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Roles in an Ecosystem

Organisms in an ecosystem have different roles depending on its relationship to other organisms.

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Roles in an Ecosystem

What roles do coral reef organisms have?

Corals are not rocks or plants, but little animals that live in a carbonate shell they create. They have a symbiotic relationship with zooxanthellae, tiny photosynthesizing organisms. The zooxanthellae provide food for the coral and the coral provides a safe home for the zooxanthellae. Together they form the base of a complex ecosystem.

Roles in Ecosystems

There are many different types of ecosystems. Despite this, every ecosystem has the same general roles that living creatures fill.  For example, every ecosystem must have some organisms that produce food in the form of chemical energy. These organisms are primarily algae in the oceans, plants on land, and bacteria at hydrothermal vents.

Producers and Consumers

The organisms that produce food are extremely important in every ecosystem. Organisms that produce their own food are called producers. There are two ways of producing food energy, but will consider just one:

  • Photosynthesis: plants on land, phytoplankton in the surface ocean, and some other organisms.

Organisms that use the food energy that was created by producers are named consumers. There are many types of consumers:

  • Herbivores eat producers directly. These animals break down the plant structures to get the materials and energy they need.
  • Carnivores eat animals; they can eat herbivores or other carnivores.
  • Omnivores eat plants and animals as well as fungi, bacteria, and organisms from the other kingdoms.

A llama grazes on a terrace near Machu Picchu, Peru

Feeding Relationships

There are many types of feeding relationships (Figure below) between organisms. A predator is an animal that kills and eats another animal, known as its prey. Scavengers are animals, such as vultures and hyenas, that eat organisms that are already dead. Decomposers break apart dead organisms or the waste material of living organisms, returning the nutrients to the ecosystem.

(a) Predator and prey; (b) Scavengers; (c) Bacteria and fungi, acting as decomposers.

Relationships Between Species

Species have different types of relationships with each other. Competition , one type of relationship, occurs between species that try to use the same resources. When there is too much competition, one species may move or adapt so that it uses slightly different resources. It may live at the tops of trees and eat leaves that are somewhat higher on bushes, for example. If the competition does not end, one species will die out.

Choose which type of relationship is described by each of the images and captions below (Figure below).

(a) The pollinator gets food; the plant’s pollen gets caught in the bird’s feathers so it is spread to far away flowers. (b) The barnacles receive protection and get to move to new locations; the whale is not harmed. (c) These tiny mites are parasitic and consume the insect called a harvestman.


  • carnivore: Animals that only eat other animals for food.
  • competition: A rivalry between two species, or individuals of the same species, for the same resources.
  • consumer: An organism that uses other organisms for food energy.
  • decomposer: An organism that breaks down the tissues of a dead organism into its various components, including nutrients, that can be used by other organisms.
  • herbivore: An animal that only eats producers.
  • omnivore: An organism that consumes both producers and other consumers for food.
  • predator: A symbiotic relationship between two species in which one species benefits and one species is harmed.
  • prey: An animal that could be killed and eaten by a predator.
  • producer: An organism that converts energy into chemical energy that it can use for food. Most producers use photosynthesis but a very small number use chemosynthesis.
  • scavenger: Animals that eat animals that are already dead.


  • Herbivores eat plants, carnivores eat meat, and omnivores eat both.
  • Predators are animals that eat a prey animal. Scavengers eat organisms that are already dead. Decomposers break down dead plants and animals into component parts, including nutrients.
  • Relationships between species can be one of competition.


Use this resource to answer the questions that follow.


1. What is competition?

2. What is predation?

3. How do stable communities develop?

5. What is succession?

6. What causes competition?


1. After a producer produces food energy, follow its path until it ends up being used by another producer.

2.  Explain what happens when two species compete for the same respurces.

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