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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.