The interaction between predators and prey, including parasites and hosts, not only influences the adaptations that species develop but also their population dynamics. Some predator–prey populations are linked and oscillate in response to one another. What controls such patterns of cycling?

Consider the classic example of predator–prey cycling in the Canada lynx (Lynx canadensis) and the snowshoe hare (Lepus americanus), which makes up a large portion of the lynx diet. Long-term records from Hudson’s Bay Company, a Canadian retailer with a long history in the fur-trading business, show that the numbers of lynx and hares trapped over time reflect a regular cycle in the population sizes of each species, with peak abundances occurring every 10 years but falling to low abundances in the intervening years (Figure 55.9). In addition, the peak in the lynx population lags behind that of the hare population by 2–3 years. Models of predator–prey interactions suggest that this cycling is the result of a tight coupling between prey and predator: as prey increase in abundance, predators do as well, causing prey numbers to decline as a result of increased mortality. When prey decline, so do predators, causing both predator and prey populations to decrease. At some point, the prey population starts to increase again due to the lack of predation, which then allows the predator population, which lags behind, to increase as well. However, in the case of lynx and hare populations, it is likely more complicated. A long-term study by Charles Krebs and colleagues showed that if lynx are absent or food is added for the hares, predator and prey populations continue to cycle, suggesting that the underlying cause of the cycling may be controlled by environmental factors such as large-scale variability in climate.

Question

The peak number of lynx usually occurs after the peak number of hares. One reason this might occur is that as hare abundance rises, lynx have more prey and thus can produce more offspring. However, these offspring are not born immediately, so the rise in lynx numbers lags behind the rise in hare numbers.