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Examining the Effects of Pollution on Ecosystems

Duration 2 to 3 class periods
Grade Level 10-12
Key Terms/
Relative abundances
Species abundances
Life Science
Physical Science


dead fishStudents recognize that hazardous waste may have far-reaching impacts on ecosystems and these impacts are not always easy to identify. Students become familiar with several types of tests used to measure the environmental effects of hazardous waste pollution. In the process, they learn that no single assessment procedure is applicable to all ecosystems and no single test is adequate to assess pollution impacts on an entire ecosystem. They examine a case study and discuss the limitations of current ecosystem assessment methods for establishing cause-and-effect relationships, especially for mixtures of chemicals in the environment.


The impact of hazardous waste on the environment is thought to be widespread and in some areas severe. Establishing cause-and-effect relationships between exposure and ecosystem damage is a major challenge. An ecosystem-such as a marsh-is a highly complex structure, consisting of all living organisms in a given area and their interactions not only among themselves but also with the environment. Even a mature ecosystem-one that has achieved stability over time-is constantly adapting to changes. Some of these changes are due to natural influences such as animal migration patterns, weather, erosion, and sedimentation. Other changes, however, are the result of habitat encroachment and human pollution. This pollution is often in the form of complex mixtures of chemicals in widely varying concentrations.

Ecosystems are complex and dynamic (ever changing). This makes linking any one effect to a specific cause very difficult. Conditions cannot be controlled sufficiently to allow the effects of individual pollutants to be observed. Only recently have scientists begun to focus attention on finding ways to determine the major effects of hazardous waste on ecosystems.

Researchers have built laboratory models of ecosystems to study environmental pollution in controlled settings. But models can provide only simple representations of real ecosystems that contain thousands of living species. They cannot provide adequate measures of the diversity of species and the complexity of the relationships among all the living organisms that make ecosystems unique.

There is no single best strategy or design for assessing environmental pollution that is appropriate for every situation. The characteristics of the area and the specific objectives and issues of concern must be considered in determining how to proceed. Nevertheless, scientists generally use four major categories of tests to assess the impact of hazardous waste on ecosystems:

  • Chemical and physical tests to measure contaminant levels, pH, oxygen levels, and other environmental conditions
  • Toxicity tests to determine if the pollution can or is causing adverse biological effects in ecosystem species
  • Biomarkers to indicate actual exposure
  • Field surveys.

These ecological assessments are important tools in Federal and state government efforts to clean up hazardous waste contamination under the Superfund Program.

For additonal information on ecosystems and pollution, see the Suggested Reading list found at the end of the Haz-Ed materials. Other Haz-Ed materials related to this topic include Fact Flash 2: The Superfund Cleanup Program.


  1. Assemble the following materials:

    • Copies for each student of Student Handout 1, Major Categories of Tests for Ecological Assessment, found at the end of this activity
    • Copies for each student of Student Handout 2, Case Study: Tidal Bay Ecological Assessment.

  2. Read the student handouts to prepare your lecture.

  3. Distribute copies of Student Handout 2 and assign students to read the case study for homework.

NOTE: In this lesson, students will encounter a large number of scientific terms and phrases. Depending on the grade level in which you use the lesson and the skill level of your students, you may need to spend extra time introducing unfamiliar vocabulary and preparing your students for this lesson. Many terms are defined in the Glossary found at the end of the Haz-Ed materials, but a textbook may be helpful.


Class #1

  1. Ask the class to define an ecosystem. Then ask the class to distinguish between an ecosystem and the environment. (An ecosystem is a specialized community, including all the component organisms, that forms an interacting system, for example, a marsh. An environment is the totality of conditions surrounding an organism.)

  2. Organize the students in groups of 3 or 4 students each, and ask each group to write down how hazardous waste released into the environment can affect plants (flora) or animals (fauna) in an ecosystem. Ask them to list as many possibilities as they can think of in 10 minutes.

  3. Ask each group how many ideas they wrote down.

  4. Ask the group with the fewest ideas to lead off by naming one effect of pollution. Ask the rest of the class if they agree that the named effect can result from pollution. Ask those who agree to explain why they agree. Ask those who do not agree to explain their reasons.

