Biology Reference
In-Depth Information
1954. By the early 1960s, they had become abundant. By 1966, the small-
est mature
Daphnia retrocurva
was 1.05 mm in length. This cladoceran
reproduces parthenogenetically for most of the year, and clones vary in
body size. Thus, it appears that alewife predation was selecting for small,
early maturing clones.
In Belgium, another cladoceran,
Daphnia magna
,was found to show
rapid evolutionary changes in vertical migration behavior, and possible in
morphology, to changing predation patterns due to the introduction of
plankton-feeding fish to a small pond (Cousyn et al. 2001).The pond,
constructed in 1970, acquired a natural population of
Daphnia magna
.For
the first 3 yr, only benthic-feeding fish were stocked. Then, for about a
decade, planktivorous fish were stocked in large numbers. Finally, after the
mid-1980s, stocking was gradually reduced and finally stopped in 1993.
Thus, the predation regime on
Daphnia
changed from light to heavy and
then back to light over about 23 yr. Cousyn and his coinvestigators were
able to recover resting eggs of
Daphnia
from sediment deposits dating
from these three periods, and because these eggs are very long-lived,
establish laboratory populations of parthenogenetic offspring derived
from individual eggs.
The vertical migration behavior of
Daphnia magna
from the three
periods of different predation intensity showed marked differences when
tests were conducted in water containing chemical signals produced by
fish. Most
Daphnia
from the period of heavy predation by plankton-feed-
ing fish migrated downward in the water column in response to light,
whereas almost all of those from periods of low predation intensity did
not. These differences were quite significant, and clearly genetic in their
basis. In addition, the size of the resting eggs produced by animals during
the period of heavy predation was smaller than those of animals from
periods of low predation, a difference that probably also had a genetic
basis. Frequencies of microsatellite alleles, considered to be neutral in
terms of selection by predators, for animals through the three predation
periods showed no change, indicating that factors such as genetic drift
could not account for the genetic changes. Thus, evolutionary changes
had occurred in
Daphnia magna
over periods of only about 7-10 yr as pre-
dation intensity at first increased and then decreased.
Another good example of response by a native invertebrate comes
from New Zealand, where many alien predators have been introduced.
Wetas (Orthoptera: Stenopalmatidae) are large, flightless cricket-like ani-
mals that occur in forest habitats in New Zealand. Introduced mammalian
predators that feed on wetas include black rats (
Rattus rattus
), kiores (
Rat-
