Chapter Introduction Core Case StudyTropical Rain Forests Are Disappearing 3.1Earth’s Life-Support System

Chapter Introduction
Core Case StudyTropical Rain Forests Are Disappearing
3.1Earth’s Life-Support System
3.1aEarth’s Life-Support System Has Four Major Components
3.1bThree Factors Sustain the Earth’s Life
3.2Ecosystem Components
3.2aEcosystems Have Several Important Components
3.2bSoil Is the Foundation of Life on Land
3.3Energy in an Ecosystem
3.3aEnergy Flows through Ecosystems in Food Chains and Food Webs
3.3bSome Ecosystems Produce Plant Matter Faster Than Others Do
3.4Matter in an Ecosystem
3.4aNutrients Cycle Within and Among Ecosystems
3.4bThe Water Cycle
3.4cThe Carbon Cycle
3.4dThe Nitrogen Cycle
3.4eThe Phosphorus Cycle
3.5How Do Scientists Study Ecosystems?
3.5aStudying Ecosystems Directly
3.5bLaboratory Research and Models
3.5cFour Laws of Ecology
Tying It All TogetherTropical Rain Forests and Sustainability
Chapter Review
Critical Thinking
Doing Environmental Science
Data Analysis
Chapter Introduction
Cleared area of tropical rainforest in Uganda (Africa)
Prill/ Shutterstock.com
ore Case StudyTropical Rain Forests Are Disappearing
Learning Objective
LO 3.1State three reasons why we need to care about the ongoing loss of rain forests.
Tropical rain forests are found near the earth’s equator and contain an amazing variety of life. These lush forests are warm year round and have high humidity because it rains almost daily. Rain forests cover only 7% of the earth’s land but contain up to half of the world’s known plant and animal species found on land. The diversity of species in these forests makes them excellent natural laboratories in which to study ecosystems—communities of organisms that interact with one another and with the physical environment of matter and energy in which they live.
To date, human activities have destroyed or degraded more than half of the earth’s rain forests (see chapter-opening photo). People continue clearing the forests to grow crops, graze cattle, and build settlements (Figure 3.1). Ecologists warn that without protection, most of these ecologically important forests will be gone or severely degraded by the end of this century.
Figure 3.1
Natural capital degradation: Satellite image of the loss of tropical rain forest, cleared for farming, cattle grazing, and settlements, near the Bolivian city of Santa Cruz between June 1975 (left) and May 2003 (right). This is the latest available view of the area but forest degradation has continued since 2003.
Left: United Nations Environment Programme; United Nations Environment Programme
Why should we care that tropical rain forests are disappearing? Scientists give three reasons. First, clearing these forests causes the extinction of many plant and animal species by destroying the habitats where they live. The loss of key species in these forests can have a ripple effect that leads to the extinction of other species they help support.
Second, destroying these forests contributes to atmospheric warming and speeds up climate change. How does this occur? Eliminating large areas of trees faster than they can grow back means that there are fewer plants using photosynthesis to remove some of the excess carbon dioxide  emitted into the atmosphere when carbon-containing fossil fuels are burned. The resulting increased levels of  in the atmosphere contributes to atmospheric warming and climate change, which you will learn more about in Chapter 19.
Third, large-scale loss of tropical rain forests can change regional weather patterns in ways that can prevent the regrowth of rain forests in cleared or degraded areas. When this irreversible ecological tipping point is reached, tropical rain forests in such areas become drier and less-diverse tropical grasslands.
In this chapter, you will learn how tropical rain forests and other ecosystems work, how human activities are affecting them, and how we can help sustain them.
3.1Earth’s Life-Support System
LO 3.1ADescribe the four main systems, or spheres, that make up the earth’s life-support system.
LO 3.1BExplain how the flow of energy from the sun and the greenhouse effect are connected within the biosphere.
LO 3.1CExplain the roles that solar energy, nutrient cycling, and gravity play in sustaining life.
3.1aEarth’s Life-Support System Has Four Major Components
The earth’s life-support system consists of four main systems (Figure 3.2) that interact with one another. They are the atmosphere (air), the hydrosphere (water), the geosphere (rock, soil, and sediment), and the biosphere (living things).
Figure 3.2
Natural capital: The earth consists of a land sphere (geosphere), an air sphere (atmosphere), a water sphere (hydrosphere), and a life sphere (biosphere).
The atmosphere is a spherical mass of air surrounding the earth’s surface that is held by gravity. Its innermost layer, the troposphere, extends about 19 kilometers (12 miles) above sea level at the equator and about 6 kilometers (4 miles) above the earth’s North and South Poles. The troposphere contains the air we breathe. It is 78% nitrogen  and 21% oxygen . The remaining 1% of air is mostly water vapor, carbon dioxide, and methane. The troposphere is the layer in which the earth’s weather occurs and where life can survive.
The stratosphere is the atmospheric layer above the troposphere. It reaches 17 to 50 kilometers (12–31 miles) above the earth’s surface. The lower stratosphere, called the ozone layer, contains enough ozone  gas to filter out about 95% of the sun’s harmful ultraviolet (UV) radiation. It acts as a global sunscreen that allows life to exist on the earth’s surface.
The hydrosphere contains all of the water on or near the earth’s surface. It is found as water vapor in the atmosphere, as liquid water on the surface and underground, and as ice—polar ice, icebergs, glaciers, and ice in frozen soil-layers called permafrost. Salty oceans that cover about 71% of the earth’s surface contain 97% of the planet’s water and support almost half of the world’s species. About 2.5% of the earth’s water is freshwater and three-fourths of that is ice.
The geosphere contains the earth’s rocks, minerals, and soil. It consists of an intensely hot core, a thick mantle of very hot rock, and a thin outer crust of rock and soil. The crust’s upper portion contains soil chemicals or nutrients that organisms need to live, grow, and reproduce. It also contains nonrenewable fossil fuels—coal, oil, and natural gas—and minerals that we extract and use.
The biosphere consists of the parts of the atmosphere, hydrosphere, and geosphere where life is found. If the earth were the size of an apple, the biosphere would be no thicker than the apple’s skin.
3.1bThree Factors Sustain the Earth’s Life
Life on the earth depends on three interconnected factors:
One-way flow of high-quality energy from the sun. The sun’s energy supports plant growth, which provides energy for plants and animals, in keeping with the solar energy principle of sustainability. As solar energy interacts with carbon dioxide , water vapor, and several other gases in the troposphere, it warms the troposphere—a process known as the greenhouse effect (Figure 3.3). Without this natural process, the earth would be too cold to support most of the forms of life we find here today.
