Day 21 Ecosystems and Communities
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Ecosystems and Communities
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Day 21 Ecosystems and Communities
- 1. Day 21 Chapter 15 November 25th Dr Amy B Hollingsworth The University of Akron Fall 2014
- 2. We only have 2 classes left! • Today – teacher evaluations, Homework CH 15 • Thursday – Turkey Day, no class • Tue Dec 2nd – Finish Chapter 15 • Thur Dec 4th – Chapter 16 • Final – Dec 11th – CBT, will be open from noon to 7pm. • Any makeup exams for excused absences will be Dec 8th – 10th, in the CBT, by appointment only
- 3. Chapter 15: Ecosystems and Communities Organisms and their environments by Mark Manteuffel, St. Louis Community College
- 4. 15.1–15.2 Ecosystems have living and nonliving components.
- 5. 15.1 What are ecosystems? A community of biological organisms plus the non-living components with which the organisms interact.
- 8. 15.2 A variety of biomes occur around the world, each determined by temperature and rainfall.
- 9. Biomes What is the average temperature? What is the average rainfall (or other precipitation)? Is the temperature relatively constant or does it vary seasonally? Is the rainfall relatively constant or does it vary seasonally?
- 10. Biomes Temperature and precipitation dictate: • Primary productivity levels the amount of organic matter produced The numbers and types of primary producers: • are the chief determinants of the amount and breadth of other life in the region.
- 13. 15.3–15.5 Interacting physical forces create weather.
- 14. 15.3 Global air circulation patterns create deserts and rainforests.
- 18. 15.4 Local topography influences the weather.
- 20. Is it warmer or cooler in urban areas relative to nearby rural areas? “Urban Heat Islands”
- 21. Why is it so windy on streets with tall buildings?
- 22. 15.5 Ocean currents affect the weather. Global patterns of circulation in the oceans
- 24. Why do beach communities have milder weather than more inland communities? Water’s capacity to absorb and hold heat
- 25. Beaches on the east coast of the United States have warmer water than west coast beaches at the same latitudes. Why?
- 27. 15.6–15.8 Energy and chemicals flow within ecosystems.
- 28. 15.6 Energy flows from producers to consumers.
- 29. First Stop: Primary Producers
- 30. Second Stop: Primary Consumers—the Herbivores
- 31. Third Stop: Secondary Consumers— the Carnivores
- 32. Fourth Stop: Tertiary Consumers—the “Top” Carnivores
- 33. Chains or Webs? Food chain • Pathway from photosynthetic producers through the various levels of animals Food web • Involve harvesting energy from multiple stops in the food chain
- 35. Energy Flows Losses at every “step” in a food chain Inefficiency of energy transfers
- 36. 15.7 Energy pyramids reveal the inefficiency of food chains. 10% efficiency
- 37. Biomass 10% rule Where does the rest go? Expended in cellular respiration or lost as feces
- 38. In humans, why is vegetarianism more energetically efficient than meat-eating? The 10% rule in application
- 39. Why are big, fierce animal species so rare in the world?
- 42. 15.8 Essential chemicals cycle through ecosystems. The recycling of molecules
- 43. Chemical Reservoirs Each chemical is stored in a non-living part of the environment. Organisms acquire the chemical from the reservoir. The chemical cycles through the food chain. Eventually, the chemical is returned to the reservoir.
- 44. The Three Most Important Chemical Cycles 1. Carbon 2. Nitrogen 3. Phosphorus
- 46. Why are global CO2 levels rising? Fossil fuels
- 47. Global CO2 levels are rising in general, but they also exhibit a sharp rise and fall within each year. Why?
- 49. Nitrogen is like a bottleneck limiting plant growth. Fertilizers
- 52. 15.9–15.14 Species interactions influence the structure of communities.
- 53. Finally - Evaluations • I need someone to volunteer to do the survey for this class. • Be honest – what can I do to improve (seriously, besides just giving away grades like skittles) • www.ratemyprofessor.com is the best way to let future students know what’s good or bad.
- 54. What to note - • Did you like the technology for the class? • Was I helpful or caring or a big nasty jerk? • What did you think of the videos? • Do you like that I post everything online? • Do you think the material was too easy or too hard? • What did you think of lecture versus lab? • Would you take me again for a class?
