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P R I N C I P L E S O F Environmental Inquiry & Science Application Eighth Edition William P Cunningham University of Minnesota Mary Ann Cunningham Vassar College PRINCIPLES OF ENVIRONMENTAL SCIENCE: INQUIRY & APPLICATIONS, EIGHTH EDITION Published by McGraw-Hill Education, Penn Plaza, New York, NY 10121 Copyright © 2017 by McGraw-Hill Education All rights reserved Printed in the United States of America Previous editions © 2013, 2011, 2009 and 2008 No part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written consent of McGraw-Hill Education, including, but not limited to, in any network or other electronic storage or transmission, or broadcast for distance learning Some ancillaries, including electronic and print components, may not be available to customers outside the United States This book is printed on recycled paper 1 2 3 4 5 6 7 8 9 0 RMN/RMN 1 0 9 8 7 6 ISBN 978-0-07-803607-1 MHID 0-07-803607-0 Senior Vice President, Products & Markets: Kurt L Strand Vice President, General Manager, Products & Markets: Marty Lange Vice President, Content Design & Delivery: Kimberly Meriwether David Director of Development: Rose Koos Managing Director: Thomas Timp Brand Manager: Michelle Vogler Product Developer: Jodi Rhomberg Director of Digital Content Development: Justin Wyatt, Ph.D Digital Product Analyst: Patrick Diller Marketing Manager: Danielle Dodds Director, Content Design & Delivery: Linda Avenarius Program Manager: Lora Neyens Content Project Manager: Peggy J Selle Assessment Content Project Manager: Tammy Juran Buyer: Laura Fuller Designer: Tara McDermott Content Licensing Specialist (Text): Lorraine Buczek Content Licensing Specialist (Photo): Carrie Burger Cover Image: ©iStock/Getty Images Plus/RF Compositor: SPI-Global Printer: R.R Donnelley All credits appearing on page or at the end of the book are considered to be an extension of the copyright page Library of Congress Cataloging-in-Publication Data Cunningham, William P Principles of environmental science : inquiry & application / William P Cunningham, University of Minnesota, Mary Ann Cunningham, Vassar College – Eighth edition pages cm ISBN 978-0-07-803607-1 (alk paper) 1. Environmental sciences–Textbooks. I. Cunningham, Mary Ann. II. Title GE105.C865 2017 363.7–dc23 2015027521 The Internet addresses listed in the text were accurate at the time of publication The inclusion of a website does not indicate an endorsement by the authors or McGraw-Hill Education, and McGraw-Hill Education does not guarantee the accuracy of the information presented at these sites mheducation.com/highered About the Authors WILLIAM P CUNNINGHAM William P Cunningham is an emeritus professor at the University of Minnesota In his 38-year career at the university, he taught a variety of biology courses, including Environmental Science, Conservation Biology, Environmental Health, Environmental Ethics, Plant Physiology, General Biology, and Cell Biology He is a member of the Academy of Distinguished Teachers, the highest teaching award granted at the University of Minnesota He was a member of a number of interdisciplinary programs for international students, teachers, and nontraditional students He also carried out research or taught in Sweden, Norway, Brazil, New Zealand, China, and Indonesia Professor Cunningham has participated in a number of governmental and nongovernmental organizations over the past 40 years He was chair of the Minnesota chapter of the Sierra Club, a member of the Sierra Club national committee on energy policy, vice president of the Friends of the Boundary Waters Canoe Area, chair of the Minnesota governor’s task force on energy policy, and a citizen member of the Minnesota Legislative Commission on Energy In addition to environmental science textbooks, Professor Cunningham edited three editions of Environmental Encyclopedia published by ThompsonGale Press He has also authored or co-authored about 50 scientific articles, mostly in the fields of cell biology and conservation biology as well as several invited chapters or reports in the areas of energy policy and environmental health His Ph.D from the University of Texas was in botany His hobbies include birding, hiking, gardening, traveling, and video production He lives in St Paul, Minnesota, with his wife, Mary He has three children (one of whom is co-author of this book) and seven grandchildren MARY ANN CUNNINGHAM Mary Ann Cunningham is an associate professor of geography at Vassar College, in New York’s Hudson Valley A biogeographer with interests in landscape ecology, geographic information systems (GIS), and land use change, she teaches environmental science, natural resource conservation, and land-use planning, as well as GIS and spatial data analysis Field research methods, statistical methods, and scientific methods in data analysis are regular components of her teaching As a scientist and educator, she enjoys teaching and conducting research with both science students and non-science liberal arts students As a geographer, she likes to engage students with the ways their physical surroundings and social context shape their world experience In addition to teaching at a liberal arts college, she has taught at community colleges and research universities She has participated in Environmental Studies and Environmental Science programs and has led community and college field research projects at Vassar Mary Ann has been writing in environmental science for nearly two decades, and she has been co-author of this book since its first edition She is also co-author of Environmental Science: A Global Concern, now in its thirteenth edition She has published work on habitat and landcover change, on water quality and urbanization, and other topics in environmental science She has also done research with students and colleagues on climate change, its impacts, and carbon mitigation strategies Research and teaching activities have included work in the Great Plains, the Adirondack Mountains, and northern Europe, as