Predator–Prey Relationships

Introduction

Predator-prey relations refer to the interactions between two species where one species is the hunted food source for the other. The organism that feeds is called the predator and the organism that is fed upon is the prey.

There are literally hundreds of examples of predator-prey relations. A few of them are the lion-zebra, bear-salmon, and fox-rabbit. A plant can also be prey. Bears, for example, feed on berries, a rabbit feeds on lettuce, and a grasshopper feeds on leaves.

Predators and prey exist among even the simplest life forms on Earth, single-celled organisms called bacteria. The bacteria Bdellovibrio feed on other bacteria that are bioluminescent (they produce internal light due to a chemical reaction). Indeed, the study of Bdellovibrio predation has revealed a great deal of the mechanics of predation and how the predator and prey populations fluctuate in number over time in a related fashion.

Predator and prey populations respond dynamically to one another. When the numbers of a prey such as rabbits explode, the abundance at this level of the food chain supports higher numbers of predator populations such as foxes. If the rabbit population is over-exploited or drops due to disease or some other calamity, the predator population will soon decline. Over time, the two populations cycle up and down in number.

In many higher organisms, the prey can be killed by the predator prior to feeding. For example, a cheetah will stalk, run down, and kill its prey (examples include the gazelle, wildebeest, springbok, impala, and zebra). In contrast, fish and seals that are the prey of some species of shark are examples of prey that is fed on while still alive.

The key aspect of a predator-prey relationship is the direct effect that the predation has on numbers of their prey.

Historical Background and Scientific Foundations

Predators and prey have evolved together, and their relationship is ancient. For example, fossils dating back nearly 400 million years have revealed evidence that extinct animals known as Hederellids were the prey of an as yet unknown creature that killed them by drilling holes through their tubular shells.

As species developed and flourished, other species exploited them as their food. A species that has become a successful predator and has survived has developed a few or a number of strategies to acquire the prey. The predator may use speed; stealth (the ability to approach unnoticed by being quiet and deliberate in its movements, or by approaching from upwind); camouflage; a highly developed sense of smell, sight, or hearing; tolerance to poison produced by the prey; production of its own prey-killing poison; or an anatomy that permits the prey to be eaten or digested. Likewise, the prey has strategies to help it avoid being killed by a predator. A prey species can also use the aforementioned attributes listed for the predator to avoid being caught and killed.

The fitness of the prey population—the number of individuals in the population, chance of being able to reproduce, and chance of survival—is controlled by the predator population.

The ways in which predators stalk, kill, and feed on their prey can be used in a classification scheme. A so-called true predator kills the prey and then feeds on it. True predation usually does not involve harm to the prey prior to death. For example, prior to being chased down and killed by a cheetah, a gazelle is healthy. Cattle that graze on grass are not considered a predator-prey relationship, as only a portion of the grass is eaten, with the intact roots permitting re-growth of the grassy stalk to occur.

A predator and its prey can both be microscopic, as is the case with the bacterium Bdellovibrio and other Gram-negative bacteria. But, the size difference between predator and its prey can be immense. An example is the Bowhead whale, which reaches up to 65 ft (20 m) in length, but whose survival is based on straining through its baleen (bony structures in the whale’s jaw) millions of microscopic zooplankton that reach only several centimeters in length.

Predator-prey relationships can be more complex than a simple one-to-one relationship, because a species that is the predator or the prey in one circumstance can be the opposite in a relationship with different species. For example, birds such as the blue jay that prey on insects can become the prey for snakes, and the predatory snakes can be the prey of birds such as hawks. This pattern is known as a hierarchy or a food chain. The hierarchy does not go on indefinitely, and ends at what is described as the top of the food chain. For example, in some ocean ecosystems, sharks are at the pinnacle of the food chain. Other than humans, such so-called apex predators are not prey to any other species. This relationship applies only to the particular ecosystem that the apex predator is in. If transferred to a different ecosystem, an apex predator could become prey. For example, the wolf, which is at the top of the food chain in northern forests and tundra environments, could become the prey of lions and crocodiles if it were present in an African ecosystem.

