The definition of an ecosystem is a community of different species and populations of organisms interacting with each other and their environment in a particular geographical area on Earth. Ecosystems account for all relationships between living and nonliving things.
One way to describe some of the relationships in an ecosystem is through a food chain or a food web. Food chains describe a hierarchal systems or series that show and describe the relationships between organisms in terms of which organisms are eaten by those higher on the food chain.
Another way to describe what you can see on a food web is through predator-prey relationships. These relationships, also described as predation, occur when one organism (the prey) is eaten by another organism (the predator). In relation to the food chain, the organism one step higher on the hierarchy is considered a predator of the organism (or the prey) a step below them on the hierarchy.
Definition of Predation
Symbiotic relationships describe long-term and close relationships between organisms of different species. Predation is a specific type of symbiotic relationship because the predator and prey relationship is a long-term and close one within an ecosystem.
Specifically, predation is defined as one part of a symbiotic relationship when an organism is a predator against a different species of organism, called the prey, where they capture and eat that organism for energy/food.
Types of Predation
Within the term predation are specific kinds that are defined by how the predator-prey interactions and relationship dynamics work.
Carnivory. Carnivory is the first type of predation that is most commonly thought of when we think of predator and prey relationships. As the name suggests, carnivory is a type of predation that involves the predator consuming the meat of other animals or non-plant organisms. Organisms that prefer to eat other animal or insect organisms are thus called carnivores.
This type of predation and the predators that fall within this category can be further broken down. For example, some organisms must eat meat in order to survive. They're called obligatory or obligate carnivores native lions. Examples include members of the cat family, such as mountain lions, cheetahs, Africa native lions and house cats.
Facultative carnivores, on the other hand, are predators that can eat meat to survive, but they don't need it to survive. They can also eat non-animal food like plants and other types of organisms to survive. Another word for these types of carnivores is omnivores (meaning they can eat anything in order to survive). People, dogs, bears and crayfish are all examples of facultative carnivores.
Examples of carnivory include wolves eating deer, polar bears eating seals, a venus fly trap eating insects, birds eating worms, sharks eating seals and people eating meat from animals like cattle and poultry.
Herbivory. Herbivory is the a type of predation where the predator consumes autotrophs like land plants, algae and photosynthetic bacteria. Many don't consider this to be a typical predator-prey type since predation colloquially is associated with carnivory. However, since one organism is consuming another, herbivory is a type of predation.
The term herbivory is most commonly used as a descriptor for animals that eat plants. Organisms that eat plants only are called herbivores.
As with carnivory, herbivory can be divided into subtypes. Organisms that eat both plant and animal food are not considered herbivores since they don't solely eat plants/autotrophs. Instead, they're called omnivores or facultative carnivores (as was previously discussed).
The two main subtypes of herbivory are monophagous and polyphagous herbivores. Monophagous herbivory is when the predator species eats solely one type of plant. A common example would be a koala bear that only eats leaves from trees.
Polyphagous herbivores are species that eat multiple kinds of plants; most herbivores fall under this category. Examples include deer eating multiple types of grasses, monkeys eating various fruits and caterpillars that eat all types of leaves.
Parasitism. Both herbivory and carnivory require the organism being preyed upon to die in order for the predator to gain their nutrients/energy. Parasitism, however, does not necessarily require death of the prey (although it is often a side effect of the relationship).
Parasitism is defined as a relationship where one organism, called the parasite, benefits at the expense of a host organism. Not all parasitism is considered predation since not all parasites feed off of their host. Sometimes parasites use the host for protection, shelter or reproductive purposes.
In terms of predation, the parasite would be considered the predator while the host organism would be considered the prey, but the prey doesn't always die as a result of the parasitism.
A common example of this head lice. Head lice use the human scalp as a host and feed off of the blood on the scalp. This causes negative health effects (itching, scabs, dandruff, death of tissue on the scalp and more) for the host individual, but it doesn't kill the host.
Mutualism. Mutualism is another predator-prey relationship that doesn't result in the death of the prey. It describes a relationship between two organisms where both organisms benefit. Most mutualistic relationships are not examples of predation, but there are a few examples of this.
The most common example involves the endosymbiotic theory where one unicellular organism may have engulfed (a.k.a., ate) what we now know as mitochondria and chloroplasts. Current theories say that mitochondria and chloroplasts were once free-living organisms that were then eaten by larger cells.
They then became organelles and benefitted from the protection of the cell membrane while the organisms that engulfed them gained an evolutionary advantage of performing photosynthesis and cellular respiration.
Predator-Prey Relationships, Population Cycles and Population Dynamics
As you now know, predators are higher in the food chain than their prey. Most predators are considered to be secondary and/or tertiary consumers, although primary consumers that eat plants could be considered predators under the definition of herbivory.
Prey almost always outnumbers predators, which relates back to the concept of energy flow and the energy pyramid. It's estimated that only 10 percent of energy flows or is transferred between trophic levels; it makes sense that top predators are lower in numbers since there's not enough energy that can flow to that top level to support larger numbers.
Predator-prey relationships also involved what's known as predator-prey cycles. This is the general cycle:
Predators keep prey populations in check, which allows the number of predators to increase. This increase results in a decrease in prey populations as the predators consume the prey. This loss of prey then leads to a decrease in predator numbers, which allows prey to increase. This continues is a cycle that allows the ecosystem overall to stay stable.