well as in New York’s Hudson Valley, where she lives and teaches In her spare time she loves to travel, hike, and watch birds She holds a bachelor’s degree from Carleton College, a master’s degree from the University of Oregon, and a Ph.D from the University of Minnesota iii Brief Contents Understanding Our Environment Environmental Systems: Matter, Energy, and Life 26 Evolution, Species Interactions, and Biological Communities 50 Human Populations 76 Biomes and Biodiversity 96 Environmental Conservation: Forests, Grasslands, Parks, and Nature Preserves 127 Food and Agriculture 152 Environmental Health and Toxicology 180 Climate 205 10 Air Pollution 229 11 Water: Resources and Pollution 250 12 Environmental Geology and Earth Resources 281 13 Energy 302 14 Solid and Hazardous Waste 331 15 Economics and Urbanization 352 16 Environmental Policy and Sustainability 377 iv Principles of Environmental Science Contents Preface xiii Understanding Our Environment 1.6 Where Do Our Ideas About the Environment LEARNING OBJECTIVES 1 Case Study Assessing Sustainability 1.1 What is Environmental Science? Environmental science is integrative Environmental science is global Environmental science helps us understand our remarkable planet 3 Active Learning Finding Your Strengths in This Class Methods in environmental science 1.2 Major Themes in Environmental Science Environmental quality Human population and well-being Natural resources 1.3 Human Dimensions of Environmental Science How we describe resource use and conservation? Sustainability means environmental and social progress Affluence is a goal and a liability What is the state of poverty and wealth today? Indigenous peoples safeguard biodiversity 10 Exploring Science How Do We Know the State of Population and Poverty? 11 Key Concepts Sustainable development 12 1.4 Science Helps Us Understand Our World 14 Science depends on skepticism and reproducibility 14 We use both deductive and inductive reasoning 15 The scientific method is an orderly way to examine problems 15 Understanding probability reduces uncertainty 16 Active Learning Calculating Probability 16 Experimental design can reduce bias 16 Exploring Science Understanding sustainable development with statistics 17 Science is a cumulative process 18 What is sound science? 18 Uncertainty, proof, and group identity 19 1.5 Critical Thinking 19 Critical thinking helps us analyze information 20 We all use critical thinking to examine arguments 20 Critical thinking helps you learn environmental science 20 Come From? 21 Environmental protection has historic roots 21 Resource waste triggered pragmatic resource conservation (stage 1) 21 Ethical and aesthetic concerns inspired the preservation movement (stage 2) 22 Rising pollution levels led to the modern environmental movement (stage 3) 22 Environmental quality is tied to social progress (stage 4) 23 Conclusion 24 Data Analysis Working with Graphs 25 Environmental Systems: Matter, Energy, and Life 26 LEARNING OUTCOMES 26 Case Study Working to Rescue an Ecosystem 27 2.1 Systems Describe Interactions 28 Systems can be described in terms of their characteristics 29 Feedback loops help stabilize systems 29 2.2 Elements of Life 30 Matter is recycled but doesn’t disappear 30 Elements have predictable characteristics 30 Electric charges keep atoms together 31 Acids and bases release reactive H+ and OH– 32 Organic compounds have a carbon backbone 32 Cells are the fundamental units of life 34 Nitrogen and phosphorus are key nutrients 34 Exploring Science A “Water Planet” 35 2.3 Energy 35 Energy occurs in different types and qualities 35 Thermodynamics describes the conservation and degradation of energy 36 2.4 Energy for Life 36 Green plants get energy from the sun 37 How does photosynthesis capture energy? 38 2.5 From Species to Ecosystems 38 Organisms occur in populations, communities, and ecosystems 39 Food chains, food webs, and trophic levels link species 39 Active Learning Food Webs 39 CO N T EN TS v Exploring Science Remote Sensing, Photosynthesis, and Material Cycles 40 Ecological pyramids describe trophic levels 41 2.6 Biogeochemical Cycles and Life Processes 41 The hydrologic cycle 41 The carbon cycle 42 The nitrogen cycle 43 Key Concepts How energy and matter move through systems? 44 Phosphorus eventually washes to the sea 46 The sulfur cycle 47 Conclusion 47 Data Analysis Examining Nutrients in a Wetland System 49 Evolution, Species Interactions, and Biological Communities 50 LEARNING OUTCOMES 50 Case Study Natural Selection and the Galápagos Finches 51 3.1 Evolution Leads to Diversity 52 Natural selection and adaptation modify species 52 Limiting factors influence species distributions 53 A niche is a species’ role and environment 54 Speciation leads to species diversity 55 Key Concepts Where species come from? 56 Taxonomy describes relationships among species 58 3.2 Species Interactions 59 Competition leads to resource allocation 59 Predation affects species relationships 60 Predation leads to adaptation 61 Symbiosis involves cooperation 61 Keystone species play critical roles 62 Exploring Science Say Hello to Your 90 Trillion Little Friends 63 3.3 Population Growth 64 Growth without limits is exponential 64 Carrying capacity limits growth 64 Environmental limits lead to logistic growth 65 Species respond to limits differently: r- and K-selected species 66 Active Learning Effect of K on Population Growth Rate (rN) 66 3.4 Community Diversity 67 Diversity and abundance 67 Patterns produce community structure 68 What Can You Do? Working Locally for Ecological Diversity 68 Resilience seems related to complexity 70 3.5 Communities Are Dynamic and Change over Time 72 Are communities organismal or individualistic? 72 Succession describes community change 72 Some communities depend on disturbance 73 Conclusion 74 Data Analysis Competitive Exclusion 75 vi CO N T E N TS Human Populations 76 LEARNING OUTCOMES 76 Case Study Population Stabilization in Brazil 77 4.1 Past and Current Population Growth Are Very Different 78 Human populations grew slowly until recently 78 Active Learning Population Doubling Time 79 4.2 Perspectives on Population Growth 79 Does environment or culture control human population growth? 