"HOW ECOSYSTEMS WORK: ENERGY FLOW and NUTRIENT CYCLES" TEACHER'S GUIDE

Transcription

1 "HOW ECOSYSTEMS WORK: ENERGY FLOW and NUTRIENT CYCLES" TEACHER'S GUIDE I. INTRODUCTION "How Ecosystems Work: Energy Flow and Nutrient Cycles" is part of the exciting six part Basics of Ecology series. The Basics of Ecology series in turn is part of the 30 part Basics of Biology series which includes the following six part series; Basics of Biodiversity, Basics of Cell Biology, Basics of Genetics, and Basics of Anatomy. The programs in the Basics of Biology series are designed to cover most the topics brought up in high school biology courses. "How Ecosystems Work: Energy Flow and Nutrient Cycles" looks at the processes that are fundamental to all ecosystems. First the concepts of primary productivity, trophic levels, food chain, energy pyramids and the flow of energy through ecosystems are introduced. The program then explains how carbon, nitrogen, phosphorous and water cycle through ecosystems. "How Ecosystems Work: Energy Flow and Nutrient Cycles" concludes by explaining how human activities can disrupt nutrient cycles and throw them out of balance leading to accelerated eutrophication in lakes in the case of phosphorous imbalances and global warming in the case of carbon imbalances. "How Ecosystems Work: Energy Flow and Nutrient Cycles" - like all thirty programs in the Basics of Biology series- contains a student study guide, a multiplechoice test and a crossword puzzle that are stored as PDF files on the DVD. For information on other programs in the series or if you have questions or comments call or us at info@greatpacificmedia.com. II. STUDENT OBJECTIVES After viewing How Ecosystems Work, students will be able to: * Discuss how nutrients cycle through the living and non-living worlds. * Explain how solar energy flows through an ecosystem, from plants to decomposers. * Explain the concept of net primary productivity. * Explain the various trophic levels from photosynthetic producers to tertiary consumers. * Detail the role of decomposers and detritivores in ecosystems.

2 * Explain the cycling of carbon, nitrogen and other nutrients through ecosystems. * Detail the hydrologic cycle. * Explain the term Greenhouse Effect. cycles. * Discuss the impact of human activities on ecosystems in terms of disrupting nutrient * Describe the complex feeding relationships that exist in most ecosystems. * Describe the process of accelerated eutrophication. III. VIEWING THE VIDEO It is best that students view the program in its entirety first, without interruption. Then, as time allows, go back to the sections that correspond with your lesson plan for discussion. You may also want to pass out the study question worksheet that is provided with this teacher's guide for the students to fill in as they watch the program. IV. PRESENTATION OF THE PROGRAM The suggestions listed below will help you prepare a useful lesson plan to accompany the presentation of the program. Teacher's Preparation 1. Preview the program and familiarize yourself with this Teacher's Guide. Review the vocabulary list, script narration, and discussion questions, as well as the student study question worksheet and crossword puzzle included with this program. 2. For your convenience, major sections of the program are divided by intertitles. Each intertitle introduces the upcoming section and will provide you with a natural place to pause for discussion. Note where these intertitles appear throughout the program and plan your lesson accordingly. Student's Preparation 1. Discuss the topics presented in the program with students before showing them the video. This will help you discover what your students already know about the subject as well as create student interest about the topic. 2. Also introduce the vocabulary words on the board or overhead projector. Discuss any of the words with which your students are not familiar. The crossword puzzle can also be used to familiarize your students with terms introduced in the program that may be new

3 to them. V. VOCABULARY WORDS Autotroph Carnivore Consumer Decomposers Denitrifying Bacteria Detritus Feeders Energy Pyramid Food Chain Food Web Greenhouse Effect Herbivore Heterotroph Legumes Primary Productivity Nitrogen Fixation Nutrient Cycle Primary Consumer Producer Reservoir Secondary Consumer Sedimentary Cycle Tertitary Consumer VI. SUGGESTED DISCUSSION QUESTIONS Discuss these questions after watching "How Ecosystems Work: Energy Flow and Nutrient Cycles" 1. Are you aware of any ecosystems where the producers aren't capable of photosynthesis? Where all the organisms live in darkness? 2. What do you think the effect of global warming would be on primary productivity, biodiversity and the water cycle? 3. Why is it that a lake full of nutrients like phosphates might actually have less biodiversity than one with fewer nutrients? 4. How would the Earth's ecosystems be different if the transfer of energy between trophic levels was more efficient? Less efficient? 5. Why do think there aren't any animal-like heterotrophs?? 6. What are the biggest limits on primary productivity in the ecosystems near your school 7. What evidence of the disruption of nutrient cycles by human activity do you see near your school? 8. What sacrifices might you be willing to make to combat global warming? A nuclear power plant near your house? Driving a smaller car? Other? 9. What do you see as the biggest threats of global warming? Do see any positive ramifications to global warming? water? 10. Would you rather cycle through an ecosystem as carbon, nitrogen, phosphorous or VII. NARRATION FOR "HOW ECOSYSTEMS WORK: ENERGY FLOW

