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Chapter 34: Ecosystems and Human Interferences 34-1 The Nature of Ecosystems An ecosystem contains biotic (living) components and abiotic (nonliving) components. The biotic components of ecosystems are the populations of organisms. The abiotic components include inorganic nutrients, water, temperature, and prevailing wind. 34-2 Biotic Components of an Ecosystem Autotrophs are producers that produce food for themselves and for consumers. Most are photosynthetic organisms but some chemosynthetic bacteria are autotrophs. Heterotrophs are consumers that take in preformed food. 34-3 Biotic components 34-4 Consumers may be: Herbivores – animals that eat plants, Carnivores – animals that eat other animals, Omnivores, such as humans, that eat plants and animals, or Decomposers, bacteria and fungi, that break down dead organic waste. Detritus is partially decomposed organic matter in the soil and water; beetles, earthworms, and termites are detritus feeders. 34-5 Consumers 34-6 Energy Flow and Chemical Cycling Every ecosystem is characterized by two phenomena: 1) Energy flows in one direction from the sun to producers through several levels of consumers, and 2) Chemicals cycle when inorganic nutrients pass from producers through consumers and returned to the atmosphere or soil. 34-7 Nature of an ecosystem 34-8 Only a small portion of energy and nutrients made by autotrophs is passed on to heterotrophs, and only a small amount is passed to each succeeding consumer; much energy is used at each level for cellular respiration and much is lost as heat. Ecosystems are dependent on a continual supply of solar energy. The laws of thermodynamics support the concept that energy flows through an ecosystem. 34-9 Energy balances 34-10 Energy Flow The feeding relationships in an ecosystem are interconnected in a food web. Generally, the upper portion of a food web is a grazing food web, based on living plants, and the lower portion is a detrital food web, based on detritus and the organisms of decay. 34-11 Forest food webs 34-12 Trophic Levels A trophic level is all the organisms that feed at a particular link in a food chain. A diagram that link organisms together by who eats whom is called a food chain. A grazing food chain: Leaves → caterpillars → tree birds → hawks A detrital food chain: Dead organic matter → soil microbes → worms 34-13 Food chain 34-14 Ecological Pyramids The shortness of food chains can be attributed to the loss of energy between trophic levels. Generally, only about 10% of the energy in one trophic level is available to the next trophic level. This relationship explains why so few carnivores can be supported in a food web. 34-15 The flow of energy with large losses between successive trophic levels can be depicted as an ecological pyramid that shows trophic levels stacked one on the other like building blocks. Usually a pyramid shows that biomass and energy content decrease from one trophic level to the next, but an inverted pyramid occurs where the algae grow rapidly and are consumed by long-lived aquatic animals. 34-16 Ecological pyramid 34-17 Global Biogeochemical Cycles All organisms require a variety of organic and inorganic nutrients. Since pathways by which chemicals cycle through ecosystems involve both biotic and abiotic components, they are known as biogeochemical cycles. Biogeochemical cycles often contain reservoirs, such as fossil fuels, sediments, and rocks that contain elements available on a limited basis to living things. 34-18 Exchange pools are components of ecosystems like the atmosphere, soil, and water—which are ready sources of nutrients for the biotic community that uses the chemicals. Nutrients cycle among the members of the biotic component of an ecosystem and may never enter an exchange pool. Nutrients flow between terrestrial and aquatic ecosystems. 34-19 Model for chemical cycling 34-20 The Water Cycle In the water, or hydrologic cycle, the sun’s rays cause fresh water to evaporate from the oceans, leaving the salts behind. Vaporized fresh water rises into the atmosphere, cools, and falls as rain over oceans and land. Precipitation, as rain and snow, over land results in bodies of fresh water plus groundwater, including aquifers. 34-21 Water is held in lakes, ponds, streams, and groundwater. Evaporation from terrestrial ecosystems includes transpiration from plants. Eventually all water returns to the oceans. Groundwater “mining” in the arid West and southern Florida is removing water faster than underground sources can be recharged. 