Food-web Dynamics in a Marine Ecosystem
Graduate Partners in Science Education
Learning Objective(s):
AZ DOE---Grade 7, Strand 4, Life Science, Concept 3
PO1: “Compare food chains in a specified ecosystem and their corresponding food web”
PO3: “Analyze the interactions of living organisms with their ecosystems (limiting factors, carrying capacity)”
CORE IDEA LS2—“Ecosystems: Interactions, Energy, and Dynamics”
LS2.A*—“Interdependent Relationships in Ecosystems:
  1. Organisms, and populations of organisms, are dependent on their environmental interactions both with other living things and with nonliving factors. (MS-LS2-1)
  2. In any ecosystem, organisms and populations with similar requirements for food, water, oxygen, or other resources may compete with each other for limited resources, access to which consequently constrains their growth and reproduction. (MS-LS2-1)
  3. Growth of organisms and population increases are limited by access to resources. (MS-LS2-1)
  4. Similarly, predatory interactions may reduce the number of organisms or eliminate whole populations of organisms. Mutually beneficial interactions, in contrast, may become so interdependent that each organism requires the other for survival. Although the species involved in these competitive, predatory, and mutually beneficial interactions vary across ecosystems, the patterns of interactions of organisms with their environments, both living and nonliving, are shared. (MS-LS2-2)”
LS2.C*—“Ecosystem Dynamics, Functioning, and Resilience:
  1. Ecosystems are dynamic in nature; their characteristics can vary over time. Disruptions to any physical or biological component of an ecosystem can lead to shifts in all its populations. (MS-LS2-4)
  2. Biodiversity describes the variety of species found in Earth’s terrestrial and oceanic ecosystems. The completeness or integrity of an ecosystem’s biodiversity is often used as a measure of its health. (MS-LS2-5)”
*From:
Materials:
  1. Three 10-20 gallon fish tanks
  2. Several magnets of different sizes
  3. Several paperclips of different colors
  4. Sticky notes in different colors (if possible to match the paper clips)
  5. 3 boards, markers, and erasers
  6. Paper towels
  7. Timer
  8. Notebooks
  9. Pencils

10 min. / Engage
  1. Start by showing a video, such as this one: (short animated video of a marine food web, 1:37min long).
  2. Continue by showing a diagram of a marine food web and explaining differences between food chains and food webs while engaging students with questions. Some examples are:
  1. Explain that interactions among marine organisms can become very complex, and that the health of marine ecosystems depends on its ability to maintain its biodiversity and function.

20min / Explore
  1. Have the experiment set up and explain the procedures and rules to the students.
  1. Each paper clip color will represent a type of prey species (see details below)
  2. Each magnet size will represent a type of predator species (see details below)
  3. Each predator can only eat selected species (specialized eating; see details below)
  4. The prey and predator species will simulate a particular ecosystem
  5. Fishing times are 1 minute, and they have to stop when prompted
  6. There will be 2 rounds of fishing
  7. There will be 3 groups simulating 3 potential results from the same activity
  1. The tank filled with water, paper clips will be mixed and put inside.
  2. ROUND 1: Students will each grab a different sized magnet (smaller) and go “fishing” for clips.
  3. After a minute, they will stop and count how many prey they caught and of which kind, and record their results.They must keep track of prey species caught even if they are not the food item for that specific predator (this will be used to build a food web in step 7).
  4. ROUND 2: Students will repeat but this time, they will place the smaller magnets inside the tank and use the bigger ones to do the “fishing”.
  5. Students will stop after 1 minute and once again tally their results, this time including any predator species they might have also caught.
  6. With the data gathered, students will build a structural map of the food web of their marine ecosystem on the board by using the sticky notes (representing the different types of organisms), and drawing arrows with the markers. Please see description below for more details. Can calculate averages of several runs to come up with mean catch values.
  7. Lastly, all 3 groups will compare their results (maps).