Editor's Notes
- Picture a lush garden: some greenery, a bit of debris from rotting wood, and abundant wildlife. Grazing animals abound, while predators feed on other animals and their eggs. Parasites are poised, looking for hosts, and, just below the surface, scavengers find meals among the organic detritus. But now, imagine that the entire scene gets up and walks away! The “camera” in your mind pulls back to reveal that the entire scene is playing out on the back of a beetle no more than two inches long (Figure 15-1 A small-scale ecosystem). The host of this mini-ecosystem is a beetle from New Guinea called the large weevil. The weevil is camouflaged from its predators by lichens—fungi and photosynthetic algae living together—while the lichens are given a safe surface on which to live. And the garden of lichens supports a wide range of other organisms, from tiny mites to a variety of other microscopic invertebrates, some free-living and some parasitic.
- What is important is that the two essential elements of an ecosystem are present: the biotic environment and the physical (abiotic) environment (Figure 15-2 What makes up an ecosystem?). The biotic environment consists of all the living organisms within an area and is often referred to as a community. The physical environment, often referred to as the organisms’ habitat, consists of: the chemical resources of the soil, water, and air, such as carbon, nitrogen, and phosphorus, and the physical conditions, such as the temperature Biologists view communities of organisms and their habitats as “systems” in much the same way that engineers might, hence the term ecosystem. Biologists monitor the inputs and outputs of the system, tracing the flow of energy and various molecules as they are captured, transformed, and utilized by organisms and later exit the system or are recycled. They also study how the activities of one species affect the other species in the community—whether the species have a conflicting relationship, such as predator and prey, or a complementary relationship, such as flowering plants and their pollinators. Regardless of size, the same principles of ecosystem study apply: observe and analyze organisms and their environments, while monitoring everything that goes into and comes out of the system: salinity (salt level), moisture, humidity, and energy sources.
- Biomes cover huge geographic areas of land or water—the deserts that stretch almost all the way across the northern part of Africa, for example. Terrestrial (land) biomes are defined and usually described by the predominant types of plant life in the area. But looking at a map of the world’s terrestrial biomes, it is clear that they are mostly determined by the weather.
- Specifically, when defining terrestrial biomes, we ask four questions about the weather: What is the average temperature? What is the average rainfall (or other precipitation)? Is the temperature relatively constant, or does it vary seasonally? Is the rainfall relatively constant, or does it vary seasonally?
- By knowing the answers to these questions, it is possible to predict, with great accuracy, the type of biome. This is because temperature and precipitation dictate the primary productivity levels, or the amount of organic matter produced, primarily through photosynthesis. And the numbers and types of primary producers—the organisms responsible for the primary productivity, such as grasses, trees, and agricultural crops—are, in turn, the chief determinants of the amount and breadth of other life in the region.
- For example, where it is always moist and the temperature does not vary across the seasons, tropical rainforests develop. And where it is hot but with strong seasonality that brings a “wet” season and a “dry” season, savannas or tropical seasonal forests tend to develop. At the other end of the spectrum, in dry areas with a hot season and a cold season, temperate grasslands or deserts develop. Figure 15-3 (Terrestrial ecosystem diversity) shows examples of the nine chief terrestrial biomes; all are determined, in large part, by the precipitation and temperature levels.
- Aquatic biomes are defined a bit differently, usually based on physical features such as salinity, water movement, and depth. Chief among these environments are 1) lakes and ponds, with non-flowing fresh water; 2) rivers and streams, with flowing fresh water; 3) estuaries and wetlands, where salt water and fresh water mix in a shallow region characterized by exceptionally high productivity; 4) open oceans, with deep salt water; and 5) coral reefs, highly diverse and productive regions in shallow oceans (Figure 15-4 Aquatic ecosystem diversity).
- If the terrestrial biomes of the world are determined by the temperature and rainfall amounts and seasonality, what determines those features? In other words, what makes the weather? We investigate next how the geography and landscape of the planet—from the shape of the earth and its orientation to the sun to patterns of ocean circulation—cause the specific patterns of weather that create the different climate zones and the biomes characteristic of each. Then, we’ll see how energy and chemicals are made available for life to flourish in these biomes.
- What determines the temperature and rainfall in a particular part of the world? As we’ll see, differences in both ultimately result from one simple fact: The earth is round. Let’s first explore how the earth’s curvature influences temperature.
