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Ecosystem Cycles

Each of the first four volumes provides 25 probes with easy-to-follow steps for uncovering and addressing students’ ideas by promoting learning through conceptual change instruction. Probes cover topics such as physical, life, and Earth and space science; the nature of science; and unifying themes. Each volume on page 23 provides topic-specific probes. These invaluable books include teacher materials that explain content, identify links to standards, and suggest grade-appropriate ways to present materials so students learn the concepts accurately. Teachers, professional development coordinators, and college science and preservice faculty will find these resources essential and exciting.

Ecosystem Cycles

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Purpose

The purpose of this assessment probe is to elicit students’ ideas about the transfer of matter and energy in ecosystems. The probe is designed to reveal whether students recognize that only matter is cycled through an ecosystem.

Related Concepts

Ecosystem, cycling of matter, flow of energy

Explanation

Felicia has the best answer: “I think only matter cycles through an ecosystem.” Matter and energy both move through an ecosystem. However, only matter cycles back and forth between organisms and the environment; energy moves only in one direction, with much of it being dissipated into the environment as heat. Both matter and energy can be transferred from one organism to another or from an organism to the environment; but only matter cycles within ecosystems, being used in various forms as it moves through food webs, water, soil, and the atmosphere.

The cycles in which elements or molecules move through living and nonliving components in an ecosystem are referred to as biogeochemical cycles. Examples of common elements or molecules that go through biogeochemical cycles include water, carbon, oxygen, nitrogen, phosphorus, and sulfur. The important thing to remember is that matter cycles, energy flows. Thus, an ecosystem has a constant need for energy input via photosynthetic organisms capturing the Sun’s energy.

Curricular and Instructional Considerations

Elementary Students

In the elementary grades, students learn that some materials, including once-living organisms, are recycled by the earth. They know that ecosystems include living and nonliving matter and that all living things require a source of energy. They learn that the Sun is the major source of energy in most ecosystems on Earth and that all food can be traced back to plants. They can follow a food chain, from plants, to animals that eat plants, to animals that eat other animals, and to the decomposers that consume dead organisms and recycle once-living material. Understanding matter cycling at a molecular level and energy flow can wait until middle school. The water cycle is the biogeochemical cycle students learn about in elementary grades.

Middle School Students

Most of the emphasis in the middle grades is on following matter as it is transferred through food webs and into nonliving components of ecosystems. By the end of eighth grade, students should begin to link energy flow ideas to their knowledge of the cycling of matter in ecosystems. They should be able to follow the unidirectional transfer of energy as one organism eats another and know that energy does not cycle back into the ecosystem. They build on their previous knowledge of the water cycle by learning about other biogeochemical cycles, including the carbon, oxygen, and nitrogen cycles. Middle school students should also know that matter and energy are not created or destroyed as they move through an ecosystem and connect this idea to two major laws in science: the law of the conservation of mass and the law of the conservation of energy.

High School Students

High school students have sufficient knowledge of matter and energy to link these conversions to what happens in living and nonliving systems. Although there is no need to account for all the energy, students should know that most of the energy transferred from one organism to another is lost to the environment as heat and that this energy is not available for reuse. Knowledge of biogeochemical cycles is integrated across biology, chemistry, and Earth science, including the idea that some matter can remain in chemical reservoirs for a long time before it is cycled again.

Administering the Probe

This probe is most appropriate for middle and high school students. Make sure students know what a cycle is before using this probe.

Related Ideas in National Science Education Standards (NRC 1996)

K–4 Organisms and Their Environments

  • All animals depend on plants. Some animals eat plants for food. Other animals eat animals that eat the plants.

5–8 Populations and Ecosystems

  • For ecosystems, the major source of energy is sunlight. Energy entering ecosystems as sunlight is transferred by producers into chemical energy through photosynthesis. That energy then passes from organism to organism in food webs.

5–8 Structure of the Earth System

  • Water, which covers the majority of the Earth’s surface, circulates through the crust, oceans, and atmosphere in what is known as the “water cycle.”

9–12 The Interdependence of Organisms

  • Energy flows through ecosystems in one direction, from photosynthetic organisms to herbivores to carnivores and decomposers.

9–12 Matter, Energy, and Organization in Living Systems

  • As matter and energy flows through different levels of organization of living systems—cells, organs, organisms, communities—and between living systems and the physical environment, chemical elements are recombined in different ways. Each recombination results in storage and dissipation of energy into the environment as heat. Matter and energy are conserved in each change
Related Ideas in Benchmarks for Science Literacy (AAAS 2009)

K–2 Interdependence of Life

  • Animals eat plants or other animals for food and may also use plants (or even other animals) for shelter and nesting.

3–5 Flow of Matter and Energy

  • Almost all kinds of animals’ food can be traced back to plants. • Some source of “energy” is needed for all organisms to stay alive and grow.

6–8 Flow of Matter and Energy

  • Energy can change from one form to another in living things.
  • Organisms that eat plants break down the plant structures to produce the materials and energy they need to survive. Then they are consumed by other organisms.
  • Over a long time, matter is transferred from one organism to another repeatedly and between organisms and their physical environment. As in all material systems, the total amount of matter remains constant, even though its form and location change.

