Fundamentals of Ecosystem Science PDF

FUNDAMENTALS OF ECOSYSTEM SCIENCE FUNDAMENTALS OF ECOSYSTEM SCIENCE KATHLEEN C. WEATHERS, DAVID L. STRAYER, AND GENE E. LIKENS AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKY...

FUNDAMENTALS OF ECOSYSTEM SCIENCE FUNDAMENTALS OF ECOSYSTEM SCIENCE KATHLEEN C. WEATHERS, DAVID L. STRAYER, AND GENE E. LIKENS AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORD PARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO Academic Press is an imprint of Elsevier Academic Press is an imprint of Elsevier 225 Wyman Street, Waltham, MA 02451, USA 525 B Street, Suite 1800, San Diego, CA 92101-4495, USA 84 Theobald’s Road, London WC1X 8RR, UK Copyright r 2013 Elsevier, Inc. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the Publisher. 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To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. ISBN: 978-0-12-088774-3 For information on all Academic Press publications visit our website at http://store.elssevier.com Printed in the United States of America 13 14 15 16 9 8 7 6 5 4 3 2 1 Table Of Content Preface Part I: Introduction 1. Introduction to Ecosystem Science Part II: Ecological Energetics 2. Primary Production: The Foundation of Ecosystems 3. Secondary Production and Consumer Energetics 4. Organic Matter Decomposition Part III: Biogeochemistry 5. Element Cycling 6. The Carbon Cycle 7. The Nitrogen Cycle 8. The Phosphorus Cycle Part IV: Synthesis 9. Revisiting the Ecosystem Concept: Important Features That Promote Generality and Understanding 10. Ecosystems in a Heterogeneous World 11. Controls on Ecosystem Structure and Function Part V: Case Studies 12. From Global Environmental Change to Sustainability Science: Ecosystem Studies in the Yaqui Valley, Mexico 13. Ecology of Lyme Disease 14. Understanding Ecosystem Effects of Dams 15. Acid Rain 16. Streams and Their Valleys Part VI: Frontiers 17. Frontiers in Ecosystem Science Appendix: A Primer on Biologically Mediated Redox Reactions in Ecosystems Glossary Index  Preface This book provides an introduction to approaches, and as a result, multiple the content, ideas, and major findings of “voices” will be evident throughout the contemporary ecosystem science. We wrote book. We believe that this diversity reflects the book primarily for beginning graduate some of the myriad perspectives and students and advanced undergraduates but approaches that are fruitfully brought to it should also be useful to a broad range of bear on the field of ecosystem science. academic scientists and resource managers, The book contains six major sections. and even to dedicated amateurs who seek The opening chapter introduces the concept an introduction to the field. Ecosystem of the ecosystem, explores some of the science is a rigorous, quantitative science; consequences of this concept, describes we assume that readers of the book will the intellectual tools of the science, and have had an introductory class in ecology briefly reviews the history of this young and basic understanding of chemistry and science. Chapters 2 through 8 lay the foun- math. The book deliberately covers multi- dation for the study of ecosystems, and ple approaches to understanding ecosys- cover the two major branches of ecosystem tems (e.g., the use of experiments, theory, science: energetics (Chapters 2 4) and bio- cross-system comparisons), in multiple geochemistry (Chapters 5 8). These chap- environments (terrestrial, freshwater, and ters present the core content of ecosystem marine; managed, built, and natural ecosys- science—the movement and fate of energy tems), across all parts of the world and materials in ecosystems—in some (although many examples come from the detail. In the synthetic Chapters 9 11, authors’ experience in North America). we revisit major themes that cut across mul- The origins of this book stem from tiple areas of study in ecosystem science. an intensive two-week Fundamentals of Authors of these chapters review the power Ecosystem Ecology class (the FEE class) and utility of the ecosystem concept, the that we have taught to graduate students roles of heterogeneity in space and time, from around the world every year or two and the importance of various types of con- at the Cary Institute of Ecosystem Studies trols in ecosystems. Chapters 12 16 take since 1989. We, and many of the chapter ecosystem science into the real world by authors, have played central roles in the illustrating, through five case studies, the development, evolution, and running of the value of ecosystem science in identifying FEE class since its origin. and solving a range of environmental pro- We decided upon an edited book for sev- blems. The book closes with Chapter 17, eral reasons, not the least of which was its co-authored by several current Cary Institute genesis in this team-taught course. While postdoctoral associates, which lays out we shepherded and integrated the chapters some challenges and needs for the future. and their contents, we also deliberately Today’s ecosystem science is evolving rap- allowed—and even encouraged—multiple idly, with major new discoveries and ideas ix x PREFACE emerging every year. The ultimate shape ecosystem science; their comments sub- and contributions of this science remain to stantially improved the book. We thank be discovered. the authors of various chapters for their This book benefited from the persistent scholarship, patience, goodwill, and and hard work of the Academic Press commitment to bringing this project to fru- team, especially Jill Cetel, Candice Janco, ition. The Cary Institute’s assistant, Matt and multiple graphic artists. We were Gillespie, was an enormous help as well. also fortunate to have received helpful Finally, generations of FEE students were and critical reviews of chapters from and continue to be an impetus colleagues, including Clifford Ochs and and inspiration to us and the field of eco- several anonymous reviewers who teach system science.  C H A P T E R 1 Introduction to Ecosystem Science Kathleen C. Weathers, David L. Strayer, and Gene E. Likens Cary Institute of Ecosystem Studies, Millbrook, NY Humans have devised many intellectual systems to understand and manage the com- plicated world in which we live, from physics to philosophy to economics. In this book, we present one such intellectual system, ecosystem science, that tries to make sense of the complex natural world and help us better manage it. As you will see, ecosystems can be highly varied in size and character, from a little pool of water in a tree cavity, to a red- wood forest, to a neighborhood in a city, to a frigid river, to the entire globe (Figure 1.1). Nevertheless, a common set of tools and ideas can be used to analyze and understand these varied and complicated systems. The results of these analyses are both intellectually satisfying and useful in managing our planet for the benefit of humankind and nature. Indeed, because of the growing demands placed on the living and nonliving resources by humans, it could be argued that ecosystem science is one of the essential core disciplines needed to understand and manage the modern planet Earth. This book defines the ecosystem, describes the chief characteristics of ecosystems and the major tools that scientists use to analyze them, and presents major discoveries that scientists have made about ecosystems. It also lays out some of the important questions for the future. This book is not specifically about ecosystem management, but throughout the book some of the management implications of ecosystem science are described. WHAT IS AN ECOSYSTEM? An ecosystem is the interacting system made up of all the living and nonliving objects in a spec- ified volume of space. This deceptively simple definition both says much and leaves out much. First, as with other systems (Box 1.1), ecosystems contain more than one object, and those objects inter- act. More surprisingly, living and nonliving objects are given equal status in ecosystem sci- ence. A particle of clay and the plant drawing its nutrition from that clay are both parts of Fundamentals of Ecosystem Science. 3 © 2013 Elsevier Inc. All rights reserved. 4 KATHLEEN C. WEATHERS ET AL. (a) (b) (c) (d) (e) (f) (g) FIGURE 1.1 Some examples of ecosystems: (a) the frigid Salmon River, Idaho; (b) a residential neighborhood in Baltimore, Maryland; (c) a biofilm on a rock in a stream; (d) a section of the southern ocean containing a phyto- plankton bloom; (e) a redwood forest in the fog in California; (f) a tree cavity; (g) the Earth (Photocredits: 1a - John Davis; 1b - Baltimore Ecosystem Study LTER; 1c - Colden Baxter; 1d - US government, public domain; 1e -Samuel M. Simkin; 1f -Ian Walker; 1g - NASA, http://visibleearth.nasa.gov/). an ecosystem, and therefore equally valid objects of study. This viewpoint contrasts with physiology and population ecology, for example, in which the organism is the object of study, and the nonliving environment is conceived of as an external influence on the object of study. Finally, the definition implies that ecosystems have definite boundaries, but does not tell us how we might go about setting or finding the boundaries to an ecosystem. There are some unexpectedly powerful advantages to this simple definition. First, by including all living and nonliving objects in a specified space, it is possible to use the tool of mass balance to follow the movement and fate of materials (Box 1.2). Material that comes into an ecosystem must either stay in the ecosystem or leave—there is simply no I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 5 BOX 1.1 SOME NONECOLOGICAL SYSTEMS Thinking about some of the many famil- other banks. Systems may be described iar examples of nonecological systems may according to their controls as well. Gravity help you