- Wherever you are, begin walking toward the equator. As you get closer, does it get hotter or colder? Hotter, of course. What is responsible for the increased warmth at the equator? This is most easily explained with a drawing. The sun shines most directly on the equator. That means that a given amount of solar energy hitting the earth at the equator is spread out over a relatively small area. Away from the equator, the earth curves toward the north and south poles. Because of this curvature, the same amount of solar energy hitting the earth at the poles is spread out over a much larger area and also travels a greater distance through the atmosphere, which absorbs or reflects much of the heat. Because the energy is spread out over a larger area, there is less warmth at any one point on the earth’s surface. It’s similar to the fact that, at noon, the sun’s rays hit the earth at a more direct angle than they do later in the day. That is why the sun provides less warmth late or early in the day. It is also why the risk of sunburn and skin cancer is greatest around noon and the nearer you are to the equator. Figure 15-5 The curvature of the earth. The sun shines more directly on the equator. Closer to the poles, the curvature of the earth causes sunlight to be spread out over a larger area, reducing the heart in any one spot.
- Global patterns of rainfall can be predicted just as easily, by taking into account that warm air holds more moisture than cold air. We’ll start at the equator again. This is where the greatest warming power of the sun hits the earth, and some of that energy radiates back, warming the air. This starts a three-step process that ends with rain. 1) Hot air rises; 2) as it rises high into the atmosphere, it cools; 3) because cool air holds less moisture, as the air rises, clouds form and the moisture that can no longer be held in the air falls as rain (Figure 15-6 Rainmaking). The equator is hot, but it is also very wet.
- The high, cold air that just rained on the equator at 0° latitude is high in the atmosphere and expands outward, away from the equator, to about 30° north and south. Here—about a third of the way from the equator toward each pole—the cold air, which is heavier than warm air, begins to fall downward toward the earth. The falling air becomes warmer as it gets closer to the earth’s surface, which radiates back some of the heat from the sun. And as the air gets warmer, it can hold more and more moisture, making it less likely to rain. That is, moisture can be held in the air rather than being released as rain. In fact, the falling and rapidly heating air can hold so much moisture that it sometimes sucks up moisture from the land itself. For this reason, at around 30° north and south around the world, there is very little rainfall, the ground is very dry, and deserts form (Figure 15-7 Desert formation). As the air falls near 30° north and south and hits the earth, it spreads equally toward the equator and toward the poles. Moving toward the poles, a similar pattern is repeated. The air moving over the surface of the earth absorbs radiated heat and moves along the surface toward the poles, gradually getting warmer, until, at around 60° latitude, it begins to rise because of its accumulated heat. Again, the rising air loses its moisture as rainfall. And so, at 60° latitude, two-thirds of the way toward the poles from the equator, it’s not particularly warm, but there is a great deal of rain. Not surprisingly, at these latitudes lie huge temperate forests with extensive plant growth. And finally, around the poles, air masses again descend. And as they do, because they can hold more moisture, very little rain falls. The poles are cold but with little precipitation—they resemble a frozen desert.
- Why is it so windy on the sidewalk around tall buildings? Why does it rain all the time on one side of some mountain ranges, while deserts form on the other side? And is it actually warmer in the city than in the country? The answers to these questions hinge on the fact that physical features of the land, its topography—including features created by humans—can have dramatic but predictable effects on the weather. Let’s explore some of these features, beginning with natural landscapes.
- High altitudes have lower temperatures. With increasing elevation, the air pressure drops—this is because the weight of the atmosphere becomes lower as altitude increases. And when pressure is lower, the temperature drops. For each 1000 meters above sea level, the temperature drops by about 6°C. Rain shadows create deserts. When wind blows against a mountain, the air rises to get over the top. But because the air cools as it rises, it can’t hold as much moisture. So clouds form and rain falls. After the air eventually passes over the top of the mountain, it can fall back down toward lower elevations. As it falls, it becomes warmer, and because warm air can hold more moisture than cold air, there is rarely any rain on the backside of the mountain. In fact, often the air will pull moisture from the ground, intensifying the already dry conditions, creating rain shadow deserts (Figure 15-8 The rain shadow effect). Along the west coast of the United States, the Sierra Nevada Mountains and the Cascade Mountain range are responsible for the Mojave Desert in California and the Great Sandy Desert in Oregon.
