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Content Benchmark L.8.C.2
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Students know how to characterize organisms in any ecosystem by their functions.  E/S

An ecosystem is defined as a system of interacting living organisms (biotic) and nonliving components (abiotic).  Therefore, an ecosystem will include components such as all plants, animals, fungi, bacteria, soil, rocks, water, and the atmosphere.  Figure 1 shows an example of a pond ecosystem.  It illustrates how the abiotic factors such as water, sunlight, oxygen, and nutrients interact with the biotic factors including the fish, plants, crayfish, and decomposers to create a healthy stable environment.  

Figure 1. An example of an aquatic ecosystem.

For a discussion on the abiotic factors including how energy and matter are cycled through food webs see MS TIPS Benchmark L.8.C.1

Biotic Roles in an Ecosystem
Every organism in an ecosystem has a role as a producer, a consumer, or a decomposer. Primary producers (also called autotrophs) are responsible for converting energy from an unusable form (radiant energy) to a usable form (chemical energy). Green plants on land and algae in aquatic environments are the major types of primary producers. A small class of autotrophic organisms, called chemotrophs, obtains energy by the oxidization of molecules in the environment. Non-autotrophic organisms are consumers (also called heterotrophs). Consumers are organisms that must obtain nutrients and energy from other living organisms. The often overlooked participants in an ecosystem are the decomposers which obtain energy from the remains of dead plants and animals.  During the process of decomposition, they release the nutrients trapped in the dead tissues so that it can be reused by other members of the ecosystem.

A food web, as shown in Figure 2, is a biological tool that illustrates the relationship of organisms within an ecosystem as well as the flow of energy from the primary producer to the top consumer and/or decomposer.  In this food web, the algae are an example of primary producers.  If you follow the arrows, the energy in the algae is transferred to a primary consumer such as the Daphnia, Sedge or TadpoleDecomposers are the organisms that derive energy from the decayed material.  Therefore, the decomposers in the diagram are the bacteria.

Figure 2.  A typical food web. 

A symbiotic relationship is a close interaction between two different species within an ecosystem. The three general types of symbiosis are mutualism, commensalism, and parasitism. The following table shows the affect of the symbiotic relationship on each organism using "+" to denote a benefit to the species, "-" denotes a negative consequence to the species, and a “0” represents no effect on the species. As Table 1 depicts, mutualism is a type of symbiotic relationship where both organisms benefit from the interaction. In commensalism, one organism benefits and the other organism does not experience a positive or a negative impact. During parasitism, the host is harmed as the parasite benefits.   

Table 1. Types of Symbiosis


Species A

Species B










Examples of mutualism, commensalism, and parasitism are illustrated in Figure 3.  Pollination is a classic example of mutualism (Figure 3a).  Both organisms benefit as the flower provides food to the pollinator in the form of nectar or pollen in exchange for pollen dispersal by the bee.  Commensalism is demonstrated by the clown fish and the anemone (Figure 3b).  The fish derives protection from predators from the anemone but the anemone is unaffected by the fish.  Tapeworms are parasitic and live inside the digestive tract of a host organism (Figure 3c). The worm obtains nutrients from the host to grow and reproduce. Consequently, the host loses nutrients, energy, and can suffer from tissue damage from the infestation.


Figure 3.  Examples of symbiosis using the narrow definition.  
(Left) Mutualism between a bee and flower (From
(Center) Commensalism between a clownfish and sea anemone (From
(Right) Parasitism of a tapeworm and its host.  (From

Predator/Prey Relationships
Predation involves one species feeding on another species.  The predatory species are intricately connected to the prey population.  The populations are dependent on each other.  As the predator species obtains food, the prey has a negative interaction (death) with the predator. However, overall the prey population is improved by the predator.  The predator often removes the weak, sick, or injured individuals from the prey population.  The predator species also keeps the prey population in check with available resources.  This makes the prey species stronger overall.   When predators are removed from an ecosystem, the prey population dramatically increases in numbers and often starves through lean seasons.  For example, the removal of wolves from forested areas resulted in an explosion in the deer populations.  Many deer starved over winter due to lack of food to feed the increased population.  Prey and predator populations therefore are dependent upon one another.  If the prey population increases, the predator population will increase.  If the prey population decreases, again the predator population will follow (see Figure 4).

