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Production: Instructor's Guide to Activities

Notes on the Food Web Slide Show

The slide show presentation on the food web and trophic states is designed to run under Auto desk, Animator Pro player program (ANIPLAY.EXE). Each step is a single frame and is intended to be advanced manually. See the technical notes on how to operate ANIPLAY.EXE in the Supporting Materials section of this module. The individual frames are described below. The slide show builds the complexities of trophic levels in the food web in a stepwise procedure. If computers are not available to run the slide show, print-outs of each frame are included in the Supporting Materials section from which overheads or copies can be made.

Although this model of the food web is a gross abstraction of reality, it contains enough detail to demonstrate how human interactions with the system have effects throughout the environment. Humans interact directly and indirectly with both the compartments and the flows. A description of each graphic frame follows:

Graphic frame 1

Geochemical reactions alter the gaseous composition of the air contained in the voids of rocks. This process created the atmosphere. This process was and continues to be important in creating the atmosphere. In the absence of living organisms, gas exchange between the porous surface material and the atmosphere is driven by changes in air pressure or the addition or loss of water. The classical definition of soil (the medium of plant growth) does not apply to soils at this stage. Severe human impacts made some sites revert to this abiotic state (or nearly so). The most visible examples are the effects of acid fallout from metal smelter stacks (for example in Ducktown, Tennessee and Sudbury, Ontario). Graphic frame 2 Vegetation may be composed of a suite of organisms from simple algae and large plants. Different plants in different amounts at different stages of development extract nutrients from the soil at different rates. Thus the uptake rate depends on the composition of the assemblage. Respiration rates, growth, and fallout are also dependent on the composition of vegetation. The external factors of solar radiation, temperature, and rainfall regimes regulate all transfer rates.

The respiration of plants is the primary mechanism for removing carbon dioxide from the atmosphere and replenishing the oxygen supply. Photosynthesis fixes carbon from atmospheric carbon dioxide into plant organic matter and releases the oxygen back into the atmosphere. Fire (very rapid oxidation) releases the carbon sequestered by the plant.

The array of organisms that make up the vegetation is controlled by a genetic materials budget that involves immigration, local evolution, and local extinction rates. The diversity of these plants also depends on how well they are able to share local nutrient and energy resources.

The litter volume on the soil surface depends on fallout and decomposition rates. Initially, litter decomposition was a purely mechanical/chemical process; with the evolution of organisms, decomposition became a biogeochemical process.
Graphic frame 3 Micro-organisms (from insects to bacteria) contribute to the nutrient cycle by converting nutrients from plant litter to a form that plants can use again for regrowth.

Environmental changes, direct human actions that change the litter composition, cultivation that stirs or buries the litter, and the use of insecticides or other chemicals that change the environmental chemistry alter the mix and populations of micro-organisms.
Graphic frame 4 Herbivores (aphids, grasshoppers, gophers, elephants) add a trophic level to the food web. The array of organisms that make up the herbivore trophic level is also controlled by a genetic materials budget that involves immigration, local evolution, and local extinction rates. The diversity of these animals also depends on how well they share the local nutrient and energy resources.

The amount of biomass supported at this level is limited by the amount of primary production (vegetation) that is suitable for the available animals. We commonly judge habitat quality for single grazers by our observation of their preferences, but that can be misleading. Grazers (like gazelles) are adaptable to new types of forage, provided that the forage meets the grazers' palatability, digestive, and nutrient requirements.

Graphic frame 5 Carnivores (wolves, cats, hawks, etc.) add yet another trophic level to the food web. The array of organisms in the carnivore trophic level is also controlled by a genetic materials budget that is the result of immigration, local evolution, and local extinction rates. The diversity of these animals also depends on how well they share the local nutrient and energy resources. The amount of biomass supported at this level is limited by the amount of herbivore production that is suitable for the available animals. Assessing habitat quality for a single carnivore is more complex than assessing it for a grazer because carnivores require habitats in which the food source (i.e., their prey) is sufficiently exposed. Carnivores are adaptable to new types of food sources, provided that these food sources meet their palatability, digestive, and nutrient requirements.
Graphic frame 6 In some places a second level of carnivores provides another reservoir for sequestering nutrients briefly before returning them to the surface litter. These animals, like the eagle, which eats carnivorous fish as well as herbivorous rodents, usually are not solely dependent on first-level carnivores.