Cycling of nutrients through parts of the biosphere. Nutrients are chemicals that organisms need to survive. Because the earth does not get significant inputs of matter from space, its fixed supply of nutrients must be recycled to support life. This is in keeping with the chemical cycling principle of sustainability.
Gravity allows the planet to hold on to its atmosphere and enables the movement and cycling of chemicals through air, water, soil, and organisms.
Figure 3.3
Greenhouse Earth. High-quality solar energy flows from the sun to the earth. It is degraded to lower-quality energy (mostly heat) as it interacts with the earth’s air, water, soil, and life forms, and eventually some of it returns to space. Certain gases in the earth’s atmosphere retain enough of the sun’s incoming energy as heat to warm the planet as a result of the greenhouse effect.
National Geographic Visual Atlas of the World. Washington, DC: National Geographic Society, 2008.
3.2Ecosystem Components
LO 3.2AExplain the relationships among the biosphere, ecosystems, communities, populations, and organisms using an organism of your choice (which could even be you).
LO 3.2BList two consumer organisms that feed on a common producer organism of your choice.
LO 3.2CDescribe the diet of a detritus feeder and a decomposer and explain their role in chemical cycling.
LO 3.2DWrite the chemical equations that represent photosynthesis and aerobic respiration.
LO 3.2EList six components of soil.
LO 3.2FExplain how topsoil supports terrestrial life.
3.2aEcosystems Have Several Important Components
Ecology is the science that focuses on how organisms interact with one another and with their nonliving physical environment of matter and energy. Scientists classify matter into levels of organization ranging from atoms to galaxies. Ecologists study five levels of matter—the biosphere, ecosystems, communities, populations, and organisms—all shown and defined in Figure 3.4.
Figure 3.4
Ecology focuses on the top five of these levels of the organization of matter in nature.
The biosphere and its ecosystems are made up of living (biotic) and nonliving (abiotic) components (Figure 3.5). Living components include plants, animals, and microbes. Nonliving components include water, air, nutrients, rocks, heat, and solar energy.
Figure 3.5
Key living (biotic) and nonliving (abiotic) components of an ecosystem in a field.
Ecologists assign each organism in an ecosystem to a feeding level, or trophic level, based on its source of nutrients. Organisms are classified as producers and consumers based on whether they make (produce) or find (consume) their food.
Producers are organisms, such as green plants, that make the nutrients they need from compounds and energy obtained from their environment. In the process known as photosynthesis, green plants capture solar energy that falls on their leaves. They use it to combine carbon dioxide and water to form carbohydrates such as glucose , which they store as a source of chemical energy. In the process, they emit oxygen  gas into the atmosphere. This oxygen keeps us and most other animal species alive. The following chemical reaction summarizes the overall process of photosynthesis.
About 2.8 billion years ago, producer organisms called cyanobacteria, most of them floating on the surface of the ocean, started carrying out photosynthesis and adding oxygen to the atmosphere. After several hundred million years, oxygen levels reached about 21%—high enough to keep humans and other oxygen-breathing animals alive.
Learning from Nature
Scientists hope to make a molecular-sized solar cell by mimicking how a leaf uses photosynthesis to capture solar energy. These artificial leaf films might be used to coat the roofs, windows, or walls of a building and provide electricity for most homes and other buildings.
On land, most producers are green plants such as trees and grasses. In freshwater and ocean ecosystems, algae and aquatic plants growing near shorelines are the major producers. In open water, the dominant producers are phytoplankton—mostly microscopic organisms that float or drift in the water.
Some producer bacteria live in dark and extremely hot water around fissures on the ocean floor. Their source of energy is heat from the earth’s interior, or geothermal energy. They are an exception to the solar energy principle of sustainability.
The other organisms in an ecosystem are consumers that cannot make their food. They get the nutrients they need by feeding on other producers, or other consumers or on the wastes and remains of producers and consumers.
There are several types of consumers. Primary consumers, or herbivores (plant eaters), are animals that eat mostly green plants. Examples are caterpillars, giraffes, and zooplankton (tiny sea animals that feed on phytoplankton). Carnivores (meat eaters) are animals that feed on the flesh of other animals. Some carnivores, including spiders, lions (Figure 3.6), and most small fishes, are secondary consumers that feed on the flesh of herbivores. Other carnivores such as tigers, hawks, and killer whales (orcas) are tertiary (or higher-level) consumers that feed on the flesh of herbivores and other carnivores. Some of these relationships are shown in Figure 3.5. Omnivores such as pigs, rats, and humans eat both plants and animals.
Figure 3.6
This lioness (a carnivore) is feeding on a freshly killer zebra (an herbivore) in Kenya, Africa.
nelik/ Shutterstock.com
Critical Thinking
When you ate your most recent meal, were you an herbivore, a carnivore, or an omnivore?
Decomposers are consumers that get nourishment by breaking down (decomposing) the wastes or remains of plants and animals. These nutrients return to the soil, water, and air for reuse by producers. Most decomposers are bacteria and fungi, such as molds and mushrooms. Other consumers, called detritus feeders, or detritivores, feed on the wastes or dead bodies (detritus) of other organisms. Examples are earthworms, soil insects, hyenas, and vultures (Figure 3.7).
Figure 3.7
The vultures and Marabou storks, eating the carcass of an animal that was killed by another animal, are detritivores.
javarman/ Shutterstock.com
Detritivores and decomposers can transform a fallen tree trunk into simple inorganic molecules that plants can absorb as nutrients (Figure 3.8). In natural ecosystems, the wastes and dead bodies of organisms are resources for other organisms in keeping with the chemical cycling principle of sustainability. Without decomposers and detritivores, many of which are microscopic organisms, the planet’s land surfaces would be buried in plant and animal wastes, dead animal bodies, and garbage.
Figure 3.8
Various detritivores and decomposers (mostly fungi and bacteria) can “feed on” or digest parts of a log and eventually convert its complex organic chemicals into simpler inorganic nutrients that can be taken up by producers.
Producers, consumers, and decomposers use the chemical energy stored in glucose and other organic compounds to fuel their life processes. In most cells, this energy is released by aerobic respiration, which uses oxygen to convert glucose and other organic compounds back into carbon dioxide and water. The overall chemical reaction for the aerobic respiration is shown in the following equation:
Some decomposers, such as yeast and some bacteria get the energy they need by breaking down glucose (or other organic compounds) in the absence of oxygen. This form of cellular respiration is called anaerobic respiration, or fermentation. Instead of carbon dioxide and water, the products of this process are compounds such as methane gas , ethyl alcohol , acetic acid (, the key component of vinegar), and hydrogen sulfide (, a highly poisonous gas that smells like rotten eggs). Note that all organisms get their energy from aerobic or aerobic respiration, but only plants carry out photosynthesis.
