p. 148 # 17, 20, 21, 24 p. 148 # 17, 20, 21, 24 17. What would happen to a population of predators if there was a sudden increase in food for the prey? Explain. 17. If there is a sudden increase in food for a population of prey, the prey population will increase. This means there will be more food for the predators, which will then also increase. 20. How is the relationship between parasites and their hosts similar to a predator-prey relationship? 20. The relationship between parasites and their hosts is similar to a predator-prey relationship because in both cases the two species involved limit one another’s population growth. p. 148 # 17, 20, 21, 24 21. How would a drop in the water level of a river affect a fish population living in that river? 21. A drop in the water level of a river would affect a fish population in that river because the fish population’s density increases. This makes the fish population more susceptible to density-dependent limiting factors. 24. How can you account for the fact that the human population has grown more rapidly during the past 500 years than it has at any other time in history? 24. The human population began growing more rapidly 500 years ago due to conditions that made survival more likely, thereby decreasing the death rate. Reliable food supply, improved sanitation, and better medical care are three factors that caused this change.