understand how ecosystems are and rotational dynamics control the described and compared. A system is just a motions of the planets, and the copper collection of more than one interacting atom is controlled by strong and weak object. A few familiar systems include the atomic forces, whereas the Federal Reserve group of planets rotating around the sun as System is controlled by the decisions of its a system (the solar system); the group of Board of Governors (who, in turn are cho- electrons, protons, and neutrons forming an sen by a president who is elected by the atom; and the system of banks that controls voters of the United States). All of these the money supply of the United States (the descriptions allow us to understand how Federal Reserve System). Just as with eco- each system works. Perhaps more impor- systems, we can describe these systems by tantly, they let us compare one system to their structures, their functions, and the fac- another—our solar system with those of tors that control them. other stars; the copper atom with the cad- A description of system structure often mium atom; our current banking system in begins with the number and kinds of the United States with that of France, objects in the system. Thus, we might note or with that of the United States in the that our solar system contains eight or nine nineteenth century. Ecosystem scientists planets; or that the copper atom has 29 likewise describe ecosystems in various electrons, 29 protons, and 35 neutrons; or ways to understand them better, and to that the Federal Reserve System contains a allow comparisons across ecosystems. seven-member Board of Governors, 12 Systems science, the general field of banks, and 26 branch banks. Systems have understanding all kinds of systems, is well functional properties as well—the copper developed. Many of the conceptual frame- atom exchanges electrons with other atoms works for ecosystem science are those of in chemical reactions, and the Federal system science (e.g., Hogan and Weathers Reserve System exchanges money with 2003). other place for the material to go. Mass balance offers a convenient quantitative tool for measuring the integrated activity of entire, complicated systems without having to mea- sure the properties and interactions of each of its parts. It also allows ecosystem scientists to estimate the size of a single unknown flux by difference. Consequently, it will become evident throughout the book that ecosystem scientists often use the powerful tool of mass balance. Second, defining an ecosystem as we have done makes it possible to measure the total activity of an ecosystem without having to measure all the parts and exchanges within the ecosystem. This is sometimes referred to as a “black-box” approach, because we can I. INTRODUCTION 6 KATHLEEN C. WEATHERS ET AL. BOX 1.2 MASS BALANCE To see just how useful the tool of mass that was bounded by the lakeshore, the balance can be, suppose we are trying to overlying air, and the bedrock deep beneath evaluate whether a lake ecosystem is taking the lake sediments. Using mass balance, we up or releasing phosphorus. We could try note that the amount of phosphorus being to measure all the exchanges between parts retained by the lake ecosystem is simply of the ecosystem (e.g., the uptake of phos- the amount of phosphorus going into the phorus by phytoplankton and rooted lake minus the amount that is leaving plants; the consumption and excretion of the lake. Now we just have to measure the phosphorus by the animals that eat phyto- exchanges across the ecosystem boundary plankton and plants; the release of phos- (stream water and ground water going into phorus during the decay of phytoplankton, and out of the lake; rain, snow, and parti- plants, and animals; and dozens of other cles falling on the lake; and any animals exchanges), then simply sum up all of entering and leaving the lake; hard these measurements. It would take an enor- enough!) to calculate whether the lake is mous amount of work to measure all the taking up or releasing phosphorus. In the exchanges, and our final answer would case of Mirror Lake, New Hampshire be fraught with large uncertainties. Alterna- (Figure 1.2), almost 40% of incoming phos- tively, we could define a lake ecosystem phorus is retained by the lake. Precipitation FIGURE 1.2 Average phosphorus 3.7 inputs and outputs in kilograms/year to t Mirror Lake, NH. Total average inputs 5 6.7 Tr le kilograms/year; total average outputs 5 4.1 ib 1 in ut.2 y ar ar kilograms/year. Inputs
outputs 5 2.7 ut.1 y in ir b 0 kilograms/year or 39.7% retention of phos- etl T phorus in the lake. (Data from Winter and Likens 2009.) Tributary inlet 1.4 Outflow Ground water in 1.7 0.3 Ground water out 2.4 I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 7 measure the function (input and output) of a box (the ecosystem) without having to know what is in the box (Figure 1.3). Sometimes ecologists debate whether it is philosophically possible to predict the properties of a complex system by studying its parts (reductionism) or whether it is necessary to study intact systems (holism). It is not necessary to accept the philosophical claims of holism, though, to recognize that studies of whole systems may be a much more efficient way than reductionism to understand ecosystems. Such a holistic approach to ecosystems is a powerful tool of ecosystem science, and is often combined with reductionist approaches to develop insights into the functioning and controls of ecosystems. Third, the definition gives the investigator complete flexibility in choosing where to set the boundaries of the ecosystem in time and space. The size, location, and timescale at which ecosystems are defined can therefore precisely match the question that the scientist is trying to answer. Boundaries often are drawn at places where fluxes are easy to measure (e.g., a single point on a stream as it leaves a forested-watershed ecosystem) or so that fluxes across the boundary are small compared to cycling inside the ecosystem (e.g., a lake shore). Nevertheless, boundaries are required to make quantitative measures of these fluxes. It is true that ecosystems frequently are defined to be large (e.g., lakes and water- sheds hectares to square kilometers in size) and are studied on the scale of days to a few years, but there is nothing in the definition of ecosystem that requires ecosystems to be defined at this scale. Indeed, as you will see, an ecosystem may be as small as a single rock or as large as the entire Earth, and can be studied for time periods as long as hun- dreds of millions of years. Fourth, defining an ecosystem to contain both living and nonliving objects recognizes the importance of both living and nonliving parts of ecosystems in controlling the func- tions and responses of these systems. There are examples throughout the book in which living organisms, nonliving objects, or both acting together determine what ecosystems look like (structure) and how they work (function). Furthermore, the close ties and strong interactions between the living and nonliving parts of ecosystems are so varied and so strong that it would be very inconvenient to study one without the other. Thus, the ECOSYSTEM BOUNDARY ORGANIC COMPARTMENT ATMOSPHERIC Litter COMPARTMENT Biological uptake Meteorologic Meteorologic Windblown particulates of gases and aerosols Living Herbicore Dead Geologic Geologic and gases above and Biological release of Plant Carnivore Biomass Biologic Biologic below ground gases and organic aerosols Omnivore Biomass INPUT INPUT Detritivore W et leaching, throughfall, an Dry deposition stemflow, exudation d dr Inorganic Mineralization, aerosols yd Re ep Biological lea os BIOSPHERE BIOSPHERE uptake se itio of n ga se s OUTPUT OUTPUT AVAILABLE NUTRIENT SOIL AND ROCK Weathering COMPARTMENT MINERAL Meteorologic Meteorologic COMPARTMENT on in Geologic Geologic Formation of exchange soil Biologic Biologic secondary minerals sites solution INTRASYSTEM CYCLE FIGURE 1.3 Two views of the same ecosystem. The left side shows some of the parts inside an ecosystem and how they are connected, as well as the exchanges between the ecosystem and its surroundings, whereas the right side shows a black-box approach in which the functions of an ecosystem (i.e., its inputs and outputs) can be stud- ied without knowing what is inside the box. (Modified from Likens 1992.) I. INTRODUCTION 8 KATHLEEN C. WEATHERS ET AL. inclusion of living and nonliving objects in ecosystems has practical as well as intellectual advantages. Finally, we note one further property of ecosystems—they are open to the flow of energy and materials. It might be theoretically possible to define particular examples of ecosystems that are closed systems, not exchanging energy or materials with their sur- roundings, but nearly all ecosystems as actually defined have important exchanges of energy and materials with their surroundings. Indeed, such exchanges are one of the cen- tral subjects of ecosystem science. We note in particular that most ecosystems depend on energy inputs from external sources, either as energy from the sun or as organic matter brought in from neighboring ecosystems. Now consider briefly what is missing from the definition. We have already noted that the definition does not specify the time or space scales over which an ecosystem is defined, or where exactly the boundaries are placed. Ecosystems are not required to be self-regulating, permanent, stable, or sustainable. They are not required to have any partic- ular functional properties. For example, they need not be in balance or efficient in the way that they process materials. Our definition does not require ecosystems to have a purpose. Although ecosystems change over time, the basic definition does not suggest anything about the