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10.2: Roles in an Ecosystem

Difficulty Level: At Grade Created by: CK-12
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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. Climate conditions determine which ecosystems are found in a particular location. A biome encompasses all of the ecosystems that have similar climate and organisms.

Different organisms live in different types of ecosystems because they are adapted to different conditions. Lizards thrive in deserts, but no reptiles are found in any polar ecosystems. Amphibians can't live too far from the water. Large animals generally do better in cold climates than in hot climates.

Despite this, every ecosystem has the same general roles that living creatures fill. It’s just the organisms that fill those niches that are different. 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:

  • Photosynthesis: plants on land, phytoplankton in the surface ocean, and some other organisms.
  • Chemosynthesis: bacteria at hydrothermal vents.

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 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. Each niche can only be inhabited by one species.

Some relationships between species are beneficial to at least one of the two interacting species. These relationships are known as symbiosis and there are three types:

  • In mutualism, the relationship benefits both species. Most plant-pollinator relationships are mutually beneficial. What does each get from the relationship?
  • In commensalism, one organism benefits and the other is not harmed.
  • In parasitism, the parasite species benefits and the host is harmed. Parasites do not usually kill their hosts because a dead host is no longer useful to the parasite. Humans host parasites, such as the flatworms that cause schistosomiasis.

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.


  • 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 or one of symbiosis, in which one or both species benefits. Mutualism, commensalism, and parasitism are the three types of symbiotic relationships.


Use this resource to answer the questions that follow.


1. What is competition?

2. What is predation?

3. What is symbiosis?

4. How do stable communities develop?

5. What is succession?

6. What is a niche?

7. What does a niche include?

8. What causes competition?


1. Compare and contrast the two different ways of producing food energy, photosynthesis and chemosynthesis.

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

3. What kind of symbiotic relationship do zooxanthellae and corals have?

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carnivore Consumer that only eats other animals for food.
consumer An organism that uses other organisms for food energy.
herbivore Consumer that only eats producers such as plants or algae for food.
omnivore Consumer that eats both producers and other consumers for food.
scavenger Decomposer that consumes dead organisms and other wastes.

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Sep 11, 2016
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