Figure 4.  Predator populations fluctuate in response with prey populations. 


Populations compete for limited resources such as water, energy, living space, and mates.  Competition is the driving force behind natural selection.  Competition within a species may result in the better adapted individual (better able to get food, water, territory, mates, etc) eliminating the less fit competitor.  This is better known as the “Survival of the Fittest”.

For more detailed information on natural selection see MS TIPS Benchmark L.8.D.2 and MS TIPS Benchmark L.8.D.3

Interspecific (between species) competition negatively effects both competing populations.  Resources are finite.  If only one species existed in an area, the number of individuals in the population would be greater than if the limiting resource was divided between two species.  Both populations will be reduced in number due to competition for limiting resources.  Lions and cheetahs hunt similar prey and consequently, they must divide the available prey between both populations.  Therefore, both the lion and cheetah populations are lower than if only one cat species existed in the area.  The same mechanism can result in the extinction of a species.  If one species is out competed by another it is forced to adapt to using other available resources or the population will decrease until no individuals remain.  Figure 5a and 5b show a population of Paramecium living in isolation and both thrive but when place together (Figure 5c) one species out competes the other resulting in the decline and eventual elimination of the less fit species.  However, the better fit species was also be negatively affected.  P. aurelia showed a slower growth rate in the mixed culture than in the single culture. 

Figure 5.  Competition between two species of Paramecium.

In the short term, ecosystems may appear to be stable but are in a constant state of flux. Minor changes occur as one species slowly replaces another in response to such factors as climate changes, human impact, or introduction of alien species. This process is called ecological succession. In land environments, succession is driven by the plants. If the plant species change, the herbivores respond, and the predators follow.  Succession continues until the most mature, stable community evolves. This community is called the climax community. The climax community is the most stable and will remain unless upset by a catastrophic event like a flood, fire, clear cutting, volcanic eruption, etc.

There are two main types of succession, primary and secondary. Primary succession (Figure 6) occurs when no soil is present. Therefore, the organisms that migrate into the area must contribute to soil formation before other, larger plants can move in. Weathering begins breaking down the bare rock into smaller particles, and then pioneer species can move in. Lichens are a good example of a pioneer species. Lichens attach themselves to rocks with root-like rhizoids. They secrete acids onto the rock surface, dissolving the rocks. This begins the formation of soil. Once there is a thin layer of soil, small herbaceous plants (like grasses) move in and further the formation of soil. This process continues until a mature soil profile is created and the climax community is established.

Figure 6.  Primary succession. 

Secondary succession that takes place when soil is already present, usually following a catastrophic event that partially or completely removes the existing vegetation. Secondary succession occurs much faster than primary succession because soil already exists, a healthy seed bank in the soil may be present, or root stocks of previous plants may still be viable. Secondary succession usually begins with fast growing herbaceous ground cover that stabilizes the soil. Ultimately, the slower growing climax shrubs and trees re-grow and dominate the community again.

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Content Benchmark L.8.C.2

Students know how to characterize organisms in any ecosystem by their functions.  E/S

Common misconceptions associated with this benchmark

1. Students incorrectly include only predators and prey but not producers in self -constructed food webs.

According to Gallegos et al. (1994), students classify organism as carnivores or herbivores and their classification is based on size and ferocity.  Students overlooked the importance of producers and often left them out of food webs.  Food webs show the flow of energy not the flow of food.   Photosynthetic organisms capture unusable radiant energy and convert it into chemical energy.  This chemical energy can then be passed to herbivores and carnivores (consumers).

