Chapter 55
When given the opportunity, lizards regulate their body temperature to maintain a
temperature optimal for physiological functioning. Would lizards in open habitats exhibit
different escape behaviors from lizards in shaded forest? (Figure 55.3)
Answer: Very possibly. How fast a lizard runs is a function of its body temperature.
Researchers have shown that lizards in shaded habitats have lower temperatures and thus
lower maximal running speeds. In such circumstances, lizards often adopt alternative
escape tactics that rely less on rapidly running away from potential predators.
If resources became more abundant, would you expect smaller or larger species to
increase in population size more quickly? (Figure 55.10)
Answer: Because of their shorter generation times, smaller species tend to reproduce
more quickly, and thus would be able to respond more quickly to increased resources in
the environment.
Suppose you wanted to keep meadow grass in your room as a houseplant. Suppose, too,
that you wanted to buy an individual plant that was likely to live as long as possible.
What age plant would you buy? How might the shape of the survivorship curve affect
your answer? (Figure 55.12)
Answer: Based on the survivorship curve of meadow grass, the older the plant, the less
likely it is to survive. It would be best to choose a plant that is very young to ensure the
longest survival as a house plant. A survivorship curve that is shaped like a Type I curve,
in which most individuals survive to an old age and then die would also lead you to select
a younger plant. A type III survivorship curve, in which only a few individuals manage to
survive to an older age, would suggest the selection of a middle-aged plant that had
survived the early stages of life since it would also be more likely to survive to old age.
Would natural selection favor producing many small young or a few large ones? (Figure
55.15)
Answer: It depends on the situation. If only large individuals are likely to reproduce (as is
the case in some territorial species, in which only large males can hold a territory), then a
few large offspring would be favored; alternatively, if body size does not affect survival
or reproduction, then producing as many offspring as possible would maximize the
representation of an individual's genes in subsequent generations. In many cases,
intermediate values are favored by natural selection.
Why does the growth rate converge on zero? (Figure 55.18)
Answer: Because when the population is below carrying capacity, the population
increases in size. As it approaches the carrying capacity, growth rate slows down either
from increased death rates, decreased birthrates, or both, becoming zero as the population
hits the carrying capacity. Similarly, populations well above the carrying capacity will
experience large decreases in growth rate, resulting either from low birthrates or high
death rates, that also approach zero as the population hits the carrying capacity.
Why might birthrates be density-dependent? (Figure 55.20)
Answer: There are many possible reasons. Perhaps resources become limited, so that
females are not able to produce as many offspring. Another possibility is that space is
limited so that, at higher populations, individuals spend more time in interactions with
other individuals and squander energy that otherwise could be invested in producing and
raising more young.
What would happen if researchers supplemented the food available to the birds? (Figure
55.21)
Answer: The answer depends on whether food is the factor regulating population size. If
it is, then the number of young produced at a given population size would increase and
the juvenile mortality rate would decrease. However, if other factors, such as the
availability of water or predators, regulated population size, then food supplementation
might have no effect.
Suppose experimenters artificially kept the hare population at a high and constant level;
what would happen to the lynx population? Conversely, if experimenters artificially kept
the lynx population at a high and constant level, what would happen to the hare
population? (Figure 55.24)
Answer: If hare population levels were kept high, then we would expect lynx populations to stay
high as well because lynx populations respond to food availability. If lynx populations were
maintained at a high level, we would expect hare populations to remain low because increased
reproduction of hares would lead to increased food for the lynxes.
Based on what we have learned about population growth, what do you predict will
happen to human population size? (Figure 55.25)
Answer: If human populations are regulated by density-dependent factors, then as the
population approaches the carrying capacity, either birthrates will decrease or death rates
will increase, or both. If populations are regulated by density-independent factors, then if
environmental conditions change, then either both rates will decline, death rates will
increase, or both.
What will the population distributions look like in 20 years? (Figure 55.27)
Answer: The answer depends on whether age-specific birth and death rates stay
unchanged. If they do, then the Swedish distribution would remain about the same. By
contrast, because birthrates are far outstripping death rates, the Kenyan distribution will
become increasingly unbalanced as the bulge of young individuals enter their
reproductive years and start producing even more offspring.
Which is a more important cause of resource depletion, overpopulation or
overconsumption? (Figure 55.29)
Answer: Both are important causes and the relative importance of the two depends on
which resource we are discussing. One thing is clear: The world cannot support its
current population size if everyone lived at the level of resource consumption of people in
the United States.