4 and NUTRIENT CYCLES" All organisms living in an ecosystem such as the African savanna require energy and nutrients in order to carry out essential life activities such as growth, respiration and reproduction. On the savanna as in nearly every ecosystem on Earth sunlight provides the energy that powers life, while nutrients and the other critical building blocks of life such as carbon, nitrogen, phosphorous, and water enter the ecosystem from the non-living environment, the atmosphere and the earth or sea, from which the ecosystem arises. Solar energy continuously bombards the Earth day after day, year after year providing an essentially limitless source of energy for the Earth s ecosystems. That energy flows through ecosystems, starting with the plants that convert the light energy to chemical energy via photosynthesis. Energy captured by plants is then transferred to the herbivores that graze on the plants. From there part of the energy is transferred to predators as they eat the herbivores. Scavengers also obtain part of the energy that flooded into the ecosystem as sunlight as they pick at the remains left by predators. Finally decomposers breaking down what s left to the molecular level obtain the last remaining bits of energy from what started as beams of light. It is fortunate for life on Earth that the flow of energy from the sun is essentially limitless, because as energy flows from plant to decomposer each organism along the way releases part of the energy they capture as heat to the atmosphere, energy that is forever lost for biological purposes within the ecosystem. Thus, for the flow of energy and for life itself to continue in a ecosystem, an outside source must constantly be injecting it with new energy. Unlike solar energy the nutrients essential to life are limited. The Earth only has so much carbon or phosphorus or nitrogen available for living organisms. For example, each year photosynthesis in plants and other organisms captures approximately 1/7th of the carbon available in the atmosphere, if biological processes such as cellular respiration didn t return most of this captured carbon back to the atmosphere, life as we know it on Earth would rapidly end, as there would be no carbon in the atmosphere for plants and other photosynthetic organisms to carry out life processes such as capturing the energy in sunlight. So in order for life on Earth to continue nutrients must be constantly recycled by the Earth s ecosystems. Thus, energy flows through ecosystems while nutrients cycle through them. The processes by which energy flows through and nutrients are cycled within an ecosystem play a major role in shaping the complex interactions that occur between populations within the Earth s great living communities. Let s now take a look at how the energy and nutrients crucial to life travel through ecosystems and the interactions their travels generate. How Ecosystems Works: Energy Flows Through Ecosystems- Primary Producers Capture Outside Energy Ninety-three million miles away, as the sun fuses hydrogen into helium,

5 tremendous quantities of energy are released, in the form of light and other electromagnetic waves. The solar energy reaching the Earth each day is roughly 100 million times greater than the energy released by a nuclear bomb. Upon reaching the Earth, clouds and dust in the atmosphere and the Earth s surface reflect much of this energy back into space. Most of the remaining energy is absorbed as heat by the Earth and atmosphere. After reflection and absorption about 1 percent of the solar energy reaching Earth is left to power nearly all life on Earth. Of this 1 %, plants and other producers capture less than 3%. Thus the teeming life on Earth is supported by less than 0.03% of the energy reaching it from the sun. The process that converts this light energy into the chemical energy that powers nearly all ecosystems is photosynthesis. In photosynthesis, pigments such as chlorophyll contained in plants, plantlike protists, and cyanobacteria absorb certain wavelengths of light energy and use it to combine carbon dioxide and water into sugar, a compound that stores energy in its chemical bonds. Some of the energy stored in the sugar is used to power other chemical reactions, that convert sugars into starches, cellulose, fats, vitamins, pigments, and proteins. Photosynthetic organisms are called autotrophs, or producers, since they produce food not only for themselves but for nearly all other life as well. The organisms that rely on the high-energy molecules made by autotrophs are called heterotrophs, or consumers. How Ecosystems Works: Energy Flows Through Ecosystems- Measuring Primary Productivity The amount of life, or biomass, an ecosystem can support is determined by the amount of chemical energy generated by producers. The total energy that photosynthetic organisms make available to other members of the community is called net primary productivity. Net primary productivity can be measured in units of energy (calories), or as the dry weight of organic material per unit area per year. The productivity of an ecosystem is influenced by environmental variables, such as temperature range and the availability of sunlight, water, and nutrients. In the arctic tundra, for example, temperature and sunlight limit productivity, while in deserts it is water and nutrients that limit productivity. In temperate wetlands and tropical rain forests where energy and nutrients are abundant productivity is high. Overall the primary producers in all the Earth s ecosystems use solar energy and nutrients to produce an impressive170 billion tons of organic material per year. How Ecosystems Works: How Energy Flows Through Ecosystems- Trophic Levels Ecologists categorize living things according to their role in the flow of energy through an ecosystem. The flow of energy through an ecosystem begins with photosynthetic producers and continues on through several levels of consumers. Each category organism forms a trophic level. Photosynthetic producers, from marsh grasses to thorny acacia trees form the first trophic level. Herbivores from caterpillars and grasshoppers to giraffes and elephants, that feed exclusively on producers are considered primary consumers, and form the second trophic level. Flesh-eating spiders, birds,

