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Rat Attack

Classroom Activity


Activity Summary
Students use an online interactive to explore what happens to one forest ecosystem when a resource pulse occurs.

Learning Objectives
Students will be able to:

  • understand that an ecosystem encompasses both biotic (organisms) and abiotic components (such as light, nutrients, and moisture).

  • describe the interactions among the components of one forest ecosystem.

  • predict how a forest ecosystem might change when a resource pulse occurs.

Suggested Time
One class period

Materials

Multimedia Resources

Additional Materials

Background

Like all ecosystems, a forest ecosystem is a complex biological and physical system. A forest ecosystem comprises:

  • plants, both woody and non-woody
  • soils (and the organisms living within)
  • animals (including humans) that live and visit there
  • weather and climate (temperature, precipitation) and other abiotic factors, such as light, nutrients, elevation or altitude, and pollution

All of these components—climate, soils, plants, and animals—interact with and influence each other. To understand how ecosystems behave, scientists study energy transfer, cycling of nutrients and elements, and interactions among the organisms within the system.

Each group of organisms in a forest ecosystem has a specific role. The main ecological functional groups are primary producers, primary consumers (herbivores), secondary consumers and higher consumers (predators and parasites), and decomposers. The study of feeding habits among organisms helps scientists understand energy transfer through food webs as well as shifts in population densities when the availability of a food resource changes or another perturbation occurs in the system.

Complex interrelationships between ecosystem organisms are often difficult to unravel. The availability of resources—which can include food, inorganic nutrients, water, number of prey, and living space—plays a role in determining the density of populations of organisms within the community, especially of the consumers. Furthermore, these relationships can change over time and space (area in which organisms exist).

Changes in resource availability often cause changes in population densities. Such changes can reveal important interrelationships among ecosystem components. For example, ecologists have been able to observe increases and decreases in population densities when a short-lived pulse—a superabundance of a resource food and/or nutrients—suddenly occurs. Although infrequent, these occasions of pulsed resources happen throughout nature in both terrestrial and aquatic ecosystems and can involve food (such as seeds), inorganic nutrients, water, prey, or detritus.

Two examples of resource pulses are masting, a synchronous flowering and massive production of seeds in some forest ecosystems, and the El-Niņo Southern Oscillation-induced nutrient upwelling in aquatic coastal ecosystems. Both phenomena dramatically and rapidly increase the levels of primary production available to primary consumers in the ecosystem. These fleeting and infrequent pulses of rapidly increased amounts of food, followed by a depletion of that food, set off a "boom and crash" cycle of increases and subsequent decreases. By observing organisms' responses to the pulsed resource, ecologists can determine both bottom-up and top-down effects on the food web.


Procedure

Before the Lesson

  1. Bookmark or download the Population Explosion Flash Interactive on student computers and on a teacher computer that is connected to a projection screen. (If you or your students have problems opening the downloaded version, drag the file into a browser window.)

  2. Make a copy of the Acorns Aplenty handout for each student.

The Lesson

  1. Ask students what makes up an ecosystem. Prompt them to think of both biotic and abiotic factors. Write responses on the board. (Biotic factors refer to all the living organisms in the ecosystem. Abiotic factors refer to nonliving factors—such as climate, weather, and temperature.)

  2. Ask students to think about the biotic and abiotic factors they named and use that information to identify different types of ecosystems. How are these ecosystems similar and different? (Answers will vary and should include examples of terrestrial and aquatic ecosystems. Students can also suggest "nontraditional" ecosystems like their schoolyard or neighborhood. Point out that humans are part of many ecosystems.)

  3. Display the Oak Tree Forest Ecosystem teacher sheet on an overhead. Remind students that these populations represent only a small number of the species that live in the oak tree forest ecosystem. (Make sure students understand that the pictures are representative of populations of that species and not of an individual organism.) Ask students how they think the organisms might interact with each other in this forest ecosystem. (Answers will vary but should include some feeding relationships and should mention trees existing at the bottom of the food chain.) With the class, explore which organisms in the ecosystem are primary producers, primary consumers, secondary consumers, higher consumers, and decomposers. (These terms describe feeding relationships between species, which are often more complex than those represented in simplified food chains and food webs. For example, in their role as primary consumers, songbirds eat berries, fruit, and seeds. They also eat plant-eating insects, making them secondary consumers as well. In almost all ecosystems, primary producers are photosynthetic organisms such as plants, algae, and cyanobacteria. Primary consumers are animals that eat primary producers; secondary consumers are animals that eat primary consumers; higher consumers eat secondary consumers and/or consumers from higher feeding levels in the food chain; decomposers eat dead plants and animals.) What abiotic factors might affect this ecosystem? (Some abiotic factors affecting this ecosystem would be rainfall, soil nutrients, soil architecture, sunlight, temperature, forest size, air, or soil pollutants.)

