). For instance, a white grub (Family: Diplostomatidae)
occurred at high prevalence and intensity in fish from Malpais Spring
and Mound Spring, but was absent from fish examined from Salt Creek and
Lost River (Stockwell and Collyer unpublished data). The life cycle of
white grubs includes snails (Physa sp.), fish, and the definitive host
is typically herons (Hoffman 1967). The absence of the white grub at the
two saline sites is apparently due to the absence of snails (Physa sp.)
at these sites. Recent experimental work has shown that Physid snails
can not tolerate salinity above 7ppt (Stockwell unpublished data). Environmental
differences also affect the distribution of other parasites. For instance,
two additional parasite species occurred only in Salt Creek pupfish; Heterophyidae
species 1 and 2 (Rogowski and Stockwell 2006) (Stockwell and Collyer unpublished
data).
A subsequent habitat survey revealed the presumptive host of these
parasites, a recently described springsnail species (Juturnia tularosa;
Hershler et al. 2002) that occurs at high abundance in Salt Creek. Whether
this springsnail species is actually part of the life cycle of the two
trematode species is yet to be determined. These parasites may be costly
to their hosts. Indeed, a population decline have observed at Mound Spring
in 1995 coincided with parasite outbreaks (Pittenger 1996). Three lines
of evidence suggest that parasitism by Diplostomatids are costly. First,
seasonal differences in parasite intensities suggest that heavy parasite
burdens reduce over-winter survivorship at Malpais Spring and Mound Spring
(Collyer and Stockwell 2004). Second, parasite load was inversely correlated
with fat storage (Stockwell unpublished data). Third, recent experimental
work conducted at North Dakota State University has shown that this parasite
is costly in terms of mortality risk, fat storage and growth rates (Collyer
and Stockwell 2004). These data are consistent with the idea that parasite
load may regulate pupfish populations as indicated by the population crashes
at Mound Spring.
Flow regime also varies among the sites and may influence fish body
shape. Recent work revealed that the Salt Creek and Lost River fish are
streamlined, whereas Malpais Spring and Mound Spring fish are deep bodied
(Collyer 2003; Collyer et al. 2005). These body shapes correspond with
the flow conditions in the local habitas. Salt Creek and Lost River have
periodic high flows, whereas Malpais Spring and Mound Spring have no appreciable
flow. A common garden experiment showed body shape to be heritable, providing
evidence for a case of contemporary evolution (sensu Stockwell et al.
2003) for the Mound Spring population (Collyer 2003; Collyer et al. 2005).
These environmental data show that the ecological characteristics vary
considerably between the two native sites as well as between the two sites
with introduced populations of pupfish. Thus, translocations may result
in altering the evolutionary trajectory of the targeted taxum.
Translocation efforts are typically made to assure that the "refuge"
habitat is representative of the targeted taxa's native habitat. For this
strategy to be successful, extensive ecological information will be required
from both the native and targeted "refuge" habitats. Even under
the "best case" scenarios, one or more important ecological
factors are likely to differ between the native site and the refuge site,
which could potentially lead to rapid evolution in the refuge site (Stockwell
and Weeks 1999; Stockwell et al. 2003).
Thus, rapid evolution may ultimately lead to a refuge population that
is not adapted to its native environment. This issue is of particular
concern, because the establishment of at least one additional refuge population
of White Sands pupfish is a possible conservation strategy (Stockwell
et al. 1998).
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