High School Earth Science/Ecosystems

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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.

Lesson Objectives[edit]

  • 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.

Biological Communities[edit]

Figure 18.1: The horsetail Equisetum is a primitive plant.
Figure 18.3: A giraffe is an example of a vertebrate.

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. The living organisms within an ecosystem are its biotic factors. Living things include bacteria, algae, fungi, plants (Figure 18.1) and animals. Animals include invertebrates (Figure 18.2), animals without backbones and vertebrates (Figure 18.3), animals with backbones.

Figure 18.2: Insects are among the many different types of invertebrates.

Physical and chemical features are abiotic factors. Abiotic factors include resources living organisms need like light, oxygen, water, carbon dioxide, good soil, and nitrogen, phosphorus and other nutrients. Abiotic factors also include environmental features that are not materials or living things, like 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 hopefully 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. This niche can be filled by a lion in a savanna, a wolf in the tundra, or a tuna in the oceans.

Every species fills a niche, and niches are almost always filled in an ecosystem.

An organism's habitat is where it lives. The important characteristics of a habitat include climate, the availability of food, water and other resources, and other factors, such as weather. A habitat may be a hole in a cactus or the underside of a fern in a rainforest. It may be a large area of savanna.

Roles in Ecosystems[edit]

There are many different types of ecosystems (Figure 18.4). A few examples of some ecosystems are a rainforest, chaparral, tundra, and desert. These words are the same words used for biomes. This is because climate conditions determine which ecosystems are found in which location. A particular biome encompasses all of the ecosystems that have similar climate and organisms.

Figure 18.4: Coral reefs are complex and beautiful ecosystems.
Figure 18.5: Tube worms have a symbiotic relationship with chemosynthetic bacteria. The bacteria provide the worms with food and the worm tubes provide the bacteria with shelter.

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 deep sea hot springs.

The organisms that produce food are extremely important in every ecosystem. The most fundamental distinction between types of organisms is whether they are able to produce their own energy or not. Organisms that produce their own food are called producers. In contrast, organisms that use the food energy that was created by producers are named consumers.

There are two types of producers. Nearly all producers take energy from the Sun and make it into chemical energy (food) by the process of photosynthesis. Photosynthesizing organisms use carbon dioxide (CO2) and water (H2O) to produce sugar (C6H12O6) and oxygen (O2). This food can be used immediately or stored for future use.

A tiny group of producers create usable chemical energy from chemicals, without using any sunlight. At the bottom of the ocean, at deep-sea hot springs known as hydrothermal vents, a few types of bacteria break down chemicals to produce food energy. This process is called chemosynthesis (Figure 18.5).

There are many types of consumers. Herbivores eat producers directly (Figure 18.6). These animals break down the plant structures to get the materials and energy they need. Many other consumers eat animals. These are known as carnivores. Carnivores can eat herbivores or they can eat other carnivores. Omnivores eat plants and animals, as well as fungi, bacteria and organisms from the other kingdoms.

Figure 18.6: Deer are herbivores.

There are many types of feeding relationships between organisms. A predator is an animal that kills and eats another animal (Figure 18.7). The animal it kills is its prey.

Figure 18.7: This South China Tiger is a predator.
Figure 18.8: Fungi are decomposing this tree.

Scavengers are animals that eat organisms that are already dead. Vultures and hyenas are just two types of scavengers. Decomposers break apart dead organisms or the waste material of living organisms, returning the nutrients to the ecosystem. Many decomposers are bacteria, but there are others as well, including fungi (Figure 18.8). Decomposers are recyclers; they make nutrients from dead organisms available for living organisms.

Flow of Energy in Ecosystems[edit]

Energy cannot be created or destroyed. Energy can only be changed from one form to another. This is such a fundamental law in nature that it has its own name: The Law of Conservation of Energy. Plants do not create chemical energy from nothing. Instead, they create chemical energy from abiotic factors that include sunlight. So they transform solar energy into chemical energy. Organisms that use chemosynthesis start with chemical energy to create usable chemical energy. After the producers create the food energy, it is then passed on to consumers, scavengers, and decomposers.