For more information on energy flow through an ecosystem go to

A website that examines students’ misconceptions in ecology can be found at

The Journal of Research in Science Education can be found at the following link
This is a pay site.  You can pay $29.95 to access journal articles.
Gallegos L, Jerezano ME, Flores F (1994) Preconceptions and relations used by children in the construction of food chains. Journal of Research in Science Teaching 31: 259-272.

2. Students incorrectly identify decomposers as unimportant.

Students have little knowledge of decomposers.  If asked, students cannot classify an organism as a decomposer.  They have little understanding of the role of a decomposer, and have trouble visualizing decomposition.  They have experience of moldy oranges and rotting logs but cannot connect that experience to food webs and nutrients cycles.  Decomposers and scavengers release bound nutrients in dead organisms and waste back into the ecosystem to be reused.  Without decomposers, life would cease to exist when all available resources are bound in the undecomposed bodies of dead organisms.

For more information on decomposers and scavengers go to the following links and

3. Students believe that an ecological disturbance like a fire has permanent and irreparable effect on an ecosystem.

Students view fire, clear cutting, and other dramatic changes in an ecosystem as disastrous. While these changes have dramatic effects on the ecosystem, the changes are not permanent. Ecosystems are in a constant state of flux.  The populations of organisms are always adjusting to slight variations in their ecosystem.  It is difficult to assess these changes as they are slow and small in a stable ecosystem.   Secondary succession is the slow repair of a devastated ecosystem where the vegetation has been removed.  Plants will immediately start to regrow and changes can be seen almost immediately.

For additional discussion on succession see HS TIPS Benchmark L.12.C.2

For more information on succession and example case studies see

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Content Benchmark L.8.C.2

Students know how to characterize organisms in any ecosystem by their functions.  E/S

Sample Test Questions

1st Item Specification:  Identify producers, consumers, decomposers and their functions in an ecosystem.

Depth of Knowledge Level 1

  1. What distinguishes producers from other organisms?
    1. Producers consume a variety of foods.
    2. Producers feed at different trophic levels.
    3. Producers are autotrophs.
    4. Producers are heterotrophs.
  1. Most of the minerals within an ecosystem are recycled and returned to the environment by the direct activities of organisms known as
    1. producers.
    2. consumers.
    3. scavengers.
    4. decomposers.

Depth of Knowledge Level 2

  1.  Carefully examine the food web in the figure below.

From ExamView test Generator for Modern Biology, Holt, Rinehart, and Winston, 2006.

A drought has caused the producer populations to significantly decrease.  Which of the following statements describes an immediate effect caused by the decrease of producers?

    1. The grouse population would increase.
    2. The grasshopper population would decrease.
    3. The seed populations would increase.
    4. The grizzly bear populations would decrease.
  1. A factory has been releasing pollution into a nearby river.  A chemical in the runoff has been killing the fungi populations in the nearby forest.  In response to the decrease in fungi, the plant growth will
    1. decrease which will in turn decrease competition between herbivores.
    2. increase due to the increased recycling of nutrients back into the soil.
    3. decrease due to decreased recycling of nutrients back into the soil.
    4. increase due to decreased competition with between the plants and fungi.

2nd Item Specification:  Understand how different populations interact within an ecosystem.

Depth of Knowledge Level 1

  1. In a forest, two different insect eating birds prefer to nest in different trees. This behavior allows the birds to avoid
    1. predators.
    2. parasites.
    3. competition.
    4. succession.
  1.  An overpopulation of squirrels in a forest will most likely lead to
    1. a decrease in squirrel predators like fox and owls.
    2. an increase in competition between squirrels.
    3. an increase in the number of acorns available for food.
    4. a decrease in disease transmission between squirrels.

Depth of Knowledge Level 2

  1. Protists are single-celled organisms that feed on bacteria and yeast.  In a test tube, a single species of protists grew and flourished.  Another test tube had two species of protists, one species died within 16 days, while the other survived.  This observation illustrates
    1. competition.
    2. predation.
    3. mutualism.
    4. commensalism.
  1. Carefully examine the predator/prey population graph below of Paramecium and yeast.  A Paramecium is a single-celled organism that feeds on yeast. 