34-22 The water cycle 34-23 The Carbon Cycle In the carbon cycle, a gaseous cycle, organisms exchange carbon dioxide with the atmosphere. Shells in ocean sediments, organic compounds in living and dead organisms, and fossil fuels are all reservoirs for carbon. Fossil fuels were formed during the Carboniferous period, 286 to 360 million years ago. 34-24 The carbon cycle 34-25 Carbon Dioxide and Global Warming The transfer rate , the amount of a nutrient that moves from one compartment of the environment to another, can be altered by human activities, allowing more carbon dioxide to be added to the atmosphere. Atmospheric carbon dioxide has risen from 280 ppm to 350 ppm due to burning of fossil fuels and forests. Besides CO2, nitrous oxide and methane are also greenhouse gases. 34-26 Similar to the panes of a greenhouse, these gases allow the sun’s rays to pass through but hinder the escape of infrared (heat) wavelengths. Buildup of more of these “greenhouse gases” could lead to more global warming. The effects of global warming could include a rise in sea level, affecting coastal cities, and a change in global climate patterns with disastrous effects. 34-27 Earth’s radiation balances 34-28 The Nitrogen Cycle Nitrogen makes up 78% of the atmosphere but plants are unable to make use of this nitrogen gas and need a supply of ammonium or nitrate. The nitrogen cycle, a gaseous cycle, is dependent upon a number of bacteria. During nitrogen fixation, nitrogen-fixing bacteria living in nodules on the roots of legumes convert atmospheric nitrogen to nitrogen-containing organic compounds available to a host plant. 34-29 Cyanobacteria in aquatic ecosystems and free-living bacteria in the soil also fix nitrogen gas. Bacteria in soil carry out nitrification when they convert ammonium to nitrate in a two-step process: first, nitrite-producing bacteria convert ammonium to nitrite and then nitrate-producing bacteria convert nitrite to nitrate. During denitrification, denitrifying bacteria in soil convert nitrate back to nitrogen gas but this does not quite balance nitrogen fixation. 34-30 The nitrogen cycle 34-31 Nitrogen and Air Pollution Human activities convert atmospheric nitrogen to fertilizer which when broken down by soil bacteria adds nitrogen oxides to the atmosphere at three times the normal rate. Humans also burn fossil fuels which put nitrogen oxides (NOx) and sulfur dioxide (SO2) in the atmosphere. 34-32 Nitrogen oxides and sulfur dioxide react with water vapor to form acids that contribute to acid deposition. Acid deposition is killing lakes and forests and also corrodes marble, metal, and stonework. Nitrogen oxides and hydrocarbons (HC) react to form photochemical smog, which contains ozone and PAN (peroxyacetylnitrate), oxidants harmful to animal and plant life. 34-33 Acid deposition 34-34 A thermal inversion, where these pollutants are trapped under warm, stagnant air concentrates pollutants to dangerous levels. Nitrous oxide is not only a greenhouse gas, but contributes to the breakdown of the ozone shield that protects surface life from harmful levels of solar radiation. 34-35 Thermal inversion 34-36 The Phosphorus Cycle The phosphorus cycle is a sedimentary cycle. Only limited quantities are made available to plants by the weathering of sedimentary rocks; phosphorus is a limiting inorganic nutrient. The biotic community recycles phosphorus back to the producers, temporarily incorporating it into ATP, nucleotides, teeth, bone and shells, and then returning it to the ecosystem via decomposition. 34-37 The phosphorus cycle 34-38 Phosphorus and Water Pollution Phosphates are mined for fertilizer production; when phosphates and nitrates enter lakes and ponds, eutrophication occurs. Many kinds of wastes enter rivers which flow to the oceans; oceans are now degraded from added pollutants. If pollutants are not decomposed, they may increase in concentration as they pass up the food chain, a process called biological magnification. 34-39 Chapter Summary An ecosystem includes autotrophs that make their own food and heterotrophs that take in preformed food. Solar energy enters biotic communities via photosynthesis, and as organic molecule pass from one organism to another, heat is returned to the atmosphere. 34-40 Chemicals cycle within and between ecosystems in global biogeochemical cycles. Biogeochemical cycles are gaseous (carbon cycle, nitrogen cycle) or sedimentary (phosphorus cycle). The addition of carbon dioxide (and other gases) to the atmosphere is associated with global warming. 34-41 The production of fertilizers from nitrogen gas is associated with acid deposition, photochemical smog, and temperature inversions. Fertilizer also contains mined phosphate; fertilizer runoff is associated with water pollution. Certain pollutants undergo biological magnification as they pass through the food chain. 34-42