6 hyenas, lions and other insectivores and carnivores that feed primarily on herbivores; are called secondary consumers and form the third trophic level. Some carnivores such as eagles and bears occasionally eat other carnivores or insectivores, and when doing so they form the fourth trophic level: tertiary consumers How Ecosystems Works: Energy Flows Through Ecosystems- Food Webs are Complex The feeding relationships of an ecosystem, are often illustrated by food chains in which representatives of a trophic level are shown eating a representative of the trophic level below them. In a prairie community, for example, grasses the major producers are eaten by grasshoppers the primary consumers. Grasshoppers in turn are eaten by field mice, secondary consumers in the prairie food chain. Field mice are then eaten by rattlesnakes-tertiary consumers on the food chain, and finally rattlesnakes are eaten by hawks the quaternary consumers at the top of our hypothetical prairie food chain. But though food chains illustrate the general flow of energy through a community. The relationships they depict are largely simplifications; rattlesnakes don t just eat mice and hawks don t just eat rattlesnakes, both for example eat prairie dogs and other members of the prairie community such as prairie chickens and jackrabbits. The actual feeding relationships within a given community are much more accurately by food webs. For example, bald eagles, are secondary consumers when they eat herbivores like a snowshoe rabbit, tertiary consumers when eating salmon and secondary, tertiary, or quaternary consumers when eating ptarrmigan depending on whether the ptarmigan had been eating vegetation, herbivorous insects, or insect eating spiders. Brown bears which are omnivores complicate matters even further. Acting as primary consumers when they eat grasses and berries, secondary consumers when eat herbivores like arctic ground squirrels, and as tertiary and quaternary consumers when they eat salmon and spider eating ptarmigans respectively. Though complicated and incomplete the process of diagramming a food web enables one to begin to understand the complex relationships that exist in living communities. For simplicities sake, however, some of the most important members of a living community - decomposers and detritivores - often aren t represented on food webs. Decomposers and detritivores are nevertheless critical to the functioning of living communities as every member of a community upon its death is consumed at least in part by these often unnoticed inhabitants of living communities. How Ecosystems Works: Energy Flows Through Ecosystems- Decomposers and Detritivores Decomposers and detritivores are among the most important strands in actual food webs. By liberating nutrients for reuse, they form a vital link in the nutrient cycles of ecosystems. Decomposers are primarily fungi and bacteria, which digest food outside their bodies, absorb the nutrients they need, and release the remaining nutrients into the soil or water. Most detritivores are small, often unnoticed organisms that live on the refuse of life: molted exoskeletons, fallen leaves, wastes, and dead bodies. Detritivores form an extremely complex network of organisms that include earthworms, nematode