What is the most likely reason for the increase in Paramecium population between days 9 and 11?

    1. An increasing food supply between days 7 and 9.
    2. An equal sized predator and prey populations between days 7 and 9.
    3. A decreasing prey population between days 5 and 7.
    4. A decreasing food supply between day 11 and 12.

3rd Item Specification:  Understand different types of relationships (e.g. symbiotic, mutual, parasitic/host) in an ecosystem.

Depth of Knowledge Level 1

  1. Which statement describes all symbiotic relationships? A relationship where both organisms
    1. benefit from the relationship.
    2. have no impact on one another.
    3. live in a close association with one another.
    4. have a negative effect on the other.
  1. The Honeyguide, a bird, and the badger both eat honey. The Honeyguide cannot open a bee hive and the badger cannot find the hive. The Honeyguide leads the badger to the hive and the badger breaks open the hive so both can eat the honey.  This type of relationship is best described as
    1. competition.
    2. parasitism.
    3. mutualism.
    4. commensalism.

Depth of Knowledge Level 2

  1. The symbiotic relationship between a flower and the insect that feeds on its nectar is an example of
    1. mutualism because the flower provides the insect with food, and the insect pollinates the flower.
    2. commensalism because the insect lives off the nectar but the flower does not benefit.
    3. parasitism because the insect harms the flower by removing the nectar.
    4. predation because the insect feeds on the flower and the flower dies.
  1. Some plants form a relationship with bacteria. The bacteria are protected by the roots where they convert nitrogen gas to a usable form of nitrogen needed by the plant. Which of the following statements explains this relationship?
    1. Commensalism because the plant is not harmed or benefited from the relationship but the bacteria is benefited.  
    2. Parasitism because the plant is harmed by the bacteria infection in their roots. 
    3. Commensalism because the plant benefits by associating with the bacteria but the bacteria is not affected.   
    4. Mutualism because the plant derives a benefit by associating with the bacteria and the bacteria also benefits. 

4th Item Specification:  Understand the process of succession (e.g. producers enter ecosystem first, followed by consumers)

Depth of Knowledge Level 1

  1. What is one difference between primary and secondary succession?
    1. Primary succession is slow and secondary succession is rapid.
    2. Secondary succession begins on soil and primary succession begins on rock.
    3. Primary succession modifies the environment and secondary succession does not.
    4. Secondary succession begins with lichens and primary succession begins with trees.
  1.  A lava flow covers a mature forest.  Which of the following is most likely to be the sequence of plants that regrow in the area?
    1. Lichens, grasses, shrubs, trees
    2. Grasses, flowering groundcover, shrubs, trees
    3. Flowering groundcover, grasses, lichens, trees
    4. Lichens, shrubs, grasses, trees

Depth of Knowledge Level 2

  1. Use the information below to answer the following question.

Location #1:
A plowed field → grasses → shrubs → trees

Location #2:
Bare rock → lichens → grasses → shrubs → trees

            Which of the following are the pioneer species in each location?

    1. Grasses and shrubs
    2. Shrubs and trees
    3. Lichens and grasses
    4. Trees and grasses
  1. Use the diagram below to answer the following question. 

Figure from Examgen test generating program

The boxes above represent a different stage in succession. Put the boxes in order.

  1. I, II, III, IV
  2. II, IV, I, III
  3. IV, II, I, III
  4. III, IV, II, I

Constructed Response L.8.C.2

1. An old growth forest was destroyed by a wildfire.  Overtime the forest returned. 

  1. Summarize the process that returned the mature forest from the destroyed environment. 
  2. Construct a food web of the resulting mature forest that includes 2 producers, 2 consumers from different trophic levels, and 1 decomposer. 
  3. Describe the role and importance of the producers, consumers, and decomposers that you chose to include in the ecosystem.