- Asphalt, cement, and tops of buildings absorb heat, raising the temperature. Modern landscapes also influence the weather, creating “urban heat islands,” metropolitan areas where the asphalt and concrete lead to greater absorption, rather than reflection, of solar energy. When energy from the sun hits concrete or pavement or the dark roof of a building, some of it is reflected, heating the air around it, and most of the rest is absorbed and then released at night. This is very different from what happens when sunlight hits trees or other plant life. The solar energy evaporates water in the leaves. The ground surface doesn’t get much hotter, and neither does the air. It’s not surprising, then, that cities tend to be 1° to 6°C warmer than surrounding rural areas. And not only is it hotter in cities, but the rising warm air also alters rainfall patterns both in and around cities.
- Tall buildings channel wind downward. Tall buildings are responsible for the perpetual winds you feel when walking on a city sidewalk. Here’s why. Winds blow more strongly when they are higher above the earth, freed from the earth’s frictional drag (from plants, dirt, rocks, and water) that slows wind as it gets near the surface. When these strong, elevated winds suddenly encounter tall buildings, they are deflected. Some of the wind goes up and over the building, but much of it is pushed downward, reaching double or even triple its initial speed by the time it reaches street level.
- Weather is affected not just by circulating air masses. It is also affected by the oceans. Just as there are global patterns of air circulation that produce broad climate patterns across the globe, there are also global patterns of circulation in the oceans.
- Water is continuously moving and mixing due to a combination of forces, including wind, the earth’s rotation, the gravitational pull of the moon, temperature, and salt concentration. There are several large, circular patterns of flow in the world’s oceans, as illustrated in Figure 15-10 (Ocean currents influence weather).
- Much of water’s effect on weather stems from its great capacity to absorb and hold heat—a capacity 10,000 times greater than air. This means that temperatures fluctuate much more in air than in water. During a day at the beach, the air temperature can go from mild to very hot and back to mild over the course of a day, while the heat of the sun will have a tiny, almost negligible effect on the water temperature. Thus, during summers, much of the heat of the sun is absorbed by the ocean water in coastal towns, rather than causing hot air temperatures. Conversely, during cold winter months, heat from the water can reduce the coldness of the air.
- One of the strongest ocean currents is the Gulf Stream. It travels north through the Caribbean, bringing warm water up the east coast of the United States and then across the Atlantic Ocean toward Europe. Because the current begins in a warm part of the globe, close to the equator, it is still warm when it reaches the east coast of the United States and then Europe. The warm water also warms the climate in these areas. In fact, if it weren’t for the Gulf Stream, much of Europe—given its high latitudes—would have a climate more like Canada’s. Because all ocean currents in the northern hemisphere rotate in a clockwise direction, water reaching the beaches of California, unlike water reaching east coast beaches, has just come from the north, near Alaska, where it gets very cold.
- Every two to seven years, a dramatic climate change driven by ocean currents, called El Niño, causes a sustained surface temperature change in the central Pacific Ocean (Fig. 15-11). It is blamed for flooding, droughts, famine, and a variety of other extreme climate disruptions. We can more easily understand El Niño if we contrast it with the more common climate pattern. In a typical year: A steady wind blows westward across the Pacific Ocean from South America toward southeast Asia and Australia. These winds push the warm surface water away from the coast of South America and raise the sea level in southeast Asia and Australia by about half a meter. The warmth of the water heats the air above it, causing the air to rise and produce great tropical rainstorms. (To experience this effect, fill your bathtub with hot water and feel the room get warmer and moister.) Off the coast of South America, an upwelling of colder water from deep in the ocean replaces the water that is pushed west. The cold upwelling cools the air above it, causing extremely dry weather. The deep water brings up rich nutrients from below, enabling plankton to flourish and in turn nourish huge populations of fish. The cooler water cools the air above and causes unusually dry weather. This frequently leads to droughts and, in extreme years, causes famines.
- All life on earth is made possible because energy flows perpetually from the sun to the earth.
- The sun is where our pathway of energy flow begins. Most of the energy is absorbed or reflected by the earth’s atmosphere or surface, but about 1% of it is intercepted and converted to chemical energy through photosynthesis. That intercepted energy is then transformed again and again by living organisms, making about four stops as it passes through an ecosystem. Let’s examine what happens at each stop, known as trophic levels.