7 worms, millipedes, certain insects, protists, and crustaceans, and even a few large vertebrates such as vultures. These organisms consume dead organic matter, extract some of the energy stored within it, and excrete it in a still further decomposed state. Their excretory products serve as food for other detritus feeders and decomposers, until most of the stored energy has been utilized. Once-living organisms are reduced to simple molecules such as carbon dioxide and water that are returned to the atmosphere, and minerals and organic acids that are returned to the soil. In some ecosystems, such as deciduous forests, more energy passes through the detritus feeders and decomposers than through the primary, secondary, and tertiary consumers combined. If the detritus feeders and decomposers were to disappear suddenly, communities would gradually be smothered by accumulated wastes and dead bodies. The nutrients stored in these bodies would be unavailable and the soil would become poorer and poorer until plant life could no longer be sustained. As a result, energy would cease to enter the community and the higher trophic levels dependent on the energy captured by plants would disappear as well. How Ecosystems Works: Energy Flows Through Ecosystems-Energy Flows Between Trophic Levels A basic law of thermodynamics states that the use of energy is never completely efficient. Light bulbs, computers, televisions, cars, in fact anything that consumes energy gives off part of that energy as heat. For example, as cars convert the chemical energy stored in gasoline to the energy of movement, approximately 75% of the energy is immediately lost as heat. So too in living systems; has we humans exercise vigorously, the energy of muscular contraction produces large amounts of heat as a by-product. The firing of neurons in the brain; the germination of seeds; the thrashing of the flagella of a protist; in fact nearly all biological processes give off heat as a by product. Just as the use of energy by living organisms is inefficient so is the transfer of energy from one trophic level to the next. When a primary consumer such as a caterpillar feeds on a producer like a shrub, only a portion of the solar energy originally trapped by the scrub is available to the caterpillar. This is because some energy originally captured by the scrub was lost to the atmosphere as heat as the scrub carried out various metabolic processes. Further, some of these metabolic processes involved building of molecules such as cellulose, which the caterpillar cannot breakdown and obtain energy from. Therefore, only a fraction of the energy captured by the scrub at the first trophic level is available to caterpillar at the second trophic level. In turn, part of that energy that is obtained by the caterpillar is released as heat to the environment; or used to power crawling and the gnashing of mouthparts; or to construct the caterpillars indigestible chitinous exoskeleton. All this energy is unavailable to the bird in the third trophic level that eats the caterpillar. The bird, in turn, uses much of the energy captured from the caterpillar to maintain body temperature, power flight, and build indigestible feathers, beak, and bone. Energy unavailable to the eagle at the fourth trophic level that consumes the bird.

8 How Ecosystems Works: Energy Flows Through Ecosystems- The 10% Law According to the 10% Law derived from studies of a variety of ecosystems, the energy transfer between trophic levels in most ecosystems is roughly 10% efficient. This means that the energy stored in primary consumers (herbivores) is only about 10% of the energy stored in the bodies of producers. The bodies of secondary consumers, in turn, possess roughly 10% of the energy stored in primary consumers. In other words, for every 100 calories of solar energy captured by grass, only about 10 calories are converted into herbivores, and only 1 calorie into carnivores. Energy pyramids like this one illustrate the distribution of energy between trophic levels in ecosystems graphically. The unequal distribution of energy between trophic levels in ecosystems is reflected in species populations on the African savanna. Here one notices that the predominant organisms are plants; which have the most energy available to them because they can trap it directly from sunlight. The most abundant animals are herbivores feeding on plants, while carnivores like lions are relatively rare. The inefficient transfer of energy between trophic levels also has important implications for human food production. The lower the trophic level utilized, the more food energy available to human populations; in other words far more people can be fed on grain than on the meat produced by grain fed cattle. How Ecosystems Works: Nutrients Cycle Through Ecosystems In contrast to solar energy which continuously flows into ecosystems from the sun, nutrients are, as mentioned earlier, scarce and so must be recycled. For practical purposes, the same pool of nutrients has been supporting life for over 3 billion years. Nutrients are the chemical building blocks of life. Macro-nutrients, such as water, carbon, hydrogen, oxygen, nitrogen, phosphorus, and calcium are required by organisms in large qualities. Micro-nutrients, such as zinc, molybdenum, iron, selenium, and iodine, are required only in trace quantities. Nutrient cycles describe the pathways these substances follow as they move from the living to the nonliving portions of ecosystems and back again. The major source, or reservoir, of important nutrients is generally in the nonliving environment. For example, the atmosphere is the major reservoir for carbon and nitrogen while rock is the reservoir for phosphorus. Let s look briefly at how carbon, nitrogen, phosphorus and water cycle through ecosystems. How Ecosystems Works: Nutrients Cycle Through Ecosystems-Carbon Cycles Through Atmosphere All organic molecules are formed out of a framework of carbon atoms. Carbon enters food webs through producers, which trap CO 2 from the atmosphere during photosynthesis. CO 2 makes up 0.033% of the gas in the atmosphere. Some of the CO 2 captured by producers is returned to the atmosphere through cellular respiration, while some is incorporated into their bodies and passed on to herbivores. Herbivores, in turn,