nature or direction of that change. It might seem like a shame not to include such interesting attributes in a definition of ecosystem (O’Neill 2001), and indeed some ecologists have included such attributes in their definitions, but we think it is neither nec- essary nor helpful to include them in a basic definition. They may, however, be useful hypotheses and the subject of fruitful research projects. For instance, we might hypothe- size that as forest ecosystems recover from disturbances like fire or clear-cutting, they retain a higher proportion of nutrient inputs. This viewpoint is quite different than saying that ecosystems are systems that tend to maximize efficiency of use of limiting nutrients. WHAT ARE THE PROPERTIES OF ECOSYSTEMS? All systems have characteristic properties that allow us to describe them and compare them with other similar systems (Box 1.1). How might we describe the properties of ecosystems? What Is in an Ecosystem? We might begin simply by listing the contents of an ecosystem. Plants and animals occur in most ecosystems. As we will see later in the book, the number and kinds of plants and animals can have a strong influence on ecosystem function. Many ecosystems also contain people. Historically, many ecologists treated humans as being outside of the eco- system, or deliberately studied ecosystems without people, but it has become increasingly common to treat people and our institutions as parts of ecosystems (e.g., Pickett et al. 2001, 2011; see Chapter 17). Certainly the structure and function of many modern ecosys- tems cannot be understood without considering human activities. I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 9 Almost all ecosystems contain microbes (bacteria and fungi); although not as conspic- uous as plants and animals, their activities are vital to ecosystem functioning. Viruses occur in most ecosystems, and may play important roles as regulators of plant, animal, and microbial populations. Ecosystems also contain water and air, which are themselves resources for many organisms and also serve as media in which organisms and nonliv- ing materials can be transported. Finally, ecosystems contain an enormous variety of nonliving materials, organic and inorganic, solid and dissolved. These nonliving materi- als, including such disparate items as dead wood, clay particles, bedrock, oxygen, and dissolved nutrients, interact with the living biota and exercise strong influences on the character and functioning of ecosystems. Thus, the total inventory of an ecosystem can be very long; it might contain thousands or millions of kinds of items, living and nonliving. Ecosystems Have Structure This complexity allows for an essentially infinite number of possible descriptions of ecosystem structure. Nevertheless, only a few descriptions of ecosystem structure are commonly used by the scientists who study ecosystems. Often ecosystems are described by the numbers and kinds of objects that they contain, focusing on key materials or organisms. Thus, we may describe an ecosystem as having a plant biomass of 300 g/m2, or a deer population of 5/km2, or a nitrogen content of 200 kg/ha. Sometimes ecosystem scientists describe ecosystems by the ratios of key elements such as the nitrogen : phos- phorus ratio of a lake ecosystem. If we were interested in the role of biological communi- ties in regulating ecosystem function, we would refer to the biodiversity (especially the species richness) of the organisms in the ecosystem. We may be interested in the spatial variation, as well as the mean value, of any such key variables (see Chapter 10). Thus, we may describe ecosystems as being highly patchy as opposed to relatively homoge- neous in nitrogen content or biodiversity. Finally, scientists often describe ecosystems by their size or location (e.g., latitude, altitude, biogeographic realm, or distance from the coast). Ecosystems Perform Functions In the broadest sense, ecosystems consume energy and transform materials. As with all systems subject to the second law of thermodynamics, some of the useful energy that comes into ecosystems in forms such as solar radiation, chemical energy (e.g., organic matter), or mechanical energy (e.g., wind) is degraded to heat and becomes unable to perform further work. In particular, living organisms need a continual source of energy to maintain biochemical and physiological integrity, as well as to perform activities such as swimming, running, and flying. Curiously, although these biological energy transfor- mations are only a part of the energy transformations that occur in an ecosystem, most studies of energy flow through ecosystems treat only forms of energy that can be cap- tured and used by living organisms (i.e., solar radiation and chemical energy), and ignore such purely abiotic processes as the conversion of kinetic energy to heat by flow- ing water. Organisms can capture solar energy or chemical energy from inorganic I. INTRODUCTION 10 KATHLEEN C. WEATHERS ET AL. compounds (photosynthesis and chemosynthesis, respectively), store energy, obtain energy from other organisms (e.g., predation), or convert energy into heat (respiration). Patterns of energy flow through ecosystems can be of direct interest to humans who har- vest wild populations, and can tell ecosystem scientists a good deal about how different ecosystems function. Ecosystems also transform materials in various ways. Materials that come into the eco- system may be taken up by some part of the ecosystem and accumulate. In some cases, this accumulation may be temporary so that the ecosystem acts as a sort of capacitor, releasing the material at a later time. The lag time between atmospheric deposition of sul- fate onto a terrestrial ecosystem and its export in stream water from that system is an example. Ecosystems may also be a source of material, releasing their internal stores to neighboring systems. Weathering of soils and bedrock is a prime example. Finally, and perhaps most interesting, ecosystems transform materials by changing their chemical and physical states (see Chapter 5). Nitric acid contained in rainwater falling on a forest soil may react with the soil and form calcium nitrate in soil water. The nitrate in the solution may then be taken up by a plant and incorporated into protein in a leaf. At the end of the growing season, the leaf may fall into a stream where it is eaten by an insect and chopped into small leafy bits, which then wash out of the ecosystem. The description of chemical and biological transformations by ecosystems forms the field of biogeochemistry (Schlesinger 1997; see Chapter 5), a major part of modern ecosystem sciences (and this book). Many biogeochemical functions are important to humans (e.g., the removal of nitrate by riparian forests in the Mississippi River basin), as well as essential to under- standing how different ecosystems work. Ecosystems often are described by their functions as well as their structures. One of the most common functional descriptions of ecosystems is whether the system is a source or a sink of a given material; that is, whether the inputs of that material to the ecosystem are less or more, respectively, than the outputs of that material from the ecosystem. In the special case of energy flow through ecosystems, the degree to which an ecosystem is a source or a sink is described by the P/R (gross photosynthesis to respiration) ratio for the system. At a steady state, ecosystems with a P/R ratio less than 1 must import chemical energy (usually organic matter) from neighboring ecosystems and are called heterotrophic; those with a P/R ratio greater than 1 export chemical energy to neighboring ecosystems and are called autotrophic. Another useful functional description is the residence time of a given material in an ecosystem; that is, the average amount of time that a material spends in an ecosystem. Residence time is calculated by dividing the standing stock of the material in the ecosystem by its input rate. Recently, people have begun to formally recognize that ecosystem structures and functions may have economic value. For instance, ecosystems provide lumber, they purify water and air, they regulate the prevalence of human diseases, and they provide pollination for crop plants. These and many other goods and services provided by eco- systems are now commonly called “ecosystem services”—the benefits that people derive from ecosystem structures and functions (e.g., Millennium Ecosystem Assessment 2005; Kareiva et al. 2011). Developing ways to estimate quantitatively the value of ecosystem services is an important and developing field at the intersection of ecology and economics. I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 11 Ecosystem Structure and Function Are Controlled by Many Factors Unlike systems like the solar system, the dynamics of which are controlled by just a few factors, ecosystem structure and function depend on many factors. Ecosystem scientists have learned much about how ecosystems are controlled, and much of the remainder of this book will be concerned with this subject (see Chapter 11). Ecosystem structure and function often are affected by organisms (including humans), either through trophic activi- ties such as herbivory, predation, and decomposition, or through engineering activities (Jones et al. 1994) such as burrowing, shelter construction (beaver dams), and the like (see Box 11.1 in Chapter 11). Likewise, the nonliving parts of ecosystems often control eco- systems by determining supplies and movement of air, water, key nutrients, and other materials. Temperature is another abiotic factor that has strong effects on ecosystems. Finally, because most ecosystems are open and exchange energy and materials with the ecosystems that surround them or that preceded them, the