- First stop: primary producers. When it comes to energy flow, all the species in an ecosystem can be placed in one of two groups: producers or consumers. Plants (along with some algae and bacteria) are, as we noted earlier, the primary producers. They convert light energy from the sun into chemical energy through photosynthesis. We use another word to describe that chemical energy: food.
- Second stop: primary consumers—the herbivores. Cattle grazing in a field, gazelle browsing on herbs, insects devouring the leaves of a crop plant—these are the primary consumers in an ecosystem, the animals that eat plants. Plant material such as cellulose can be difficult to digest. Consequently, most herbivores—animals that eat plants—need a little help in digesting the plants they eat. Primary consumers, from termites to cattle, often have symbiotic bacteria living in their digestive system. These microorganisms break down the cellulose, enabling the herbivore to harness the energy held in the chemical bonds of the plants’ cells.
- Third stop: secondary consumers—the carnivores. Energy originating from the sun is converted into chemical energy within a plant’s tissue. The herbivore that eats the plant breaks down the chemical bonds, releasing the energy. This energy fuels the herbivore’s growth, reproduction, and movement, but the energy doesn’t remain in the herbivore forever. Carnivores, such as cats, spiders, and frogs, are animals that feed on herbivores. They are also known as secondary consumers. As they eat their prey, some of the energy stored in the chemical bonds of the carbohydrate, protein, and lipid molecules is again captured and harnessed for their own movement, reproduction, and growth.
- Fourth stop: tertiary consumers—the “top” carnivores. In some ecosystems, energy makes yet another stop: the tertiary consumers, or “top carnivores.” These are the “animals that eat the animals that eat the animals that eat the plants.” They are several steps removed from the initial capture of solar energy by a plant, but the general process is the same. A top carnivore, such as a tiger, eagle, or great white shark, consumes other carnivores, breaking down their tissue and releasing energy stored in the chemical bonds of the cells. As in each of the previous steps, the top carnivores harness this energy for their own physiological needs.
- This path from primary producers to tertiary consumers is called a food chain. The food chain pathway from photosynthetic producers through the various levels of animals, though, is a slight oversimplification. In actuality, food chains are better thought of as food webs because many organisms are omnivores and can occupy more than one position in the chain. When you eat a simple meal of rice and chicken, after all, you’re simultaneously a carnivore and an herbivore. On average, about 30 to 35% of the human diet comes from animal products and the remaining 65 to 70% comes from plant products. Many other animals, from bears to cockroaches, also have diets that involve harvesting energy from multiple stops in the food chain.
- In every ecosystem, as energy is transformed through the steps of a food chain, organic material accumulates in the form of animal waste and dead plant and animal matter. Decomposers, usually bacteria or fungi, and detritivores, including scavengers such as vultures, worms, and a variety of arthropods, break down the organic material, harvesting energy still stored in the chemical bonds (Figure 15-13). Because the decomposers are able to break down a much larger range of organic molecules, they are distinguished from the detritivores. Both groups, nonetheless, release many important chemical components from the organic material than can eventually be recycled and utilized by plants and other primary producers.
- Energy flows from one stop to the next in a food chain, but not in the way that runners pass a baton in a relay race. The difference is that, at every step in the food chain, much of the usable energy is lost as heat. An animal that eats five pounds of plant material doesn’t convert that into five new pounds of body weight. Not by a long shot. In the next section, we’ll see how this inefficiency of energy transfers ensures that most food chains are very short.
- Why are big, fierce animals so rare? And why are there so many more plants than animals? The answers are closely related to our earlier observation that an animal consuming five pounds of plant material does not gain five pounds in body weight from such a meal. The actual amount of growth such a meal can support is far, far less. And when that herbivore is consumed by a carnivore, the carnivore, too, can convert only a small fraction of the energy it consumes into its own tissue. The fraction turns out to be about 10%, and it is fairly consistent across all levels of the food chain.
- This means that only about 10% of the biomass—the total weight of all the living organisms in a given area—of plants in an ecosystem is converted into herbivore biomass. So the herbivore consuming five pounds of plant material is likely to gain only about half a pound in new growth, while the remaining 90% of the meal is either expended in cellular respiration or lost as feces. Similarly, a carnivore eating the herbivore converts only about 10% of the mass it consumes into its own body mass. Again, 90% is lost to metabolism and feces. And the same inefficiency holds for a top carnivore, as well. Let’s explore how this 10% rule limits the length of food chains and is responsible for the rarity of big, fierce animals outside your window and across the world.
