Biology Reference
In-Depth Information
areas with pike cichlids to areas without this predator had declined in sur-
vival ability.
In addition to the tests with wild-caught guppies, O'Steen et al. (2002)
reared guppies from the natural and introduced high and low predation
stream situations in the laboratory for two generations.The second-gener-
ation guppies were then tested to determine if their survival abilities when
exposed to pike cichlids were retained. In all tests, guppies from high pre-
dation stream areas, regardless of whether they were natural or introduced,
survived better than guppies from low predation sources. These results
clearly show that natural selection modified genetically based traits related
to predator avoidance within relatively few generations.
Introduced predatory fish may also have secondary impacts on native
predatory species through food competition.The fish fauna of the North
American Great Lakes was altered enormously by the invasion of preda-
tory fish such as the sea lamprey (
Petromyzon marinus
), alewife (
Alosa
pseudoharengus
), and rainbow smelt (
Osmerus mordax
) (Cox 1999). The
lamprey, a predator on large fish, decimated populations of large predatory
fish, driving some species to extinction. The alewife and smelt, both
plankton feeders, competed with other native fish for small prey.
One of the native fish to survive the impacts of these introductions is
the bloater (
Coregonus hoyi
). The larvae of bloaters feed on small zoo-
plankton in pelagic waters, but shift to benthic feeding on larger inverte-
brates as they mature. Following the explosion of alewife populations in
the 1960s, a major shift in feeding pattern occurred (Crowder and Craw-
ford 1984). By 1979 and 1980, sampling of bloater populations showed
that they had shifted from small zooplankton to larger benthic prey about
2 yr earlier than before the alewife explosion. Analysis of foraging behav-
ior of bloaters showed that they were more efficient at harvesting benthic
prey than are alewives (Crowder and Binkowski 1983).
In addition, within less than 20 yr, a morphological shift in the feed-
ing apparatus of bloaters occurred. By 1979, the number of gill rakers of
adult bloaters had declined by about 15% (Crowder 1984). These struc-
tures serve as filters that retain particles in the digestive tract, preventing
their passage into the gill chamber and eventually the outside water.This
shift was correlated with the earlier switch to larger benthic prey. The
most likely reason for the shift is competition for small zooplankton with
the alewife.
Fewer examples of such evolutionary responses exist for terrestrial ver-
tebrates.The spread of the brown-headed cowbird (
Molothrus ater
) to vir-
tually all temperate areas of North America from its original range in the
