Lesson Objectives

  • Discuss the importance of chemical and physical factors to living organisms.
  • Describe the role of different species in an ecosystem.
  • Describe the function of an ecosystem, and how different species fill different roles in different ecosystems.
  • Describe energy transfer from the lowest to the highest trophic level in a chain, including energy loss at every trophic level.
  • Discuss how materials are cycled between trophic levels and how they can enter or leave a food web at any time.


  • abiotic
  • biotic
  • carnivore
  • commensalism
  • community
  • competition
  • consumer
  • decomposer
  • ecosystem
  • food chain
  • food web
  • habitat
  • herbivore
  • mutualism
  • niche
  • nutrients
  • omnivore
  • parasitism
  • population
  • predator
  • prey
  • producer
  • scavenger
  • species
  • symbiosis
  • trophic level


An ecosystem is made up of the living creatures and the nonliving things that those creatures need within an area. Energy moves through an ecosystem in one direction. Nutrients cycle through different parts of the ecosystem and can enter or leave the ecosystem at many points.

Biological Communities

A population consists of all individuals of a single species that occur together at a given place and time. A species is a single type of organism that can interbreed and produce fertile offspring. All of the populations living together in the same area make up a community. An ecosystem is all of the living things in a community and the physical and chemical factors that they interact with.

In an Ecosystem

The living organisms within an ecosystem are its biotic factors (Figure below). Living things include bacteria, algae, fungi, plants , and animals, including invertebrates, animals without backbones, and vertebrates, animals with backbones.

(a) The horsetail Equisetum is a primitive plant. (b) Insects are among the many different types of invertebrates. (c) A giraffe is an example of a vertebrate.

Physical and chemical features are abiotic factors. Abiotic factors include resources living organisms need such as light, oxygen, water, carbon dioxide, good soil, and nitrogen, phosphorous, and other nutrients. Abiotic factors also include environmental features that are not materials or living things, such as living space and the right temperature range.


Organisms must make a living, just like a lawyer or a ballet dancer. This means that each individual organism must acquire enough food energy to live and reproduce. A species’ way of making a living is called its niche. An example of a niche is making a living as a top carnivore, an animal that eats other animals, but is not eaten by any other animals (Figure below). Every species fills a niche, and niches are almost always filled in an ecosystem.

The top carnivore niche is filled by lions on the savanna, wolves in the tundra, and tuna in the oceans.


An organism’s habitat is where it lives (Figure below). The important characteristics of a habitat include climate, the availability of food, water, and other resources, as well as other factors, such as weather.

Birds living in a saguaro cactus. A habitat may be a hole in a cactus or the underside of a fern in a rainforest. It may be rocks and the nearby sea.

Roles in Ecosystems

There are many different types of ecosystems, some of which were described in the biomes discussion in the Climate chapter (Figure below). As with biomes, climate conditions determine which ecosystems are found in which location. A particular biome encompasses all of the ecosystems that have similar climate and organisms.

Coral reefs are complex and beautiful ecosystems.

Different organisms live in each different type of ecosystems. Lizards thrive in deserts, but no reptiles can survive at all in polar ecosystems. 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, described in the Earth’s Atmosphere chapter and elsewhere.
  • Chemosynthesis: bacteria at hydrothermal vents as discussed in the Earth’s Oceans chapter.

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

  • Herbivores eat producers directly (Figure below). 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.

Deer are herbivores.

Feeding Relationships

There are many types of feeding relationships (Figure below) between organisms: predators that feed on prey, scavengers, and decomposers.

(a) A predator is an animal that kills and eats another animal, known as its prey. (b) Scavengers are animals, such as vultures and hyenas, that eat organisms that are already dead. (c) Decomposers break apart dead organisms or the waste material of living organisms, returning the nutrients to the ecosystem. Bacteria and fungi are decomposers.

Flow of Energy in Ecosystems

Remember from the Earth’s Atmosphere chapter that plants create chemical energy from abiotic factors that include solar energy. Chemosynthesizing bacteria create usable chemical energy from unusable chemical energy. The food energy created by producers is passed to consumers, scavengers, and decomposers.

Trophic Levels

Energy flows through an ecosystem in only one direction. Energy is passed from organisms at one trophic level or energy level, to organisms in the next trophic level. Which organisms do you think are at the first trophic level (Figure below)?

Producers are always the first trophic level, herbivores the second, the carnivores that eat herbivores the third, and so on.

Most of the energy – about 90% – at a trophic level is used at that trophic level. Organisms need it for locomotion, heating themselves, and reproduction. So animals at the second trophic level have only about 10% as much energy available to them as do organisms at the first trophic level. Animals at the third level have only 10% as much available to them as those at the second level.

