Title: Unit 6 Learning Module
Course: BIS 105 Environmental Biology > Section: BIS 105 Environmental Biology

Unit 6 Learning Module

Predation
Copyright
Adapted from CK-12 Foundation, - Biology I Honors (CA DTI-3) by Douglas Wilkin .
Adapted from CK-12 Foundation, - Biology .
Adapted from OpenStax College - Threats to Biodiversity .

What Is a 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.

Predation

Predation is a relationship in which members of one species (the predator) consume members of other species (the prey). The predator always lowers the prey’s fitness. It does this by keeping the prey from surviving, reproducing, or both. Predator-prey relationships are essential to maintaining the balance of organisms in an ecosystem.

There are different types of predation, including:

- parasitism

- true predation

- herbivory

Parasitism is a type of symbiotic relationship and will be described later in the symbiosis concept.

True predation is when a predator kills and eats its prey. Some predators of this type, such as jaguars, kill large prey. They tear it apart and chew it before eating it. Others, like bottlenose dolphins or snakes, may eat their prey whole. In some cases, the prey dies in the mouth or the digestive system of the predator. Baleen whales, for example, eat millions of plankton at once. The prey is digested afterward. True predators may hunt actively for prey, or they may sit and wait for prey to get within striking distance.

The lions and cape buffalo pictured below are classic examples of predators and prey. In addition to the lions, there is another predator in this figure. Can you find it? The other predator is the cape buffalo. Like the lion, it consumes prey species, in this case species of grasses. Predator-prey relationships account for most energy transfers in food chains and webs.

An adult male lion and a lion cub feed on the carcass of a South African cape buffalo.

Predation and Populations

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 the 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.

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.

In herbivory, the predator eats part of the prey but does not usually kill it. You may have seen cows grazing on grass. The grass they eat grows back, so there is no real effect on the population. In the ocean, kelp (a type of seaweed) can regrow after being eaten by fish. Insects are the most widespread type of herbivore. In response, plants have evolved a variety of defenses to protect against herbivory. Spines, thorns, and toxic or distasteful chemicals are strategies plants have developed to reduce being grazed upon.

Predators play an important role in an ecosystem. For example, if they did not exist, then a single species could become dominant over others. Herbivores on a grassland keep grass from growing out of control. Predators can be keystone species. These are species that can have a large effect on the balance of organisms in an ecosystem. For example, if all of the wolves are removed from a population, then the population of deer or rabbits may increase. If there are too many deer, then they may decrease the amount of plants or grasses in the ecosystem. Decreased levels of producers may then have a detrimental effect on the whole ecosystem. In this example, the wolves would be a keystone species. Some starfish species are keystone species in the intertidal zone along Washington's Pacific coastline. The starfish prey on mussels, which have no other natural predators. If starfish are removed from the community, mussel populations would have explosive growth, which in turn would drive out most other species and drastically alter 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.