9–12 Flow of Matter and Energy

  • The chemical elements that make up the molecules of living things pass through food webs and are combined and recombined in different ways. At each link in a food web, some energy is stored in newly made structures but much is dissipated into the environment. Continual input of energy from sunlight keeps the process going.
  • At times, environmental conditions are such that land and marine organisms reproduce and grow faster than they die and decompose to simple carbon-containing molecules that are returned to the environment. Over time, layers of energy-rich organic material inside the Earth have been chemically changed into great coal beds and oil pools.

Related Research

  • Smith and Anderson (1986) found that almost all of the 12-year-old students in their sample were aware that there are cyclical processes in ecosystems. However, most of the students thought in terms of sequences of cause-and-effect events where matter was either created or destroyed and then the sequence repeated. They failed to recognize oxygen and carbon dioxide cycles or processes involving food. Their understanding of matter cycling was fragmented (Driver et al. 1994).
  • Leach et al. (1992) found that some students fail to recognize that matter is conserved in the processes of photosynthesis, assimilation of food, decay, and respiration. Furthermore, they had difficulty distinguishing between food, matter, and energy.
  • Gayford’s (1986) study of 17- and 18-yearold biology students revealed that many students thought that energy flows from place to place and is stored like a material. They thought energy was either created or destroyed in biological processes rather than converted and conserved.
  • Many children think that dead things simply disappear when they decay and are not cycled back into the environment (Driver et al. 1994).

Suggestions for Instruction and Assessment

  • Combine this probe with “Earth’s Mass” and “Rotting Apple” from Uncovering Student Ideas in Science, Vol. 3: Another 25 Formative Assessment Probes (Keeley, Eberle, and Dorsey 2008). Both of these probes address the concept of matter cycling by decomposers.
  • Having students account for where the matter and energy go may help them see that matter can cycle from organisms to the environment and from the environment to organisms, but that energy flows only in one direction. Instruction that uses charts of the flow of matter through an ecosystem and emphasizes the reasoning involved with the entire process may enable students to develop more accurate conceptions (NRC 1996).
  • Be sure to combine an understanding of matter cycling and energy flow with conservation of matter and energy in ecological cycles. However, be aware that the term conserved in an energy context can confuse students. Students may think conservation means the energy is still available for biological activity. Teachers should reiterate that the energy that is released as matter moves up the trophic pyramid is conserved—it does not simply disappear and cease to exist. However, organisms cannot hold onto that energy to do work. Rather, the energy is lost because it is converted into a form (heat) that most organisms cannot use efficiently. So even though total energy is conserved, the useful energy is lost from the working system.
  • Understanding that the breakdown and reassembly of molecules during matter transformation needs to precede the idea of matter cycling. This is particularly important when using processes such as photosynthesis and cellular respiration to explain carbon and oxygen cycling.
  • Combine the idea of decomposers with the concept of recomposers. This may help students recognize that these organisms not only break down matter into basic components but also make these components available for further use by organisms and the environment. However, make sure students know that recomposer is not a term used in biology; it is merely used to conceptualize what happens as a result of decomposition.

Related NSTA Science Store Publications, NSTA Journal Articles, NSTA SciGuides, NSTA SciPacks, and NSTA Science Objects

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1. (See “Flow of Matter in Ecosystems” map, pp. 76–77.) Washington, DC: AAAS.

American Association for the Advancement of Science (AAAS). 2001. Atlas of science literacy. Vol. 1. (See “Flow of Energy in Ecosystems” map, pp. 78–79.) Washington, DC: AAAS.

Koba, S., with A. Tweed. 2009. Hard-to-teach biology concepts: A framework to deepen student understanding. Arlington, VA: NSTA Press.

Trautmann, N. 2003. Decay and renewal. Arlington, VA: NSTA Press.

Related Curriculum Topic Study Guides (in Keeley 2005) “Cycling of Matter in Ecosystems” “Flow of Energy Through Ecosystems”

References

American Association for the Advancement of Science (AAAS). 2009. Benchmarks for science literacy online. http://www.project2061.org/publications/bsl/online/index.php

Driver, R., A. Squires, P. Rushworth, and V. WoodRobinson. 1994. Making sense of secondary science: Research into children’s ideas. London: RoutledgeFalmer.

Gayford, C. 1986. Some aspects of the problems of teaching about energy in school biology. European Journal of Science Education 8 (4): 443–450.

Keeley, P. 2005. Science curriculum topic study: Bridging the gap between standards and practice. Thousand Oaks, CA: Corwin Press and Arlington, VA: NSTA Press.

Keeley, P., F. Eberle, and C. Dorsey. 2008. Uncovering student ideas in science, vol. 3: Another 25 formative assessment probes. Arlington, VA: NSTA Press.

Leach, J., R. Driver, P. Scott, and C. Wood-Robinson. 1992. Progression in conceptual understanding of ecological concepts by pupils age 5–16. Leeds, UK: University of Leeds, Centre for Studies in Science and Mathematics Education.

National Research Council (NRC). 1996. National science education standards. Washington, DC: National Academies Press.

Smith, E., and C. Anderson. 1986. Alternative student conceptions of matter cycling in ecosystems. Paper presented to the National Association of Research in Science Teaching, San Francisco, California.

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