Page 97 - Environment: The Science Behind the Stories

Figure 4.4 Predator–prey systems occasion-
160,000
ally show paired cycles, in which changes in a
population of one species help drive changes
140,000
Numbers of hares and lynx 120,000 Hare Lynx of snowshoe hares and the lynx that prey on them in
in a population of the other. A classic case is that
Canada. These data come from fur-trapping records
100,000
of the Hudson Bay Company and represent numbers
80,000
of each animal trapped. Data from MacLulich, D.A., 1937.
Fluctuation in the numbers of varying hare (Lepus americanus).
60,000
40,000
sity of Toronto Press.
20,000 University of Toronto Studies in Biology Series 43, Toronto: Univer-
0
1850 1860 1870 1880 1890 1900 1910 1920 1930 1940
Year

Several types of interactions the Great Lakes and Hudson River. Zebra mussels also consume
are exploitative the smaller types of zooplankton, and this predation has dimin-
ished zooplankton populations by up to 70% in Lake Erie and
In competitive interactions, each participant exerts a negative the Hudson River. Most predators are also prey, however. Zebra
effect on other participants, because each takes resources the mussels have become a food source for muskrats; crayfish; a
others could have used. This is reflected in the two minus signs number of North American diving ducks; and a variety of fish,
shown for competition in Table 4.1. In other types of interac- including flounder, sturgeon, eels, carp, and freshwater drum.
tions, some participants benefit while others are harmed; that Predation can sometimes drive cyclical population dynam-
is, one species exploits the other (note the +/– interactions in ics. An increase in the population size of prey creates more
Table 4.1). Such exploitative interactions include predation, food for predators, which may survive and reproduce more
parasitism, and herbivory. effectively as a result. As the predator population rises, intensi-
fied predation drives down the population of prey. Diminished
Predators kill and consume prey numbers of prey in turn cause some predators to starve, and the
predator population declines. This allows the prey population
Every living thing needs to procure food, and for most animals, to rise again, starting the cycle anew (Figure 4.4).
that means eating other living organisms. Predation is the process Predation also has evolutionary ramifications. Individual
by which individuals of one species—the predator—hunt, cap- predators that are more adept at capturing prey will likely live
ture, kill, and consume individuals of another species, the prey. longer, healthier lives and be better able to provide for their off-
Interactions among predators and prey structure the food webs spring. Natural selection (pp. 68–71) will thereby lead to the evo-
we will examine shortly, and they help shape community compo- lution of adaptations (p. 68) that make predators better hunters.
sition by influencing the relative numbers of predators and prey. Prey face an even stronger selective pressure—the risk of immedi-
Predation by zebra mussels on phytoplankton has reduced ate death. As a result, predation pressure has driven the evolution
phytoplankton populations by up to 90%, according to studies in of an elaborate array of defenses against being eaten (Figure 4.5).

A gecko’s camouflage A yellowjacket signals When threatened, a
coloration hides it from that it is dangerous. caterpillar swells its
predators. tail, with false
eyespots, looking like
a snake’s head.

(a) Crypsis (b) Warning coloration (c) Mimicry
Figure 4.5 Natural selection to avoid predation has resulted in fabulous adaptations. Some prey spe-
cies use cryptic coloration (a) to blend into their background. Others are brightly colored (b) to warn predators
96 they are toxic, distasteful, or dangerous. Others use mimicry (c) to fool predators.

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