79 Technology increases carrying capacity for humans 80 Population growth could bring benefits 81 4.3 Many Factors Determine Population Growth 81 How many of us are there? 81 Key Concepts How big is your footprint? 82 Fertility varies among cultures and at different times 84 Mortality offsets births 85 Life expectancy is rising worldwide 85 What Do You Think? China’s One-Child Policy 86 Living longer has profound social implications 87 4.4 Fertility Is Influenced by Culture 87 People want children for many reasons 87 Education and income affect the desire for children 89 4.5 A Demographic Transition Can Lead to Stable Population Size 89 Economic and social conditions change mortality and births 90 Many countries are in a demographic transition 90 Two ways to complete the demographic transition 91 Improving women’s lives helps reduce birth rates 91 4.6 Family Planning Gives Us Choices 92 Humans have always regulated their fertility 92 Today there are many options 92 4.7 What Kind of Future Are We Creating Now? 92 Conclusion 94 Data Analysis Population Change over Time 95 Biomes and Biodiversity 96 LEARNING OUTCOMES 96 Case Study Forest Responses to Global Warming 5.1 Terrestrial Biomes Tropical moist forests are warm and wet year-round 97 98 100 Active Learning Comparing Biome Climates 101 Tropical seasonal forests have annual dry seasons 101 Tropical savannas and grasslands are dry most of the year 101 Deserts are hot or cold, but always dry 101 Temperate grasslands have rich soils 102 Temperate scrublands have summer drought 102 Temperate forests can be evergreen or deciduous 103 Boreal forests lie north of the temperate zone 103 Tundra can freeze in any month 104 5.2 Marine Environments 105 Active Learning Examining Climate Graphs 105 Open ocean communities vary from surface to hadal zone 106 Tidal shores support rich, diverse communities 106 5.3 Freshwater Ecosystems 108 Lakes have extensive open water 108 Wetlands are shallow and productive 108 Streams and rivers are open systems 109 5.4 Biodiversity 110 Increasingly we identify species by genetic similarity 110 Biodiversity hot spots are rich and threatened 110 5.5 Benefits of Biodiversity 110 Biodiversity provides food and medicines 111 Biodiversity can aid ecosystem stability 112 Aesthetic and existence values are important 112 5.6 What Threatens Biodiversity? 112 HIPPO summarizes human impacts 112 Habitat destruction is usually the main threat 112 Key Concepts What is biodiversity worth? 114 Invasive species are a growing threat 116 Exploring Science What’s the Harm in Setting Unused Bait Free? 117 What Can You Do? You Can Help Preserve Biodiversity 119 Pollution poses many types of risk 119 Population growth consumes space, resources 120 Overharvesting depletes or eliminates species 120 5.7 Biodiversity Protection 122 Hunting and fishing laws protect useful species 122 The Endangered Species Act protects habitat and species 122 Recovery plans aim to rebuild populations 122 Landowner collaboration is key 123 The ESA has seen successes and controversies 123 Many countries have species protection laws 124 Habitat protection may be better than species protection 124 Conclusion 125 Data Analysis Confidence Limits in the Breeding Bird Survey 126 Environmental Conservation: Forests, Grasslands, Parks, and Nature Preserves 127 LEARNING OUTCOMES 127 Case Study Palm Oil and Endangered Species 128 6.1 World Forests 129 Boreal and tropical forests are most abundant 129 Active Learning Calculating Forest Area 130 Forests provide essential products 130 Tropical forests are being cleared rapidly 131 Saving forests stabilizes our climate 133 Temperate forests also are at risk 133 What Do You Think? Protecting Forests to Prevent Climate Change 135 Key Concepts Save a tree, save the climate? 136 Exploring Science Using Technology to Protect the Forest 138 What Can You Do? Lowering Your Forest Impacts 139 6.2 Grasslands 140 Grazing can be sustainable or damaging 141 Overgrazing threatens many rangelands 141 Ranchers are experimenting with new methods 142 6.3 Parks and Preserves 142 Many countries have created nature preserves 143 Not all preserves are preserved 144 Marine ecosystems need greater protection 145 Conservation and economic development can work together 146 Native people can play important roles in nature protection 146 Exploring Science Saving the Chimps of Gombe 147 What Can You Do? Being a Responsible Ecotourist 148 Species survival can depend on preserve size and shape 149 Conclusion 149 Data Analysis Detecting Edge Effects 151 Food and Agriculture 152 LEARNING OUTCOMES 152 Case Study Farming the Cerrado 7.1 Global Trends in Food and Hunger Food security is unevenly distributed Active Learning Mapping Poverty and Plenty Famines have political and social roots 7.2 How Much Food Do We Need? A healthy diet includes the right nutrients Overeating is a growing world problem More production doesn’t necessarily reduce hunger Biofuels have boosted commodity prices Do we have enough farmland? 7.3 What Do We Eat? Rising meat production is a sign of wealth Seafood, both wild and farmed, depends on wild-source inputs Biohazards arise in industrial production Active Learning Where in the World Did You Eat Today? 7.4 Living Soil Is a Precious Resource What is soil? Healthy soil fauna can determine soil fertility CO N T EN TS 153 154 154 156 156 157 157 157 158 159 159 160 160 161 162 162 163 163 163 vii Your food comes mostly from the A horizon 164 How we use and abuse soil? 165 Water is the leading cause of soil loss 165 Wind is a close second in erosion 166 7.5 Agricultural Inputs 166 High yields usually require irrigation 166 Fertilizers boost production 167 Modern agriculture runs on oil 167 Key Concepts How can we feed the world? 168 Pesticide use continues to rise 170 7.6 How Have We Managed to Feed Billions? 