structure and function of an ecosystem can be strongly affected by its spatial and temporal context (see Chapter 10). Ecosystems Change Through Time Ecosystems change through time (see Chapters 10 and 11). These changes may be grad- ual and subtle (the millennial losses of minerals from a weathering soil) or fast and dramatic (a fire sweeping through a forest). Both external forces (changes in climate or nutrient inputs) and internal dynamics (aging of a tree population, accumulation or deple- tion of materials in a soil or a lake) are important in driving temporal changes in ecosys- tems. In some cases, changes are directional and predictable (e.g., soil weathering, the filling of a lake basin), while in other cases changes may be idiosyncratic and difficult to predict (e.g., the arrival of an invasive species, disturbance by a hurricane). Understanding and predicting how ecosystems change through time is of great theoretical and practical interest, and is a major part of contemporary ecosystem science. How Do We Classify or Compare Ecosystems? Thus, ecosystem scientists use structure, function, control, and temporal dynamics to classify and compare ecosystems. For instance, it is common to see ecosystems described as rich in nitrogen (structure), sinks for carbon (function), fire-dominated (control), or recently disturbed (dynamics). All of these attributes of ecosystems can provide useful frameworks to classify ecosystems, and ultimately to organize and interpret the vast amount of information that scientists have collected about ecosystems. Similar descriptions and classifications are evident throughout the book. WHY DO SCIENTISTS STUDY ECOSYSTEMS? Scientists have been motivated to study ecosystems for several reasons. To begin with, if ecosystems truly are the “basic units of nature” on Earth, any attempt to understand our planet and the products of evolution on it must include ecosystem science as a central I. INTRODUCTION 12 KATHLEEN C. WEATHERS ET AL. theme. Indeed much of ecosystem science has been motivated by simple curiosity about how our world and how systems—whether ecological, social, or socio-ecological—work. Many salable products such as timber and fish are taken directly from “wild” ecosystems, so many early ecosystem studies were done to try to better understand the processes that supported these products and ultimately increase their yields. Especially in the past 20 years, we have come to realize that the valuable products of nature include far more than obviously salable products like timber and fish. Wild ecosystems also provide us with clean air and water, opportunities for recreation and spiritual fulfillment, protection from diseases, and many more “ecosystem services” (e.g., Millennium Ecosystem Assessment 2005; Kareiva et al. 2011). Human economies and well-being are wholly embedded in and dependent on wild ecosystems. Thus, many contemporary ecosystem studies are con- cerned with how ecosystems provide this broad array of services, how human activities reduce or restore the ability of ecosystems to provide these services, and ultimately trying to reconcile the growing demands of human populations with the needs of both nature and ourselves for functioning ecosystems. HOW DO ECOSYSTEM SCIENTISTS LEARN ABOUT ECOSYSTEMS? Depending on the problem that they are studying, ecosystem scientists use a wide vari- ety of approaches and an array of simple to sophisticated tools to measure different aspects of ecosystem structure and function. We offer a few examples here; however, new approaches and tools emerge every year, and with them come more ways to open black boxes in ecosystem science (see Chapter 17). Approaches for Learning about Ecosystems There are multiple approaches by which scientists can understand ecosystem structure, function, and development, both qualitatively and quantitatively. Five approaches are especially important in ecosystem science, including (1) natural history or observation, (2) theory and conceptual models, (3) long-term study, (4) cross-ecosystem comparison, and (5) experiments (modified from the lists of Likens 1992; Carpenter 1998). These approaches are complementary to one another (Table 1.1), and are best used in combina- tion. Almost every scientific question of any size or importance requires the use of two or more of these approaches to get a satisfactory answer. Natural History A good deal can be learned about ecosystems simply from watching them and docu- menting what is observed in some fashion. Do fallen leaves decay in place, wash away into a stream, or burn in episodic fires? Is the soil deep and rich, or shallow and rocky? Does it freeze in the winter? As a result, our understanding of an ecosystem often is based on simple observations of its natural history. Indeed, without such careful observations, even the most sophisticated studies can go astray by formulating nonsensical questions or I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 13 omitting key observations or measurements. Not surprisingly, careful natural history stud- ies (such as Forbes’ “The Lake as a Microcosm,” discussed later) were important precur- sors to modern ecosystem science. Although these forerunners of ecosystem science often included speculation about ecosystem processes, they did not have the technical means to easily measure such functions as net ecosystem productivity or nutrient cycling, or to quantify trophic transfers. Long-Term Studies Long-term studies (i.e., those lasting for more than 3 to 5 years—the length of most grants or the time it takes to earn a PhD!) are relatively rare in ecology as a whole. However, long-term studies are especially good at providing insight into slow processes (e.g., changes associated with forest succession), subtle changes (e.g., changing chemistry of precipitation), rare events (e.g., effects of hurricanes or insect outbreaks), or processes controlled by multiple interacting factors (e.g., fish recruitment; Likens 1989; Lindenmayer and Likens 2010; and see the Long-Term Ecological Research Program (LTER) of the National Science Foundation, http://www.lternet.edu). Sometimes long-term understand- ing can be obtained by short-term analyses of materials that record history, such as soil or sediment cores, otoliths (fish ear-stones), or written historical records. For example, analy- sis of pollen, diatoms, pigments, geochemistry, and minerals in lake sediment can reveal the history of terrestrial vegetation, phytoplankton, soils, and lake level—in short, the his- tory of the development of the linkages between terrestrial and aquatic ecosystems. It is from long-term studies or their surrogates that scientists have documented climatic, atmo- spheric, geochemical, and organismal changes over decades to billions of years. Cross-Ecosystem Comparison Comparative studies have served two important roles in ecosystem science. Most sim- ply, scientists often have measured some variable associated with ecosystem structure or function across a series of ecosystems to identify typical values of that variable, show how it varies among types of ecosystems, and generate hypotheses about what factors might TABLE 1.1 Strengths and limitations of approaches to understanding ecosystems. Natural history observations and understanding underpin all of these approaches. Approach Some Strengths Some Limitations Theory Flexibility of scale; integration; deduction of Cannot develop without linkage to testable ideas observation, experiment Long-term Temporal context; detection of trends and surprises; Potentially site specific, difficult to observation test hypotheses about temporal variation determine cause Comparison Spatial or inter-ecosystem context; detection of spatial Difficult to predict temporal change or pattern; test hypotheses about spatial variation response to perturbation Ecosystem Measure ecosystem response to perturbation; test Potentially site specific; potentially difficult experiment hypothesis about controls and management of to rule out some explanations; hard to do ecosystem processes After Carpenter (1998). I. INTRODUCTION 14 KATHLEEN C. WEATHERS ET AL. control that variable. An example of such an analysis is shown in Table 2.1 in Chapter 2. Alternatively, scientists often test whether some factor controls an ecosystem by compar- ing ecosystems that differ in that factor and not (to the extent possible) in any other rele- vant characteristic (Cole et al. 1991). For instance, if we wanted to test whether phosphorus inputs control primary production in lakes, we might try to measure primary production in a series of 10 lakes of similar size, depth, and terrain that differ in their phosphorus inputs. In practice, it often is difficult to find such a perfect series of well- matched study sites. Experiments Experiments, whether conducted in the laboratory or in the field, are powerful ways to reveal controls on ecosystem structure and function (Likens 1985; Carpenter et al. 1995). There are no rules about the size of experimental units: manipulations have been made across hundreds of square kilometers (e.g., iron fertilization experiments conducted in the ocean) and within square centimeters. Often, the goal of experiments is to measure an ecosystem’s response to a change in a single variable while holding all others as constant as possible. For example, to understand whether phytoplankton in lakes were controlled by phosphorus or by other nutrients such as nitrogen and carbon, scientists in the Experimental Lakes area of Canada added phosphorus, nitrogen, and carbon to one-half of a lake (cut in two by a mas- sive curtain) and just nitrogen and carbon to the other half. They then compared responses— such as the amount of primary production—in each half of the lake to see what effects the treatments had (see Chapter 