Chapter 14

Predation and Herbivory

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Canada lynx and snowshoe hare. For nearly 100 years, ecologists have been examining the regular fluctuations in the populations of these species to determine the causes.
Photo by Tom & Pat Leeson.

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CHAPTER CONCEPTS

  • Predators and herbivores can limit the abundance of populations.
  • Populations of consumers and consumed populations fluctuate in regular cycles.
  • Predation and herbivory favor the evolution of defenses.

A Century-long Mystery of the Lynx and the Hare

For centuries, naturalists, hunters, and trappers have noticed that populations of many species often experience large fluctuations, and that some species fluctuate at regular intervals. In 1924, the ecologist Charles Elton drew attention to regular population fluctuations in many species of high-latitude animals in Canada, Scandinavia, and Siberia. In particular, he focused on snowshoe hares (Lepus americanus) and Canada lynx (Lynx canadensis). Elton examined data that had been compiled from the Hudson’s Bay Company, a Canadian firm that had been purchasing pelts from trappers for more than 70 years. He assumed that the number of purchased pelts over time reflected the abundance of the two species. Elton and his fellow ecologists were fascinated by regular cycles of high and low density among lynx and hare populations that occurred approximately every 10 years. The 10-year cycles in the abundance of lynx and hares were clear, but the mechanisms causing these cycles have been debated for nearly a century.

“The 10-year cycles in the abundance of lynx and hares were clear, but the mechanisms causing these cycles have been debated for nearly a century.”

There have been numerous hypotheses for these 10-year cycles. When Elton wrote his classic paper, some biologists hypothesized that animals possessed a “physiological rhythm” that caused both the lynx and the hare to reproduce abundantly in some years and sparingly in others. Elton rejected this hypothesis because it was very unlikely that such a rhythm would be synchronized for all individuals of different ages and for all individuals across large regions. Instead, he favored an explanation related to the 9- to 13-year cycle of sunspots, which are periods of increased solar activity. If the cycle of sunspots could substantially affect the climate and, therefore, the growing conditions of the plants that hares consumed, it could explain the cycles of the hares. The lynx cycle, which occurred about 2 years later than the hare cycle, was thought to reflect the fact that lynx primarily consume hares. When hares are abundant, lynx have more food and therefore reproduce more in subsequent years, but when hares are rare, the lynx reproduce poorly and many of them starve, which causes the lynx population to decline.

Since Elton’s original work, ecologists have determined that although the sunspot cycle is similar in length to the hare cycle, it has never closely matched the timing of the hare cycle. Nor have they been able to find a climate-driven mechanism that connects the sunspot cycle to the hare cycle. With these hypotheses eliminated, researchers turned their attention to competition and predation. For many decades a lively debate focused on the possibility that the hare cycles were caused by the hares exceeding their carrying capacity, which could explain the observation that hare reproduction declines as the hare population grows. Another hypothesis suggested that lynx predation caused the cycles. When lynx were rare, the hares survived better. As lynx became more numerous, they began consuming hares faster than the hares could reproduce, thereby causing the hare population to decline.

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It seemed impossible to determine the answer without conducting some experiments. From 1976 to 1985, a large experiment manipulated the presence or absence of supplemental food for the hares. Although the supplemental food increased the carrying capacity for the hare population, it still cycled in synchrony with populations that were not fed. This suggested that the hare population does not decline due to a lack of food. However, if the hares do not experience a lack of food, why does their rate of reproduction decline at higher densities?

In a subsequent experiment, researchers built large fences so they could manipulate the presence or absence of supplemental food and the presence or absence of lynx predation. Both excluding lynx and adding food increased the peak population size of the hares, but the hare populations still cycled. However, the fence that excluded the lynx did not exclude other predators, including owls and hawks, which continued to kill hares. Among the hares that died, more than 90 percent died from predation and few from starvation, which further confirmed that predation rather than food availability contributed to the cyclical decline of hares.

A new insight came in 2009 when researchers discovered that the declining rate of hare reproduction under high hare densities is caused by high densities of predators, which induce high levels of stress in the hares. Eventually, the stress of the predation threat becomes so high that the hares experience reduced reproduction. Once the hare population declines, there are many fewer predators so the hare stress level is much reduced and hare reproduction returns to a high level. In short, while the abundance of food can affect the number of hares in the population, it appears that the lynx-hare population cycles can be attributed to a combination of direct predation and the indirect effects of predator stress that cause reduced hare reproduction.

The century-long investigation of the lynx-hare cycles illustrates that consumers and the resources that they consume can interact in complex and interesting ways. In this chapter, we will examine how predators and herbivores can affect the populations of the species they consume, including how the abundance of predators and prey populations can cycle over time, how consumers catch their prey, and how prey defend themselves.

SOURCES: C. S. Elton, Periodic fluctuations in the numbers of animals: Their causes and effects, British Journal of Experimental Biology 2 (1924): 119–163.

M. J. Sheriff, The sensitive hare: Sublethal effects of predator stress on reproduction in snowshoe hares, Journal of Animal Ecology 78 (2009): 1249–1258.

C. J. Krebs, Of lemmings and showshoe hares: The ecology of northern Canada, Proceedings of the Royal Society B. 278 (2011): 481–489.

Most species consume resources and serve as a resource for other species to consume. For example, plants and algae consume nutrients, water, and light; these resources allow plants and algae to photosynthesize and grow. While plants and algae are alive, they are consumed by herbivores, parasites, and pathogens. After they die, these producers are consumed by detritivores and decomposers. Similarly, animals consume a wide variety of other organisms at the same time that they are subject to consumption by carnivores, parasites, and pathogens. After they die, animals are consumed by scavengers, detritivores, and decomposers. As you can see, a tremendous number of interactions occur among species in nature. These interactions, critical to the composition of species in different communities, are the subjects of the next four chapters.

In this chapter we will focus on interactions between predators and their prey and between herbivores and producers. We will examine the conditions under which predators and herbivores can limit the population sizes of the species they consume. We will also look at models of predators and herbivores to help us understand how populations of these consumers fluctuate in relation to the populations of the species they consume. We will conclude the chapter by exploring how predators and herbivores have favored the evolution of defenses in prey and plants.

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