171 The green revolution has increased yields 171 Genetic engineering has benefits and costs 172 Most GMOs are engineered for pesticide production or pesticide tolerance 173 Is genetic engineering safe? 173 7.7 Sustainable Farming Strategies 174 Soil conservation is essential 174 Groundcover, reduced tilling protect soil 175 Low-input sustainable agriculture can benefit people and the environment 175 What Do You Think? Shade-Grown Coffee and Cocoa 176 7.8 Consumer Action and Farming 177 You can be a locavore 177 You can eat low on the food chain 177 Conclusion 177 Data Analysis Mapping Your Food Supply 179 Environmental Health and Toxicology LEARNING OUTCOMES 205 180 Case Study How Dangerous Is BPA? 181 8.1 Environmental Health 182 Global disease burden is changing 182 Emergent and infectious diseases still kill millions of people 183 Conservation medicine combines ecology and health care 185 Resistance to antibiotics and pesticides is increasing 186 What Can You Do? Tips for Staying Healthy 187 8.2 Toxicology 188 How toxics affect us? 188 Endocrine hormone disrupters are of special concern 189 Key Concepts What toxins and hazards are present in your home? 190 8.3 Movement, Distribution, and Fate of Toxins 192 Solubility and mobility determine when and where chemicals move 192 CO N T E N TS Climate 205 LEARNING OUTCOMES 180 viii Exposure and susceptibility determine how we respond 192 Bioaccumulation and biomagnification increase chemical concentrations 193 Persistence makes some materials a greater threat 193 Chemical interactions can increase toxicity 195 8.4 Mechanisms for Minimizing Toxic Effects 195 Metabolic degradation and excretion eliminate toxics 195 Repair mechanisms mend damage 195 8.5 Measuring Toxicity 195 We usually test toxic effects on lab animals 196 There is a wide range of toxicity 196 Active Learning Assessing Toxins 197 Acute versus chronic doses and effects 197 Detectable levels aren’t always dangerous 198 Low doses can have variable effects 198 Exploring Science The Epigenome 199 8.6 Risk Assessment and Acceptance 200 Our perception of risks isn’t always rational 200 How much risk is acceptable? 201 Active Learning Calculating Probabilities 201 8.7 Establishing Public Policy 202 Conclusion 203 Data Analysis How Do We Evaluate Risk and Fear? 204 Case Study Shrinking Florida 206 9.1 What Is the Atmosphere? 207 The atmosphere captures energy selectively 208 Evaporated water stores and redistributes heat 209 Ocean currents also redistribute heat 210 9.2 Climate Changes over Time 210 Ice cores tell us about climate history 211 What causes natural climatic swings? 211 El Niño/Southern Oscillation is one of many regional cycles 212 9.3 How Do We Know the Climate Is Changing Faster Than Usual? 213 Active Learning Can you explain key evidence on climate change? 213 Scientific consensus is clear 214 Rising heat waves, sea level, and storms are expected 214 The main greenhouse gases are CO2, CH4, and N2O 215 What consequences we see? 217 Ice loss produces positive feedbacks 217 Controlling emissions is cheap compared to climate change 219 Why are there disputes over climate evidence? 219 Key Concepts Climate change in a nutshell: How does it work? 220 Exploring Science How Do We Know That Climate Change Is Human-Caused? 222 9.4 Envisioning Solutions 223 International protocols have tried to establish common rules 224 A wedge approach has multiple solutions 224 Wind, water, and solar could save the climate 225 What Do You Think? Unburnable carbon 226 What Can You Do? Climate Action 226 Local initiatives are everywhere 226 Carbon capture saves CO2 but is expensive 227 Conclusion 227 Data Analysis Examining the IPCC Fifth Assessment Report (AR5) 228 10 Air Pollution 229 LEARNING OUTCOMES 229 Case Study The Great London Smog 10.1 Air Pollution and Health The Clean Air Act regulates major pollutants Active Learning Compare Sources of Pollutants Conventional pollutants are abundant and serious Hazardous air pollutants can cause cancer and nerve damage Mercury is a key neurotoxin Indoor air can be worse than outdoor air 10.2 Air Pollution and Climate What Do You Think? Cap and Trade for Mercury Pollution? Air pollutants travel the globe CO2 and halogens are key greenhouse gases The Supreme Court has charged the EPA with controlling greenhouse gases CFCs also destroy ozone in the stratosphere CFC control has had remarkable success 10.3 Environmental and Health Effects Acid deposition results from SO4 and NOx Urban areas endure inversions and heat islands Smog and haze reduce visibility 10.4 Air Pollution Control The best strategy is reducing production Clean air legislation is controversial but extremely successful Trading pollution credits is one approach 10.5 The Ongoing Challenge Pollution persists in developing areas Change is possible Key Concepts Can we afford clean air? Conclusion Data Analysis How Polluted Is Your Hometown? 230 231 232 233 233 235 236 236 236 237 237 238 239 239 240 240 241 242 243 243 243 244 245 245 245 245 246 248 249 11 Water: Resources and Pollution 250 LEARNING OUTCOMES 250 Case Study A Water State of Emergency 11.1 Water Resources How does the hydrologic cycle redistribute water? Major water compartments vary in residence time Groundwater storage is vast and cycles slowly Surface water and atmospheric moisture cycle quickly Active Learning Mapping the Water-Rich and Water-Poor Countries 11.2 How Much Water We Use? “Virtual water” is exported in many ways Some products are thirstier than others Industrial uses include energy production Domestic water supplies protect health 11.3 Dealing with Water Scarcity Drought, climate, and water shortages What Do You Think? Water and Power Groundwater supplies are being depleted Diversion projects redistribute water Questions of justice often surround dam projects Would you fight for water? 