  4. If you are doing this lesson in the fall, bring in some acorns with caps for students to study. What are the parts of the acorn? (The cap that connects the acorn to the tree and the nut within the cap.) What do students notice about the size of acorns in relation to other seeds, like those of sunflowers or dandelions? (Acorns are much larger than sunflower or dandelion seeds. They are too heavy to be dispersed by the wind, so other organisms—such as birds, mice, squirrels, pigs, bears, and deer—are needed to help disperse them.)

  5. Start the Population Explosion Flash Interactive. Review the Cast with each student (place your cursor over each organism to get information about it). This information will provide students with background on the organisms in the oak forest. Students will use what they learn to uncover the relationships between and among the organisms. Use questions to facilitate understanding of the interactions of these species. For example, ask students to identify consumers of the gypsy moth. Are they primary consumers, secondary consumers, or higher consumers? (Mice are consumers of the gypsy moth; because gypsy moth pupa eat tree leaves, mice are secondary consumers.) Have students defend their reasoning.

  6. Ask students what they think would happen to the populations in this ecosystem if the oak trees were to produce a superabundance of acorns, an event known as a resource pulse. Inform students that a resource pulse is an ecological phenomenon in which a large amount of a resource (in this case, food) is rapidly introduced into an ecosystem. Pulsed-resource inputs to ecosystems are rapid, short-lived, and infrequent. On occasion, several species of trees, such as oak and bamboo, synchronously flower and put out an unusually large amount of seeds—a pulsed-response called masting. Explain that the synchronous nature of the mast makes large amounts of seeds available to consumers in the ecosystem. In oak tree forest ecosystems, this means that there will be lots of acorns (seeds) available for primary consumers. Ask students why masting would be considered an adaptive strategy. (So much seed is produced that predators cannot eat it all; this allows the plant to successfully produce offspring.)

  7. Have students predict any changes they might expect to see in population densities of the oak tree forest within two or three years after the mast. Would they expect to see all of the changes happening right away? Why or why not? Ask students to make a drawing representing what they think will happen. (Answers will vary but should at least include the mouse population increasing. Some students may predict that there will be a delay in some responses.)

  8. After you review the Cast, have students explore the interactive on their computers.

  9. When they are finished, assign each student one of the following sets of organisms to work on, and distribute the Acorns Aplenty handout to each student:

    Set 1Set 3Set 5
    Oak Tree/AcornAdult SongbirdsAdult Ticks
    MiceFledgling SongbirdsLarval Ticks
    Songbird EggsTick Eggs

    Set 2Set 4
    RaptorsAdult Gypsy Moths
    DeerMoth Pupae

  10. Group students into teams according to their organism set. Tell the class each team will work together as expert ecologists to fill in the chart and tell a two-year story of the population "ups and downs" of their organisms. They should be able to explain the causes of any changes in their organisms over time. Students can review the interactive to locate information that will help teams fill out their table. Each team should come up with a story summarizing what happens to its organisms over time. Then each team member should create a copy of the the completed story that they have agreed on with their team members.

  11. After all teams have completed their assignment, organize the class into new teams, each of which should contain enough experts so that all 12 organisms in the ecosystem have representation within each team. Have the new teams combine organism stories and put together a comprehensive narrative of what happened with the entire forest ecosystem over the two years, including the outcome of each organism at the end of each year.

