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Ecosystems | Lesson 4 - Ecosystem Services and Changes in the Environment

Lesson 4: Carbon Pools and Fluxes in Changing Ecosystems

NOTE: This lesson introduces a quantitative model of carbon pools and fluxes, then uses that model to explain how ecosystems change over time. This is challenging content that addresses NGSS High School Performance Expectations. You may want to skip it with middle school classes.

In Lesson 2 students identified the pattern of biomass distribution (the biomass pyramid) in a meadow ecosystem. In Lesson 3 they explained why that pattern exists by tracing matter and energy and connecting scales. In Lesson 4, students explain changes in ecosystems by keeping track of carbon pools and carbon fluxes. Carbon fluxes are the rates (mass/time) at which carbon transforming processes occur.

Guiding Question

How and why do carbon pools in ecosystems change over time?

Activities in this Lesson

  • Activity 4.1: Tiny Pool and Flux Game (30 min)
  • Activity 4.2: Carbon Pools and Constant Flux Simulation (30 min)
  • Activity 4.3: How Fluxes Change and Carrying Capacity (40 min)
  • Activity 4.4: Seasonal Changes and Ecosystem Disturbances (40 min)

Objectives

  1. Explain how changes in size of carbon pools are governed by fluxes into and out of carbon pools.

NGSS Performance Expectations

Middle School

  • Matter and Energy in Organisms and Ecosystems. MS-LS2-1. Analyze and interpret data to provide evidence for the effects of resource availability on organisms and populations of organisms in an ecosystem.
  • Interdependent Relationships in Ecosystems. MS-LS2-2. Construct an explanation that predicts patterns of interactions among organisms across multiple ecosystems.
  • Matter and Energy in Organisms and Ecosystems. MS-LS2-3. Develop a model to describe the cycling of matter and flow of energy among living and nonliving parts of an ecosystem.
  • Matter and Energy in Organisms and Ecosystems. MS-LS2-4. Construct an argument supported by empirical evidence that changes to physical or biological components of an ecosystem affect populations.
  • Matter and Energy in Organisms and Ecosystems. MS-LS1-6. Construct a scientific explanation based on evidence for the role of photosynthesis in the cycling of matter and flow of energy in and out of organisms.
  • Human Impacts. ESS3-4. Construct an argument supported by evidence for how increases in human population and per-capita consumption of natural resources impact Earth’s systems.

High School

  • Interdependent Relationships in Ecosystems. HS-LS2-1. Use mathematical and or computational representations to support explanations of factors that affect carrying capacity of ecosystems and different scales.
  • Matter and Energy in Organisms and Ecosystems. HS-LS2-4. Use mathematical representations to support claims for the cycling of matter and flow of energy among organisms in an ecosystem.
  • Matter and Energy in Organisms and Ecosystems. HS-LS2-5: Develop a model to illustrate the role of photosynthesis and cellular respiration in the cycling of carbon among the biosphere, atmosphere, hydrosphere, and geosphere.
  • Earth’s Systems. HS-ESS2-6. Develop a quantitative model to describe the cycling of carbon among the hydrosphere, atmosphere, geosphere, and biosphere.

Background Information

In Lesson 4 students explain carbon movement in ecosystems in terms of pools and fluxes. A pool size is constant when the fluxes into and out of it are the same or balanced – i.e., when the net flux is zero. Pool sizes change when in and out fluxes are not balanced. High school students explain carrying capacity as the maximum, steady state amount of biomass an ecosystem can support. One of the factors that determines carrying capacity is the rate of photosynthesis. When an ecosystem reaches its carrying capacity, the rate of photosynthesis and cellular respiration are essentially the same. Students explain how different types of disturbances affect carrying capacity.

Activity 4.1 introduces students to a 2-carbon pool model of an ecosystem where organic carbon is in biomass and inorganic carbon is in the atmosphere in the form of carbon dioxide. The hands-on activity uses a very simple system to introduce students to the effects of fluxes on pool sizes. They discover that balanced fluxes, that is situations where the net flux is zero, result in unchanging pool sizes. Through graphing exercises, students associate the value on the y-axis (the amount of carbon) with pool size and the slope of the line (carbon movement per unit of time) with flux rates.

In Activity 4.2, students use a computer simulation of a larger 2-pool system to find that the rules still hold. When the net flux into and out of a pool is zero, the pool size does not change and conversely, when the net flux is not zero, the pool size will change.

In Activity 4.3, students use a more sophisticated model of carbon pools and fluxes to explain that carrying capacity is affected by the maximum photosynthesis rate. In this model, the flux rates of cellular respiration and photosynthesis, rather than being constant, depend on the pool size.

In Activity 4.4 students use the 2-pool simulation to explore the effects of various disturbances on ecosystems. Press disturbances are long-term changes to the environment that may affect an ecosystem’s carrying capacity. Pulse disturbances are one-time events that ecosystems often recover from.

Key carbon-transforming processes: photosynthesis, cellular respiration, combustion

Ecosystems Unit Map