Biology Reference
In-Depth Information
tionary dynamics are presumably like those that have occurred in many
coastal areas invaded by sticklebacks.
In western Canada, threespine sticklebacks have colonized many
coastal streams and lakes in postglacial time, where they experience pre-
dation by a diversity of invertebrate and vertebrate predators (Reimchen
1994).The number of lateral plates tends to vary with the type of preda-
tor that predominates. Sticklebacks in lakes that lack larger predatory fish
but have avian predators tend to possess only three or four plates.Where
both predatory fish and birds are present, the number of plates tends to
average seven. Threespine sticklebacks often experience seasonally vari-
able intensities of predation by fish, birds, and dragonfly larvae (Reimchen
and Nosil 2002). Temporal shifts in selection pressures and morphology
associated with these different types of predators can often be recognized.
These patterns of shifting selection may help maintain genetic variability
in defense morphology in many populations.
At Drizzle Lake in the Queen Charlotte Islands of western Canada,
both predatory fish and birds are present, but the seasonal pattern of pre-
dation on sticklebacks by these two predator groups varies. Predation by
cutthroat trout (
Oncorhynchus clarki
) tends to be concentrated in summer,
whereas bird predation can be heavy throughout the year. Only if the lake
becomes ice-covered does bird predation cease. Thus, the mean number
of lateral armor plates for fish, especially subadults, tends to vary season-
ally.When bird predation is heavy, the mean number of plates drops, and
when trout predation predominates, it rises. It appears likely that a large
number of plates gives protection against fish predators, but it may reduce
the ability of sticklebacks to escape diving birds. Clearly, the predation
regime can impose strong selection on the number of armor plates over
short time periods. Other studies have shown that threespine sticklebacks
can respond quickly to change in the predation regime. In northern Rus-
sia, for example, Ziuganov (1995) introduced sticklebacks with heavy
plating to a freshwater pond that was predator-free. After only eight gen-
erations, he found that about 17% of the pond population showed
reduced plate numbers.
Another variable feature of stickleback morphology is the presence or
absence of a pelvic girdle, which supports ventral spines (Ziuganov and
Zotin 1995). In the ninespine stickleback (
Pungitius pungitius
), this struc-
ture is present in coastal marine waters and streams with predatory fish
but reduced or absent in ponds without such predators. Some of these
ponds were isolated about 50 yr earlier by human activities, and it appears