11.4 Water Conservation and Management Everyone can help conserve water What Can You Do? Saving Water and Preventing Pollution Communities are starting to recycle water 11.5 Water Pollutants Pollution includes point sources and nonpoint sources Biological pollution includes pathogens and waste Nutrients cause eutrophication Inorganic pollutants include metals, salts, and acids Exploring Science Inexpensive Water Purification Organic chemicals include pesticides and industrial substances Is bottled water safer? Sediment is one of our most abundant pollutants 11.6 Persistent Challenges Developing countries often have serious water pollution Groundwater is especially hard to clean up Ocean pollution has few controls 11.7 Water Treatment and Remediation Impaired water can be restored Nonpoint sources require prevention How we treat municipal waste? Municipal treatment has three levels of quality Natural wastewater treatment can be an answer Remediation can involve containment, extraction, or biological treatment Key Concepts Could natural systems treat our wastewater? CO N T EN TS 251 252 252 253 254 255 255 255 256 256 257 257 257 258 259 260 260 261 262 263 263 263 264 264 264 265 266 267 268 268 269 269 270 270 271 272 273 273 273 274 274 274 275 276 ix Predation leads to adaptation Predator-prey relationships exert selection pressures that favor evolutionary adaptation Predators become more efficient at searching and feeding, and prey become more effective at escape and avoidance Prey organisms have developed countless strategies to avoid predation, including toxic or bad-tasting compounds, body armor, extraordinary speed, and the ability to hide Plants have evolved thick bark, spines, thorns, or distasteful and even harmful chemicals in tissues—poison ivy and stinging nettle are examples In response, animals have found strategies for avoiding spines, eating through thick bark, or tolerating chemicals Arthropods, amphibians, snakes, and some mammals produce noxious odors or poisonous secretions that cause other species to leave them alone Speed is a common defense against predation On the Serengeti Plain of East Africa, the swift Thomson’s gazelle and even swifter cheetah are engaged in an arms race of speed and endurance The cheetah has an edge in a surprise attack, because it can accelerate from to 72 kph in seconds But the gazelle often escapes because the cheetah lacks stamina A general term for this close adaptation of two species is coevolution Species with chemical defenses often display distinct coloration and patterns to warn away enemies (fig 3.14) Species also display forms, colors, and patterns that help them hide Insects that look exactly like dead leaves or twigs are among the most remarkable examples (fig 3.15) Predators also use camouflage to conceal themselves as they lie in wait for their next meal In a neat evolutionary twist, certain species that are harmless resemble poisonous or distasteful ones, gaining protection against predators that remember a bad experience with the actual toxic organism This is called Batesian mimicry, after the English naturalist H W Bates (1825−1892) Many wasps, for example, have bold patterns of black and yellow stripes to warn off potential predators (fig. 3.16a) A harmless variety of longhorn beetle has evolved to look and act like a wasp, tricking predators into avoiding it (fig 3.16b) Similarly, the benign viceroy butterfly has evolved to closely resemble the distasteful monarch butterfly When two unpalatable or dangerous species look alike, we call it Müllerian mimicry (after the biologist Fritz Müller) When predators learn to avoid either species, both benefit Symbiosis involves cooperation In contrast to predation and competition, some interactions between organisms can be nonantagonistic, even beneficial (table 3.2) In such relationships, called symbiosis, two or more species live FIGURE 3.15 This walking stick is highly camouflaged to blend in with the forest floor, a remarkable case of selection and adaptation (a) Wasp (b) Beetle FIGURE 3.16 In Batesian mimicry, a stinging wasp (a) has bold yellow and black bands, which a harmless long-horned beetle mimics (b) to avoid predators TABLE 3.2 Types of Species Interactions FIGURE 3.14 Poison arrow frogs of the family Dendrobatidae display striking patterns and brilliant colors that alert potential predators to the extremely toxic secretions on their skin Indigenous people in Latin America use the toxin to arm blowgun darts INTERACTION BETWEEN TWO SPECIES EFFECT ON FIRST SPECIES EFFECT ON SECOND SPECIES Mutualism + + Commensalism + Parasitism + – Predation + – Competition ± ± (+ beneficial; − harmful; neutral; ± varies) CHAPTE R Evolution, Species Interactions, and Biological Communities 61 FIGURE 3.17 Coevolution has led to close evolutionary relationships between many species, as in this star orchid and the specially adapted hawk moth that pollinates it intimately together, with their fates linked Symbiotic relationships often involve coevolution Many plants and pollinators have forms and behaviors that benefit each other Many moths, for example, are adapted to pollinate particular flowering plants (fig 3.17) Symbiotic relationships often enhance the survival of one or both partners In lichens, a fungus and a photosynthetic partner (either an alga or a c yanobacterium) combine tissues to mutual benefit A symbiotic relationship such as this, in which both species clearly benefit, is also called mutualism (fig 3.18) Competition and predation were long thought to drive most adaptation and speciation, but ecologists increasingly recognize the frequency and importance of in cooperative and mutualistic relationships You have trillions of symbiotic microorganisms living in and on your body (Exploring Science, p 63) The interdependence of coral polyps and algae in coral reefs is a globally important form of mutualism, in which the polyp provides structure and safety for algae, while the photosynthetic algae provide nutrients to the coral polyp as it builds a coral reef system Another widespread mutualistic relationship is that between ants and acacia trees in Central and South (a) Symbiosis (b) Mutualism America Colonies of ants live inside protective cover of hollow thorns on the acacia tree branches Ants feed on nectar