The study of mercury provides a good example of bioaccumulation and biomagnification of the chemicals in the ecosystem. Environment Canada has a simplified worksheet reinforcing the idea of biomagnification. The activity below outlines a popular simulation game illustrating bioaccumulation and biomagnification. The activity was developed by Project Wild and is available on the web from a number of different sources. We have adapted this exercise from a unit on food chains and bioaccumulation published by the Fish and Wildlife Service.
- Construction paper squares: For each student, prepare 20 white and 10 colored squares to represent ‘food’
- Provide one paper bag per ‘grasshopper’ (see below – students will be grouped into grasshoppers, shrews, and hawks)
Review the terms food chain, food web, ecosystem, bioaccumulation and biomagnification. Since the students will be assuming roles of grasshoppers, shrews, and hawks, it would be useful to use grasshoppers, shrews, and hawks within the examples of food chains and food webs that you provide for students.
Playing the Game
Take students outside, to a gymnasium, or large open space. Since you will be scattering bits of paper around, it is important that it not be a windy space.
Divide the class into 3 groups: grasshoppers, shrews and hawks - with approximately 3 times as many shrews as hawks and grasshoppers as shrews (ex. in a class of 26 students, there would be 2 hawks, 6 shrews and 18 grasshoppers), you may wish to mark them for easy identification using arm ties or bandanas.
- Give each grasshopper a bag to represent its 'stomach' and then have the students look the other way while you spread the paper squares of 'food' all over the playing area.
- Instruct the grasshoppers to enter the playing area and to quickly gather as much food as they can in 30 seconds while the hawks and shrews sit on the sidelines and watch.
- After 30 seconds, while the grasshoppers continue to look for food, allow the shrews to enter the playing area to catch the grasshoppers by tagging them. This stage of the game should last for a short amount of time (enough for some grasshoppers to be “caught”)–the right amount of time depends on the size of the playing field and number of students. When a grasshopper is tagged by a shrew, he/she must give the bag of food to the shrew and then sit down.
- While shrews are still trying to catch grasshoppers (and after a few have been caught), allow the hawks to enter the playing area to catch the shrews. The same rules apply - when a shrew is caught it must give the bag of food to the hawk and sit down.
- Call an end to the game. You should time this so that there are still a few grasshoppers and shrews in the game. Ask students to circle together and look at the food bags they have, or report if they were eaten. Count the number of grasshoppers and shrews left and the food squares (white and colored) in their stomachs. Ask the hawks to empty out their bags and count the number of white and colored squares.
- Inform the students that the colored squares represent mercury. Have students determine how many of the lower trophic level organisms they ate, how many squares they ended up with, and the number of colored (mercury) squares that they acquired into their system.
- Next, head back to the classroom to do some follow-up fact finding.
Class Discussion of Game Results
The general idea that you want to have emerge from this game is that animals higher on the food chain end up with a greater amount of mercury. So, the important number for each student is the number of colored squares.
You should be aware that the “model” of bioaccumulation implied in this game is inaccurate in one very important respect: food never gets “used” to sustain the metabolism of grasshoppers, shrews, or hawks. What this means is that the percentage of mercury—the ratio of colored squares to all squares—will not increase as you move up the food chain. In a more realistic model we would “use up” white squares over time, which would do even more to magnify the mercury as it moves up the food chain. You don’t need to explain this to the students at the outset—but you do need to be aware of it yourself. What this means is that you want to have the students just focus on the count of colored squares (quantity of mercury) rather than on the ratio of colored squares to white squares.
Keeping this in mind, here are some “data” that you should have the students record.
- The number of surviving organisms of each type.
- The average number of grasshoppers eaten by each shrew and the average number of shrews eaten by each hawk.
- The total amount of mercury contained in each organism.
- The average amount of mercury absorbed by each species.
- The ratios of absorbed mercury as one moves up the food chain.
You might have the students portray the average amounts of mercury graphically.