To summarize, ecosystems and the biosphere are sustained by the one-way energy flow from the sun and the nutrient cycling of key materials—in keeping with two of the scientific principles of sustainability (Figure 3.9).
Figure 3.9
Natural capital: The main components of an ecosystem are energy, chemicals, and organisms. Nutrient cycling and the flow of energy—first from the sun, then through organisms, and finally into the environment as low-quality heat—link these components.
3.2bSoil Is the Foundation of Life on Land
Soil is a complex mixture of rock pieces and particles, mineral nutrients, decaying organic matter, water, air, and living organisms that support plant life, which supports animal life. Life on land depends on roughly 15 centimeters (6 inches) of topsoil. The minerals that make up your muscles, bones, and most other parts of your body come almost entirely from soil.
Soil is one of the most important components of the earth’s natural capital. It purifies water and supplies most of the nutrients needed for plant growth. Through aerobic respiration, organisms living in soil remove some of the carbon dioxide in the atmosphere and store it as organic carbon compounds, thereby helping to control the earth’s climate.
Soil formation begins when physical, chemical, and biological processes called weathering break down bedrock into small pieces. Various forms of plant and animal life begin living on the weathered particles. Their wastes and decaying bodies add organic matter and minerals to the slowly forming soil. Decomposers and detritivores break down fallen leaves and wood (Figure 3.8) and add organic matter and nutrients to the soil. Air (mostly nitrogen and oxygen) and water occupy pores or spaces between soil particles. Over hundreds to thousands of years, various types of life build up distinct layers of mineral and organic matter on a soil’s original bedrock.
Most mature soils contain several horizontal layers or horizons. A cross-sectional view of the horizons of a soil is called a soil profile (Figure 3.10, right). The major horizons in a mature soil are O (leaf litter), A (topsoil), B (subsoil), and C (weathered parent material), which build up over the parent material. Each layer has a distinct texture, composition, and thickness that vary with the soils formed in different climates and biomes such as deserts, grasslands, and forests (Figure 3.11). Soil forms faster in wet, warm climates.
Figure 3.10
Natural capital: Generalized soil formation and soil profile.
Critical Thinking:
What role do you think the tree in this figure plays in soil formation? How might the soil formation process change if the tree were removed?
Figure 3.11
Natural capital: Soil profiles of the principal soil types typically found in five types of terrestrial ecosystems.
The roots of most plants and the majority of a soil’s organic matter are found in the soil’s two upper layers: the O-horizon of leaf litter and the A-horizon of topsoil. In a fertile soil, these two layers teem with bacteria, fungi, earthworms, and numerous small insects, all interacting by feeding on and decomposing one another. The leaf litter and topsoil layers are also habitats for larger animals such as snails, reptiles, amphibians, and burrowing animals such as moles.
Every handful of topsoil contains billions of bacteria and other decomposer organisms. They break down some of the soil’s complex organic compounds into a mixture of the partially decomposed bodies of dead plants and animals, called humus. A fertile soil that produces high crop yields has a thick topsoil layer with a lot of humus mixed with minerals from weathered rock.
Moisture in topsoil dissolves nutrients needed for plant growth. The resulting solution is drawn up by the roots of the plants and transported through their stems into the leaves of plants. This movement of nutrients from the topsoil to plant leaves and to insects and other animals that eat the leaves is part of the chemical cycling process essential for the earth’s life. Healthy soils retain more water and help reduce the severity of drought by releasing some of the water into the atmosphere. Figure 3.12 shows the movement of plant nutrients in soils.
Figure 3.12
Pathways of plant nutrients in soils.
The color of topsoil indicates how useful it is for growing crops or other plants. Black or dark brown topsoil is rich in nitrogen and organic matter. A gray, bright yellow, or red topsoil is low in organic matter and needs the addition of nitrogen to support most crops.
The B-horizon (subsoil) and the C-horizon (parent material) contain most of a soil’s inorganic matter, mostly broken-down rock, consisting of various mixtures of sand, silt, clay, and gravel. The C-horizon sits on the soil’s parent material, which is often bedrock.
Soils can include particles of three different sizes: very small clay particles, medium-size silt particles, and larger sand particles. The relative amounts of these different sizes and types of these mineral particles, the composition of organic materials, and the amount of space between the particles determine the texture of a soil. A soil’s texture affects how rapidly water flows through it (Figure 3.13).
Figure 3.13
Natural capital: The size, shape, and degree of clumping of soil particles determine the number and volume of spaces for air and water within a soil. Water can flow more easily through soils with more spaces (left) than through soils with fewer spaces (right).
Soil is a renewable resource but it is renewed very slowly and becomes a nonrenewable resource if we deplete it faster than nature can replenish it. The formation of just 2.5 centimeters (1 inch) of topsoil can take hundreds to thousands of years. Removing plant cover from soil exposes its topsoil to erosion by water and wind. This explains why protecting and renewing topsoil is a key to sustainability. You will learn more about soil erosion and soil conservation in Chapter 12.
3.3aEnergy Flows through Ecosystems in Food Chains and Food Webs
Chemical energy, stored as nutrients in the bodies and wastes of organisms, flows through ecosystems from one trophic (feeding) level to another in food chains and food webs. A sequence of organisms with each one serving as a source of nutrients or energy for the next level of organisms is called a food chain (Figure 3.14).
Figure 3.14
In a food chain, chemical energy in nutrients flows through various trophic levels.
Critical Thinking:
Think about what you ate for breakfast. At what level or levels on a food chain were you eating?
Every use and transfer of energy by organisms from one feeding level to another involves a loss of some high-quality energy to the environment as low-quality energy in the form of heat, as required with the second law of thermodynamics. A graphic display of the energy loss at each trophic level is called a pyramid of energy flow. Figure 3.15 illustrates this energy loss for a food chain, assuming a 90% energy loss for each level of the chain.
Figure 3.15
Generalized pyramid of energy flow showing the decrease in usable chemical energy available at each succeeding trophic level in a food chain or food web. This model assumes that with each transfer from one trophic level to another, there is a 90% loss of usable energy to the environment in the form of low-quality heat. (Calories and joules are used to measure energy. .)