8). This whole-lake experiment helped to demonstrated that phosphorus was a major factor controlling algal productivity in lakes. Theory and Conceptual Models As in other sciences, ecosystem scientists routinely use theory and conceptual models. Such theories and models are highly varied in structure and purpose (Canham et al. 2003; Pickett et al. 2007). Models may be as simple as a statistical regression (see Chapter 11) or a box-and-arrow diagram drawn on a napkin, or as complex as a detailed simulation model (Figure 1.4). Models are highly flexible, can cover scales of time and space that are difficult to study using other approaches, and often can provide quick answers at low cost. They also are very useful as a way to organize facts and ideas; to generate, sharpen, or narrow hypotheses; and to guide research activities. Scientists often make rapid prog- ress by tightly coupling theory and models to other approaches. What Do Ecosystem Scientists Measure? Ecosystem scientists are inherently interested in the connections between structure and function of ecosystems and how they develop over time. Thus, many of the examples of measurements or values in this book are related to structure and function, such as biomass of a species, or rates of carbon cycling. They are what is often found on the x or y axes of graphs, or are used as treatments or are measured as responses in experiments. Ecosystem structure is sometimes measured by variables such as leaf area index or the number of trophic levels in a lake (see Chapter 11). Productivity (Chapters 2 and 3), rates of I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 15 decomposition (Chapter 4), or mineralization (Chapter 7) over time or space, or the accu- mulation of some element of interest can be indicators of ecosystem function. Ecosystem development is often described by changes in structure, function, and their relationship over time (e.g., linked changes in soil and vegetation over millennia; Ewing 2002). Many, if not most, of our measurements of ecosystem function are indirect. Sometimes we can measure function directly, such as measurement of gas exchange, but these mea- surements are almost inevitably made on a tiny fraction of the ecosystem (e.g., individual leaves within a grassland or bottle of water from a lake). To estimate a flux over a larger area of a grassland, for example, an ecosystem scientist might deploy eddy covariance instruments that measure carbon dioxide, water, temperature, and wind speed and direc- tion continuously at a place within the grassland. From these measures, a model can be used to infer carbon dioxide flux into or out of the ecosystem. Land management, natural disturbance Atmospheric Deposition Cations and Anions Atmospheric CO2 SUBLIMATION TRANSLOCATION Snowpack CO2 N2O LITTERFALL NOx ET INFILTRATION RUNOFF PHREEQC soil reactions UPTAKE SOM DECOMP. MINERALIZATION Cation Exchange Soil Organic Aqueous reactions C,N,P,S NO3– NITRIF./DENITRIF. Mineral denudation NH4+ CO2 CO2 dissolution DOC, Cations, Anions, CEC LEACHING ANC, pH, BC, Cl, Al, SO42– Aquifer PHREEQC stream reactions BASEFLOW Aqueous reactions Mineral denudation CO2 dissolution Stream Flow CO2 FIGURE 1.4 DayCent-Chem model processes. DayCent-Chem was developed to address ecosystem responses to combined atmospheric nitrogen and sulfur deposition. DayCent-Chem operates on a daily time step and com- putes atmospheric deposition, soil water fluxes, snowpack and stream dynamics, plant production and uptake, soil organic matter decomposition, mineralization, nitrification, and denitrification (left side of figure) while utiliz- ing PHREEQC’s (an aqueous geochemical equilibrium model) low-temperature aqueous geochemical equilibrium calculations, including CO2 dissolution, mineral denudation, and cation exchange, to compute soil water and stream chemistry (right side of figure). ET 5 evapotranspiration; DOC 5 dissolved organic carbon; CEC 5 cation exchange capacity; ANC 5 acid neutralizing capacity; BC 5 base cations (Ca, Mg, K, Na). The model requires considerable site- specific environmental data to run. (From Hartman et al. 2009, Figure 1.3.) I. INTRODUCTION 16 KATHLEEN C. WEATHERS ET AL. Scientists often choose indirect measures because they are easier to make across larger parts of a system or across more systems. As another example, the measurement of chloro- phyll-a is often used as an indicator of primary productivity in aquatic ecosystems. However, chlorophyll-a is not a direct measure of productivity, rather it is a measure of the presence of a pigment used in photosynthesis, and the photosynthetic process is the source of building biomass. Likewise, the carbon : nitrogen (C:N) ratio in soil is often used as an indicator of litter or soil quality, and is often used to predict decomposition rates, or rates of nitrogen cycling (see Chapters 4 and 5). To make these indirect measures useful, empirical relationships between direct and surrogate measures must be established– quantifying these relationships is an active area of research (see Chapter 17). Some Tools in the Ecosystem Scientist’s Toolbox Ecosystem scientists try to answer a diverse range of questions about a wide array of characteristics of the most varied kinds of ecosystems, using any of several scientific approaches. It will therefore come as no surprise that ecosystem scientists use an enormous number of specific scientific techniques in their investigations, some simple, some sophisti- cated, some developed within the discipline, and some borrowed and adapted from other disciplines. These techniques are far too numerous to list and discuss in an introductory textbook. Nonetheless, several tools are worth introducing here because they are character- istic of ecosystem science and will appear repeatedly in the coming chapters. Balances: Mass and Charge Mass balance (Box 1.2) is a major tool in ecosystem science, especially for ecosystems of which the boundaries are defined by their watersheds. The laws of thermodynamics tell us that matter and energy are not created or destroyed. When both inputs and outputs of energy or matter can be measured relatively completely and accurately it is possible to construct a mass balance and infer processes. For example, an unbalanced watershed mass balance suggests that either the element of interest is being retained in (inputs. outputs) or leaking from (outputs. inputs) the ecosystem (see Chapters 5 and 9). The watershed mass balance approach was pioneered in the 1960s by scientists at the Hubbard Brook Experimental Forest, New Hampshire (Bormann and Likens 1967), and has been used powerfully around the world to understand the abiotic and biotic movement of a suite of elements through ecosystems. The other powerful “balance” tool that ecosystem scientists use is charge balance. In water, the charges held by positive ions (cations) and negative ions (anions) must bal- ance each other. That is, for every anion (such as chloride) in an aqueous solution, there must be a corresponding cation (such as sodium). Why is this tool so useful? Charge bal- ance tells us, for example, that when an anion moves through a forest soil from ground water into a stream, it must be accompanied by a corresponding cation (see Chapter 5). The sum of all the negative charges brought by anions must be balanced by the same number of positive charges. Charge balance also makes it possible to check whether the major ions in a water sample have been measured correctly; a charge imbalance tells us that a measurement error has been made or that we have not quantified all the cations or anions that are important in a sample. I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 17 Tracers As useful as balances are as tools, they tell us about the bulk (or net) movements of materials through ecosystems, and rarely allow us to distinguish among different path- ways of material movement within ecosystems. All nitrogen atoms look alike to a mass balance. Tracers are tools that allow ecosystem scientists to distinguish among particular pathways of material movement by labeling just some of the atoms or molecules of inter- est. Ecosystem scientists have used several tracer methods, which have been enormously powerful in understanding how ecosystems work. Radioisotopes (Box 1.3) were some of the first tracers used in ecosystem science. Radioisotopes can be detected and quantified at very low concentrations, so they make excellent tracers, and have had many applications in ecosystem science. In the mid-twentieth century, ecosystem scientists added small amounts of radioisotopes to ecosystems to trace the movement of water and the uptake and movement of carbon and limiting nutrients through ecosystems. Radioisotopes are no longer added to ecosystems as tracers because of associated health risks, but they continue to be used widely in laboratory studies and measurements (e.g., to measure microbial production; see Chapter 3). They also are used in “natural abun- dance” studies where ecosystem scientists use the very low natural abundance of BOX 1.3 ECOLOGICAL TRACERS: ISOTOPES Most elements exist in several forms that isotopes have been used in specialized stud- contain different numbers of neutrons (but ies (see Figure 1.4). the same number of protons and electrons, The concentration of stable isotopes is and basically the same chemical properties). usually expressed in a “del” (δ) notation For example, about 99% of the carbon on that compares the abundance of the heavier Earth is 12C, which contains six protons, six isotope to that of the lighter isotope. electrons, and six neutrons, but about 1% of Thus, the abundance of 13C in a sample the carbon is 13C, which contains seven neu- is expressed as: trons. A tiny amount (B0.0000000001%) of the carbon is 14C, which has eight neutrons.