10 min / Explain
  1. Once more, compare food chains and food webs using their maps
  2. Explain that simulations can go in different ways due to randomness.
  3. Explain that resources are limited (an ecosystem can only hold so much; limiting factors), and organisms are competing for them to survive and reproduce. Therefore, populations cannot grow out of control (carrying capacity, or K).
  4. Explain that there are limiting factors that shape the level of success an organism has when foraging for food (i.e. size, mouth piece, prey detection mechanisms, food availability, environmental conditions, etc.)
  5. Explain that predator-prey interactions maintain the population in equilibrium when nothing else is altering the ecosystem (touch more on this on the “Expand” section).

10min / Expand
  1. Following up on the last point of the “Explain” section, Explain to students how major disturbances an ecosystem suffers due to human activities can completely change the way the ecosystem functions, and its biodiversity.
  2. To illustrate this, use the example of a fisherythat harvests a particular species from one of the ones in their map. Use a really large magnet in the water tank.
  3. Ask students to go to their maps and remove the selected species (instructor can select which one) from their map, and see what happens to the connections they had made.
  4. Ask students: “what happens to your ecosystem when that species is removed? What happens to the biodiversity, and how does it reflect the state of health of that ecosystem?”
  5. After the discussion, explain why it is important that we make sure we only support sustainable fisheries, to help keep problems like this from happening.

10min / Evaluate
  1. After the activity, ask students to ask questions and then write for 5 min what they learned from the activity and what they enjoyed the most/least and why, followed by a paired or group-share.
  2. Go over the papers to detect any potential misconceptions that can be addressed on the spot before the lesson is over while they share their thoughts in pairs or groups.

Notes for instructors:

  1. Magnets need to be sized in a way that will allow bigger ones to pick up smaller ones (as well as other paper clips), and smaller ones to pick up paper clips. This will allow the simulation to be more accurate. The large magnet must be large enough to collect a lot of clips at one time (such as a fishery).
  2. The different colors representing the different species of prey are in themselves predators as well, which will be defined and given to the students when building their maps. This will facilitate the simulation but will ensure the map is relevant to the activity (see description below).

FOOD WEB EXAMPLE:SIMPLIFIED ALASKAN MARINE FOOD WEB

SIMULATION ITEM / QUANTITY PER TANK / REPRESENTS
Green clips / 100 / Plankton
Blue clips / 75 / Herring
Red clips / 50 / Salmon
Small magnet / 8 / Seals
Medium magnet / 2-3 / Orcas
Large magnet / 1 / Fishery (Humans)

DESCRIPTION OF EXERCISE:

  1. Prey item per predator species:
  1. Plankton can only be consumed by Herring and Salmon
  2. Herring can only be consumed by Salmon, Seals, and the Fishery
  3. Salmon can only be consumed by Seals, Orcas, and the Fishery
  4. Seals can only be consumed by Orcas
  5. Orcas are apex predators: no one consumes them in this simulation
  6. The examples that can be used for Fishery removal are Herring and Salmon
  1. Constructing the food web:
  1. When students are tallying their “catch” after the first fishing exercise, their small magnets (representing seals) will technically only count the blue and red clips (representing the herring and salmon). However, they might also catch green clips (representing plankton). Although this is not a prey item for seals, herring and salmon do need it to survive. Therefore, students should take note of it when they are building their food web map, and add it to the bottom of the tropic levels.
  2. For the second round, students will repeat the tallying and building of the food web but this time adding the medium magnets (orcas) as predators. The same thing applies in this case with regards to what is caught during the exercise.
  1. Effect of overfishing on marine food webs:
  1. During the “Expand” section, it might be useful to have students count how many prey species are left (herring or salmon), and compare those numbers to the ones they originally tallied in their catch. Most likely, these numbers might not be enough to feed the predator population, and therefore will cause a decline in their numbers. This can help show the effect of overfishing on marine ecosystems.

Lesson created by Yaiyr Astudillo-Scalia