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Content Benchmark L.8.C.2

Students know how to characterize organisms in any ecosystem by their functions.  E/S

Answers to Sample Test Questions

  1. C, DOK Level 1
  2. D, DOK Level 1
  3. B, DOK Level 2
  4. C, DOK Level 2
  5. C, DOK Level 1
  6. B, DOK Level 1
  7. A, DOK Level 2
  8. A, DOK Level 2
  9. C, DOK Level 1
  10. C, DOK Level 1
  11. A, DOK Level 2
  12. D, DOK Level 2
  13. B, DOK Level 1
  14. A, DOK Level 1
  15. C, DOK Level 2
  16. C, DOK Level 2

Constructed Response L.8.C.2 Score Rubric:





3 points


Response addresses all parts of the question clearly and correctly.

  1. The first plants to return are grasses and other ground cover plants.  These plants stabilize the soil. Shrubs replace the ground over as the area matures. Next, trees replace the shrub community. Lastly, the forest is returned as the climax community. 
  2. Generic food web provided above. Students should have specific organisms representing each trophic level. Food web must include arrows indicating the direction of the energy transfer.
  3. Producers capture sunlight energy and convert it to usable organic energy. The energy from the producers is transferred through the food web through the various levels of consumers. Decomposers live off the energy and recycle the nutrients in dead plants and animals.

2 points

 Response addresses all parts of the question and includes only minor errors.

1 point

 Response does not address all parts of the question.

0 points

 Response is totally incorrect or no response provided

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Content Benchmark L.8.C.2

Students know how to characterize organisms in any ecosystem by their functions.  E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student understanding of this benchmark.

1. Trophic / Food Web Activity

This website allows students to practice placing animals in appropriate trophic levels and then seeing the completed food web. It includes decomposers and scavengers which are often overlooked by students.

To access this activity, go to

2. Online Food Web Lesson

This website requires students to begin by review essential vocabulary associated with food webs.  It then has students analyze a food web and determine what essential element is missing form the food web (decomposers).  Last, students research and construct their own habitat food web.  This lesson can be used to reinforce food web content or adapted as an assessment tool.

To access this website, go to

3. Interaction Between Species Lesson Plan

This NOVA Online website provides a lesson plan for investigating symbiotic relationships.  Students research online and classify pairs of organisms into each type of symbiotic relationship.  This lesson is tied to “Volcanoes of the Deep” video to explain tubeworm and sulfur-oxidizing bacteria at deep sea thermal vents but the video is not required to complete this activity.  The video can be purchased from this site for $19.95.  NOVA and submitting teachers have provided lesson ideas using this video to teach several ecological concepts.

To access this website, visit

4. A role-playing experience for middle and high school students to explore human interference in an ecosystem and secondary succession.

This lesson is titled “Firestorm: Thinking Critically about Environmental Issues.”  This lesson was developed to help “students gain critical thinking skills and build an understanding of environmental decision-making processes.  These are important elements in the pursuit of environmental literacy. That’s why the Environmental Literacy Council has developed “Firestorm: Thinking Critically about Environmental Issues.”  Firestorm is a role-playing simulation designed to give students authentic experience in the process of making important decisions about the environment—gathering and analyzing information; judging the reliability of information sources; understanding multiple, complex perspectives; and forming opinions and making recommendations based on solid knowledge of ecosystems and different approaches to environmental management.  Firestorm invites students to take on the roles of the people and organizations who have a stake in most environmental decisions: scientists, environmentalists, businesses, outdoor enthusiasts, homeowners and nature lovers. Students will learn the varied perspectives that different stakeholders place on the value of natural resources. Although Firestorm centers on a fictionalized controversy about the role of prescribed burning in forest management, teachers can use the exercise to lead their students in the exploration of broader social questions about environmental issues.” 

To access this lesson, follow the link

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Benchmark Related Vocabulary

Food chain