that is produced in glands at the leaf bases and eat protein-rich structures that are produced on leaflet tips The acacias thus provide shelter and food for the ants What the acacias get in return? Ants aggressively defend their territories, driving away herbivorous insects that might feed on the acacias Ants also trim away vegetation that grows around the tree, reducing competition by other plants for water and nutrients This mutualistic relationship thus affects the biological community around acacias, just as competition or predation shapes communities Commensalism is a type of symbiosis in which one member clearly benefits and the other apparently is neither benefited nor harmed Many mosses, bromeliads, and other plants growing on trees in the moist tropics are considered commensals (fig 3.18c) These epiphytes are watered by rain and obtain nutrients from leaf litter and falling dust, and often they neither help nor hurt the trees on which they grow Parasitism, a form of predation, may also be considered symbiosis because of the dependency of the parasite on its host Keystone species play critical roles A keystone species plays a critical role in a biological community that is out of proportion to its abundance Originally, keystone species were thought to be only top predators—lions, wolves, tigers— which limited herbivore abundance and reduced the herbivory of plants Scientists now recognize that less-conspicuous species also play keystone roles Tropical fig trees, for example, bear fruit year-round at a low but steady rate If figs were removed from a forest, many fruit-eating animals (frugivores) would starve in the dry season when fruit of other species is scarce In turn, the disappearance of frugivores would affect plants that depend on them for pollination and seed dispersal The effect of a keystone species on communities ripples across multiple trophic levels (c) Commensalism FIGURE 3.18 Symbiosis refers to species living together: for example, lichens (a) consist of a fungus, which gives structure, and an alga or cyanobacterium, which photosynthesizes Mutualism is a symbiotic relationship that benefits both species, such as a lichen or a parasite-eating red-billed oxpicker and a parasiteinfested impala (b) Commensalism benefits one species but has little evident effect on the other, as with a tropical tree and a free-loading bromeliad (c) 62 Principles of Environmental Science EXPLORING Science Say Hello to Your 90 Trillion Little Friends H ave you ever thought of yourself as a biological community or an ecosystem? Researchers estimate that each of us has about 90 trillion bacteria, fungi, protozoans, and other organisms living in or on our bodies The largest group—around kg w orth— inhabit your gut, but there are thousands of species living in every orifice, gland, pore, and crevice of your anatomy Although the 10 trillion or so mammalian cells make up more than 95 percent of the volume of your body, they represent less than 10 percent of all the cell types that occupy that space Because most of the other species with which we coexist are microorganisms, we call the collection of cells that inhabit us our microbiome The species composition of your own microbial community will be very similar to that of other people and pets with whom you live, but each of us has a unique collection of species that may be as distinctive as our fingerprints As you’ll learn elsewhere in this chapter, symbiotic relationships can be mutualistic (both benefit), commensal (one benefits while the other is unaffected), or parasitic (one harms the other) We used to think of all microorganisms as germs to be eliminated as quickly and thoroughly as possible Current research suggests, however, that many of our fellow travelers are beneficial, perhaps even indispensable, to our good health and survival Your microbiome is essential, for example, in the digestion and absorption of nutrients Symbiotic bacteria in your gut supply essential nutrients (important amino acids and short-chain fatty acids), vitamins (such as K and some B varieties), hormones and neurotransmitters (such as serotonin), and a host of other signaling molecules that communicate with, and modulate, your immune and metabolic systems They help exclude pathogens by competing with them for living space, or by creating an environment in which the bad species can’t grow or prosper In contrast, the inhabitants of different organs can have important roles in specific diseases Oral bacteria, for example, have been implicated in cardiovascular disease, pancreatic cancer, rheumatoid arthritis, and preterm birth, among other things Symbionts in the lung have been linked to cystic fibrosis and chronic obstructive pulmonary disease (COPD) And the gut community seems to play a role in obesity, diabetes, c olitis, susceptibility to infections, allergies, and chronic problems A healthy biome seems to be critical in controlling chronic inflammation that triggers many important long-term diseases As is the case in many ecosystems, the diversity of your microbiome may play an important role in its stability and resilience Having a community rich in good microbes will not only help you resist infection by pathogens but will allow faster recovery after a catastrophic event People in primitive or rustic societies who eat a wide variety of whole grains, raw fruits and vegetables, and unprocessed meat and dairy products tend to have a much greater species blend than those of us who have a diet full of simple sugars and highly processed foods Widespread use of antibiotics to treat illnesses, as well as chronic low levels of antimicrobials, preservatives, and stabilizers in our food, toothpaste, soap, and many other consumer products also limits diversity in our symbiotic community A growing problem in many places is antibiotic-resistant, hospital-acquired infections One of the most intractable of these is Clostridium difficile, or C diff, which infects 250,000 and kills 14,000 Americans every year An effective treatment for this superpathogen is