    Continue the discussion by asking each group in turn to add to the list. Have a student write the ideas on the chalkboard as they are mentioned. Some possible effects that could be listed include:

    • Gaps in vital food chains or nutrient cycles
    • Reproductive problems (such as eggshell thinning or loss of nesting materials)
    • Developmental effects (such as malformed chick beaks)
    • Tumors (such as fish tumors)
    • Critical organ damage (such as liver, kidney, or skin lesions)
    • Immune system dysfunction (leading to, for example, viral infections in dolphins)
    • altered individual or population growth rates
    • Changes in population and community organization
    • Loss of total biomass (flora and fauna)
    • Relative loss of taxa or species abundance in defined areas (such as fish kills, amphibian mortality, macroinvertebrate depletion)
    • Loss of species diversity.

    Some of the students' ideas may overlap because one adverse ecosystem effect can impact another since life in ecosystems is interdependent. For example, reproductive and developmental problems in birds may ultimately cause a decrease in species abundance and diversity, which in turn may alter community organization.

    Some of the students' ideas may overlap because one adverse ecosystem effect can impact another since life in ecosystems is interdependent. For example, reproductive and developmental problems in birds may ultimately cause a decrease in species abundance and diversity, which in turn may alter community organization.

    Students may not come up with many ideas. In a way this reflects the current state-of-the-science in ecological assessment. It can be difficult to determine what is occurring or could occur in an ecosystem as a result of pollution; effects are often very subtle unlike those of habitat loss, which usually tend to be very obvious.

    Leave the list on the chalkboard.

  5. Distribute the Student Handout, Major Categories of Tests for Ecological Assessment. You may wish to have students discuss the various types of tests in more depth before proceeding.

  6. Ask the students which category or categories of tests could be used to study each of the adverse effects listed on the chalkboard. Have students explain the choices they make. Encourage the rest of the class to comment on various answers.

Class #2

  1. Give each student a copy of the Student Handout, Case Study: Tidal Bay Ecological Assessment. Give them about 30 minutes to read it in class or as homework. Questions and guidelines are in the text to assist them in their review. Ask them to answer as many of the questions as possible.

  2. For the remainder of the class period, discuss the case study and the students' answers to the questions. An Instructor's Answer Key is included at the end of this lesson for your use.

Extensions (Optional)

  • After allowing each group to add to the list of ideas about how hazardous waste can adversely effect ecosystems, ask students to rank the items according to their importance. Have them discuss their choices.
  • Point out during the discussion that sometimes investigators limit ecological components of concern to commercially important species (e.g., blue crabs in the Chesapeake Bay). Have students discuss how this might influence public attitudes with respect to proposed environmental regulations or legislation.

Instructor's Answer Key - Handout 1
Case Study: Tidal Bay Ecological Assessment

  1. What are the benefits of comparing contaminant concentrations and biological impacts in Tidal Bay sediments with those of a reference area?

    By expressing all chemical and biological measures as changes (increases or decreases) relative to a "normal" ecosystem (Shipshape), comparisons can be made that provide a sound basis for identifying and quantifying effects. Comparing results with a reference area allows investigators to determine not only what is not "normal" in the study area, but also how much weight to place on the changes.

  2. What are some of the limitations (problems) associated with the use of a reference area and with the choice of Shipshape Inlet as this area?

    Shipshape Inlet differs in sediment type from Tidal Bay, and although it may be the least polluted area of those studied in the basin, it is hardly a pristine environment unaltered by urbanization and industrialization. Furthermore, comparing a complex biological response such as benthic macroinvertebrate community change with a reference site requires reducing the data to a single value(s), which results in a substantial loss of data.

  3. Can you think of another approach that would work?

    If Tidal Bay contained only one or possibly two specific wastes, the contaminant concentrations and biological measures of their impact on the ecosystem could be compared with toxicity and risk levels published in the literature or in government databases. It is not known, however, how complex chemical mixtures interact to possibly increase or decrease the effects of individual chemicals. Further, the exact combination of chemicals in Tidal Bay may be unique. So, under these circumstances, the use of a reference area is probably the best choice.

  4. What impact do you think the presence of multiple types of hazardous waste will have on the ability of investigators to establish a cause-and-effect relationship between specific chemicals and adverse biological changes in Tidal Bay?

