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tus exulans ), hedgehogs ( Erinaceus europaeus ), stoats ( Mustela erminea ), and
feral cats ( Felis catus ). These predators have eliminated ground-dwelling
wetas on mainland areas and near-shore islands, but a number of prima-
rily tree-dwelling wetas ( Hemideina spp.) have survived both on the main-
land and on islands that have few or no introduced predators (Moller
1985). On islands with mammalian predators, wetas are strongly arboreal,
whereas on islands without such predators they are commonly seen on
the ground. In areas with mammalian predators, they also shelter in tree
holes with constricted openings that prevent predator access, whereas on
predator-free islands they commonly shelter in more open sites. These
behavioral tendencies are apparently genetic, as they are maintained by
animals in the laboratory (Rafanut 1995).
Native parasitoids have in several cases expanded their host use to
include introduced host species. The native North American braconid
wasp Macrocentrus ancylivorous , for example, has expanded its parasitism to
include the oriental fruit moth ( Grapholita molesta ), an alien orchard pest.
Parasitoid insects are also among the species most frequently introduced
for biological control of invertebrate pests, as we noted in chapter 10. In
at least one case, involving pea aphids and an introduced parasitoid (Huf-
bauer 2001), the aphid hosts have shown evolutionary adaptation.We shall
examine this case in detail in chapter 17.
Evolutionary Adaptation to Predators by
Native Vertebrates
The bullfrog ( Rana catesbeiana ), native to eastern North America, has been
introduced to many areas of western North America, where it has
become a serious predator of many native amphibians and reptiles (Cox
1999). Both adult bullfrogs and large tadpoles are predators on the tad-
poles of other frogs. In California and Oregon, the red-legged frog ( Rana
aurora ) has been extirpated in many locations by bullfrog predation.
In Oregon, bullfrogs were introduced in the early 1930s and have
invaded many, but not all, habitats occupied by red-legged frogs. Kiesecker
and Blaustein (1997) examined the behavior and survival of red-legged
frogs from populations that coexisted with bullfrogs versus those from
locations without bullfrogs. Red-legged frogs show antipredator behavior,
typically hiding or a reduction of activity, in response to chemical cues of
predators such as fish. When red-legged frog tadpoles were exposed to
chemical cues of bullfrogs, individuals from populations that coexisted
with bullfrogs showed strong antipredator responses, whereas those from
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