fecal transplants Either a sample of the microbiome from a healthy person is implanted either directly through a feeding tube into the patient’s stomach or frozen, encapsulated pellets of feces are delivered orally In one trial, 18 of 20 patients who received fecal transplants recovered from C diff Similarly, obese mice given fecal transplants from lean mice lose weight, while lean mice that receive samples of gut bacteria from obese mice gain weight The microbiome may even regulate mood and behavior When microbes from easygoing, adventurous mice are transplanted into the gut of anxious, timid mice, they become bolder and more adventurous So, it may pay to take care of your garden of microbes If you keep them happy, they may help keep you happy as well Intestinal bacteria, such as these, help crowd out pathogens, aid in digestion, supply your body with essential nutrients, and may play a role in obesity, diabetes, colitis, allergies, and chronic inflammation, along with a host of other critical diseases CHAPTE R Evolution, Species Interactions, and Biological Communities 63 in several meters of housefly bodies Luckily housefly reproduction, as for most organisms, is constrained in a variety of ways—scarcity of resources, competition, predation, disease, accident The housefly merely demonstrates the remarkable amplification—the biotic potential— of unrestrained biological reproduction (c) Sea otters protect kelp ecosystem by preying on urchins (a) Kelp shelter fish, seals, and other species FIGURE 3.19 Sea otters protect kelp ecosystems on the Pacific coast by eating sea urchins, which could otherwise destroy the kelp (b) Sea urchins graze on kelp Off the northern Pacific coast, a giant brown alga (Macrocystis pyrifera) forms dense “kelp forests,” which shelter fish and shellfish species from predators, allowing them to become established in the community Within this kelp forest are also sea urchins, which graze on the kelp on the seafloor, and sea otters, which eat the sea urchins When sea otters have been eliminated—by trapping or by predation, for example—the urchins overgraze and diminish the kelp forests, potentially causing collapse of this complex system (fig 3.19) Because of their critical role in supporting the entire kelp forest, otters are seen as a classic example of a keystone species Keystone functions have been documented for vegetationclearing elephants, predatory ochre sea stars, and frog-eating salamanders in coastal North Carolina Even microorganisms can play keystone roles In many temperate forest ecosystems, groups of fungi that are associated with tree roots (mycorrhizae) facilitate the uptake of essential minerals When fungi are absent, trees grow poorly or not at all 3.3 POPULATION GROWTH Apart from their interactions with other species, organisms have an inherent rate of reproduction that influences population size Many species have the potential to produce almost unbelievable numbers of offspring Consider a single female housefly (Musca domestica), which can lay 120 eggs In 56 days those eggs become mature adults, and each female—suppose half are female—can lay another 120 eggs At this rate, there can be seven generations of flies in a year, and that original fly would be the proud grandparent of 5.6 trillion offspring If this rate of reproduction continued for 10 years, the entire earth would be covered 64 Principles of Environmental Science Growth without limits is exponential Understanding population dynamics, or the rise and fall of populations in an area, is essential for understanding how species interact and use resources As discussed in chapter 2, a population consists of all the m embers of a single species living in a specific area at the same time The growth of the housefly population just described is exponential, having no limit and possessing a distinctive shape when graphed over time An exponential growth rate (increase in numbers per unit of time) is expressed as a constant fraction, or exponent, which is used as a multiplier of the existing population The mathematical equation for exponential growth is dN = rN dt Here d means “change,” so the change in number of individuals (dN) per change in time (dt) equals the rate of growth (r) times the number of individuals in the population (N) The r term (intrinsic capacity for increase) is a fraction representing the average individual contribution to population growth If r is positive, the population is increasing If r is negative, the population is shrinking If r is zero, there is no change, and dN/dt = A graph of exponential population growth is described as a J curve (fig 3.20) because of its shape As you can see, the number of individuals added to a population at the beginning of an exponential growth curve can be rather small But the numbers begin to increase quickly with a fixed growth rate For example, when a population has just 100 individuals, a percent growth rate adds just individuals For a population of 10,000, that percent growth adds 200 individuals The exponential growth equation is a very simple model; it is an idealized description of a real process The same equation is used to calculate growth in your bank account due to compounded interest rates; achieving the maximum growth potential would require that you never withdraw any money But, in fact, some money probably will be withdrawn Similarly, not all individuals in a population survive, so actual growth rates are something less than the full biotic potential Carrying capacity limits growth In the real world there are limits to growth Around 1970, ecologists developed the concept of carrying capacity to mean the number or biomass of animals that can be supported (without harvest) in a Biotic potential 150 Overshoot Dieback S o hare e Snowshoe C Canada lynx Population size 125 Pelts 100 Carrying capacity J curve 75 50 25 0 Time FIGURE 3.20 A J curve, or exponential growth curve, leads to repeated overshoot and dieback cycles The environment’s ability to support the species (carrying capacity) may diminish as overuse