Predator-Prey Interactions

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Predator-prey interactions

The relationship between predators and their prey within an ecosystem is often a quite complex array of offenses and defenses. Predation is the consumption of one living organism by another. Predators must have ways of finding, catching, and eating their prey (the offensive strategies), and prey organisms must have ways of avoiding or discouraging this activity (the defensive strategies). Predator-prey interactions are often simply thought of as one animal, a carnivore, catching and eating another animal, another carnivore or an herbivore. The interrelationships of predators and their prey is much more involved than this and encompasses all levels within an ecological food chain/web , from plant-herbivore systems to herbivore-carnivore systems to three-way interactions of interdependent plant-herbivore-carnivore systems.

Even though a plant may not be killed outright, removing leaves, stems, roots, bark, or the sap from plants will reduce its fitness and ultimately its ability to survive. The younger, more tender leaves and stems of a plant are typically consumed by herbivores. Because these are sites of active growth, the plants send stores of nutrients from roots and other tissues to these developing regions. When they are eaten the plant is losing a disproportionate amount of nutrients that will be difficult to replenish. Severe defoliation results in a tremendous loss of vigor and the loss of the ability to photosynthesize and store energy reserves for the dormant season. Without this energy store available, the plants are more vulnerable to insect or disease attack the following year.

Plants have responded to herbivory in a variety of ways. Some plants have developed spines or thorns, whereas others have developed the ability to produce chemical products which deter herbivory by making the plant unpalatable, barely digestible, or toxic. Many herbivores have found ways to circumvent these defenses, however. Some insects are able to absorb or metabolically detoxify these chemicals from the plants. Other insects have learned to cut trenches in the leaves of plants before they eat them, which stops the flow of these chemical substances to the leaves. Other animals have developed digestive modifications, such as bacterial communities that break down cellulose, to enable them to ingest less nutritious, but more abundant, vegetation.

A classic example of predator-prey interactions in carnivores is the relationship of the lynx, snowshoe hare, and woody browse of northern coniferous forests. The hare feeds on buds and twigs of conifer, aspen, alder, and willow trees. Excessive browsing, as the hare population increases, decreases the subsequent year's growth, initiating a decline in the hare population. The lynx population follows that of the snowshoe hare, having greater reproductive success when more prey are available, then declining as hare populations drop. This relationship is cyclical and follows its course over a ten-year period.

The prey of carnivores, both herbivores and other carnivores, have developed a variety of defensive mechanisms to avoid being detected, caught, or eaten. Some employ chemicals, such as odorous secretions, stored toxic substances ingested from plants, or synthesized venom or poisons. Other animals have taken to hiding from predators through cryptic coloration, patterns, shapes, and postures. Mimicry, widespread in the animal kingdom, is a system whereby one animal, a palatable species , mimics an inedible species, and is thus avoided by the predator. Additional means of predator avoidance or discouragement involves the employment of armor coats or shells, the use of quills or spines, or living in groups. However, predators can benefit prey populations by removing sick or inferior individuals. Subtle, long-term interactions, in fact, can benefit both predator and prey as they co-evolve together into more highly adapted or finelytuned forms.

See also Evolution; Population biology

[Eugene C. Beckham ]