- Given the 10% efficiency with which herbivores convert plant biomass into their own biomass, how much plant biomass is necessary to produce a single 1200-pound (500- kg) cow? On average, that cow would need to eat about 12,000 pounds (5000 kg) of grain in order to grow to weigh 1200 pounds. But that 1200-pound cow, when eaten by a carnivore, could only add about 120 pounds of biomass to the carnivore, and only 12 pounds to a top carnivore. That’s a huge amount of plant biomass required to generate only a very tiny amount of our top carnivore, which explains why big, fierce animals are so rare (and why vegetarianism is more energetically efficient than meat-eating). After all, multiply that 5000 kg of grain by several hundred (or thousands, more appropriately) to get an idea of how much plant matter would be necessary to support even a small population of top carnivores: Millions of kilograms of grain can support only a few top carnivores.
- How much would be required to support an even higher link in the food chain? Ten times as much, so much that there may not be enough land in the ecosystem to produce enough plant material. And even if there were, the area required would be so large that the “top, top carnivores” might be so spread out and so busy trying to eat enough that they would be unlikely to encounter each other in order to mate. Hence, the 10% rule limits the length of food chains.
- We can illustrate the path of energy through the organisms of an ecosystem with an energy pyramid in which each layer of the pyramid represents the biomass of a trophic level. In Figure 15-14 (Inefficiencies in the transfer of biomass), we can see that, for terrestrial ecosystems, the biomass (in kilograms per square meter) found in the photosynthetic organisms, at the base of the pyramid, is reduced significantly at each step, given the incomplete utilization by organisms higher up the food chain.
- Figure 15-15 (Relative biomass of producers and consumers) illustrates the huge variation in primary productivity across a variety of ecosystems. It is highest in tropical rain forests, marshes, and algal beds, and lowest in deserts, tundra, and the open ocean. In each case, the shapes of the energy and biomass pyramids are similar. With a smaller base, though, the ability of an ecosystem to support higher levels of the food chain is reduced. One dramatic exception is seen in some aquatic ecosystems where the primary producers are plankton. Because plankton have such short life spans and rapid reproduction rates, a relatively small biomass can support a large biomass of consumers, giving rise to an inverted pyramid (see the bottom pyramid in Figure 15-15). From an energy perspective, though, the pyramid still resembles that seen in terrestrial ecosystems.
- What is necessary for life? Energy and some essential chemicals top the list. New energy continually comes to earth from the sun, fulfilling the first need. And everything else is already here. The chemicals just cycle around and around, using the same pathway taken by energy—the food chain. Plants and other producers generally take up the molecules from the atmosphere or the soil. Then, the chemicals move into animals as they consume plants or other animals, and thus move up the food chain. And, as the plants and animals die, detritivores and decomposers return the chemicals to the abiotic environment. From a chemical perspective, life is just a continuous recycling of molecules.
- Chemicals cycle through the living and non-living components of an ecosystem. Each chemical is stored in a non-living part of the environment called a reservoir. Organisms acquire the chemical from the reservoir, the chemical cycles through the food chain, and eventually, it is returned to the reservoir. This is a useful overview, but we can get a deeper appreciation of the functioning of ecosystems and the ecological problems that can occur when they are disturbed—particularly by humans—by investigating a few of these cycles in more detail.
- Here, we’ll explore the three most important chemical cycles: carbon, nitrogen, and phosphorus.
- Carbon is found largely in four compartments on earth: the oceans, the atmosphere, terrestrial organisms, and fossil deposits. Plants and other photosynthetic organisms obtain most of their carbon from the atmosphere, where carbon is in the form of carbon dioxide (CO2). As we saw in Chapter 4, plants and some microorganisms utilize carbon dioxide in photosynthesis, separating the carbon molecules from CO2 and using them to build sugars. Carbon then moves through the food chain as organisms eat plants and are themselves eaten (Figure 15-16 Element cycling: carbon). The oceans contain most of the earth’s carbon. Here, many organisms use dissolved carbon to build shells (which later dissolve back into the water after the organism dies). Most carbon returns to its reservoir as a consequence of organisms’ metabolic processes. Organisms extract energy from food by breaking carbon-carbon bonds, releasing the energy stored in the bonds, and combining the released carbon atoms with oxygen. They then exhale the end product as CO2.