Predator-prey relationships involve detection of the prey, pursuit and capture of the prey, and feeding. Adaptations such as camouflage can make a prey species better able to avoid detection. By blending into the background foliage or landscape and remaining motionless, an insect or animal offers no visual cue to a predator since it mimics its surroundings. There are many examples of mimicry in predator-prey relationships. Some moths have markings on their outer wings that resemble the eyes of an owl or that make the creature look larger in size. Insects popularly known as walking sticks appear similar to the twigs of the plants they inhabit. Another insect species called the praying mantis appears leaflike. As a final example, the stripes on a zebra are a different form of camouflage that exploits animals’ tendency to herd together. The vertical stripes cause individual zebras in a herd to blend together when viewed for a distance. To a predator like a lion, the huge shape is not recognized as a potential source of food.

Camouflage can also be a strategy used by a predator to avoid detection by prey. An example is the polar bear, whose white color blends in with snow, reducing the likelihood that the bear will be detected as it approaches its prey. In this case, the same strategy and color can be utilized by young seals, since their color allows them to be invisible as they lie on the snowy surface.

WORDS TO KNOW

ECOSYSTEM: The community of individuals and the physical components of the environment in a certain area.

FOOD CHAIN: A sequence of organisms, each of which uses the next lower member of the sequence as a food source.

FOOD WEB: An interconnected set of all the food chains in the same ecosystem.

HABITAT: The natural location of an organism or a population.

SELECTION PRESSURE: Factors that influence the evolution of an organism. An example is the overuse of antibiotics, which provides a selection pressure for the development of antibiotic resistance in bacteria.

The opposite of camouflage can occur. A prey can be vividly colored or have a pattern that is similar to another species that is poisonous or otherwise undesirable to the predator. This sort of strategy, which is known as aposematism, is meant to repel a potential predator based on the predator’s previous undesirable experience with the genuine noxious species.

A successful predator must judge when pursuit of a prey is worth continuing and when to abandon the chase. This is because the pursuit requires energy. A predator that continually pursues prey without a successful kill will soon become exhausted and will be in danger of starvation. Predatory species such as lions are typically inactive during the hot daytime hours, when prey is often also resting, but become active and hunt at night when conditions are less energy taxing and prey is more available. Similarly, bats emerge at night to engage in their sonar-assisted location of insects that have also emerged into the air.

When supplied with food in a setting such as a zoo, predators will adopt a sedentary lifestyle. Predation is an energy-consuming activity that is typically done only when the creature is hungry or to supply food for offspring. In settings such as an aquarium, predators and prey will even co-exist.

Being a prey does not imply that the creature is completely helpless. The prey may escape from the predator by strategies such as mimicry, or can simply outrun or hide from the predator. Some species act coordinately to repel a predator. For example, a flock of birds may collectively turn on a predator such as a larger bird or an animal such as a cat or dog to drive off the predator.

This mobbing type of repulsion can be highly orchestrated. For example, when attacked by an animal such as a dog, mockingbirds have been observed to coordinate their attack, with some birds flying close to the animal’s face with others pestering it from the rear when it lunges in response. As well, some bird species use different calls, which are thought to be a specific signal to other birds in the vicinity to join the attack. Even birds of a different species may respond to such a call.

The fluctuation in the numbers of a predator species and its prey that occurs over time represents a phenomenon that is known as population dynamics. The dynamics can be modeled mathematically. The results show that a sharp increase in the numbers of a prey species (an example could be a rabbit) is followed soon thereafter by a smaller increase in numbers of the relevant predator (in this case the example could be the fox). As the prey population decreases due to predator killing, the food available for the predators is less, and so their numbers subsequently decline. With the predator pressure reduced, the numbers of the prey can increase once again and the cycle goes on. The result is a cyclical rising and falling of the numbers of the prey population, with a slightly later cyclical pattern of the predator.

A famous predator-prey model is the Lotka-Volterra version. The two equations were formulated in the mid-1920s by Italian mathematician Vito Volterra (1860–1940) to explain the decline in a fish population observed in the Adriatic Sea during World War I (1914–1918). At the same time, American mathematician Alfred Lotka (1880–1949) was using the equations to explain the behavior of some chemical reactions. Their efforts were recognized as the Lotka-Volterra model, which represents one of the first examples of ecological modeling.