Energy flows through an ecosystem in only one direction. Energy enters the ecosystem with the producers. In nearly all ecosystems, sunlight is the original energy source. This energy is passed from organisms at one trophic level or energy level, to organisms in the next trophic level. Producers are always the first trophic level, herbivores the second, the carnivores that eat herbivores the third, and so on.

An average of 90% of the energy that reaches a trophic level is used to power the organisms at that trophic level. They 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. They use about 90% of what they receive, and so those at the third level have only 10% as much available to them as those at the second level. This 10% rule continues up the trophic levels, so much less energy is available at the next higher trophic level in an ecosystem.

Figure 18.9: A simple food chain from a Swedish lake. Not pictured: algae eaten by the shrimp; Shrimp are eaten by a small fish, a bleak, which is eaten by a perch, which is eaten by a northern pike, which is eaten by an osprey.

The set of organisms that pass energy from one trophic level to the next is described as the food chain (Figure 18.9). In this simple depiction, all organisms eat at only one trophic level. Animals at the 3rd trophic level only eat from the 2nd trophic level and those at the 2nd eat only from the 1st. But many omnivores feed at more than one trophic level, with plants and animals in their diets.

Since only 10% of the energy is passed up the food chain, each level can support fewer organisms. A top predator, like a jaguar, must have a very large range in which to hunt so that it can get enough energy to live. Top carnivores are quite rare relative to herbivores for this reason. The result of this is that the number of organisms at each trophic level looks like a pyramid. There are many more organisms at the base of the pyramid, at the lower trophic levels than at the top of the pyramid, the higher trophic levels.

Food chains usually have only four or five trophic levels because 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. Organisms at higher trophic levels also tend to be larger than those at lower levels. The reason for this is simple: a whale must be able to eat a plankton, but the plankton does not have to be able to eat the whale. Sometimes multiple smaller predators will act together to take down a larger prey, so the organisms at the higher level are smaller than those at the lower level. This is true of a pack of wolves, which acts together as one to hunt a moose.

Since some organisms feed at more than one trophic level, the food chain does not adequately describe the passage of energy in an ecosystem. The more accurate representation is a food web (Figure 18.10). A food web recognizes that many organisms eat at multiple trophic levels. A food web includes the relationships between producers, consumers and decomposers.

Figure 18.10: An arctic food web. Besides the living organisms, some abiotic components (nitrogen, mineral salts) and nonliving parts (dung) are included.

All organisms depend on two global food webs that are interconnected. The base of one is phytoplankton, microscopic ocean producers. These tiny organisms are eaten by zooplankton. The zooplankton are tiny animals which in turn are eaten by small fish and then larger fish. Land plants form the base of the second food web. They are eaten by herbivores, that are eaten by carnivores and so on. Birds or bears that live on land may eat fish, which connects the two 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[edit]

The flow of matter in an ecosystem is not like energy flow. Matter can enter an ecosystem at any level and can leave at any level. It cycles freely between trophic levels and between the ecosystem and the physical environment. Nutrients are ions that are crucial to the growth of living organisms. Nutrients, like 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 like carbohydrates, lipids, proteins and nucleic acids.

Nutrients may enter an ecosystem from the breakdown of rocks and minerals. They enter the soil and are taken up by plants. Nutrients can be brought in from other regions, perhaps carried to a lake by a stream. 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. Nutrients can blow out of an ecosystem on the wind.

Decomposers play a key role in making nutrients available to organisms. After scavengers eat dead organisms, they almost always leave some parts of the dead animal or plant behind. Decomposers complete the process of breaking down dead organisms. They convert dead organisms into nutrients and carbon dioxide, which they respire into the air. These left over nutrients are then available for other organisms to use. Without decomposers, life on Earth would not be able to continue. Dead tissue would remain as it is and eventually nutrients would run out. Decomposers break apart tissue and return the nutrients to the ground. Without decomposers, life on earth would have died out long ago.