    Ideally, characterization of ecological impacts from hazardous waste is supported by definitive cause-and-effect relationships between specific chemicals and biological endpoints. Almost no information is available for establishing cause and effect for chemical mixtures, however, so they will not be able to determine specific cause and effect relationships. In lieu of a standardized approach for assessing ecological impacts of complex chemical mixtures, the Tidal Bay investigators developed relative measures of effect based on the reference area.

  5. Do you feel these measurements are relevant to this aquatic ecosystem?

    A number of biological measures are used to quantify the pollution impact on Tidal Bay. These include several toxicity tests, benthic community composition, and fish histopathology. All of these measures can be justified on ecological grounds. For example, amphipods are crustaceans that reside in Tidal Bay and are important prey for higher trophic-level species like fish. Also, they are relatively sensitive to toxic chemicals and are highly likely to be exposed to contaminants because they burrow in and feed on sediment material.

    Oysters also are considered useful indicators of ecological effects because they are very sensitive to toxic chemicals. The oyster test is a standardized test of developmental effects, which provides a broader view of adverse effects than lethality tests alone.

    Benthic macroinvertebrate species also are valuable indicators of toxicity because they live in direct contact with sediments, are relatively stationary, and are important components of aquatic food chains. Many fish and crab that live near the sediment feed on benthic organisms and are exposed to contaminants through the food chain.

    Note: although the investigators avoided limiting ecological components of concern to commercially important species or to those selected for the sake of political expediency, the ecological significance of the effects observed in the bioassay tests is not explained in terms of the entire ecosystem of Tidal Bay.

  6. Are these measurements likely to furnish the kind of data required to fulfill the purpose of the assessment? If not, how would you change the approach?

    The use of multiple chemical and biological tests (such as sediment chemistry, sediment toxicity, benthic macroinvertebrate assemblages, tissue residues resulting from bioaccumulation, and fish liver histopathology) provides a powerful weight-of-evidence approach to identify pollution problems in an ecosystem. They also provide the kind of data needed to define the extent of hazardous waste contamination in estuarine sediments and the magnitude of damage to benthic organisms and fish.

  7. Investigators characterized degradation of benthic macroinvertebrate communities in terms of a decrease in the abundance of total amphipods, molluscs, polychaetes, or total macrofauna. Many conditions can influence the overall abundance of benthic macroinvertebrates, including an algae bloom that depletes oxygen in the water. Did the investigators' report consider all factors that could have altered macroinvertebrate numbers?
  8. While some species may decrease in abundance due to chemical pollutants, other, more pollution-tolerant species are likely to increase. This makes changes in abundance at a major taxon level or at the total macrofauna level an unreliable indicator of ecosystem health. Generally speaking, a chemical pollution problem is the only condition that will render a waterway totally devoid of macroinvertebrates. However, the investigators did not study the levels of macroinvertebrate species in detail, possibly because of the extra costs involved. Precise and careful analyses of macroinvertebrate samples is time consuming and expensive. Also, they did not report looking at other possible causes for macroinvertebrate depletion.

  9. Could apparent effects thresholds be determined for bioaccumulation and histopathology in fish? Why do you suppose investigators did not do this?

    Apparent effects thresholds could have been established for bioaccumulation and histopathology in fish, but the purpose of the apparent effects thresholds was to rank specific problem areas within the bay. The fish indicators reflect a wide area of conditions. Also, there is a lot of uncertainty associated with how much hazardous waste the fish have been exposed to in the water and food chain and for how long. Thus, it is difficult to link the bioaccumulation and histopathology data directly to chemical concentrations in specific sediment samples.

  10. What are some major strengths of the apparent effects thresholds and what are some limitations?

    The apparent effects thresholds method is a plausible approach for dealing with problems created by contamination and uncertain cause-and-effect relationships. It uses empirical relationships to get around difficulties like bioavailability and synergistic and antagonistic relationships among chemical mixtures. The approach is limited for several reasons: it does not describe cause-and-effect relationships, it is site-specific (specific to certain areas), does not take into account data on bioavailability of chemicals in organ tissues, and lacks independent validation.

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