9 return some of the carbon back to the atmosphere as CO 2 through cellular respiration and while incorporating some of it into their body tissues. The transfer of carbon continues in much the same way through predators, detritus feeders, and decomposers, until ultimately most the carbon captured by producers is returned to the atmosphere as CO 2. However, some carbon cycles through the environment more slowly. For example, mollusks extract carbon dioxide dissolved in water and combine it with calcium to form calcium carbonate CaCO 3, from which they construct their shells. The shells of dead mollusks often collect in deposits like these cockles shells on the shores of Shark Bay in western Australia. Deposits like these are often transformed into limestone rich formations like those in the Pinnacles Desert south of Shark Bay. As these limestone formations dissolve gradually overtime much of much of the carbon rich calcium carbonate CaCO 3 finds its way back into the ocean or other bodies of water where chemical reactions transform it into molecular forms that can be used as a carbon source by different aquatic organisms. Another long-term carbon cycle produces fossil fuels as the carbon found in the organic molecules of ancient plant and animal remains is transmuted by temperature, pressure and vast expanses of time into coal, oil, or natural gas. Thus the energy of prehistoric sunlight is trapped in fossil fuels. This energy is released by combustion. So is carbon in the form of CO 2. Human activities, including burning fossil fuels and cutting and burning the Earth's great forests where much carbon is stored, thus increase the amount of carbon dioxide in the atmosphere which as we will see later can have serious consequences for the biosphere. How Ecosystems Works: Nutrients Cycle Through Ecosystems- Nitrogen Also Cycles Through Atmosphere Nitrogen is a critical component of the amino acids, proteins, nucleic acids essential to all living organisms. As the Earth s atmosphere is about 79% nitrogen gas (N 2 ) on the surface it would appear easy for living organisms to obtain, but neither plants nor animals can use nitrogen gas directly. Instead, plants require ammonia (NH 3 ) or nitrates (NO 3 -). Plants and the rest of living world depend on nitrogen fixing bacteria, which combine atmospheric nitrogen with hydrogen to produce the ammonia required by plants. Some of these bacteria live independently in water and soil, while others exist in symbiotic associations with plants called legumes (a group including soybeans, clover, and peas) where they live in special swellings on the roots. Decomposer bacteria can also produce ammonia from amino acids and urea found in dead bodies and wastes. Much the ammonia produced by these bacteria is used by other bacteria as an energy source and converted into nitrates in the process. Nitrates are also produced by electrical storms and by other forms of combustion that cause nitrogen to react with atmospheric oxygen. In human-dominated ecosystems such as farm fields, gardens, and suburban lawns, ammonia and nitrates are supplied by chemical fertilizers; which are produced by using the energy in fossil fuels to artificially "fix" atmospheric nitrogen.

