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of grasshopper mice ( Onychomys leucogaster ) from Colorado populations
that had experienced a plague outbreak showed 75% survival when
infected with plague, whereas individuals from an Oklahoma population
that had been plague-free showed only 27% survival (Thomas et al. 1988).
Similarly, California ground squirrels ( Spermophilus beecheyi ) from an area
with a plague history showed 67% survival compared to 39% survival for
animals from a plague-free area (Meyer 1942).
Adaptation of Animals to Alien Ecosystem Engineers
Ecosystem engineers are species that exert a dominating influence on
the physical or structural environment in an ecosystem. The beaver
( Castor canadensis ) is such a species. By their cutting of woody plants
and damming of streams, beavers are capable of transforming a shaded
forest stream environment into one of open ponds, marshes, and
shrublands.
In New England, where the beaver was extirpated from most areas fol-
lowing European settlement, it is now reinvading many areas. Although
not an alien species in the strict sense, the ecosystem impacts of beaver
activity are substantial, and the reinvasion of the species is influencing the
evolution of at least one species, the wood frog ( Rana sylvatica ). Skelly and
Freidenberg (2000) have examined the thermal biology of wood frogs in
forested wetlands and in beaver wetlands in Connecticut.The beaver wet-
lands were created by reinvasion of beavers about 36 yr before. These
wetlands constitute a much warmer aquatic environment for egg and
tadpole development than do forested wetland areas. When eggs from
forested and beaver wetlands were incubated under conditions of the
forest environment, the development of eggs of the forest animals was
faster. On the other hand, tests of tolerance of high water temperatures
showed that tadpoles from beaver wetlands tolerated significantly warmer
conditions. The critical upper temperature for tadpoles from the beaver
wetlands was 0.4°C higher than that for tadpoles from the forest wet-
lands. Skelly (2004) has shown that compensatory microevolutionary
adjustments can be made by populations of the wood frog to differing
thermal environments separated by tens to hundreds of meters. These
adjustments are rapid and appear to be made within a time frame meas-
ured in decades. Thus, the extirpation and reinvasion of the beaver have
both led to changes in the thermal biology of the wood frog, as in all
probability have landscape changes directly caused by human settlement.
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