Organism Interactions

  • Organisms within an ecosystem interact with one another in many different ways. These interactions play an important role in the survival of the organisms and the function of the ecosystem. Organisms can affect one another directly, through a shared resource, or through common enemies. Some interactions are harmful to the organisms involved, whereas others provide benefits for one or both of the organisms.

Food Chains & Food Webs

Food chains or food webs describe the eating relationships between species and represent the flow of energy through an ecosystem. The Sun provides energy to producers, such as plants. Producers convert this energy into a form that can be consumed by animals. Animals that eat producers are then consumed by animals at higher trophic levels. Eventually all organisms are broken back down into nutrients by decomposers. Producers use these nutrients to convert solar energy into a usable form and the food chain continues. Food chains and food webs describe interactions between organisms at different trophic levels.

A Walk in the Forest


Symbiosis is an interaction between individuals of different biological species. At least one of the organisms receives a benefit from the interaction. The other organism can either receive a benefit, be harmed, or not be affected in any way.
A symbiotic interaction involves a close relationship between the two organisms involved. There are three main kinds of symbiotic relationships: commensalism, mutualism, and parasitism.
external image 6309Symbiosis.png
Commensalism is a kind of symbiotic relationship where one of the organisms receives a benefit and the other is not significantly harmed or helped by the interaction. Often the organism that is receiving a benefit can either find food more easily because of the interaction or is protected by the other organism.
For example, cattle egrets eat insects in pastures and fields. Large grazing herbivores, such as cattle and horses, stir up insects as they graze on grass in fields. Cattle egrets often follow behind the grazing herbivores and eat the insects that are stirred up. The cattle egret is receiving the benefit of being able to find food easily. The grazing herbivores are not affected by the presence of the egrets. This is an example of commensalism.
external image 6309CattleEgret.jpg
Cattle egrets follow behind large grazing herbivores and eat the insects that are stirred up. This is a form of commensalism.
Mutualism is a kind of symbiotic relationship in which both organisms involved receive a benefit. Flowers and their pollinators are one of the most common forms of mutualism. Plants cannot reproduce without being pollinated. Many kinds of plants depend on insects, such as moths, bees, wasps, and beetles, to perform pollination. Plants that rely on pollinators attract the pollinator by the shape, color, or smell of their flowers. The pollinator receives nectar or pollen as a food source from the flower. As the pollinator receives the food, it picks up pollen on its legs and body that can be spread to another plant. When it visits a second plant of the same species, the pollen from the first plant is transferred to the reproductive organs of the second plant and pollination can occur. Both organisms are receiving a benefit from this interaction. The pollinator is receiving access to a food source and the plant is able to reproduce because of this relationship.
external image 6309Bee.jpg
Bees are pollinators that receive nectar or pollen from flowering plants. They also aid in the pollination of the plant, which makes the relationship mutualistic.
Parasitism is a kind of symbiotic relationship in which one organism receives a benefit and the other organism is harmed by the interaction. The organism that receives a benefit is known as a parasite. The organism that is harmed by the relationship is known as the host. The host species is usually impaired slowly over a long period of time. Parasites can live either inside or on the outside of their host.
Common external parasites include fleas and mosquitoes which feed on the blood of their hosts. Internal parasites, such as tapeworms, live inside the body of their host and absorb nutrients from the host’s body. In both cases, the parasite is receiving nutrients at the expense of the host. The host can no longer use these nutrients for its own life processes.



Organisms within an ecosystem often must compete for limited resources. Organisms can compete for food, water, space, nesting areas, and access to mates. Competition occurs because resources within nature are limited and there is not enough for all of the populations within an ecosystem to grow indefinitely. The competition between species for resources will determine the population growth that is possible for those species. Species are limited by the number of resources they have access to.
Competitive exclusion occurs when two species are competing for the same resource. In this case, one of the two competitors will always eventually take over the resource completely and the other species will be forced to find a new location to gather resources or will become extinct.
When organisms are competing for territory they will often display aggressive behavior known as territorial aggression. The organism that is in control of a specific territory will use aggression to warn off any potential competitors. If another organism tries to take over the territory, the two organisms will use aggression to determine the winner.