10 The nitrogen incorporated into the amino acids, proteins, nucleic acids, and vitamins of plants eventually finds its way into the bodies of either primary consumers, detritus feeders, or decomposers. As it is passed through the food web, most of the nitrogen is liberated in wastes and dead bodies, which decomposer bacteria convert back to nitrates and ammonia that recycle though the soil and water. However, a small amount of nitrogen gas is returned back to the atmosphere by denitrifying bacteria found in anaerobic conditions in mud, bogs, and estuaries. These denitrifying bacteria break down nitrates to obtain their oxygen molecules and in the process release the nitrogen molecules back to the atmosphere. How Ecosystems Works: Nutrients Cycle Through Ecosystems- Phosphorus Cycle a Sedimentary Cycle Phosphorus is a crucial component of biological molecules, including the energy transfer molecules (ATP) and (NADP), nucleic acids, and the phospholipids of cell membranes. It is a major component of the teeth and bones of vertebrates. The reservoir of phosphorus for most ecosystems is crystalline rock, where it is found as phosphate (PO 4 3- ). Since phosphorus does not enter the atmosphere, the phosphorus cycle is referred to as a sedimentary cycle. As phosphate-rich rocks are exposed and eroded, phosphate ions are dissolved in rainwater. Dissolved phosphate is readily absorbed through the roots of plants, and by other autotrophs such as photosynthetic protists and cyanobacteria. From these producers, phosphorus it passes through a food web. At each level, organisms excrete excess phosphate. Ultimately, decomposers return the phosphorus remaining in dead bodies back to the soil and water in the form of phosphate. Here it may be reabsorbed by autotrophs, or it may become bound to sediment and eventually reincorporated into rock. Some of the phosphate dissolved in fresh water is carried to the oceans. Although much of this phosphate ends up in marine sediments, some is absorbed by marine producers and eventually incorporated into the bodies of invertebrates and fish. Some of these, in turn, are consumed by seabirds, who excrete large quantities of phosphorus back onto the land. Guano deposited by seabirds along the western coast of South America, along with phosphate-rich rock, are the major sources for the phosphates incorporated into agricultural fertilizers. In some ecosystems, especially freshwater ones like lakes and rivers, overall productivity is limited by the availability of phosphates. Run-off from agriculture fields and stockyards, municipal sewage discharges, and other sources can create imbalances in these aquatic ecosystems by carrying large quantities of phosphates them. The imbalance caused by the increased phosphate levels often leads to what ecologists refer to as accelerated eutrophication. Huge blooms of algae and other photosynthetic organisms form, increasing the demand for dissolved oxygen in the body of water. Eventually anaerobic conditions develop that result in the massive die off of animal life. The smell of decay rises from these bodies of water as anaerobic bacteria release methane and other gases as they decompose the dead organisms that litter the body of water. As result of the

11 problems caused by accelerated eutrophication many states have banned phosphate detergents and taken other measures to insure excessive levels of phosphates don t enter aquatic ecosystems. A comparable situation exists in marine environments such as estuaries where nitrates are often the limiting factor in primary productivity. Run-off from agricultural fields, livestock feedlots, and other nitrogen rich sources can flow into rivers and streams feeding into estuaries. The high levels of nitrates again set off a process of accelerated eutrophication like that caused by phosphates in freshwater. The results are comparable. High nitrate levels are a problem in San Francisco Bay, the Mississippi Delta near New Orleans, and other estuaries throughout the world that receive large amounts of agricultural run-off. How Ecosystems Works: Nutrients Cycle Through Ecosystems- The Water Cycle The water or hydrologic cycle, differs from most other nutrient cycles in that water usually remains chemically unchanged throughout the cycle. The major reservoirs for water are the oceans, which cover about three quarters of the Earth's surface and contain over 97% of the available water. The hydrologic cycle is driven by solar energy, which causes water to evaporate into the atmosphere, and by gravity, which draws the water back to earth in the form of precipitation (rain, snow, sleet, dew). Most of the evaporation of water occurs over the oceans, and much of this water simply returns to the oceans as precipitation. Water that does fall on the land takes a variety of paths. A percentage returns to the atmosphere as the result of evaporation from lakes and streams and the land itself. Much runs off the land back to the oceans via river networks, while a small amount enters underground reservoirs called aquifers. The rest is taken up by living organisms, most of which are about 70% water. Much of the water absorbed by plants, is returned to the atmosphere as a result of evaporation from their leaves. But a small amount of the absorbed water is combined with carbon dioxide during photosynthesis to produce high-energy molecules like glucose. Eventually these high energy molecules are broken down during cellular respiration and the water and CO 2 they contain released back into the atmosphere. Herbivores, carnivores, and other heterotrophs acquire water from their food and by drinking it where they can find it. Like plants, heterotrophs return much of the water they acquire back to the atmosphere via cellular respiration and evaporation and additionally through excretion. Although the hydrologic cycle would continue in the absence of life on Earth, the distribution of life and the composition of biological communities depends on and, to a great extent is determined by, the patterns of precipitation and evaporation that exist on our planet. For example, the hydrologic cycle differs considerably in deserts and rainforests and this is reflected in the composition of their respective living communities. In deserts a lack of water limits biological productivity, while in rainforests where water is abundant biological productivity is much higher and any limits placed on biological productivity are the result of a scarcity of nutrients.