Other examples include the Kermack-McKendrick model and the Jacob-Monod model (used to model predation of one bacterial species on another).

Impacts and Issues

Predator-prey relations are an important driving force to improve the fitness of both predator and prey. In terms of evolution, the predator-prey relationship continues to be beneficial in forcing both species to adapt to ensure that they feed without becoming a meal for another predator. This selection pressure has encouraged the development and retention of characteristics that make the individual species more environmentally hardy, and thus collectively strengthens the community of creatures that is part of various ecosystems.

For example, lions that are the fastest will be most successful in catching their prey. Over time, as they survive and reproduce, the number of fast lions in the population will increase. Similarly, the superior attributes that enable prey species to survive will be passed on to succeeding generations. Over time, the fitness of the prey population will also increase. Left to operate naturally, the predator-prey relation will be advantageous for the fitness of both species in relation to how they compete against other species in the same ecosystem. However, since each species improves, their relationship with each other remains unchanged, and the challenge remains to kill or escape from being killed.

The fossil record of Hederellids, which date back almost 400 million years, indicate that the survival race between predator and prey has been a driver of evolution perhaps since evolution began. If so, the predator-prey relationship is fundamentally important to life on Earth.

Predator-prey relationships are also vital in maintaining and even increasing the biological diversity of the particular ecosystem, and in helping to keep the ecosystem stable. This is because a single species is kept under control by the species that uses it for food. Without this population check, a species such as a rabbit could explode in numbers, which can destroy the ability of the ecosystem to support the population. A well-known example is the introduction of rabbits to Australia. An initial population of 24 rabbits was introduced in 1788 to permit hunting. In the absence of natural predators, the population rose unchecked, and by 1859 the numbers exceeded tens of millions. The ecological pressure of this immense population has decimated vegetation, leading to erosion, and the over-competition for food has caused the extinction of plants and nearly 10% of the country’s natural mammalian species.

The predator-prey balance of an ecosystem can be disrupted by other changes to the ecosystem including climate related changes such as drought, or human activities that include urban development, foresting, and overuse of resources.

For example, a 2007 study from the Scripps Institution of Oceanography chronicled how overfishing of sharks by people has disrupted the food chain in Caribbean waters. Depriving the food chain of its apex predator causes carnivorous fish that are their usual prey to increase in number, and they in turn decimate the populations of other fish including parrotfish that feed on the algae that grows on the coral in the region. The explosive algal growth can smother the coral.

Modeling of predator-prey population dynamics can be useful in indicating whether the population of a species could tax the capacity of a particular ecosystem to support their numbers. For example, allocating licenses to hunt deer and elk is based on a census of the populations, and modeling. It can be that the reduction of the deer and elk population during annual fall hunting seasons enables the survivors to better make use of the available resources. As well, the information is useful in avoiding the issuing of too many licenses, which could result in a dramatic and harmful reduction in the animal population. Put another way, information on population dynamics is valuable in conservation strategies.

Knowledge of predator-prey relations can be exploited in controlling the numbers of a pest or diseases. For example, a strategy that is being explored in Africa to control the spread of malaria is the release of female mosquitoes that are incapable of breeding. In this case, the mosquito, which can transfer the bacterium responsible for malaria between animals and people or person-to-person when it takes a blood meal, represents the predator and the source of the blood is the prey. By circumventing the production of a new generation of mosquito, the population plummets, leaving insufficient mosquitoes to widely disseminate the disease.

See Also Commercial Fisheries; Ecosystem Diversity; Endangered Species; Extinction and Extirpation; Habitat Loss; Habitat Alteration; Human Impacts; Silent Spring; Species Reintroduction Programs

BIBLIOGRAPHY

Books

Bolen, Eric, and William Robinson. Wildlife Ecology and Management. New York: Benjamin Cummings, 2008.

Chiras, Daniel D., John P. Reganold, and Oliver S. Owen. Natural Resource Conservation: Management for a Sustainable Future. New York: Prentice-Hall, 2004.

Molyneaux, Paul. Swimming in Circles: Aquaculture and the End of Wild Oceans. New York: Thunder’s Mouth Press, 2006.

Brian D. Hoyle