13 C  ! 12 C sample Some isotopes are stable, while others are δ CðmÞ 5
13 C  13 2 1 3 1000 radioactive (i.e., they spontaneously decay 12 C standard into other elements or isotopes). In the case of carbon, 12C and 13C are stable isotopes, The standard in this case is Vienna whereas 14C is a radioisotope that decays Pee Dee Belemnite (a particular kind of fos- into nitrogen (14N) with a half-life of 5730 sil). Negative δ values indicate that the years. Some isotopes that commonly make heavier isotope is less abundant in the sam- an appearance in ecosystem science include ple than in the standard, while positive δ the radioisotopes 3H (tritium), 14C, 32P, and values indicate that the heavier isotope is 35 S, and the stable isotopes 2H (deuterium), more abundant in the sample than in the 13 C, 15N, 18O, and 34S, although many other standard. I. INTRODUCTION 18 KATHLEEN C. WEATHERS ET AL. radioisotopes to trace the movement of materials through ecosystems, rather than adding radioisotopes to ecosystems. For example, Caraco and her colleagues (2010) observed that the concentration of 14C in organic matter washed into the Hudson River from the soils of its watershed was very different from that of organic matter produced by photosynthesis within the river. They could therefore use 14C to trace movement of terrestrial organic matter through the Hudson River food web, and show that modern zooplankton were being supported in part by carbon that was captured by primary production thousands of years ago (Figure 1.5). Stable isotopes have largely taken the place of radioisotopes as tracers outside the labo- ratory (Box 1.3). Although much more difficult to measure and often expensive to use, stable isotopes do not present a health risk to humans and wildlife. Stable isotopes are available for many elements of ecological interest, including hydrogen, nitrogen, carbon, oxygen, sulfur, and others. Stable isotopes often are added to ecosystems (or to laboratory experiments) and followed as they move through the system. For example, Templer and her colleagues (2005) added a stable isotope of nitrogen, 15N, to forest plots in the Catskill 100 Modern Modern FIAV FIAV terrestrial terrestrial 0 SAV SAV Phytoplankton Phytoplankton –100 δ14C (‰) –200 Zooplankton Zooplankton –300 Aged Aged terrestrial terrestrial –400 –35 –30 –25 –20 –250 –200 –150 –100 δ13C (‰) δ 2H (‰) FIGURE 1.5 Use of stable and radioisotopes to determine the source of organic matter supporting zooplank- ton in the Hudson River (Caraco et al. 2010). Isotope bi-plots show 14C vs. 13C (left side) and 14C vs. 2H (right side). Sources of carbon from modern primary production are shown near the tops of the graphs (FlAV 5 floating-leaved aquatic vegetation; SAV 5 submersed aquatic vegetation). If zooplankton were composed of carbon and hydrogen from these sources, then the data for isotopic composition of zooplankton should fall in the same region of the graph as the sources. Instead, zooplankton fall far outside this region of the graph, show- ing that they must be composed of organic matter from both modern and “aged” sources (i.e., organic matter thousands of years old from the soils of the Hudson River’s watershed). (From Caraco et al. 2010.) I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 19 Mountains, NY, and then followed it into soil, microbial biomass, understory plants, tree roots, wood, and leaves, and found that most of the nitrogen stayed in the soil. Alternatively, ecosystem scientists often use natural abundance studies of stable isotopes to follow the movement of materials through ecosystems. Substances other than isotopes can be used as tracers as well. For instance, certain fatty acids cannot be synthesized by animals and are made only by particular kinds of algae. By analyzing the fatty acid content of zooplankton and fish, we can trace the contribution of different kinds of algae throughout the food web. Caffeine, which is not readily degraded in conventional sewage treatment plants, is sometimes used as a tracer for sewage. The kinds of substances that can be used as tracers are highly varied, limited only by the inge- nuity and analytical capabilities of the investigator. Spatial Data Where are the regions of high and low productivity around the globe? How do they change over the seasons? These are questions that can now be answered largely as a result of the availability of remote sensing tools and spatially explicit data. The ability to collect, represent, and analyze spatially explicit data has risen exponentially over the past decade. Remote sensing and the georeferencing of basic data on landscape characteristics such as elevation, water bodies, land cover, and geological materials have opened the door to a description of ecosystem structure over large areas. Geographic information systems (GISs) allow analysis of the relationships between these structures and fluxes in or out of these systems. For example, the variation in atmospheric deposition across the mountain- ous terrain of Acadia National Park or Great Smoky Mountain National Park can be described by a GIS model that links empirical measurements to landscape features that are described in the GIS (Figure 1.6). Such spatially explicit models greatly enhance our ability to identify places on the landscape and times that may be subject to particularly high levels of atmospheric deposition (Weathers et al. 2006). GIS and other technologies are being used creatively and hold tremendous potential for understanding ecosystem pro- cesses across heterogeneous landscapes. Other newly emerging tools and techniques are described in Chapter 17. FROM THERE TO HERE: A SHORT HISTORY OF THE ECOSYSTEM CONCEPT IN THEORY AND PRACTICE Ecosystem science is a relatively young discipline, largely developed since the mid- twentieth century (Hagen 1992; Golley 1993; indeed, the term ecology was coined only in 1866). The concept of an ecosystem was first formally proposed by the English botanist Arthur Tansley in 1935, although related ideas were in circulation for at least a century before this. For instance, the idea of a biosphere (a region near the Earth’s surface in which living organisms are a dominant geochemical force) was outlined by the French scientist Jean-Baptiste Lamarck in 1802; the term biosphere was coined in 1875 by an Austrian geolo- gist, Eduard Suess, in describing the genesis of the Alps; and the concept of a biosphere was fully elaborated by the Russian mineralogist Vladimir Vernadsky in 1926. Other I. INTRODUCTION 20 KATHLEEN C. WEATHERS ET AL. FIGURE 1.6 Atmospheric deposition of nitrogen and sulfur for the year 2000 to Mount Desert Island study area of Acadia National Park, Maine (ACAD). Deposition estimates are based on a GIS-based empiri- cal model. (From Weathers et al. 2006.) Deposition in ACAD, 2000 (kg.ha–1.yr–1) Sulfur Nitrogen 5.5–9.2 3.0–5.0 N 9.2–12.9 5.0–7.0 12.9–16.6 7.0–9.0 16.6–20.3 9.0–11.0 0 1 2 km 20.3–24.9 11.0–13.5 important precursors to the modern idea of the ecosystem included Karl Möbius’ (1877) use of the term biocoenosis to refer to the biotic community associated with oyster beds; Stephen Forbes’ (1887) essay on “The Lake as a Microcosm,” which explored the myriad of ecological interactions that existed within a bounded area (a lake) to produce a single system; and K. Friedericks’ (1930) use of the idea of holocoens (Jax 1998). Although Vernadsky’s ideas perhaps were closest to modern ideas of the ecosystem, they were not widely influential outside of the former USSR, and none of the other early concepts really captured the idea that organisms and their abiotic environment could be integrated into a single system. In 1935, Tansley brought all of these ideas together by writing, “The fundamental con- cept appropriate to the biome [i.e., all living organisms] considered together with all the effective inorganic factors of its environment is the ecosystem.” He further stated: “It is the systems so formed which, from the point of view of the ecologist, are the basic units of nature on the face of the earth.” Tansley’s definition finally explicitly recognized the close interactivity (indeed the inseparability) of living and nonliving entities sharing the same physical space, and is remarkably similar to the definition of ecosystem that many ecolo- gists use 75 years later. Just a few years after Tansley’s paper appeared (1942), Raymond Lindeman, a young American ecologist, published a paper laying out a conceptual framework that defined trophic levels and allowed the analysis of energy flow through I. INTRODUCTION  1. INTRODUCTION TO ECOSYSTEM SCIENCE 21 ecosystems. Because modern ideas about material cycles had been