Critical Thinking:
Why is a vegetarian diet more energy efficient than a meat-based diet?
Connections
Energy Flow and Feeding People
Energy flow pyramids explain why the earth could support more people if they all ate at a low trophic level by consuming grains, vegetables, and fruits directly rather than passing such crops through another trophic level and eating the meat of herbivores such as cattle, pigs, sheep, and chickens. About two-thirds of the world’s people survive primarily by eating wheat, rice, and corn at the first trophic level, mostly because they cannot afford to eat much meat.
Ecologists can estimate the number of organisms feeding at each trophic level. Here is a hypothetical example: 100,000 blades of grass (producer) might support 30 rabbits (herbivore), which might support 1 fox (carnivore). Ecologists also measure biomass—the total mass of organisms in each trophic level—as illustrated by this hypothetical example: 1,000 kilograms (2,200 pounds) of producers might provide 100 kilograms (220 pounds) of food for herbivores, which might provide 10 kilograms (22 pounds) of food for carnivores, which might supply a top carnivore with 1 kilogram (2.2 pounds) of food.
In natural ecosystems, most consumers feed on more than one type of organism, and most organisms are eaten or decomposed by more than one type of consumer. Because of this, organisms in most ecosystems form a complex network of interconnected food chains called a food web. Food chains and food webs show how producers, consumers, and decomposers are connected to one another as energy flows through trophic levels in an ecosystem. Figure 3.16 shows an aquatic food web and Figure 3.17 shows a terrestrial food web.
Figure 3.16
A greatly simplified aquatic food web found in the southern hemisphere. The shaded middle area shows a simple food chain that is part of these complex interacting feeding relationships. Many more participants in the web, including an array of decomposer and detritus feeder organisms, are not shown here.
Figure 3.17
Greatly simplified terrestrial food web found in a temperate desert ecosystem. The shaded middle area shows a simple food chain that is part of these complex interacting feeding relationships. Many more participants in the web, including an array of decomposer and detritus feeder organisms, are not shown.
Learning from Nature
There is no waste in nature because the wastes or remains of one organism provide food for another. Scientists and engineers study food webs to learn how to reduce or eliminate food waste and some of the other wastes we produce.
3.3bSome Ecosystems Produce Plant Matter Faster Than Others Do
Scientists measure the rates at which ecosystems produce chemical energy to compare ecosystems and understand how they interact. Gross primary productivity (GPP) is the rate at which an ecosystem’s producers (such as plants and phytoplankton) convert solar energy into chemical energy stored in compounds found in their tissues. It is usually measured in terms of energy production per unit area over a given time span, such as kilocalories per square meter per year . To stay alive, grow, and reproduce, producers must use some of their stored chemical energy for their own aerobic respiration.
Net primary productivity (NPP) is the rate at which producers use photosynthesis to produce and store chemical energy minus the rate at which they use some of this stored chemical energy through aerobic respiration. NPP measures how fast producers can make the chemical energy that is stored in their tissues and that is potentially available to the consumers in an ecosystem.
Gross primary productivity is similar to the rate at which you make money, or the number of dollars you earn per year. Net primary productivity is similar to the amount of money earned per year that you can spend after subtracting your expenses such as the costs of transportation, clothes, food, and supplies.
Terrestrial ecosystems and aquatic life zones differ in their NPP as illustrated in Figure 3.18. Despite its low NPP, the open ocean produces more of the earth’s biomass per year than any other ecosystem or life zone. This happens because oceans cover 71% of the earth’s surface and contain huge numbers of phytoplankton and other producers.
Figure 3.18
Estimated annual average net primary productivity in major life zones and ecosystems expressed as kilocalories of energy produced per square meter per year .
Question:
What are the three most productive and the three least productive systems?
(Compiled by the authors using data from R. H. Whittaker, Communities and Ecosystems, 2nd ed., New York: Macmillan, 1975.)
Tropical rain forests have a high net primary productivity (NPP) because they have a large number and variety of producer trees and other plants to support a large number of consumers. When such forests are cleared (Core Case Study) or burned to make way for crops or for grazing cattle, they suffer a sharp drop in net primary productivity and lose plant and animal species.
Only the plant matter represented by NPP is available as nutrients for consumers. Thus, the planet’s NPP ultimately limits the number of consumers (including humans) that can survive on the earth. This is one of nature’s important lessons.
3.4aNutrients Cycle Within and Among Ecosystems
The elements and compounds that make up nutrients move continually through air, water, soil, rock, and living organisms within ecosystems in cycles called nutrient cycles, or biogeochemical cycles (life-earth-chemical cycles). They represent the chemical cycling principle of sustainability in action. These cycles are driven directly or indirectly by incoming solar energy and by the earth’s gravity. They include the hydrologic (water), carbon, nitrogen, and phosphorus cycles. Human activities are altering these important components of the earth’s natural capital (Figure 1.3).
Connections
Nutrient Cycles and Life
Nutrient cycles connect past, present, and future forms of life. Some of the carbon atoms in your skin may once have been part of an oak leaf, a dinosaur’s skin, or a layer of limestone rock. Your grandmother, George Washington, or a hunter–gatherer who lived 25,000 years ago may have inhaled some of the nitrogen  molecules you just inhaled.
3.4bThe Water Cycle
Water  is an amazing substance (Science Focus 3.1) that is essential for life on the earth. The hydrologic cycle, or the water cycle, collects, purifies, and distributes the earth’s fixed supply of water, as shown in Figure 3.19.
Figure 3.19
Natural capital: Simplified model of the water cycle, or hydrologic cycle, in which water circulates in various physical forms within the biosphere. The red arrows and boxes identify major effects of human activities on this cycle.
Critical Thinking:
What are three ways in which your lifestyle directly or indirectly affects the hydrologic cycle?
Science Focus 3.1
Water’s Unique Properties
Water  is a remarkable substance with a unique combination of properties:
Water exists as a liquid over a wide temperature range because of forces of attraction between its molecules. If liquid water had a much narrower range of temperatures between freezing and boiling, the oceans would probably have frozen solid or boiled away long ago.
Liquid water changes temperature slowly because it can store a large amount of heat without a large change in its own temperature. This helps protect living organisms from temperature changes, moderates the earth’s climate, and makes water an excellent coolant for car engines and power plants.
It takes a large amount of energy to evaporate water because of the attractive forces between its molecules. Water absorbs large amounts of heat as it changes into water vapor and releases this heat as the vapor condenses back to liquid water. This helps to distribute heat throughout the world and to determine regional and local climates. It also makes evaporation a cooling process—explaining why you feel cooler when perspiration evaporates from your skin.
