This is an image of a fox holding a dead animal in his mouth while walking down a gravel road.

What may be the most common way different species interact?

Biomes as different as deserts and wetlands share something very important. All biomes have populations of interacting species. Species interact in the same basic ways in all biomes. For example, all biomes have some species that prey on others for food.

Predation

Biomes as different as grasslands and estuaries share something extremely important. They have populations of interacting species. Moreover, species interact in the same basic ways in all biomes. For example, all biomes have some species that prey on other species for food. Species interactions are important biotic factors in ecological systems. The focus of study of species interactions is the community.

In ecology, a community is the biotic component of an ecosystem. It consists of populations of different species that live in the same area and interact with one another. Like abiotic factors, such as climate or water depth, species interactions in communities are important biotic factors in natural selection. The interactions help shape the evolution of the interacting species. Three major types of community interactions are predation, competition, and symbiosis.

Predation is a relationship in which members of one species (the predator) consume members of other species (the prey). The lions and zebra in Figure below are classic examples of predators and prey. In addition to the lionesses, there is another predator in this figure. Can you find it? The other predator is the zebra. Like the lionesses, it consumes prey species, in this case species of grasses. Predator-prey relationships account for most energy transfers in food chains and webs.

Image: Lionesses feed on the carcass of a zebra.

Types of Predators

The lions in Figure above are true predators. In true predation, the predator kills its prey. Some true predators, like lions, catch large prey and then dismember and chew the prey before eating it. Other true predators catch small prey and swallow it whole. For example, snakes swallow mice whole.

Some predators are not true predators because they do not kill their prey. Instead, they graze on their prey. In grazing, a predator eats part of its prey but rarely kills it. For example, deer graze on plants but do not usually kill them. Animals may also be “grazed” upon. For example, female mosquitoes suck tiny amounts of blood from animals but do not harm them, although they can transmit disease.

Predation and Populations

True predators help control the size of prey populations. This is especially true when a predator preys on just one species. Generally, the predator-prey relationship keeps the population size of both species in balance. This is shown in Figure below. Every change in population size of one species is followed by a corresponding change in the population size of the other species. Generally, predator-prey populations keep fluctuating in this way as long as there is no outside interference.

Image: As the prey population increases, the predator population starts to rise. With more predators, the prey population starts to decrease, which, in turn, causes the predator population to decline. This pattern keeps repeating. There is always a slight lag between changes in one population and changes in the other population.

Some predator species are known as keystone species, because they play such an important role in their community. Introduction or removal of a keystone species has a drastic effect on its prey population. This, in turn, affects populations of many other species in the community. For example, some sea star species are keystone species in coral reef communities. The sea stars prey on mussels and sea urchins, which have no other natural predators. If sea stars are removed from a coral reef community, mussel and sea urchin populations would have explosive growth, which in turn would drive out most other species and destroy the reef community.

Sometimes humans deliberately introduce predators into an area to control pests. This is called biological pest control. One of the earliest pests controlled in this way was a type of insect, called a scale insect. The scale insect was accidentally introduced into California from Australia in the late 1800s. It had no natural predators in California and was destroying the state’s citrus trees. Then, its natural predator in Australia, a type of beetle, was introduced into California in an effort to control the scale insect. Within a few years, the insect was completely controlled by the predator. Unfortunately, biological pest control does not always work this well. Pest populations often rebound after a period of decline.

Adaptations to Predation

Both predators and prey have adaptations to predation. Predator adaptations help them capture prey. Prey adaptations help them avoid predators. A common adaptation in both predator and prey species is camouflage, or disguise. One way of using camouflage is to blend in with the background. Several examples are shown in Figure below.

Image: Can you see the crab in the photo on the left? It is camouflaged against the sand. The preying mantis in the middle photo looks just like the dead leaves in the background. The stripes on the zebras in the right photo blend the animals together, making it hard to see where one zebra ends and another begins.

Another way of using camouflage is to look like a different, more dangerous animal. Using appearance to “mimic” another animal is called mimicry. Figure below shows an example of mimicry. The moth in the figure has markings on its wings that look like the eyes of an owl. When a predator comes near, the moth suddenly displays the markings. This startles the predator and gives the moth time to fly away.

Image: The moth on the left mimics the owl on the right. This “disguise” helps protect the moth from predators.

Some prey species have adaptations that are the opposite of camouflage. They have bright colors or other highly noticeable traits that serve as a warning for their predators to stay away. For example, some of the most colorful butterflies are poisonous to birds, so birds have learned to avoid eating them. By being so colorful, the butterflies are more likely to be noticed—and avoided—by their predators.

Predation, Natural Selection, and Coevolution

Adaptations to predation come about through natural selection. When a prey organism avoids a predator, it has higher fitness than members of the same species that were killed by the predator. The organism survives longer and may produce more offspring. As a result, traits that helped the prey organism avoid the predator gradually become more common in the prey population.

Evolution of traits in the prey species leads to evolution of corresponding traits in the predator species. This is called coevolution. In coevolution, each species is an important factor in the natural selection of the other species. Predator-prey coevolution is illustrated by rough-skinned newts and common garter snakes, both shown in Figure below. Through natural selection, newts evolved the ability to produce a strong toxin. In response, garter snakes evolved the ability to resist the toxin, so they could still safely prey upon newts. Then, newts evolved the ability to produce higher levels of toxin. This was followed by garter snakes evolving resistance to the higher levels. In short, the predator-prey relationship led to an evolutionary “arms race,” resulting in extremely high levels of toxin in newts.

Image: The rough-skinned newt on the left is highly toxic to other organisms. Common garter snakes, like the one on the right, have evolved resistance to the toxin.

Community Ecology II: Predators - Crash Course Ecology #5 (Video)

https://www.youtube.com/watch?v=mFDiiSqGB7M

Summary

A community is the biotic component of an ecosystem. It consists of populations of interacting species. Types of community interactions are predation, competition, and symbiosis.
Predation is a relationship in which members of one species consume members of other species. Predation influences population sizes and coevolution of predator and prey species.

Review

In ecology, what is a community?
Define predation and give an example of a predator and its prey.
Why do species interactions often lead to coevolution of the species involved? Give an example to illustrate your answer.
Assume that a destructive beetle was accidentally introduced to California from Europe. The beetle has no natural predators in California and is becoming a major pest. Describe how biological pest control might be used to control this beetle.

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