around since the mid- nineteenth century (Gorham 1991), much of the essential conceptual foundation for ecosys- tem science and its two major branches, material cycling and energy flow, was thus in place by 1942. However, it would take a few more decades before ecosystem studies formed a large part of ecology. What remained was for the concept of ecosystems to be publicized and widely accepted by ecologists, and for scientists to find suitable tools for studying these newly defined “ecosystems.” Of course, many scientists and techniques made important contributions to advance and shape what is now ecosystem science, but a few key contributions are worth special mention. Readers who are interested in more information about the development of the ecosystem concept and its use may want to read the detailed histories published by Hagen (1992), Golley (1993), and Kingsland (2005). A key advance in the adoption of the ecosystem concept and approach by working ecol- ogists was the appearance of a popular textbook by Eugene Odum (1953 and subsequent editions through 2004). Odum’s textbook was organized around the ecosystem concept, and was enormously influential in introducing ecosystem science to generations of ecolo- gists. This text showed with enthusiasm and clarity the possibility and value of quantita- tive, large-scale studies, how the ecosystem approach could be applied to both aquatic and terrestrial habitats, and the application of this approach for understanding complicated interactions and linkages at large scales (Likens 1992, 2001). Odum and his brother Howard T. Odum also conducted pioneering field studies showing how the ecosystem concept could be insightfully applied in nature (e.g., Odum and Odum 1955; Odum 1957). Odum’s textbook was closely followed by a high-profile article in Science by Francis Evans (1956) that recommended the ecosystem as “the basic unit in ecology.” The first Big Science initiative in ecology, the International Biological Program of 1964
1974, was organized around systems ecology and exposed hundreds of ecologists around the world to measurements of productivity, nutrient cycling, and decomposition, and the development of ecosystem models, despite controversy and criticism about the program (Committee to Evaluate the IBP 1975; Mitchell et al. 1976; Aronova et al. 2010). Thus, by the late 1960s, the basic ideas of ecosystem science were familiar to most ecologists. Among all the tools that developed with the science, we highlight two important advances here. First, radioisotopes were widely used in the 1940s through 1960s to trace movement of materials within and between ecosystems (see the earlier section “Tracers”). In the wake of the development of atomic weapons, agencies such as the United States Atomic Energy Commission (AEC) and equivalent agencies in other coun- tries were looking for peaceful uses of radioactive materials. The timing of their interest coincided with the rise of ecosystem science, and led the AEC and similar agencies to provide radioisotopes and funding for many of the early studies on the movement of materials through ecosystems (Golley 1993). Second, and more significantly, ecosystem scientists began to conduct large-scale experiments on entire ecosystems. As the essentially reductionist approach of the IBP showed, it is very difficult to understand or predict the behavior of entire complex ecosys- tems from the bottom up by measuring all of their many pieces and trying to model how the whole system will behave. Instead, a direct experimental approach can be used to cut I. INTRODUCTION 22 KATHLEEN C. WEATHERS ET AL. through the Gordian knot of ecosystem complexity, and reliably show how the system actually reacts to some perturbation. It took a few decades for such whole-ecosystem experiments to become a common and accepted tool. Perhaps because of the pervasive influence of “The Lake as a Microcosm” and the clear boundaries to lakes, many of the earliest whole-ecosystem experiments were performed on lakes (Likens 1985; Carpenter et al. 1995). For instance, models and small-scale experiments were unable to resolve a bit- ter controversy in the 1960s about whether excessive phosphorus caused lakes to become offensively eutrophic, but a whole-lake experiment was conclusive (see Figure 8.1 in Chapter 8). Likewise, by adapting the small-watershed technique from hydrology in the 1960s, ecosystem scientists could quantify inputs and outputs of materials to and from ter- restrial ecosystems and treat entire watersheds as experimental subjects (Bormann and Likens 1967; Likens et al. 1970). Perhaps more than any other tool, whole-ecosystem experiments made Tansley’s concept a practical subject of scientific study. Ecosystem experiments are now an important tool for scientists to study subjects as varied as the effects of toxins, food-web structures, disturbances, and limiting nutrients in all types of ecosystems (Table 1.1). As a result of these advances, during the period from approximately 1935 to 1975, eco- system science moved from being just an interesting concept to a central position in con- temporary ecology. Ecosystem scientists, from the roots of the discipline to the present, have worked to unravel the complexity of entire ecosystems of all sizes and forms, from a water-filled cavity in a tree, to a small vernal pool, to a large lake, to a forested watershed, to an entire city, to the total biosphere. The ecosystem concept provides a comprehensive framework for study of the interactions among individuals, populations, and communities and their abiotic environments, and for study of the change in these relationships both temporally and spatially (adapted from Likens 1992). References Aronova, E., Baker, K.S., Oreskes, N., 2010. Big science and big data in biology: From the International Geophysical Year through the International Biological Program to the Long-Term Ecological Research (LTER) network, 1957
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16. Weathers, K.C., Simkin, S.M., Lovett, G.M., Lindberg, S.E., 2006. Empirical modeling of atmospheric deposition in mountainous landscapes. Ecol. Appl. 16, 1590
1607. Winter, T.C., Likens, G.E., 2009. Mirror Lake: Interactions among air, land, and water. University of California Press, Berkeley. I. INTRODUCTION  S E C T I O N II ECOLOGICAL ENERGETICS INTRODUCTION All organisms need materials such as carbon, nitrogen, and phosphorus to build molecules, cells, and other structures, and energy to build and maintain those structures against the relentless forces of entropy. Not surprisingly, the two main branches of ecosys- tem science deal with the movement and fate of materials and energy (biogeochemistry and ecosystem energetics, respectively). This section of the book (Chapters 2 4) introduces eco- system energetics (primary production, secondary production and consumer energetics, and decomposition), and Section III deals with biogeochemistry (Chapters 5 8). Studies of energy flow through individuals, populations, communities, and ecosystems form a large part of past and present-day ecosystem science. Historically, ecosystem scien- tists studied energy flow for several reasons. Many of the earliest studies (i.e., before 1950) were motivated by the idea that the allowable harvest from a wild population (e.g., a fish- ery) would be related to the amount of energy flowing into that population, so that studies of energy flow would help to estimate sustainable yield. Although historically important, this is no longer a primary motivation for ecological energetics (but see Libralato et al. 2008 for a modern example). More generally, ecologists recognize that energy is essential for all life; thus, studies of energy flow track the movement of a key resource. Because all organisms require energy, it provides a common currency that allows ecologists to make comparisons across all organisms and habitats. That is, it allows ecologists to compare the activities of such disparate organisms as plants, mice, moose, and microbes using the same single currency that is required by all of them. Some ecologists have gone further to regard energy as the key resource, making the case that energy can be substitutable with other resources (e.g., water, nutrients) so that deficiencies in any resource can be ameliorated if enough energy is available. In this world view, which was held by a minority of ecologists, energy is the ultimate limiting resource, so pathways of energy flow might reveal pathways of control in ecosystems. Finally, energy flow often is roughly proportional to other key activities (e.g., grazing, flows of elements), so it could be argued that energy flow is the most appropriate single measure of the importance of a population (if we must reduce a population to a single number), because it roughly summarizes the multiple activities that the population performs. 