Liquid water can dissolve more compounds than other liquids. Water carries dissolved nutrients into the tissues of living organisms, flushes waste products out of those tissues, serves as an all-purpose cleanser, and helps to remove and dilute the water-soluble wastes of civilization. This property also means that water-soluble wastes can easily pollute water.
Water filters out wavelengths of the sun’s ultraviolet (UV) radiation that would harm some aquatic organisms. This allows life to exist in the upper layer of aquatic systems.
Unlike most liquids, water expands when it freezes. This means that ice floats on water because it has a lower density (mass per unit of volume) than liquid water has. Otherwise, lakes and streams in cold climates would freeze solid from the bottom up and loose most of their aquatic life. Because water expands on freezing, it can break pipes, crack a car’s engine block (if it does not contain antifreeze), break up pavement, and fracture rocks (which helps form soil).
Critical Thinking
Pick two of the properties listed above and, for each property, explain how life on the earth would be different if the property did not exist.
The sun provides the energy needed to power the water cycle. Incoming solar energy causes evaporation—the conversion of some of the liquid water in the earth’s oceans, lakes, rivers, soil, and plants to vapor. Most water vapor rises into the atmosphere, where it condenses into droplets in clouds. Gravity then draws the water back to the earth’s surface as precipitation such as rain, snow, or sleet.
Most precipitation falling on terrestrial ecosystems becomes surface runoff—water that flows into streams, rivers, lakes, wetlands, and the ocean. Some of the water evaporates back into the atmosphere, while some seeps into the upper layers of soils and is used by plants,
Water that seeps deeper into the soil is known as groundwater. Groundwater collects in aquifers, which are underground layers of water-bearing rock. Some precipitation is converted to ice that is stored in glaciers.
0.024%
Percentage of the earth’s freshwater supply that is available to humans and other species
Only about 0.024% of the earth’s vast water supply is available to humans and other species as liquid freshwater in accessible groundwater deposits and in lakes, rivers, and streams. The rest of the planet’s water is too salty, is too deep underground to extract at affordable prices, or is stored as ice in glaciers. Because water is good at dissolving many different compounds, it can easily be polluted. However, natural processes in the water cycle can purify water, which makes the cycle a vital ecosystem service.
Human activities alter the water cycle in three major ways (see the red arrows and boxes in Figure 3.19). First, people sometimes withdraw freshwater from rivers, lakes, and aquifers at rates faster than natural processes can replace it. As a result, some aquifers are being depleted and some rivers no longer flow to the ocean.
Second, people clear vegetation from land for agriculture, mining, road building, and other activities, and cover much of the land with buildings, concrete, and asphalt. This increases water runoff and reduces infiltration that would normally recharge groundwater supplies.
Third, people drain and fill wetlands for farming and urban development. Left undisturbed, wetlands provide the ecosystem service of flood control. Wetlands act like sponges to absorb and hold overflows of water from drenching rains or rapidly melting snow.
3.4cThe Carbon Cycle
Carbon is the basic building block of the carbohydrates, fats, proteins, DNA, and other organic compounds required for life. Various compounds of carbon circulate through the biosphere, the atmosphere, and parts of the hydrosphere, in the carbon cycle shown in Figure 3.20.
Figure 3.20
Natural capital: Simplified model showing the circulation of various chemical forms of carbon in the global carbon cycle. Red arrows show major harmful impacts of human activities on this cycle. (Yellow box sizes do not show relative reservoir sizes.)
Critical Thinking:
What are three ways in which you directly or indirectly affect the carbon cycle?
A key component of the carbon cycle is carbon dioxide  gas. It makes up about 0.040% of the volume of the troposphere. Carbon dioxide (along with water vapor in the water cycle) affects the temperature of the atmosphere through the greenhouse effect (Figure 3.3) and thus plays a major role in determining the earth’s climate. If the carbon cycle removes too much  from the atmosphere, the atmosphere will cool, and if it generates too much , the atmosphere will get warmer. Thus, even slight changes in this cycle caused by natural or human factors can affect the earth’s climate, which helps determine the types of life that can exist in various places, as you will learn in Chapter 7.
Carbon is cycled through the biosphere by a combination of photosynthesis by producers, which removes  from the air and water, and aerobic respiration by producers, consumers, and decomposers that add  to the atmosphere. Typically,  remains in the atmosphere for 100 years or more. Some of the  in the atmosphere dissolves in the ocean. In the ocean, decomposers release carbon that is stored as insoluble carbonate minerals and rocks in bottom sediment for long periods.
Over millions of years, some of the carbon in deeply buried deposits of dead plant matter and algae have been converted into carbon-containing fossil fuels such as coal, oil, and natural gas (Figure 3.20). Within a few hundred years, we have extracted and burned huge quantities of fossil fuels that took millions of years to form, which explains why fossil fuels are classified as nonrenewable resources. This has added large quantities of  to the atmosphere and altered the carbon cycle (see red arrows in Figure 3.20). In effect, we have been adding  to the atmosphere faster than the carbon cycle can recycle it.
As a result, levels of  in the atmosphere have been rising sharply since about 1960. There is considerable scientific evidence that this disruption of the carbon cycle is helping to warm the atmosphere and change the earth’s climate. The oceans remove some of this  from the atmosphere but as a result, the acidity of ocean waters is rising. This ocean acidification is bad news for organisms that are adapted to less-acidic ocean waters and is a serious and growing global environmental problem.
Another way in which we alter the carbon cycle is by clearing carbon-absorbing vegetation from forests, especially tropical forests (Figure 3.1), faster than they can grow back (Core Case Study). This reduces the ability of the carbon cycle to remove excess  from the atmosphere and contributes to climate change, which we discuss in Chapter 19.
3.4dThe Nitrogen Cycle
Nitrogen gas  makes up 78% of the volume of the lower atmosphere and is a crucial component of proteins, many vitamins, and DNA. However,  in the atmosphere cannot be absorbed and used directly as a nutrient by plants or other organisms. It becomes a plant nutrient only as a component of nitrogen-containing ammonia , ammonium ions , and nitrate ions .