- Fossil fuels are created when large numbers of organisms die and are buried in sediment lacking oxygen. In the absence of oxygen, at high pressures, and after very long periods of time, the organic remains are ultimately transformed into coal, oil, and natural gas. Trapped underground or in rock, these sources of carbon played little role in the global carbon cycle until humans in industrialized countries began using fossil fuels to power various technologies. Burning coal, oil, and natural gas releases large amounts of carbon dioxide, thus increasing the average CO2 concentration in the atmosphere—the current level of CO2 in the atmosphere is the highest it has been in almost half a million years. This has potentially disastrous implications, as we will see in Chapter 16.
- Although, on average, the level of CO2 in the environment is increasing, there is a yearly cycle of ups and downs in the CO2 levels in the northern hemisphere. This cycling is due to fluctuations in the ability of plants to absorb CO2. Many trees lose their leaves each fall and, during the winter months, relatively low rates of photosynthesis lead to low rates of CO2 consumption, causing an annual peak in atmospheric CO2 levels. During the summer, leaves are present, sunlight is strong, and photosynthesis (consuming CO2) occurs at much higher levels, causing a drop in the atmospheric CO2 level.
- Nitrogen is necessary to build all amino acids, the components of every protein molecule, as well as the precursors of other nitrogen-containing molecules—all critical to life. Like carbon, the chief reservoir of nitrogen is the atmosphere. But even though more than 78% of the atmosphere is nitrogen gas (N2), for most organisms, this nitrogen is completely unusable. The problem is that atmospheric nitrogen consists of two nitrogen atoms bonded tightly together, and these bonds need to be broken to make the nitrogen usable for living organisms. Only through the metabolic magic (chemistry, actually) of some soil-dwelling bacteria, the nitrogen-fixers, can most nitrogen enter the food chain. These bacteria chemically convert or “fix” nitrogen by attaching it to other atoms, including hydrogen, producing ammonia and related compounds. These compounds are then further modified by other bacteria into a form that can be taken up by plants and used to build proteins. And once nitrogen is in plant tissues, animals acquire it in the same way they acquire carbon: by eating the plants. Nitrogen returns to the atmosphere when animal wastes and dead animals are broken down by soil bacteria that convert the nitrogen compounds in tissue back to nitrogen gas.
- Because it is necessary for the production of every plant protein, and because all nitrogen must first be made usable by bacteria, plant growth is often limited by nitrogen levels in the soil. For this reason, most fertilizers contain nitrogen in a form usable by plants.
- Every molecule of ATP and DNA requires phosphorus. Virtually no phosphorus is available in the atmosphere, though. Instead, soil serves as the chief reservoir. Like nitrogen, phosphorus is chemically converted—“fixed”—into a form usable by plants (phosphate) and then absorbed by their roots. It cycles through the food chain as herbivores consume plants and carnivores consume herbivores. As organisms die, their remains are broken down by bacteria and other organisms, returning the phosphorus to the soil. The pool of phosphorus in the soil is also influenced by the much slower process of rock formation on the sea floor, its uplifting into mountains, and the eventual weathering of the rock, releasing its phosphorus.
- Like nitrogen, phosphorus is often a limiting resource in soils, constraining plant growth. Consequently, fertilizers usually contain large amounts of phosphorus. This is beneficial in the short run, but it can have some disastrous unintended consequences. As more and more phosphorus (and nitrogen) is added to soil, some of it runs off with water and ends up in lakes, ponds, and rivers. In these habitats, also, it acts as a fertilizer, making spectacular growth possible for algae. But eventually, the algae die and sink, creating a huge source of food for bacteria. The bacteria population increases and can use up too much of the dissolved oxygen, causing fish, insects, and many other organisms to suffocate and die. This process of excess nutrients leading to rapid growth of algae and bacteria, followed by large-scale die-offs, is called eutrophication. It is increasingly a problem in both small and large bodies of water, affecting more than half of the lakes in Asia, Europe, and North America. Lake Erie, on the U.S.-Canadian border, for example, has experienced eutrophication as a result of all the phosphorus- and nitrogen-containing waste water that drains into it from the extensive surrounding farmlands. Given the lower use of fertilizers in South America and Africa, eutrophication is less common there.