Biology Reference
In-Depth Information
times as great as
Littorina
in the southern Gulf. This suggests that in the
northern Gulf, where green crab populations are smaller and of more
recent arrival, crab predation has not exerted as consistent a selective force
as in the southern Gulf. The evolutionary response of
Littorina
to the
alien green crab is thus surprisingly complex in nature (Trussell and Etter
2001).
In freshwater environments, the introduction of alien fish has led to
evolutionary changes in the behavior of several invertebrates, as noted in
the introductory example. A number of aquatic insects, especially mayfly
larvae, drift-feed in the water column, probably enabling them to capture
certain algal foods that are not readily available in benthic situations.
While they are drifting, they are at increased risk of predation, especially
during the day when many insectivorous fish forage in the water column.
Thus, in streams where insect-feeding fish are present, drift-feeding by
invertebrates tends to be nocturnal. When insect-feeding fish are absent,
drift-feeding tends to occur both at night and during the day. In most
cases, differences in the pattern of drift-feeding appear to be genetic
(Flecker 1992). In some cases, mayflies from streams without fish may shift
from day-and-night drift-feeding to nocturnal feeding when transplanted
to streams with trout (Cowan and Peckarsky 1994).
In Venezuela, Flecker (1992) investigated mayfly drift-feeding in
streams with various numbers of fish that feed on drifting invertebrates.
He noted a very strong positive relationship between the diversity and
abundance of insectivorous fish and the tendency of mayflies to drift-feed
at night. In low-elevation streams with up to 20 species of insectivorous
fish, the abundance of drifting mayflies at night was 100 or more times
that during the day. In high-elevation streams that originally lacked any
insectivorous fish, he found that in streams still lacking such fish mayflies
drift-fed both day and night. In streams to which rainbow trout (
Onco-
rynchus mykiss
) had been introduced within the preceding 60 yr, however,
mayflies showed a stronger tendency to drift-feed at night.
In the North American Great Lakes, the rapidly changing biotic struc-
ture resulting from numerous introductions has probably had more evo-
lutionary impacts than are yet recognized.Wells (1970) noted that follow-
ing introduction of the alewife (
Alosa pseudoharengus
), which feeds largely
on zooplankton, many declines occurred in populations of cladocerans
and copepods. In the case of the cladoceran
Daphnia retrocurva
,however, a
reduction in body size at which individuals matured was observed. Before
the alewife became common, the smallest mature females were 1.26 mm
in length. Alewives appeared in the lake in 1949 but were still rare in