26 ECOLOGICAL ENERGETICS UNITS USED IN STUDIES OF ECOLOGICAL ENERGETICS If you’ve taken a physics class recently, you know that the proper units of energy content and flow are joules (kg-m2/s2) and watts (joules/s), respectively. It may seem confusing, then, that ecologists studying energy flow almost never express their results in terms of joules or watts. Rather, most ecologists implicitly equate energy with biomass, because biomass is the carrier of energy in organisms, and is easier to measure than energy content. Biomass thus implies energy content, and the production or destruction of biomass implies energy flow. Consequently, ecologists usually express energy in units of biomass (i.e., grams of live mass, dry mass, ash-free dry mass, or organic carbon). Other units sometimes used in ecological energetics are the mass of oxygen produced or con- sumed by photosynthesis or respiration, or calories (an obsolete unit of energy content). Table 1 shows conversions between units commonly used in ecological energetics. TABLE 1 Approximate conversion factors between energetic units used in ecological studies. Except for the conversion between joules and calories (which is exact), the conversion factors are approximate and can vary substantially among organisms and among tissues in an individual organism. Both the photosynthetic quotient and the respiratory quotient are assumed to equal 1. Units Converted From Units Converted To Joules Calories Carbon (g) Oxygen (g) Dry Mass Wet Mass Ash-Free Dry Mass Joules 1 0.239 2 3 1025 6 3 1025 5 3 1025 2.5 3 1024 4.3 3 1025 Calories 4.18 1 9 3 1025 2.5 3 1024 2 3 1024 1 3 1023 1.8 3 1024 Carbon (g) 4.5 3 104 1 3 104 1 2.7 2.2 11 1.9 Oxygen (g) 1.7 3 10 4 4 3 10 3 0.375 1 0.8 4 0.7 Dry mass 2 3 10 4 5 3 10 3 0.45 1.2 1 5 0.9 Wet mass 4 3 10 3 1 3 10 3 0.09 0.24 0.2 1 0.17 Ash-free dry mass 2.3 3 10 4 6 3 10 3 0.5 1.4 1.2 6 1 Modified from Cummins and Wuycheck (1971), Peters (1983), Benke (1993), Cattaneo and Mousseau (1995), and other sources. References Benke, A.C., 1993. Concepts and patterns of invertebrate production in running waters. Verh. Int. Ver. Theor. Angew. Limnol. 25, 15 38. Cattaneo, A., Mousseau, B., 1995. Empirical analysis of the removal rate of periphyton by grazers. Oecologia 103, 249 254. Cummins, K.W., Wuycheck, J.C., 1971. Caloric equivalents for investigations in ecological energetics. Mitt. Int. Ver. Theor. Angew. Limnol. 18, 1 158. Libralato, S., Coll, M., Tudela, S., Palomera, I., Pranovi, F., 2008. Novel index for quantification of ecosystem effects of fishing as removal of secondary production. Mar. Ecol. Prog. Ser. 355, 107 129. Peters, R.H., 1983. The ecological implications of body size. Cambridge University Press, New York.  C H A P T E R 2 Primary Production: The Foundation of Ecosystems Michael L. Pace1 and Gary M. Lovett2 1 University of Virginia, Charlottesville 2 Cary Institute of Ecosystem Studies, Millbrook, NY INTRODUCTION Primary production is the storage of energy through the formation of organic matter from inorganic carbon compounds. Primary production is carried out by autotrophic organisms. The term autotrophic is derived from the Greek words autos, meaning self, and trophikos, meaning pertaining to food. Autotrophs are “self-feeders.” Higher plants as well as some microbes (e.g., algae) are autotrophs. Plants and algae conduct the most familiar form of primary production—photosynthesis—where carbon dioxide is incorporated into organic matter using energy from sunlight. In most ecosystems primary production is car- ried out by a variety of species and the diversity of autotrophs influences primary produc- tion (e.g., Tilman et al. 2006). The accrual of organic matter by primary producers represents the first step in the capture, storage, and transfer of energy in most ecosystems. There are several reasons why ecologists consider primary production a fundamental ecosystem process. The ecosystem carbon cycle begins with the fixation of carbon (i.e., incorporation of CO2 into organic matter). Herbivores consume this organic carbon produced by autotrophs to support their growth and metabolism. Other components of the food web such as detritivores and predators also depend directly or indirectly on pri- mary production for their energy supply. Primary producers require nutrients such as nitrogen and phosphorus to build biomolecules such as proteins and nucleic acids. The uptake and cycling of nitrogen, phosphorus, and other elements accompanies primary production, and the ratio of elements that ultimately comprises primary producers influ- ences many ecological processes (Sterner and Elser 2002). The formation of organic matter Fundamentals of Ecosystem Science. 27 © 2013 Elsevier Inc. All rights reserved. 28 MICHAEL L. PACE AND GARY M. LOVETT by primary producers is also a key process of the global carbon cycle. Primary production and the short- and long-term fate of this fixed carbon influences atmospheric carbon diox- ide concentration. The study of primary production in terms of rates, controls, trophic interactions, biogeochemical cycles, and storage of the end-products of primary production is, therefore, central to ecosystem science. The results of primary production are often quite evident as, for example, the rapid growth of lawn grass during spring. In terrestrial ecosystems the accumulation of biomass by primary producers provides important structure. For example, in forests, tree growth leads to branch and root formation and the accumulation of wood. These structural ele- ments are critical components affecting many physical, chemical, and biological processes in a forest (Box 11.1). Analogous growth of marine kelp forests in the sea creates structure and habitat that support many types of organisms. Primary production may also be cryptic. Measurement of phytoplankton biomass day to day in the sea or in a lake would usually reveal little variation. It would seem that no biomass is being produced because there is no accumulation, but in this case, loss pro- cesses such as grazing by herbivores are as rapid as the increase in phytoplankton. Production might be high even though biomass of the phytoplankton does not change. In contrast, when growth rates are consistently in excess of loss rates, so-called “blooms” of phytoplankton result and can lead to massive, sometimes noxious, accumulations of algal scums. Rather than being cryptic, these scums caused by excess primary production are conspicuous and represent a serious environmental problem in many water bodies. COMPONENTS OF PRIMARY PRODUCTION Primary production is by definition a rate with units of mass per area (or volume, if measured in water) per time. For example, primary production data are often presented as grams carbon per square meter per day (g C m22 d21). The absolute amount of plant material produced in an ecosystem is sometimes referred to as production or yield (mass per unit area or volume) as, for example, the total mass of corn plants generated in a field. Time, however, is generally implicit in this use of production and yield. For example, the production of a corn field typically refers to a mass per unit area for a growing season. In this chapter the terms production and primary production will always refer explicitly to rates with the time attribute of the rate specified. Biomass is distinct from primary production. The biomass of primary producers is mass per area or volume independent of time. Biomass is often approximately correlated with primary production. However, it is possible as noted earlier to have low biomass but relatively high rates of primary production as often observed in the ocean. Alternatively, slow-growing plants may represent a substantial biomass but have relatively low rates of primary production. Primary production encompasses a number of processes that require definition and that pose problems for measurement. The components of primary production are clarified by following the flow and fates of carbon through a generalized ecosystem (Figure 2.1). Primary production begins with the fixation of CO2 into organic matter. Gross primary production (GPP) represents this first step accounting for all the carbon dioxide fixed into II. ECOLOGICAL ENERGETICS  2. PRIMARY PRODUCTION: THE FOUNDATION OF ECOSYSTEMS 29 Components of Productivity NEP CO2 GPP Oxidation (Fire or UV) Ra NPP Rh Consumers (Re = Ra+ Rh) Accumulation in biomass Detritus and exudates Decomposers Accumulation in Not sediments or soil decompo