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Answers to Thinking Critically Questions
Mader: Inquiry into Life, Twelfth Edition
Chapter 33
1. You are a farmer fighting off an insect crop pest. You know from evolutionary
biology and natural selection that using the same pesticide causes resistance to
evolve in the insect population. Knowing what you know about population
growth and regulation, what are some strategies you might use to control the
insect population?
Explanation/Answer: Because insects frequently follow exponential growth,
reducing insect population sizes causes them to enter the exponential growth
phase. Thus, reducing carrying capacity by increasing environmental resistance is
the best strategy. Predation, competition, and parasitism increase environmental
resistance and thereby reduce population sizes of affected species, and reduce the
carrying capacity of the environment. So, you could introduce natural predators
or competitors against your crop pest in addition to using pesticides. This type of
crop management is called “integrated pest management.”
2. Human population growth is perhaps our biggest environmental concern. The
more people there are, the more of the world’s resources we must use to feed,
shelter, and provide energy for ourselves. Population growth has slowed in
MDCs due to a demographic transition. However, more population growth is
predicted to occur in LDCs. Describe strategies to help stimulate a demographic
transition in LDCs.
Explanation/Answer: There are several strategies that can be employed to
stimulate a demographic transition in LDCs. One strategy includes better family
planning education and more access to birth control. However, to really stimulate
reduced population growth, the death rate must be lowered, particularly infant and
childhood mortality rates. This can be accomplished by providing more access to
health care, and pharmaceutical companies cooperating with governments to
provide medicines at reduced cost to LDCs (which is occurring on a limited scale
with some drugs). Once the death rate is reduced, that may, in part, reduce the
desire for large families. However, people still have large families in rural and
agricultural areas because more people helping to grow crops means larger crop
yield. So, industrial jobs must be created to stimulate a transition from an agrarian
society to an urbanized society. In urban areas, costs of maintaining large
families are much higher than in rural areas, so this could help decrease the
incentive for large families.
3. You find two species of insects, both using bright coloration and similar color
patterns as an antipredator defense. Design an experiment to tell whether this is
an example of Müllerian or Batesian mimicry.
Explanation/Answer: Test the two insects to see if they are poisonous, toxic, or
sting in some way. If both insects are toxic, then it is an example of Müllerian
mimicry. If one is toxic and the other isn’t, then it is an example of Batesian
mimicry. If both are not toxic, then they are probably Batesian mimics of another
species that is toxic.
4. If predators tend to reduce population densities of prey, what prevents predators
from reducing prey populations to such low levels that they drive themselves
extinct?
Explanation/Answer: Predator and prey populations often cycle, as in the case of
the lynx and snowshoe hare. A decline in the prey population usually results in a
decline in the predator population, following a lag time. Thus, with fewer prey,
there are fewer predators, which, in turn, make less of an impact on the prey
population. Then, when prey populations increase again, the food source for the
predators can increase, thereby allowing the predator population to increase.
Another possibility is that predators can switch food sources when their preferred
prey becomes scarcer. In addition, it is possible that prey populations can
“overshoot” the carrying capacity of the environment and then crash to low
numbers. Predators would then follow and decline accordingly. Thus, a decline
in prey population size need not be precipitated by an increase in predator
population size.