Predation, parasitism, and herbivory are interactions in which one species benefits at the expense of another.

In contrast to competition, some interactions benefit one participant and harm the other. Predation is a type of interaction in which one organism consumes another, its prey (Case 7: Predator–Prey). In this interaction, the predator benefits at the expense of the prey. Early experiments by the Russian biologist Georgii Gause showed that a simple system with one predator and one prey population is inherently unstable. The predator overexploits the prey, driving it to extinction, and then becomes extinct itself.

In 1958, the American scientist Carl Huffaker demonstrated that if the prey had refuges where some individuals could escape from predators, they could persist while predator populations declined. Through time, the prey population would recover and population density would rise to a point where predators would again expand and cause prey abundance to decline—and then decline again themselves (Fig. 47.6). Huffaker showed that predators and prey cycle repeatedly through periods of increasing and then decreasing density, as predators track their prey and some prey escape predation. In other words, a long-term, stable oscillation pattern can be achieved.

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HOW DO WE KNOW?

FIG. 47.6

Can predators and prey coexist stably in certain environments?

BACKGROUND In the 1950s, it was not clear whether a simple system of predators and prey could be stable and coexist indefinitely or whether both groups of species would become extinct as predators consumed all available prey. The results of experiments up to that time were equivocal, and ecologists knew that predators introduced to islands could hunt their prey to extinction.

HYPOTHESIS Ecologist Carl Huffaker hypothesized that predators and prey could stably coexist if temporary refuges were available for the prey.

EXPERIMENT Huffaker studied two kinds of mites: one that feeds on the surface of oranges, and another that preys on the orange-eating mite. Huffaker put two sets of oranges on a table, separating them with barriers of petroleum jelly, essentially making each orange its own habitat patch. He added toothpicks to one set of oranges so that the orange-eating mites could let out silk and float over the petroleum jelly barriers and, at least temporarily, escape predation.

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FIG. 47.6

RESULTS In the setup in which prey could not escape, a single cycle of increase and decline was observed: The population size of prey increased but was closely tracked by rising populations of their predators, and eventually both declined to extinction.

In experiments in which toothpicks were provided, the orange-feeding mites climbed the toothpicks and dispersed on silken threads they produced, moving away from predators and toward other oranges without mites. These populations of predators and prey went through three cycles of increase and decline, and cycles would probably have continued if oranges continued to be supplied.

CONCLUSION Predator–prey systems can be stable if there are sufficient areas available where prey can escape predators, at least temporarily.

SOURCE Information from Huffaker, C. B. 1958. “Experimental Studies on Predation: Dispersion Factors and Predator–Prey Oscillations.” Hilgardia: A Journal of Agricultural Science 27: 795–834.

Predators can limit the population sizes of their prey, preventing prey populations from increasing to the level where competitive exclusion occurs. We have seen that when species overlap in resource use and their populations increase to the point where they compete for resources, they must be separated in space or time or risk local extinction through competitive exclusion when resources become limited.

If, however, populations do not rise to densities at which resources are limiting, competition is reduced and species can overlap in niches without excluding one another. For example, in ponds throughout the southern United States, tadpoles of the large Southern Toad, the Eastern Spadefoot, and the tiny Spring Peeper all use the same food source, grazing on algae as do other tadpoles. In the presence of the two larger species, Spring Peeper tadpoles compete poorly for food (Fig. 47.7). As a result, they have low survival rates, and those individuals that do make it to maturity are often undersized because as tadpoles they did not have access to adequate nutrition.

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FIG. 47.7 Effect of predators on the outcome of competition. Newt predators in ponds prefer to eat bigger tadpoles that compete well for food, enabling smaller tadpoles that are poorer competitors to survive to adulthood. Data from P. J. Morin, 1981, “Predatory Salamanders Reverse Outcome of Competition Among Three Species of Anuran Tadpoles,” Science 212:1284–1286.
Data from P. J. Morin, 1981, “Predatory Salamanders Reverse Outcome of Competition Among Three Species of Anuran Tadpoles,” Science 212:1284–1286.

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In the presence of a predatory Red-spotted Newt, however, the tables are turned. The newt prefers to eat the larger tadpole species, and so prevents them from becoming sufficiently abundant to outcompete the smaller Spring Peeper tadpoles. The Spring Peeper tadpoles, then, have the highest survival rates. With competition for food reduced, these small tadpoles grow to their maximal body size, and become the most abundant tadpole species in the ponds.

Parasites live in close association with another species, gaining nutrition by consuming their hosts’ tissues. Unlike predators, parasites commonly do not kill their hosts, but they can reduce host fitness by tapping its resources. In this way, parasites can limit the population size of their host, keeping numbers well below the carrying capacity of the environment.

An extreme example is the fungus that infects the American chestnut, a tree once dominant in eastern North American forests. The fungus attacks the vascular system of young chestnut trees, which usually succumb when they reach 2 to 3 m in height. Today, there are only a few isolated patches of large chestnut trees in Vermont and a handful of other places that have escaped the fungus. As chestnut populations have declined, populations of oak, beech, and other trees that grow in the same forest have increased.

Herbivory, the consumption of plant parts, benefits herbivorous animals by providing some nutrients, and harms plants by directly affecting the products of photosynthesis. Plants are generally not as nutritious for animals as other animals are, and so herbivorous animals must eat more plant material to obtain the nutrients they need for survival and reproduction. As we saw in Chapter 32, plants are not passive victims of herbivores; most are well defended with chemical or physical deterrents to herbivory.