degrades habitat Moose on Isle Royales in Lake Superior seem to have exhibited this pattern 1850 1860 1870 1880 1890 1900 1910 1920 1930 Year FIGURE 3.21 Ten-year oscillations in the populations of snowshoe hares and lynx in Canada suggest a close linkage of predator and prey These data are based on the number of pelts received by the Hudson Bay Company from fur traders SOURCE: Data from D A MacLulich Fluctuations in the numbers of the Varying hare (Lepus americanus) University of Toronto Press, 1937, reprinted 1974 certain area of habitat The concept is now used more generally to suggest a limit of sustainability that an environment has in relation to the size of a species population Carrying capacity is helpful in understanding the population dynamics of some species, perhaps even humans When a population overshoots, or exceeds, the carrying capacity of its environment, resources become limited and death rates rise If deaths exceed births, the growth rate becomes negative and the population may suddenly decrease, a change called a population crash or dieback (fig 3.20) Populations may oscillate from high to low levels around the habitat’s carrying capacity, which may be lowered if the habitat is damaged Moose and other browsers or grazers sometimes overgraze their food plants, so future populations in the same habitat find less preferred food to sustain them, at least until the habitat recovers Some species go through predictable cycles if simple factors are involved, such as the seasonal light- and temperature-dependent bloom of algae in a lake Cycles can be irregular if complex environmental and biotic relationships exist Irregular cycles include outbreaks of migratory locusts in the Sahara and tent caterpillars in temperate forests— these represent irruptive population growth Often immigration of a species into an area, or emigration from an area, also affects population growth and declines Sometimes predator and prey populations oscillate in synchrony with each other One classic study employed the 2 00-year record of furs sold at Hudson Bay Company trading posts in Canada (figure 3.21 shows a portion of that record) The ecologist Charles Elton showed that numbers of Canada lynx (Lynx canadensis) fluctuate on about a 10-year cycle that mirrors, slightly out of phase, the population peaks of snowshoe hares (Lepus americanus) When the hare population is high, the lynx prosper on abundant prey; they reproduce well, and their population grows Eventually the abundant hares overgraze the vegetation, decreasing their food supplies, and the hare populations shrink For a while the lynx benefits because starving hares are easier to catch than healthy ones As hares become scarce, however, so lynx When hares are at their lowest levels, their food supply recovers and the whole cycle starts over again This predatorprey oscillation is described mathematically in the Lotka-Volterra model, named for the scientists who developed it Environmental limits lead to logistic growth Not all biological populations cycle through exponential overshoot and catastrophic dieback Many species are regulated by both internal and external factors and come into equilibrium with their environmental resources while maintaining relatively stable population sizes When resources are unlimited, they may even grow exponentially, but this growth slows as the carrying capacity of the environment is approached This population dynamic is called logistic growth because of its changes in growth rate over time Mathematically, this growth pattern is described by the following equation, which adds a feedback term for carrying capacity (K) to the exponential growth equation: (K – N) dN = rN K dt The logistic growth equation says that the change in numbers over time (dN/dt) equals the exponential growth rate (rN) times CHAPTE R Evolution, Species Interactions, and Biological Communities 65 Species respond to limits differently: r- and K-selected species Some organisms, such as dandelions and barnacles, depend on a high rate of reproduction and growth (r) to secure a place in the environment These organisms are called r-selected species because they are adapted to employ a high reproductive rate to overcome the high mortality of virtually ignored offspring These species may even overshoot carrying capacity and experience Active LEARNING Effect of K on Population Growth Rate (rN) In logistic growth, the term (K − N)/K creates a fraction that is multiplied by the growth rate (rN) Suppose carrying capacity (K) is 100 If N is 150, then is the term (K − N)/K positive or negative? Is population change positive or negative? What if N is 50? If N is 100? ANSWERS: negative, positive, no growth 66 Principles of Environmental Science K = carrying capacity Population size the portion of the carrying capacity (K) not already taken by the current population size (N) The term (K − N)/K establishes the relationship between population size at any given time and the carrying capacity (K) If N is less than K, the rate of population change will be positive If N is greater than K, then change will be negative (Active Learning, below) The logistic growth curve has a different shape than the exponential growth curve It is a sigmoidal-shaped, or S, curve (fig 3.22) It describes a population whose growth rate decreases if its numbers approach or exceed the carrying capacity of the environment Population growth rates are affected by external and internal factors External factors include habitat quality, food availability, and interactions with other organisms As populations grow, food becomes scarcer and competition for resources more intense With a larger population, there is an increased risk that disease or parasites will spread, or that predators will be attracted to the area Internal factors, such as slow growth and maturity, body size, metabolism, or hormonal status, can reduce reproductive output Often crowding increases these factors Overcrowded house mice (>1,600/m3), for instance, average 5.1 babies per litter, while uncrowded house mice (