REFLECTION/SUMMATIVE ASSESSMENT IDEAS
Once the students have worked through this idea of bioaccumulation as a group, you might want to have them work some problems individually or in small groups. The following questions couple the lessons learned from the Food Chain Game with the work on quantities introduced in Unit 1.
Question Set Activity A
- Suppose a hawk eats 1800 rodents per year (more info about hawks), and suppose that the rodents have an average mercury concentration of 0.05 µg/g and an average weight of 20 g. How much mercury (total mass) would a bird of prey eat in a lifetime of 20 years?
- If 5% of the mercury ingested is excreted from a bird of prey, and we assume that the bird weighs a kilogram at its death, what would be the concentration of mercury in that bird at that time? Please note that not all of the mercury in the prey is in the toxic methylmercury form, so this question is a hypothetical illustration of what would be a maximum accumulation.
- Assume that the bird of prey dies because it was eaten by an owl. The owl then eats two more almost identical birds of prey. How many grams of mercury has the owl ingested in these three meals alone?
The Biodiversity Research Institute has done a substantial amount of research looking at the mercury burden in bald eagles in Maine. Here is a link to a poster presentation summarizing some recent work. The mercury levels sampled among eaglets ranged from 0.08 to 1.27 ppm, with an average of 0.59 ppm. The mercury levels sampled from adult eagles ranged from 0.94 to 93.5 ppm. For adults, the average amount of mercury sampled from eagles living along rivers was 27.7 ppm; for eagles living on lakes it was 42.6 ppm.
- How does the mercury concentration measured in adult eagles compare to the concentration that we estimated as the maximum for our imaginary hawk? If it is different, what might explain the difference?
- Why do the adult eagles have so much more mercury than eaglets? What might account for the wide range of values found in adult eagles?
- Notice the higher levels of mercury in eagles living along lakes. What might explain this difference?
- Give 3 examples of other food chains involving humans in which pesticides may enter. Discuss the consequences of these pesticides entering these food chains.
- An ecologist studied the presence of a toxic chemical in a lake. She found the water had one molecule of the chemical for every billion molecules of water (1 ppb). The algae had one part per million (1 ppm) of the toxic chemical. Small animals, zooplankton, had 10 ppm. Small fish had 100 ppm. Large fish had 1000 ppm. How do you explain the increase in this toxic chemical to 1000 ppm for the large fish? Use a drawing to help support your answer. The ecologist found the chemical was mercury which had been emitted into the air many miles away from the lake. How did so much of it get into the lake?
- Who would have a greater concentration of mercury in their body- a person eating fish daily? A person who eats fish weekly? A person who eats fish only every now and then?
- Why is it recommended that you eat salmon, sardines, and smelt, but not swordfish, shark, or pickerel?
Teachers focused primarily on preparing students to investigate mercury levels in insects or fish might find that this first activity provides all of the introduction to toxicology that students need. Teachers in health occupations, however, might usefully follow this lesson with the next two lessons in the Science NetLinks Toxicology series: Toxicology 2: Finding the Toxic Dose and Toxicology 3: Toxicology and Human Health.
Biology teachers might want to connect this toxicology study to lessons on the structure of the cell. There are also potential connections to the study of anatomy.
The Society of Toxicology suggests the following topics for more extensive student work on its Topics in Toxicology page:
- Gulf War Syndrome
- Domoic Acid
Each topic is associated with a number of questions that require students to perform independent research. The questions range from factual to interpretive to evaluative. The goal is for students to gain enough information on a particular topic to be able to form an opinion about it. The site also provides Teacher’s Notes with a brief description of each of the topics and suggestions for use in a classroom toxicology discussion.
TEACHER'S CHEAT SHEET
These documents can be viewed by clicking the links below. Individual PDFs of each document can be downloaded from the "Attachments" at the bottom of this web page.
Answer Key: Question Sets
These documents can be viewed by clicking the links below. Individual PDFs of each document can be downloaded from the "attachments" at the bottom of this web page.