12 How Ecosystems Works: Imbalances in Nutrient Cycles- CO 2 and the Greenhouse Effect As we ve just seen the cycling of nutrients can have a major impact on ecosystems. However, the cycling of carbon dioxide doesn t have a major impact just on individual ecosystems but on the entire biosphere. Carbon dioxide carries out two essential roles in the biosphere. As we ve already seen it is the major source of carbon for the Earth s living organisms. But atmospheric CO 2 also acts something like the glass in a greenhouse. It allows light energy to enter the atmosphere, but absorbing and holding that energy once it has been converted to heat. This is critical in retaining heat in the atmosphere and maintaining temperatures appropriate for carrying out life processes. However, large rapid increases in CO 2 levels accompanied by proportionate increases in global temperatures could have a dramatic impact on life on Earth. Between 345 and 280 million years ago, under the unique conditions of the Carboniferous period, huge quantities of carbon were diverted from the carbon cycle when the bodies of plants and animals in large tracts of swamp and forest were buried in sediments and thus escaped the usual decomposition process. The result was that huge amounts of carbon were taken out of the carbon cycle. Over time heat and pressure converted the dead organic matter into coal, oil, and natural gas that lay buried beneath the ground largely inaccessible to the living world. Over the of the millions of years since the Carboniferous period a new equilibrium was established in the carbon cycle. However, in the last two centuries human activities have rapidly increased the level of carbon dioxide in the atmosphere and begun to shift the equilibrium point. Since the Industrial Revolution, modem cultures have increasingly relied on the energy stored in fossil fuels. As fossil fuels are burned in power plants, factories, and cars, CO 2 is released into the atmosphere. Without human intervention, carbon buried during the Carboniferous period would have slowly been returned to the atmosphere through natural processes over the course of millions of years, but industrialized societies are currently burning fossil fuels at a rate that is returning buried carbon to the atmosphere as CO 2 at least 100 times faster than would have occurred under natural conditions. In addition deforestation, resulting from the cutting down of tens of millions of acres of forests each year is also increasing CO 2 levels in the atmosphere. Deforestation is occurring principally in the tropics, where rain forests are being eliminated to make room for agricultural land. After the massive trees that make up these forests are cut the carbon stored in them is returned to the atmosphere either through burning or decomposition. Compounding the problem is the fact that these large trees are no longer alive and taking up CO 2. A result of the burning of fossil fuels and the destruction of forest land the CO 2 content of the atmosphere has increased by 25% in the past 200 years. Many scientists believe that this increase in atmospheric CO 2 is leading to the phenomenon of global warming. But controversy surrounds the issue of global warming

13 for a number of reasons. First, long term increases in mean global temperatures are difficult to establish because of the Earth s vastness, natural fluctuations in temperature, and limits to technology. But even if it is established that global warming is occurring, it is also difficult to determine how much of the warming is due to humankind s release of CO 2 into the environment and how much is due to cyclical climate changes like those that have occurred throughout the Earth s history. The final part of the controversy centers around the possible ramifications of global warming. Would global warming create world havoc by flooding coastal cities, negatively disrupting global agriculture and the Earth s ecosystems or are some of these claims overstated? Might there actually be a net positive effect for life on Earth in terms of increased primary productivity in ecosystems and on agricultural land due to increases temperature and precipitation? No matter what ones position on global warming it is obvious that more study and research is warranted, not just of global warming but of all the various ways we humans affect the flow of energy and the cycling of nutrients through the Earth s ecosystems. This so that we humans as stewards of the Earth can make better decisions not only for ourselves but for all organisms with which we share planet Earth. Summary In this program we ve seen that the flow of energy through an ecosystem usually starts with the conversion of light energy to chemical energy by plants and other photosynthetic organisms ecologists refer to as producers. The energy then flows through numerous trophic levels starting with primary consumers such as herbivores, then on to carnivores such as eagles which may act as secondary, tertiary or quaternary consumers depending the feeding habits of their prey. As displayed by energy pyramids the amount of energy available to any given trophic level is approximately only 10% of that available to the level before. The feeding relationships that transfer chemical energy from one level to another are illustrated by food chains and more precisely by food webs. The bodies of these once living organisms and the last bits of energy they contain are then consumed by decomposers and detritus feeders. Unlike energy which is basically endless and flows through ecosystems, limited nutrient resources such as carbon, nitrogen, phosphorus, and water must cycle through ecosystems so that they can be recycled and used again by other living organisms. The cycling of nutrients usually includes cycling through parts of the non-living environment such as the atmosphere or through rocks and sediments. Human activities can create changes in the cycling of nutrients that may have a dramatic impact on an ecosystem such as a lake or estuary or in the case of the carbon cycle the entire global biosphere. Fortunately, ecologists and others are constantly working on ways to better monitor the flow of energy and the cycling of nutrients through ecosystems and the entire biosphere.