These chemical forms of nitrogen are created in the nitrogen cycle (Figure 3.21) by lightning, which converts  to , and by specialized bacteria in topsoil. Other bacteria in topsoil and in the bottom sediments of aquatic systems convert  to  and nitrate ions  that are taken up by the roots of plants. Plants then use these forms of nitrogen to produce various proteins, nucleic acids, and vitamins. Animals that eat plants consume these nitrogen-containing compounds, as do detritus feeders and decomposers. Bacteria in waterlogged soil and bottom sediments of lakes, oceans, swamps, and bogs convert nitrogen compounds into nitrogen gas , which is released to the atmosphere to begin the nitrogen cycle again.
Figure 3.21
Natural capital: Simplified model showing the circulation of various chemical forms of nitrogen in the nitrogen cycle, with major harmful human impacts shown by the red arrows. (Yellow box sizes do not show relative reservoir sizes.)
Critical Thinking:
What are two ways in which the carbon cycle and the nitrogen cycle are linked?
Humans intervene in the nitrogen cycle in several ways (see red arrows in Figure 3.21). When we burn gasoline and other fuels, the resulting high temperatures convert some of the  and  in air to nitric oxide (NO). In the atmosphere, NO can be converted to nitrogen dioxide gas  and nitric acid vapor , which can return to the earth’s surface as damaging acid deposition, commonly called acid rain. Acid rain damages stone buildings and statues. It can also kill forests and other plant ecosystems, and wipe out life in ponds and lakes.
We remove large amounts of  from the atmosphere and combine it with  to make ammonia  and ammonium ions  used to make fertilizers. In addition, we add the greenhouse gas nitrous oxide  to the atmosphere through the action of anaerobic bacteria on nitrogen-containing fertilizer or animal manure applied to the soil. This greenhouse gas can warm the atmosphere and take part in reactions that deplete stratospheric ozone, which keeps most of the sun’s harmful ultraviolet (UV) radiation from reaching the earth’s surface.
People also alter the nitrogen cycle in aquatic ecosystems by adding excess nitrates . The nitrates contaminate bodies of water through agricultural runoff of fertilizers, animal manure, and discharges from municipal sewage treatment systems. This plant nutrient can cause excessive growth of algae that can disrupt aquatic systems. Our nitrogen inputs into the environment have risen sharply and are projected to continue rising (Figure 3.22).
Figure 3.22
Global trends in the inputs of nitrogen into the atmosphere from human activities, with projections to 2050.
Data Analysis:
By what percentage did the overall nitrogen input increase between 1960 and 2000? By what percentage is it projected to increase between 2000 and 2050?
Compiled by the authors using data from the Millennium Ecosystem Assessment and the Fertilizer Industry Association
3.4eThe Phosphorus Cycle
Phosphorus (P) is an element that is essential for living things. It is necessary for the production of DNA and cell membranes, and is important for the formation of bones and teeth.
The cyclic movement of phosphorus (P) through water, the earth’s crust, and living organisms is called the phosphorus cycle (Figure 3.23). Most phosphorus compounds in this cycle contain phosphate ions , which are an important plant nutrient. Phosphorus does not cycle through the atmosphere because few of its compounds exist as a gas. Phosphorus also cycles more slowly than water, carbon, and nitrogen.
Figure 3.23
Natural capital: Simplified model showing the circulation of various chemical forms of phosphorus (mostly phosphates) in the phosphorus cycle, with major harmful human impacts shown by the red arrows. (Yellow box sizes do not show relative reservoir sizes.)
Critical Thinking:
What are two ways in which the phosphorus cycle and the nitrogen cycle are linked?
As water runs over exposed rocks, it slowly erodes inorganic compounds that contain phosphate ions. Water carries these ions into the soil, where they are absorbed by the roots of plants and by other producers. Phosphate compounds are then transferred by food webs from producers to consumers and eventually to detritus feeders and decomposers.
Much of the phosphate that erodes from rocks is carried into rivers, streams, and the ocean, where phosphates can be deposited as marine sediments and remain trapped for millions of years. Over time, geological processes can uplift and expose these seafloor deposits, from which phosphate can be eroded and re-enter the phosphorus cycle.
Most soils contain little phosphate, which often limits plant growth. For this reason, people often fertilize soil by adding phosphorus as phosphate salts mined from the ground. Lack of phosphorus also limits the growth of producer populations in many freshwater streams and lakes. This is because phosphate salts are only slightly soluble in water and do not release many phosphate ions to producers in aquatic systems.
Human activities, including the removal of large amounts of phosphate from the earth to make fertilizer, disrupt the phosphorus cycle (see red arrows in Figure 3.23). Clearing tropical forests (Core Case Study) exposes and erodes the topsoil, which reduces phosphate levels in tropical soils.
Eroded topsoil and fertilizer washed from fertilized crop fields, lawns, and golf courses carry large quantities of phosphate ions into streams, lakes, and oceans. There they stimulate the growth of producers such as algae and various aquatic plants, which can upset chemical cycling and other processes in bodies of water. According to a number of scientific studies, we are disrupting the phosphorus cycle because our inputs of phosphorus into the environment (primarily for use as fertilizer) have exceeded the planet’s environmental limit for phosphorus (Science Focus 3.2).
Science Focus 3.2
Planetary Boundaries
For most of the past 10,000–12,000 years, humans have been living in an era called the Holocene. During this era, we have enjoyed a favorable climate and other environmental conditions. This general stability allowed the human population to grow, develop agriculture, and take over a large share of the earth’s land and other resources (Figure 1.9).
Most geologists contend that we are still living in the Holocene era, but some scientists disagree. According to them, when the Industrial Revolution began (around 1750) we entered an era called the Anthropocene (the era of man or humans). In this new era, our ecological footprints have expanded significantly (Figure 1.9 and Figure 1.10) and are changing and stressing the earth’s life-support system, especially since 1950.
In 2015, an international team of 18 leading researchers in their fields, led by Will Steffen and Johan Rockstrom of the Stockholm Resilience Centre, published a paper estimating how close we are to exceeding several major planetary boundaries, or ecological tipping points, because of human activities (Figure 3.A). They warn that exceeding them could change how the planet operates and could trigger abrupt and long-lasting or irreversible environmental changes. This could seriously degrade the earth’s life-support system and our economies.
Figure 3.A
Planetary boundaries for ten major components of the earth’s life-support system. A team of scientists estimated that human activities have exceeded the boundary limits for three systems (shown in red) and are close to the limits for five other systems (shown in orange). There is not enough information to evaluate the other two systems (shown in white).
