Biology 12

0

1845 1865 1875

50

1855

100

150

1885 1895 1905 1915 1925 1935

Number of pelts (in thousands)

lynx

hare

Chapter 14 Population Ecology • MHR 485

Interaction of Predator

and Prey Populations

Some populations, especially those of certain
insects, birds, and mammals, fluctuate regularly in
density. These alternating periods of high and low
populations are often referred to as population
cycles. While some small herbivorous mammals,
such as voles, have 3- to 5-year cycles, larger
herbivorous mammals, such as snowshoe hares
(Lepus americanus) and muskrats (Ondatra
zibethica), have 9- to 11-year cycles. These longer
cycles are also typical of some birds, including
ruffed grouse (Bonasa umbellus). The causes of
cycles vary with the species and from population
to population within a single species. Some may be
due to a lag in response to density-dependent
factors, as discussed in section 14.2. If such a lag
is fairly constant, a more or less regular cycle of
fluctuation above and below the population’s
carrying capacity could result.
Some of the mammal species that display
fluctuations in population density are predators
that have cycles overlapping those of their prey
(such as some of the herbivores already described).
One explanation for these cycles is the density-
dependent effect of each population on the other.
For example, some populations of Canada lynx
prey almost exclusively on snowshoe hare (see
Figure 14.24). An increase in the hare population
would reduce competition for food among the lynx
(Lynx canadensis), allowing them to increase their
reproduction rate and survive longer. The result
would be an increase in the population density of
lynx. However, the presence of a large number of
these predators would eventually cause the hare
population to decrease. This, in turn, would increase

competition among lynx for food, causing a decline

in the predator population and permitting the prey

population to expand once again.

However, there is more to this story than just the

relationship between predator and prey. Hare

populations on arctic islands where there are no

lynx also undergo a cycle, indicating that it is not

simply the effect of predators that causes hare

populations to increase or decrease. An alternative

hypothesis for fluctuations in the size of snowshoe

hare populations is that the grazing activity of a

large number of these herbivores causes serious

damage to the plants (especially willows) that they

eat. When the hare population is small, only a small

portion of each plant is consumed. The plants can

maintain high survival and reproduction rates in

this situation, resulting in high plant density. This, in

turn, allows the hare population to increase, perhaps

to a point where their grazing damages the plants,

thereby lowering plant survival and reproduction.

The hare population will then decline because of a

decrease in their food supply. A long-term research

Some of the most remarkable population cycles in the world

are those displayed by various species of cicadas (insects in

the order Homoptera). The life cycle of these species takes

place mostly underground, and requires 13 to 17 years to

complete. At the end of this period, they emerge as adults

in extremely high densities (as high as 600 per m^2 ). When

the adults lay their eggs and die, the aboveground

population shrinks to virtually nothing. The long life cycle

may be an adaptation to reduce predation. Since these

species are around for such a short time, few predators

have learned how to prey on them efficiently.

BIO FACT

Figure 14.24This graph shows the

number of Canada lynx and snowshoe

hare pelts traded annually to the

Hudson’s Bay Company over a 100-year

period in Canada’s arctic (data which

allowed biologists to estimate the

population sizes for both species). Note

that approximately every 10 years,

populations of both species become

very large, decline, and then become

large again. These cycles seem much

too regular to be explained by randomly

occurring abiotic factors.

Continued on page 488 ➥