Other examples of trophic levels include:

Carrion eaters, like vultures, consume dead animals they did not kill;
Omnivores (animals like bears, crows, and some rodents) eat both plants and animals;
Symbiotic organisms, like digestive bacteria, depend on the host animal and are required by the host to extract nutrients from food. Symbiotic organisms are not detrimental to each other. Plant versions of symbiosis include a wide variety of lichens that are formed by algae and fungi living together, neither of which can live separately;
Parasites that provide no benefit for the host animal or plant and gain no benefit from its death. Some examples are fleas, lice, schistosomes, and parasitic plants (orchid, mistletoe); and
Disease organisms (bacteria and viruses) may kill the host they depend upon for life and dispersal.

Activity 2 Nutrient Cycling Simulation Model

Goals
The primary goal of this activity is to help students see the impacts of both human-induced and non-human-induced changes in the mass and energy flows that affect the production of the biosphere. A secondary goal is for students to learn how to perform simple sensitivity analyses that test the effects of human intrusion into the biological production process by changing the nutrient storage compartments and the rates of flows between compartments. The activity allows students to examine the outcomes of changes in three very different biomes, and to compare the changes that take place in the simulation model versus those they may have predicted.

Skills

navigating and using a spreadsheet
understanding the principles behind and the usefulness of simulations
critical thinking
writing a rounded, balanced summary of impacts of human interactions with biomes

Material Requirements
A copy of the spreadsheet NUTCY4.wb1 (provided in Supporting Materials)
Spreadsheet software, preferably Quattro Pro for Windows 5.0 (or higher), or EXCEL 4.0
A copy of FOODWEB.FLC with ANIPLAY.EXE (provided in Supporting Materials); alternatively overheads made from the print-outs of the slides (originals provided)
Student Worksheet 2 (provided)
Supporting Material 2 (provided)
Suggested readings (see below)

Suggested Readings
Unit 2, Background Information (provided)
Supporting Material 2 (provided)
Relevant sections on nutrient cycling from the readings in the Appendix (provided)

Time Requirement
2 lab sessions depending on students' familiarity with spreadsheets for simulation and discussion

Task
Students should read the Background Information of Unit 2 and the additional reading on production and nutrient cycling (Supporting Material 2 and the additional readings provided in the Appendix) before they tackle this activity. The activity can be run as a demonstration with student inputs about what a selected type of environmental change might mean for the biosphere (in particular biospheric productivity). As an aid in starting the discussion, the slide show FOODWEB.FLC builds a schematic multiple trophic level model of nutrient cycling. Use that slide show to build a common knowledge base with students.

On the Student Worksheet the concepts behind the nutrient cycling model and how it represents an ecosystem are explained in more detail. The application of the model in this activity focuses on three environments, each of which has experienced radical direct human alteration or faces the threat of substantial human alteration if global warming becomes a reality. The first is the selva biome, which over the past two decades has been subject of intense scientific debate because of the rapid clearing of tropical rainforest in the Amazon Basin and in Southeast Asia (often for grazing). The global change issue here is primarily the release of carbon (sequestered by the huge standing biomass) into the atmosphere as carbon dioxide, a greenhouse gas. The second biome is the tundra which is underlain by permanently frozen soil (the term permafrost derives from the fact that the soil does not completely thaw during the summer months). The last biome is the steppe, much of which has been converted to prime agricultural land for grain crops over the past 200 years.

Running the simulation on nutrient cycling in and of itself won't be much of a challenge to most students. Detailed instructions are provided for each consecutive step students need to follow. During the testing phase of this module, students indicated that after about the third biome simulation, there was nothing new to learn from handling the spreadsheet itself. To avoid "busy work" and disinterest, divide the class into biome groups and have each group run only the 2-3 simulations for their particular biome. Coming back together afterwards, they should discuss and answer the questions asked with each run (questions accompanying the 8 simulations), compose the summary of impacts on biomes (summary paper), and then present it (possibly with print-outs of the summary graphs and tables) to the rest of the class. If the class is very big, have several groups per biome, and compare and complement each group's answers with those of others who dealt with the same biome.

Note: The NUTCY4 model was developed in Quattro Pro for Windows 5.0. It has been successfully imported into EXCEL 4.0 on a Power Mac 6100, but this transfer does not work in reverse.