  12. When teams have finished their narratives, use the following questions to guide a class discussion about the acorn masting and its effect on the ecosystem:

    • How many food chains can you identify in this ecosystem? (There are multiple food chains in this system; students may name some of the following:

      • acorns → mice → raptors
      • acorns → mice → deer ticks → Lyme bacterium →
        [+ humans]
      • oak tree → gypsy moths → mice → raptors
      • oak tree → gypsy moths → mice → deer ticks → Lyme bacterium → [+ humans]
      • oak tree → gypsy moths → songbirds → raptors
      • gypsy moth pupae → mice → raptors
      • gypsy moth pupae → mice → deer ticks → Lyme bacterium → [+ humans]
      • seeds/berries/fruit → songbirds → raptors
      • songbird eggs → mice → raptors
      • songbird eggs → mice → deer ticks → Lyme bacterium → [+ humans])

    • What patterns or trends did you observe as these populations responded directly or indirectly to the mast of acorns? Cite specific examples. (Responses might include: a) Changes in populations do not all happen at the same time. It takes time for a population to respond to a change in another population, taking into account such factors as the time needed to reproduce and the length of the life cycle. Because of this, responses of one population to a change in another can be delayed from several months to several years. b) The key player[s] initiating the booms and crashes are the oak trees and their primary consumer, the mice. c) The primary consumers [the mice] increase their rate of reproduction in response to an increased food supply [the acorns]. However, the population crashes when the acorns are eaten and the supply is depleted. The boom causes a wave of responses in other organisms, and this is followed by a crash, which then sets off a secondary set of responses. d) A boom in one population can cause a crash in another [e.g., increased predation causes a reduction in prey]. e) Populations of organisms [such as deer] migrate into and out of the forest following the pulse.)

    • Which consumer had the greatest impact in determining the densities of the various populations in the forest ecosystem after the acorn mast? Why? (The mice had the greatest impact because they had feeding relationships [eating and being eaten] with more of the other organisms in the forest. They were omnivores because they ate both plants and animals [such as gypsy pupae and songbird eggs] and were adaptable in that they could switch from one food source to another if their favorite food type was not available. Mice were also preyed upon by other organisms. In contrast, even though the deer ate acorns, they migrated out of the forest, so their impact over time was limited to bringing in ticks and increasing the tick populations.)

    • Raptors, such as owls and hawks, can switch their food source from mice to other types of foods if mice are unavailable. Predict what might happen in the oak forest ecosystem if raptors ate only small rodents like mice. (When the mouse population booms, there is plenty of food for the raptors to survive, reproduce, and feed their young. Their populations would increase, and, just when their young are hatched and ready to be fed, the raptors would be facing a depletion of mice. If the raptors could not switch their food source to non-rodents or to different types of rodents, their population would crash.)

    • How might the gypsy moth, which defoliates oak trees, affect acorn masting? (The stress on the oak tree could delay masting.)

    • Small, fragmented forests tend to have more mice and fewer other species, including predators. Because mice serve as very good reservoirs for the Lyme bacterium, smaller forests tend to have more Lyme. How might Lyme disease be reduced in these areas? Would getting rid of deer reduce Lyme? (Student answers might include connecting small patches of forests with corridors to encourage more biodiversity, vaccinating mice or people, or introducing predators that feed on mice. Eradicating deer in fragmented forests may reduce Lyme disease because a limited number of species are available on which the tick can feed; however, in larger forest areas with a higher level of biodiversity, reducing deer populations may not be as effective due to an increased availability of hosts on which the ticks can feed.)



Assessment

Use the following rubric to assess students' work.

Excellent

Satisfactory

Needs Improvement

Forest Ecosystem Organism Profile

The profile is complete. The student demonstrates a strong and accurate understanding of their organisms' niches and their importance to the forest ecosystem.

The profile is complete. The student demonstrates a satisfactory understanding of their organisms' niches and their importance to the forest ecosystem.

The profile is incomplete. The student demonstrates a limited understanding of their organisms' niches and their importance to the forest ecosystem.


Standards

The "Acorns Aplenty" activity aligns with the following National Science Education Standards.

Grades 5-8
Life Science

• Populations and ecosystems
• Diversity and adaptations of organisms

Grades 9-12
Life Science

• Interdependence of living organisms
• Matter, energy, and organization in living systems
• Behavior of organisms


Classroom Activity Author

Candace Dunlap taught high school biology in Massachusetts for 20 years. She has worked as a classroom teacher, director/supervisor of math and science, and dean of curriculum in public schools.

Teacher's Guide
Rat Attack
WATCH A PREVIEW BUY THE VIDEO WATCH THE VIDEO ONLINE PROGRAM OVERVIEW CLASSROOM ACTIVITY RELATED NOVA RESOURCES INTERACTIVE FOR STUDENTS


Interactive Population Explosion Flash Interactive (downloadable)


Video is not required for this activity



































ExxonMobil
Koch Foundation
HHMI
CPB