(Compiled by the authors using data from Johan Rockström, Paul Crutzen, and James Hansen, et al., 2009, “Planetary Boundaries: Exploring the Safe Operating Space for Humanity,” Ecology and Society, vol. 14, no. 2, p. 32.); Photo: Sailorr/ Shutterstock.com
The researchers estimated that we have exceeded or nearly exceeded several boundaries, including disruption of the nitrogen and phosphorus cycles, mostly from greatly increased use of fertilizers to produce food; biodiversity loss from replacing biologically diverse forests and grasslands with simplified fields of single crops; land system change from agriculture and urban development; and climate change from disrupting the carbon cycle, mostly by overloading it with carbon dioxide produced by the burning of fossil fuels.
However, there is an urgent need for more research to verify these findings and fill in the missing data on these planetary boundaries. This would help scientists to further evaluate how close we are to exceeding them and how exceeding them could affect humans, other species, and the earth’s life-support systems. Regardless of what we call the era we are living in, such information, combined with taking action to live more sustainably, could help us to avoid exceeding such boundaries by shrinking our ecological footprints while expanding our beneficial environmental impacts.
Critical Thinking
Select one of the planetary boundaries shown in Figure 3.A and think about how exceeding that boundary might speed our exceeding one or more other boundaries.
Learning from Nature
Scientists study the water, carbon, nitrogen, and phosphorus cycles to help us learn how to recycle the wastes we create.
3.5aStudying Ecosystems Directly
Ecologists and other scientists use several approaches to increase their scientific understanding of ecosystems. These approaches include field and laboratory research and mathematical and other types of models.
Field research involves going into forests and other natural settings to study ecosystems. Ecologists use a variety of methods for field research. They include collecting water and soil samples, identifying and studying the species in an area, observing feeding behaviors, and using global positioning system (GPS) to track the movements of animals. Most of what we know about ecosystems has come from such research (Individuals Matter 3.1). GREEN CAREER: Ecologist
Individuals Matter 3.1
Thomas E. Lovejoy—Forest Researcher and Biodiversity Educator
Luiz Rampelotto/ZUMAPRESS/Newscom
For several decades, conservation biologist and National Geographic Explorer Thomas E. Lovejoy has played a major role in educating scientists and the public about the need to understand and protect tropical forests. He has carried out research in the Amazon forests of Brazil since 1965, which focused on estimating the minimum area necessary for sustaining biodiversity in national parks and biological reserves in tropical forests. In 1980, he coined the term biological diversity.
Lovejoy served as the principal adviser for the popular and widely acclaimed public television series Nature. He has also written numerous articles and books on issues related to conserving biodiversity. In addition to teaching environmental science and policy at George Mason University, he has held several prominent posts, including director of the World Wildlife Fund’s conservation program, president of the Society for Conservation Biology, and executive director of the U.N. Environment Programme (UNEP). In 2012, he was awarded the Blue Planet Prize for his efforts to understand and sustain the earth’s biodiversity.
Scientists also use a variety of methods to study tropical forests (Core Case Study). Some erect construction cranes to reach the canopies. This, along with climbing trees and installing rope walkways between treetops, helps them identify and observe the diversity of species living or feeding in these treetop habitats.
Learning from Nature
Scientists are developing a robot modeled on the inchworm, which works its way up tree trunks by using sensors to feel for surfaces that allow for good grip. These tree-climbing robots could carry equipment up into trees for forest researchers.
Ecologists carry out controlled experiments by isolating and changing a variable in part of an area and comparing the results with nearby unchanged areas. You learned about a classic example of this in the Core Case Study of Chapter 2.
Scientists also use aircraft and satellites equipped with sophisticated cameras and remote sensing devices to scan and collect data on the earth’s surface. They use geographic information system (GIS) software to capture, store, analyze, and display such data. For example, GIS software can convert digital satellite images into global, regional, and local maps. These maps show variations in vegetation, gross primary productivity, soil erosion, deforestation, air pollution emissions, water usage, drought, flooding, pest outbreaks, and other variables.
Some researchers attach tiny radio transmitters to animals and use global positioning systems (GPSs) to track where and how far animals go. This technology is important for studying endangered species. Scientists also study nature by using cell phone cameras and mounting time-lapse cameras or video cameras on small drones and on stationary objects such as trees to capture images of wildlife. GREEN CAREERS: GIS Analyst; Remote Sensing Analyst
3.5bLaboratory Research and Models
Ecologists supplement their field research by conducting laboratory research. In laboratories, scientists create, simplified systems in containers such as culture tubes, bottles, aquariums, and greenhouses, and in indoor and outdoor chambers. In these structures, they control temperature, light, , humidity, and other variables.
These systems make it easier for scientists to carry out controlled experiments. Laboratory experiments are often faster and less costly than similar experiments in the field. However, scientists must consider how well their scientific observations and measurements in simplified, controlled systems in laboratory conditions reflect what takes place under the more complex and often-changing conditions found in nature.
Since the late 1960s, ecologists have developed mathematical models that simulate ecosystems, and they run the models on high-speed supercomputers. The models help them understand large and complex systems, such as lakes, oceans, forests, and the earth’s climate, that cannot be adequately studied and modeled in field or laboratory research. GREEN CAREER: Ecosystem modeler
Ecologists call for greatly increased research on the condition of the world’s ecosystems to see how they are changing and how well they can adapt to projected changing environmental conditions during this century. This would help scientists develop strategies for preventing or slowing their degradation.
.5cFour Laws of Ecology
Here are four basic principles or laws of ecology proposed by ecologist Barry Commoner in 1971.
Everything is connected to everything else. (Interdependence) Humans and other species are connected to and dependent on other species and on the earth for their survival. The challenge of ecology and environmental science is to identify these connections in nature and learn which ones are the most important.
Everything must go somewhere. (There is no “away.”) There is no “away” to which we can throw our wastes. Because of the law of conservation of matter, the atoms in the earth’s mater can neither be created nor destroyed. Matter can be transformed into different chemicals and materials but will always be around in some form because the atoms that makeup all matter cannot be destroyed (law of conservation of matter, Chapter 2). There is no waste in nature because the wastes of organisms are recycled and become resources (nutrients) for other organisms based on the chemical (nutrient) recycling principle of sustainability.
There is no free lunch. (Everything costs something.) Everything that we do to our life-support system has an environmental cost and often a financial one. Many people treat natural resources such as clean air, clean water, wildlife, and public lands (such as protected wilderness areas) as “free” resources that anyone can use. The cumulative effect of large numbers of people using such resources can pollute, degrade, or deplete them, and this is the tragedy of the commons (Chapter 1). Then governments and taxpayers end up paying the bills for expensive environmental cleanup, restoration, and wildlife protection that could have been prevented.
