Predation and Defense Mechanism

What counts as predation?

A predator is an organism that consumes all or part of the body of another—living or recently killed—organism, which is its prey. "Living or recently killed" distinguishes predators from decomposers, such as fungi and bacteria that break down the leftover remains of organisms that have died.

If we see a lion eating a zebra, we can feel comfortable in saying that the lion is a predator. In the broad definition, however, the zebra is too. Predator's prey can be an animal, but it can also be a plant or fungus. Nor does a predator necessarily have to kill its prey. Instead, as in a grazing cow or a bloodsucking mosquito, it may simply take a portion of the prey's body and leave it alive. Predator-prey relationship in which an animal or insect consumes a plant is called herbivory—herbi- means plant, and -vory means eating. 

Population dynamics of predators and prey

Populations of predators and prey in a community are not always constant over time. Instead, in many cases, they vary in cycles that appear to be related. The most frequently cited example of predator-prey dynamics is seen in the cycling of the lynx, a predator, and the snowshoe hare, its prey. Strikingly, this cycling can be seen in nearly 200-year-old data based on the number of animal pelts recovered by trappers in North American forests.

The population cycles of lynx and hare repeat themselves approximately every 10 years, with the lynx population lagging one to two years behind the hare population. The classic explanation is this: As hare numbers increase, there is more food available for the lynx, allowing the lynx population to increase as well. When the lynx population grows to a threshold level, however, it kills so many hares that the hare population begins to decline. This is followed by a decline in the lynx population due to scarcity of food. When the lynx population is low, the hare population begins to increase—due, at least in part, to low predation pressure—starting the cycle anew.

Defense mechanisms against predation

When we study a community, we must consider the evolutionary forces that have acted—and continue to act!—on the members of the various populations of the community. Species are not static but, rather, change over generations and can adapt to their environment through natural selection.

Predator and prey species both have adaptations—beneficial features arising by natural selection—that help them perform better in their role. For instance, prey species have defense adaptations that help them escape predation. These defenses may be mechanical, chemical, physical, or behavioral.

Mechanical defenses, such as the presence of thorns on plants or the hard shell on turtles, discourage animal predation and herbivory by causing physical pain to the predator or by physically preventing the predator from being able to eat the prey. Chemical defenses are produced by many animals as well as plants, such as the foxglove, which is extremely toxic when eaten. The millipede in the lower panel below has both chemical and mechanical defenses: when threatened, it curls into a defensive ball and makes a noxious substance that irritates eyes and skin.

Many species use their body shape and coloration to avoid being detected by predators. For instance, the crab spider has the coloration and body shape of a flower petal, which makes it very hard to see when it's standing still against the background of a real flower. Can you even see it in the picture below? It took me a minute! Another famous example is the chameleon, which can change its color to match its surroundings. Both of these are examples of camouflage, or avoiding detection by blending in with the background.

Some species use coloration in an opposite way—as a means to warn predators that they are not good to eat. For example, the strawberry poison dart frog shown below has bright coloration to warn predators that it is toxic, while the striped skunk, Mephitis mephitis, uses its bold pattern of stripes to warn predators of the unpleasant odor it produces.

Beyond these two examples, many species use bright or striking coloration to warn of a foul taste, a toxic chemical, or the ability to sting or bite. Predators that ignore this coloration and eat the organism will experience the bad taste or toxic chemicals may learn not to eat the species in the future. This type of defensive mechanism is called aposematic coloration, or warning coloration.

Some species have evolved to mimic, or copy, another species' aposematic coloration—though they themselves may not be bad-tasting or toxic. In Batesian mimicry, a harmless species imitates the warning coloration of a harmful one. If they share the same predators, this coloration protects the harmless species, even though its members do not actually have the physical or chemical defenses of the organism they mimic. For example, many nonvenomous, non-stinging insect species mimic the coloration of wasps or bees.

In Müllerian mimicry, multiple species share the same warning coloration, but all of them actually do have defenses. For example, the figure below shows pairs of foul-tasting butterflies that share similar coloration. Once a predator encounters either member of the pair and discovers its unpleasant taste, it is likely to avoid both species in the future. This similar appearance could have been evolutionarily favored because when members of the two species looked more similar, both would have tended to get eaten at lower rates—thanks to the protection provided by a predator learning to avoid either.