Science Module 2: Ecosystems & Food Webs

Transcription

1 Science Module 2: Ecosystems & Food Webs KEY STAGE 4 INTRODUCTION This module is concerned with the structure and dynamics of ecosystems. It concentrates on the dynamics of food webs, especially the losses that occur as food is transferred from one level in a food chain to the next. Comparison is made between land and ocean ecosystems. Some of these concepts are extended to a description of the carbon cycle. In designing material, we focussed on these elements of the National Curriculum for Key Stage 4 Science: Limitation of plant growth by light Interdependence of plants and animals within a habitat How predation and competition for resources affect the size of populations Quantitative description of foodwebs Role of micro-organisms in C- and N-cycles How energy is transferred through an ecosystem How the carbon cycle helps to maintain atmospheric composition We have sought to promote the following skills and aspects of scientific enquiry: Consider power and limitation of science in addressing a range of questions and ethical issues How scientific controversies can arise from different ways of interpreting evidence Communication and analysis of data using a variety of methods Judge the level of uncertainty in observations and measurements, and be able to reject anomalous data Consider whether data are sufficient to support conclusion or interpretation, and suggest improvements ICT opportunities using data handling software to create, analyse and evaluate charts and graphs ICT opportunities modelling the effects of competition and predation Below, we summarise the materials available for this module at Key Stage 4. There is a separate Users' Guide, which gives more information about each type of resource. RESOURCES 1. Teaching aids Please see the notes relating to teaching aids in the Users' Guide. The following PowerPoint slide sequences are provided to support this science module: 1

2 Plants and light in the ocean [extended] Basic description of the relationship between plant growth and light, and how this determines the types of plats growing in the ocean. Includes explicit treatment of exponential decay of light through the water column. There is a more detailed and extended study of the limits on plant growth intended for Key Stage 4 in Module1, with two associated spreadsheets and two worksheets. Ecosystems and food webs: comparing land and ocean Introduces the main features of land and ocean ecosystems, focussing on the key differences between plants in the two systems. Includes a summary of sizes of different organisms in oceanic ecosystems. Links to a quiz for Key Stages 3-4. Ecosystems and food webs: putting in some numbers A treatment of oceanic food chains that quantifies the idea of trophic efficiency and establishes its effects in short and long food chains. Links to a quiz for Key Stages 3-4, worksheets, a spreadsheet for Key Stage 3 and a more complex spreadsheet for Key Stage 4. Ecosystems and food webs: biomass and production Introduction to biomass and production in food chains. Examines the pyramid of biomass in land ecosystems and tests its application in a marine food chain. Links to a quiz for Key Stages 3-4, worksheets, a spreadsheet for Key Stage 3 and a more complex spreadsheet for Key Stage 4. Structure and function of food chains A summary of the way in which food chains work, built in part on a comparison of land and ocean ecosystems. Introduces the contrast in structure between land and open ocean systems. Defines trophic efficiency and examines its implications for short and long food chains. Assesses the utility of pyramid of biomass and concludes that pyramid of production is the only valid concept for land and ocean. Links to a quiz for Key Stages 3-4, and a worksheet and spreadsheet for Key Stage 4. 2

3 Oceans, atmosphere and climate A description of the carbon cycle, focussing on marine ecosystems. Introduces the greenhouse effect and its influence on the Earth s climate. Natural changes in greenhouse gases. The role of photosynthesis in the global carbon cycle, and the counteracting effects of respiration. Links to a quiz and a worksheet. 2. On-line activities Please see notes on quizzes and spreadsheets in the Users' Guide. Diminishing returns: Production and loss in food webs (quiz intended for Key Stages 3-4) A quiz that tests understanding of ocean food webs. Starts with questions on plankton algae and cell division. Then onto trophic efficiency and ratios between ingestion, egestion, respiration and growth. Final section on overall trophic efficiency in two food chains. Necessary information is presented in a separate window. Ten three-option multiple-choice questions, scoring 3 marks for first correct and 1 mark for second correct. Correct answer displayed automatically after two errors. Tests comprehension skills, biological knowledge, uses screen button technique. The terminology and concepts used should be familiar to students. There is a large amount of textual information, and students with visual impairment may find the text difficult to read. Foodchain (spreadsheet exercise intended for Key Stage 3 and younger Key Stage 4) A spreadsheet exercise that highlights the effects of differing food chain length and trophic efficiencies on ecosystem productivity. Students are presented with an initial example, then follow through instructions to complete datasets and plot their results. They can undertake what if experiments by testing the effects of changing variables. Tests understanding of the properties and consequences of different food chain characteristics, reinforces numerical skills, uses basic spreadsheet techniques. Foodweb scenarios (spreadsheet exercise intended for Key Stage 4) A spreadsheet exercise that highlights the effects of differing food chain length and trophic efficiencies on ecosystem productivity. Students are presented with an initial example, then follow through instructions to complete datasets and plot their results. Separate spreadsheet allows students to predict plant productivity needed to sustain predators in different food chains. They can undertake what if experiments by testing the effects of changing variables. Tests understanding of the properties and consequences of different food chain characteristics, reinforces numerical skills, uses basic spreadsheet techniques. 3

