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Predation & herbivory

Predators and prey. Adaptations of predators that help them catch prey, and adaptations of prey that help them escape predators.

Key points

  • Predation is an interaction in which one organism, the predator, eats all or part of the body of another organism, the prey.
  • Herbivory is a form of predation in which the prey organism is a plant.
  • Predator and prey populations affect each other's dynamics. The sizes of predator and prey populations often go up and down in linked cycles.
  • Predators and prey often have adaptations—beneficial features arising by natural selection—that are related to their interaction. For prey, these include various defenses and warning signals, such as bright coloration.


If you were asked to name one way that different species interact in nature, predation might be the first thing that comes to mind. After all, many of us have watched bears catching salmon, lions eating zebras, or octopuses capturing prey on the nature channel. In fact, this was the only television channel I was allowed to watch as a kid—I thought it was amazing!
In predation, a predator eats all or part of the body of its prey, with a positive (+) effect on the predator and a negative (-) effect on the prey. Nature shows on television highlight the drama of one animal killing another, but predation can also take less obvious forms. For instance, when a mosquito sucks a tiny bit of your blood, that can be viewed as a form of predation. So can herbivory, in which an animal—say, a cow or a bug—consumes part of a plant.1
In this article, we'll take a closer look at predation: the different forms it can take, how it can affect predator and prey populations, and how natural selection has shaped the features of predators and prey.

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.2
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!1 A 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.1 A 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.
Top panel: the graph plots number of animals in thousands versus time in years. The number of hares fluctuates between 10,000 at the low points and 75,000 to 150,000 at the high points. There are typically fewer lynxes than hares, but the trend in number of lynxes follows the number of hares.
Bottom panel: photographs of lynx and hare
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.
Today, ecologists no longer think that the cycling of the two populations is entirely controlled by predation. For instance, it appears that availability of plant foods eaten by the hares—which decreases when hares become too abundant, due to competition—may also be a factor in the cycle.3 Some recent studies have also shown that stress, whether due to predation, low food availability, or other density-dependent factors, may directly reduce the fecundity—reproductive output—of female hares.4 Still, the predator-prey interaction between lynx and hare is a clearly a key element of the cycle.

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.
The top left image shows the long, sharp thorns of a honey locust tree. The top right image shows the domed shell of a tortoise. The bottom left image shows the pink, bell-shaped flowers of a foxglove. The bottom right image shows a millipede curled into a ball.
Image credit: Community ecology: Figure 3 by OpenStax College, Biology, CC BY 4.0; top left , modification of work by Huw Williams; top right, modification of work by “JamieS93”/Flickr; bottom left, modification of work by Philip Jägenstedt; bottom right, modification of work by Cory Zanker
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.
A photograph of a yellow crab spider on a yellow flower.
Image credit: Community ecology: Figure 4 photograph by David Rintoul CC BY 4.0
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.
Left image shows a bright red frog sitting on a leaf. Right image shows a skunk.
Image credit: Community ecology: Figure 5 photograph by OpenStax College, Biology, CC BY 4.0; left, modification of work by Jay Iwasaki; right, modification of work by Dan Dzurisin
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.
The photograph on the left is a bumblebee on a flower with the label bumblebee, actually stings. The photograph on the right is a fly that has a similar appearance to the bumblebee and is labeled Robber fly, bumblebee mimic.
Image credit: Community ecology: Figure 6 by OpenStax College, Biology CC BY 4.0; images modified from work by Cory Zanker
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.
A photograph of 4 pairs of butterflies. Each pair of butterflies looks very similar to the other.
Image credit: Community ecology: Figure 6 by OpenStax College, Biology CC BY 4.0; image from Joron M., Papa R., Beltrán M., Chamberlain N., Mavárez J., et al.
This is just a sampling of the many adaptations that have evolved in prey species to minimize predation. Of course, predators also have their own set of adaptations to maximize the capture of prey, such as sharp claws and teeth, fast running speed, and coloring that provides camouflage, allowing the predator to lie in wait for the prey.5 In a sense, this is an evolutionary arms race in which both sides must up the ante just to stay in the game.1

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