An example of this is the relationship between the wolf and rabbit populations: as rabbit populations increase, there is more prey for wolves to eat. This allows the wolf population to increase, which means more rabbits must be eaten to support the larger population. This will cause the rabbit population to decrease.
As the rabbit population decreases, the larger wolf population can no longer be supported because of a lack of prey, which will cause death and a decrease in overall wolf numbers. Fewer predators allows more rabbits to survive and reproduce, which increases their population once again, and the cycle is back to the beginning.
Predation Pressure and Evolution
Predation pressure is one of the main influences on natural selection, which means that it also has a huge influence on evolution. Prey must evolve defenses to fight or avoid potential predators in order to survive and reproduce. In turn, predators must evolve ways to overcome those defenses in order to get food, survive and reproduce.
For prey species, individuals without these advantageous traits to avoid predation are more likely to be killed by predators, which drives natural selection of those favorable qualities for prey. For predators, individuals without advantageous traits that allow them to find and capture prey will die, which drives natural selection of those favorable qualities for predators.
Defensive Adaptations of Prey Animals and Plants (Examples)
This concept is most easily understood with examples. These are the most common examples of predation-fueled adaptations:
Camouflage. Camouflage is when organisms can use their coloring, texture and general body shape in order to blend in with their surroundings, which helps them avoid being spotted and eaten by predators.
An amazing example of this would be various species of squid that can change their appearance based on their environment to essentially become invisible to predators. Another example is the coloring of eastern American chipmunks. Their brown fur allows them to blend in to the forest floor, which makes them harder for predators to spot.
Mechanical. Mechanical defenses are physical adaptations that protect both plants and animals from predation. Mechanical defenses can make it hard or even impossible for potential predators to consume the organism, or they could cause physical harm to the predator, which makes the predator avoid that organism.
Plant mechanical defenses include things like thorny branches, waxy leaf coatings, thick tree bark and spiny leaves.
Prey animals can also have mechanical defenses to work against predation. Turtles, for example, have evolved their hard shell that makes them hard to eat or kill. Porcupines evolved spikes that make them both hard to consume and that can cause physical harm to potential predators.
Animals can also evolve the ability to outrun predators and/or to fight back (through biting, stinging, and so on) against predators.
Chemical. Chemical defenses are adaptations that allow organisms to use chemical adaptations (as opposed to physical/mechanical adaptations) to defend themselves against predation.
Many plants will contain chemicals that are toxic to predators when consumed, which leads to predators avoiding that plant. An example of this is the foxglove, which is toxic when eaten.
Animals can evolve these defenses, too. An example is the poison dart frog that can secrete toxic poison from glands on the skin. These toxins can poison and kill predators, which results in those predators usually leaving the frog alone. The fire salamander is another example: They can secrete and squirt a nerve poison out of special glands, which can injure and kill potential predators.
Other common chemical defenses include chemicals that make the plant or animal taste or smell bad to predators. This helps prey avoid predation as predators learn to avoid organisms that smell or taste bad. A prime example is the skunk that can spray a foul-smelling liquid to deter predators.
Warning Signals. While the color and look of organisms is often used as a way to blend into the environment, it can also be used as a warning to stay away to reduce predation risk.
This is called warning coloration, and it is usually bright, like poisonous frogs of the rainforest or bright stripes of venomous snakes, or bold in pattern, like the black and white stripes of the skunk. These warning colors are often accompanied with defenses like a foul smell or toxic chemical defenses.
Mimicry. Not all organisms actually evolve these types of defenses. Instead, some rely on mimicking those that do in hopes it will confuse predators.
For example, the venomous coral snake has distinctive red, yellow and black striping that acts as warning coloration against predators. Other snakes like the scarlet king snake have evolved to also have this striping, but they're actually harmless and non-venomous. The mimicry gives them protection since predators now think they're actually dangerous and should be avoided.
Predators also adapt in order to keep up with the adaptations of their prey. Predators can use camouflage in order to hide from prey and make a surprise attack, which can help them catch their prey and avoid any dangerous defenses the prey might have.
Many predators, especially large predators at higher trophic levels, evolve superior speed and strength along with other mechanical adaptations that allow them to overtake their prey. This can include the evolution of "tools" that help them overcome mechanical and chemical defenses like thicker skin, sharp teeth, sharp claws and more.
Chemical adaptations also exist in predators. Instead of using poison, venom, toxins and other chemical adaptations as defenses, many will use these adaptations for the purpose of predation. Venomous snakes, for example, use their venom to take down prey.
Predators can also evolve chemical adaptations that allow them to overcome chemical defenses of their prey. For example, milkweed is a poisonous plant to almost all herbivores and omnivores. Monarch butterflies and caterpillars, however, eat only milkweed and have evolved to not be affected by the poison. In fact, this also gives them a chemical defense as the milkweed toxins that get on the butterflies make them unappetizing to predators.
Articles Related to Predation:
- Mayo Clinic: Head Lice
- Arizona State University: Ask a Biologist: Cells Living in Cells
- Scientific American: How Do Squid and Octopuses Change Color?
- Wildlife Waystation: Animal Defense Against Predators
- Lumen: Plant Defense Mechanisms
- National Park Service: Warning Coloration
- Florida Museum: Compare Eastern Coral Snake, Scarlet Kingsnake, and Scarlet Snake
- National Wildlife Federation: Milkweed for Monarchs