Cooperation is a type of interaction in which organisms work together. Many species exhibit cooperative behavior, including horses, dolphins, lions, and ants. Animals that exhibit cooperative behavior often live, travel, and/or hunt in herds or groups. Living in these groups can provide protection for the animals and a higher success rate during hunts.
Groups of organisms that live together cooperatively are usually part of a hierarchy of leadership. Some members of the group have a higher status than other members of the group. Dominant members determine what the group will do and subordinate members follow their lead. This helps to eliminate aggression between group members and allows the group to work together for the benefit of all.
An example of an animal that lives in a cooperative group is the wolf. Wolves live in packs that usually include six or seven members. There are two leaders within the group, the alpha male and the alpha female, and these two pack members determine when the pack hunts, moves location, or stays in an area. The leaders of the pack are usually the pack members with the most experience in hunting and defending territory. The other pack members have roles within this pack to help the pack work effectively.

Protection/Defense: mimicry, false coloration, camouflage, warning colors

Adaptations can also help plants and animals protect themselves. One method of protection is camouflage, which is where the animal’s appearance helps it blend into its environment. Many stick insects, lizards, and frogs have adapted a form of camouflage that makes it hard for predators to see them.

Pictures of lizard and stick insect
Pictures of lizard and stick insect

Pictures of lizard and stick insect

The lizard and the stick insect shown are using camouflage to protect themselves against predators.

Animals can also behave in ways that help protect them. For example, snakes strike at predators, and owls spread their wings to appear larger and scare predators.

Some animals protect themselves by mimicking other animals. One example is a type of wasp that does not sting but looks similar to a stinging wasp.

Plants also have adaptations that help protect them. One example is the rose bush, which has thorns on its stems. These thorns stop predators from eating the plant and help it to survive.


Check yourself on Ecological Interactions
1) a group of organisms all in one area & all of the same species is called a(n) .
2) several different species populations living in a specific area is called a(n) .
3) in an arctic ecosystem, name a structural AND physical adaptation that organisms posses to survive the ABIOTIC harshness of the environment: physical_; behavioral
4) Name 3 biotic factors and three abiotic factors associated with the deciduous forest biome of Pennsylvania. Biotic-a)b)_c)_; Abiotic-a), b)c)
5) The organism in a food web that begins a chain by making energy from sunlight is called an autotroph or a(n) .
6) In a food web, an organism that is eating an autotroph is termed a heterotroph. Specifically, a heterotroph that is eating an autotroph is filling the role of a(n) _ in the ecosystem's food web.
7)In a food web, an organism that is eating another heterotroph is filling the role of a(n) _ in the ecosystem's food web.
8) Symbiosis means "life interacting together". There are three types of symbiosis in ecosystems that are determined by the benefit/harm relationship of the two organisms interacting. A tape worm lives inside a mammal and attaches to the wall of the small intestines. There is absorbs blood and nutrients to survive. This is an example of what type of symbiosis?
9) Symbiosis means "life interacting together". There are three types of symbiosis in ecosystems that are determined by the benefit/harm relationship of the two organisms interacting. A cattle egret bird(carnivorous insectivore) follows herbivores as they graze. As the grazing kicks up insects in the grasslands, the egret jumps to eat them. Grazers are herbivores. This is an example of what type of symbiosis?
10) Clown fish are immune to anemone stings. They hide in the sea anemones to avoid larger fish that are subject to the sting of the anemone. Conversely, the clown fish scare off the butterfly fish that eat anemones. This is an example of what type of symbiosis?
11) Red tailed hawks fly high above field using highly evolved eyesight to locate mice, rabbits, etc. They swoop in using razor sharp talons and capture these heterotrophs. Aside from heterotrophs and consumers, what specific role is the hawk filling in the ecosystem and what role is the mouse playing?
12) Native birds in Pennsylvania such as the Eastern Bluebird need special nesting holes to raise young. When the non-native English sparrow was introduced to North America, they moved in and used up many of the naturally existing nesting holes of the Eastern Bluebird. What type of interaction is this? _
13) When a pack of gray wolves meets up with another in an area of the Taiga, the males usually fight over the hunting area of the land. This aggression is an example of .
14) Structural defense mechanisms that have evolved in the appearance of organisms serve to protect them from predation in their ecosystems. Nelson is a harmless milk snake that has coloration that is very similar to a poisonous coral snake of the same ecosystem in southern USA. Snakes like Nelson benefit from this because predators that have learned to avoid the dangerous coral snake tend to leave Nelson alone as well. This an example of _.
15) Some caterpillars are shaped like a snake. This helps them avoid predation by scaring off some predators. This is an example of .
16) Some organisms have fake eye spots that confuse predators to attack a less crucial body part other than their head. This is called_.
16) A very bright yellow organisms can sometimes scare off predators. This is an example of _.
17) The walking stick has evolved a structure that looks JUST like the twigs it lives among. This safety mechanism to hide from predation is called ___.