Food Chains

The set of organisms that pass energy from one trophic level to the next is described as the food chain (Figure below). In this simple depiction, all organisms eat at only one trophic level (Figure below).

A simple food chain in a lake. The producers, algae, are not shown. For the predatory bird at the top, how much of the original energy is left?

What are the consequences of the loss of energy at each trophic level? Each trophic level can support fewer organisms. How many osprey are there relative to the number of shrimp?

What does this mean for the range of the osprey (or lion, or other top predator)? A top predator must have a very large range in which to hunt so that it can get enough energy to live.

Why do most food chains have only four or five trophic levels? There is not enough energy to support organisms in a sixth trophic level. Food chains of ocean animals are longer than those of land-based animals because ocean conditions are more stable.

Why do organisms at higher trophic levels tend to be larger than those at lower levels? The reason for this is simple: a large fish must be able to eat a small fish, but the small fish does not have to be able to eat the large fish (Figure below).

In this image the predators (wolves) are smaller than the prey (bison), which goes against the rule placed above. How does this relationship work? Many wolves are acting together to take down the bison.

Food Webs

What is a more accurate way to depict the passage of energy in an ecosystem? A food web (Figure below) recognizes that many organisms eat at multiple trophic levels.

A food web includes the relationships between producers, consumers, and decomposers.

Even food webs are interconnected. All organisms depend on two global food webs. The base of one is phytoplankton and the other is land plants. How are these two webs interconnected? Birds or bears that live on land may eat fish, which connects the two food webs.

Where do humans fit into these food webs? Humans are an important part of both of these food webs; we are at the top of a food web since nothing eats us. That means that we are top predators.

Flow of Matter in Ecosystems

Nutrients are ions that are crucial to the growth of living organisms. Nutrients – such as nitrogen and phosphorous – are important for plant cell growth. Animals use silica and calcium to build shells and skeletons. Cells need nitrates and phosphates to create proteins and other biochemicals. From nutrients, organisms make tissues and complex molecules such as carbohydrates, lipids, proteins, and nucleic acids.

The flow of matter in an ecosystem is not like energy flow. Matter enters an ecosystem at any level and leaves at any level. Matter cycles freely between trophic levels and between the ecosystem and the physical environment (Figure below).

What are the sources of nutrients in an ecosystem? Rocks and minerals break down to release nutrients. Some enter the soil and are taken up by plants. Nutrients can be brought in from other regions, carried by wind or water. When one organism eats another organism, it receives all of its nutrients. Nutrients can also cycle out of an ecosystem. Decaying leaves may be transported out of an ecosystem by a stream. Wind or water carries nutrients out of an ecosystem.

Nutrients cycle through ocean food webs.

Decomposers play a key role in making nutrients available to organisms. Decomposers break down dead organisms into nutrients and carbon dioxide, which they respire into the air. If dead tissue would remain as it is, eventually nutrients would run out. Without decomposers, life on earth would have died out long ago.

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 caption in the 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.

Lesson Summary

  • Each species fills a niche within an ecosystem. Each ecosystem has the same niches, although the same species don’t always fill them.
  • Each ecosystem has producers, consumers, and decomposers. Decomposers break down dead tissue to make nutrients available for living organisms.
  • Energy is lost at each trophic level, so top predators are scarce.
  • Feeding relationships are much more complicated than a food chain, since some organisms eat from multiple trophic levels.
  • Food webs are needed to show all the predator/prey interactions in an ecosystem.

Review Questions

  1. What is the difference between a population, a community, and an ecosystem?
  2. What is the difference between a niche and a habitat?
  3. Why are the roles in different ecosystems the same but the species that fill them often different?
  4. Why are there no producers in the deep sea ecosystem? Without producers, where does the energy come from? What is the ultimate source of the energy?
  5. Is a predator an herbivore, carnivore, or omnivore? How about a prey?
  6. Biologists have been known to say that bacteria are the most important living things on the planet. Why would this be true?
  7. Why are you so much more likely to see a rabbit than a lion when you’re out on a hike?
  8. How much energy is available to organisms on the 5th trophic level compared with those on the 1st? How does this determine how long a food chain can be?
  9. Why is a food web a better representation of the feeding relationships of organisms than a food chain?
  10. Why is energy only transferred in one way in an ecosystem, but nutrients cycle around?
  11. Why does a predator kill its prey but a parasite rarely kills its host?

Points to Consider

  • What happens if two species attempt to fill the same niche?
  • There is at least one exception to the rule that each ecosystem has producers, consumers, and decomposers. Excluding hydrothermal vents, what does the deep sea ecosystem lack?
  • Where do humans fit into a food web?
  • Most humans are omnivores, but a lot of what we eat is at a high trophic level. Since ecosystems typically can support only a few top predators relative to the number of lower organisms, why are there so many people?