4 Climate & the living planet: Exploring the carbon cycle (quiz intended for Key Stage 4) A multiple-choice quiz exploring the greenhouse effect, the global carbon cycle and carbon reservoirs, and carbon-export and recycling. Three options are presented for each of 12 questions, and two attempts are allowed for each. Marking is scaled to difficulty. Textual information is presented alongside graphics. The terminology used may be challenging for some students. There is a large amount of text, and text size may present problems for visuallyimpaired students. The quiz requires good comprehension skills, as well as basic knowledge. 3. Worksheets Please see notes on worksheets in the Users' Guide. Ecosystem template note that there are three versions of this worksheet This worksheet is designed to support practical exercises in understanding the dynamics of food chains, much as the spreadsheets Foodchain and Foodweb scenario. The templates provide a framework in which to calculate the production at different levels along four food chains, using defined values of the ratio between growth and ingestion (referred to here and elsewhere by the shorthand term efficiency ). The value of primary production for each food chain is set by the student. Different values for each food chain can be used to simulate a simple food web, where different shares of primary production flow along different food chains. The template is supplied in three versions. The first is completely blank, and could be used for any ecosystem (land or ocean). The second has names and efficiency values for the first food chain the student can complete this and then go on to fill in the other three food chains. The third version has all four food chains completed to simulate the Southern Ocean food web. In all cases, production at a particular trophic level is calculated by multiplying the efficiency by the production at the preceding trophic level (that is, to the left). The third template shows how levels can be omitted to simulate different lengths of food chain. The carbon cycle This worksheet is linked specifically to the PowerPoint slide set Oceans, atmosphere and climate and tests knowledge of that topic. It uses graphical elements from the slide set, also used in the quiz Climate & the living planet: Exploring the carbon cycle. The sheet is set out as a formal test, with ten questions requiring short (one-word or note-form) answers. The sheet tests knowledge and understanding of the global carbon cycle and its relation to biology in the oceans. Specimen answers and supporting information are provided here: 1. Carbon dioxide in the atmosphere (component 1) is the only part of the cycle involved directly in 'greenhouse' warming. 2. Human activities are included in component 3, although they can also have effects on other components (such as components 2, 4 and 5). 4

5 3. There are two carbon storage processes - also called 'carbon sinks' - one on land and one in the ocean. These sinks represent long-term isolation of carbon from the atmosphere. 4. The most important non-biotic carbon dioxide source is volcanic eruptions. 5. Forest burning reduces the size of the terrestrial carbon store (component 5), and increases emission of carbon dioxide from the land biosphere (component 2). 6. Photosynthesis removes carbon dioxide from the seawater, thereby reducing the saturation of the gas in the water and causing more carbon dioxide to diffuse in from the atmosphere. 7. Export of biomass from the surface to the deep ocean represents a carbon sink, as this carbon will spend decades to millennia remote from the atmosphere. 8. Respiration breaks down organic material to release carbon dioxide. 9. Only 1-3% of oceanic plant production is exported to the deep ocean - the rest is respired. 10. Despite the much lower biomass of plants in the ocean, total primary production is approximately equal in the two biomes. Expressed on an area basis, plant production in the ocean is about half that on land. Light in the ocean The worksheet relates to the PowerPoint slide set Plant growth in the ocean: resources and controls (Key Stage 4, Module 1), but could also be used in conjunction with Plants and light in the ocean (Keys Stages 3-4, Module 2). The sheet provides a framework in which to calculate and plot light levels at different depths for various values of water absorption (turbidity). There are two suggested scenarios one in which 2% of light is absorbed per metre and a second in which 5% is absorbed per metre. Other scenarios could be attempted. Students should understand how to complete the calculations with reference to the methods box at the bottom of the page. When they have completed their graphs, they should see a rapid decrease in light in shallow depths, followed by a more moderate decrease. They should appreciate that this is a classic exponential decay, where each value is a proportion of the preceding value. The sheet tests appreciation of models of physical processes, and simple numerical- and graphical-skills. The exercise could be extended by considering what factors contribute to light absorption, including the plankton algae themselves a good example of a negative feedback loop. A specimen answer for this worksheet is available as a separate PDF file in this module. Controlling production The worksheet presents a diagram of the controls on phytoplankton (algal) production in the ocean. The diagram is built up sequentially in the PowerPoint slide set Plant growth in the ocean: resources and controls (Key Stage 4, Module 1). The diagram is complex, but the questions concern general principles rather than detail. There are three questions, and these will need to be answered on separate sheets or in workbooks. Emphasis is 5

6 placed on interactions, especially on feedback processes. These processes are crucial to understanding the controls on plant production, and hence to describe and model the responses of the plankton to environmental change. The sheet tests understanding of ecosystems, and the ability to interpret visual- and textual-information. Specimen answers and supporting information are provided here: 1. Total production is the product of growth rate (such as the number of cell doublings per day) and the biomass. Subject to feedback processes mentioned below, a larger biomass will result in larger production. Note that in a diagram such as this, some factors act on production, influencing the rate of growth, whilst others act directly to change biomass (such as removal by grazing or sedimentation). 2. Phytoplankton biomass plays an important role in controlling turbidity in the upper ocean. This is especially important in the open ocean, where there are few other particles in the surface waters, whereas in coastal seas there are larger amounts of suspended mineral- and other particles. Increased turbidity increases the amount of light absorbed. This reduces the amount of light available for phytoplankton growth deeper down, so decreases growth rate. This is a 'negative' feedback, because increasing phytoplankton biomass become self-limiting. The absorbed light also increases the temperature slightly, which might cause a slight increase in growth rate. This is a 'positive' feedback. 3. On the diagram, regeneration of nutrients is controlled by grazing, by phytoplankton biomass and by the environmental nutrient supply. In broad terms, nutrient supply to phytoplankton is driven by large-scale patterns of concentration, affected by ocean circulation, and small-scale regeneration mechanisms. Regeneration involves several feedback processes, especially those involving grazers and other organisms responsible for recycling phytoplankton biomass. In the diagram, we have distinguished between the environmental concentration (as the 'chemical nutrient supply') - which might be measured by a chemist - and the nutrient availability - which is a measure of how much can be used by the algae. Trace elements are often important in determining how much of a nutrient can be exploited by phytoplankton. Version September 2003 Copyright National Marine Aquarium For more information, see Design by STEP - 6