Nature knows best. Over billions of years, nature has experienced catastrophic and long-lasting environmental changes, including five mass extinctions of the earth’s species. Despite these events, nature has sustained a variety of life on the earth for billions of years. Biomimicry (Core Case Study, Chapter 1) is the scientific effort to identify and copy these lessons from nature.
Observing these four laws would help us avoid going beyond ecological tipping points that could cause severe environmental degradation and economic disruption. Examples of such tipping points include:
Severe disruption of key chemical cycles
Significant reduction of life-sustaining biodiversity from excessive losses of species, the natural ecosystems where they live, and the ecosystem services these ecosystems provide
Climate change from increasing levels of  and other greenhouse gases emitted into the atmosphere by burning carbon containing fossil fuels and other human activities
Ocean acidification and disruption of marine ecosystems caused by absorption of some of the  emitted into the atmosphere, mostly from burning fossil fuels
Ozone depletion—the reduction of ozone in the stratosphere that protects life on land from the sun’s harmful ultraviolet (UV) radiation, caused by chemicals emitted into the atmosphere through human activities
Human consumption of water faster than it can be renewed by the water cycle
Increased air and water pollution resulting from failure to enact and enforce pollution control laws and regulations.
The scientific challenge is to
identify the levels of such tipping points and the projected effects of exceeding them on ecosystems, human well-being, and human economies, and
develop strategies for not exceeding estimated tipping point levels.
Big Ideas
Life is sustained by the flow of energy from the sun through the biosphere, the cycling of nutrients within the biosphere, and gravity.
Some organisms produce the nutrients they need, others survive by consuming other organisms, and still others live on the wastes and remains of organisms while recycling nutrients that are used again by producer organisms.
Human activities are altering the chemical cycling of nutrients and the flow of energy through food chains and webs in ecosystems.
.5cFour Laws of Ecology
Here are four basic principles or laws of ecology proposed by ecologist Barry Commoner in 1971.
Everything is connected to everything else. (Interdependence) Humans and other species are connected to and dependent on other species and on the earth for their survival. The challenge of ecology and environmental science is to identify these connections in nature and learn which ones are the most important.
Everything must go somewhere. (There is no “away.”) There is no “away” to which we can throw our wastes. Because of the law of conservation of matter, the atoms in the earth’s mater can neither be created nor destroyed. Matter can be transformed into different chemicals and materials but will always be around in some form because the atoms that makeup all matter cannot be destroyed (law of conservation of matter, Chapter 2). There is no waste in nature because the wastes of organisms are recycled and become resources (nutrients) for other organisms based on the chemical (nutrient) recycling principle of sustainability.
There is no free lunch. (Everything costs something.) Everything that we do to our life-support system has an environmental cost and often a financial one. Many people treat natural resources such as clean air, clean water, wildlife, and public lands (such as protected wilderness areas) as “free” resources that anyone can use. The cumulative effect of large numbers of people using such resources can pollute, degrade, or deplete them, and this is the tragedy of the commons (Chapter 1). Then governments and taxpayers end up paying the bills for expensive environmental cleanup, restoration, and wildlife protection that could have been prevented.
Nature knows best. Over billions of years, nature has experienced catastrophic and long-lasting environmental changes, including five mass extinctions of the earth’s species. Despite these events, nature has sustained a variety of life on the earth for billions of years. Biomimicry (Core Case Study, Chapter 1) is the scientific effort to identify and copy these lessons from nature.
Observing these four laws would help us avoid going beyond ecological tipping points that could cause severe environmental degradation and economic disruption. Examples of such tipping points include:
Severe disruption of key chemical cycles
Significant reduction of life-sustaining biodiversity from excessive losses of species, the natural ecosystems where they live, and the ecosystem services these ecosystems provide
Climate change from increasing levels of  and other greenhouse gases emitted into the atmosphere by burning carbon containing fossil fuels and other human activities
Ocean acidification and disruption of marine ecosystems caused by absorption of some of the  emitted into the atmosphere, mostly from burning fossil fuels
Ozone depletion—the reduction of ozone in the stratosphere that protects life on land from the sun’s harmful ultraviolet (UV) radiation, caused by chemicals emitted into the atmosphere through human activities
Human consumption of water faster than it can be renewed by the water cycle
Increased air and water pollution resulting from failure to enact and enforce pollution control laws and regulations.
The scientific challenge is to
identify the levels of such tipping points and the projected effects of exceeding them on ecosystems, human well-being, and human economies, and
develop strategies for not exceeding estimated tipping point levels.
Big Ideas
Life is sustained by the flow of energy from the sun through the biosphere, the cycling of nutrients within the biosphere, and gravity.
Some organisms produce the nutrients they need, others survive by consuming other organisms, and still others live on the wastes and remains of organisms while recycling nutrients that are used again by producer organisms.
Human activities are altering the chemical cycling of nutrients and the flow of energy through food chains and webs in ecosystems.
Critical Thinking
How would you explain the importance of tropical rain forests (Core Case Study) to people who think that such forests have no connection to their lives?
Explain
why the flow of energy through the biosphere depends on the cycling of nutrients, and
why the cycling of nutrients depends on gravity.
Explain why microbes are important. What are two ways in which they benefit your health or lifestyle? Write a brief description of what you think would happen to you if microbes were eliminated from the earth.
Make a list of the foods you ate for lunch or dinner today. Trace each type of food back to a particular producer species. Describe the sequence of feeding levels that led to your feeding.
Use the second law of thermodynamics (see Chapter 2) to explain why many poor people in less-developed countries live on a mostly vegetarian diet.
List three ways in which your life and the lives of any children or grandchildren you might eventually have would be affected if human activities continue to modify the water cycle.
What would happen to an ecosystem if
all of its decomposers and detritus feeders were eliminated,
all of its producers were eliminated, and
all of its insects were eliminated? Could an ecosystem exist with producers and decomposers but no consumers? Explain.
For each of the proposed four laws of ecology listed near the end of this chapter, find one way in which observing or breaking the law is connected to one or more of the seven possible tipping points. Explain each case.
Doing Environmental Science
Visit a nearby terrestrial ecosystem and identify its major producers, primary and secondary consumers, detritus feeders, and decomposers. Take notes and describe at least one example of each of these types of organisms. Make a simple sketch showing how these organisms might be related to each other or to other organisms in a food chain or food web. Think of two ways in which this food web or chain could be disrupted. Write a report summarizing your research and conclusions.

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