Relationships Between Species[edit]

Figure 18.11: This hummingbird hawkmoth and flower each benefit from the mutualism of their relationship.

Species have different types of relationships with each other. Competition occurs between species that are trying 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 (Figure 18.11). Most plant-pollinator relationships are mutually beneficial. The pollinator, such as a hummingbird, gets food. The plant get its pollen caught in the bird's feathers, so that pollen is spread to far away flowers helping them reproduce.

In commensalism, the relationship is beneficial to one species, but does not harm or help the other (Figure 18.12). A bird may build a nest in a hole in a tree. This neither harms nor benefits the tree, but it provides the bird and its young with protection.

Figure 18.12: The relationship between these barnacles and the humpback whale is an example of commensalism. The barnacles receive protection and get to move to new locations and the whale is not harmed.
Figure 18.13: These tiny mites are parasitic on a harvestman.

In parasitism, the parasite species benefits and the host is harmed (Figure 18.13). Parasites do not usually kill their hosts because a dead host is no longer useful to the parasite. A visible example of parasite and host is mistletoe on an oak tree. The mistletoe gains water and nutrients through a root that it sends into the tree’s branch. The tree is then supporting the mistletoe, but the tree is not killed, even though its growth and reproduction are slightly harmed by the parasite. Humans can host parasites, like the flatworms that cause schistosomiasis.

Lesson Summary[edit]

  • Each species fills a niche within an ecosystem. Each ecosystem has the same niches, although the same species doesn'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.
  • As a result, food webs are needed to show all the predator/prey interactions in an ecosystem.

Review Questions[edit]

  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?


Nonliving features of an ecosystem include space, nutrients, air, and water.
Living features of an ecosystem include viruses, plants, animals, and bacteria.
Animals that only eat other animals for food.
The creation of food energy by breaking down chemicals.
A relationship between two species in which one species benefits and the other species is not harmed.
All of the living creatures of an ecosystem; all of the populations of all of the species that live together.
A rivalry between two species, or individuals of the same species, for the same resources.
An organism that does not create its own chemical energy, but uses other organisms for food.
An organism that breaks down the tissues of a dead organism into its various components, including nutrients that can be used by other organisms.
All of the living things in a region and the physical and chemical factors that they need to live.
food chain
An energy pathway that includes all organisms that are linked as they pass along food energy, beginning with a producer and moving on to consumers.
food web
Interwoven food chains that show each organism eating from different trophic levels, which more closely reflects reality.
Where an organism lives; habitats have distinctive features like climate or resource availability.
An animal that only eats producers.
Animals without backbones.
A symbiotic relationship between two species in which both species benefit.
An organism's "job" within its community.
An organism that consumes both plants (producers) and other consumers (animals) for food.
A symbiotic relationship between two species in which there is a parasite and a host. The parasite gains nutrition from the host. The host in a parasitic relationship is harmed but usually not killed.
The process in which plants use carbon dioxide and water to produce sugar and oxygen: 6CO2 + 12H2O + solar energy C6H12O6 + 6O2 + 6H2O.
All the individuals of a species that occur together in a given place and time.
An animal that kills and eats other animals.
An animal that could be killed and eaten by a predator.
An organism that creates chemical energy to be used as food. Most producers use photosynthesis but a very small number use chemosynthesis.
Animals that eat animals that are already dead.
A classification of organisms that includes those that can or do interbreed and produce fertile offspring; members of a species share the same gene pool.
Relationships between two species in which at least one species benefits.
trophic level
Energy levels within a food chain or food web.
Animals with backbones.

Points to Consider[edit]

  • 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 vent, 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 or lower organisms, why are there so many people?

Ecosystems and Human Populations · The Carbon Cycle