xiv Contents The global nitrogen cycle 343 nic change 344 The Global Phosphorus Cycle in the Nitrogen Cycle 347 Anthropogenic Changes in the Phosphorus Cycle.... 347 The Global Sulfur Cycle 348 The Global Water Cycle 350 Anthropogenic Changes in the Water Cycle 。。。。 351 Consequences of Changes in the Water Cycle....... Summary 35 anaging and Sustaining Ecosystems Introduction 356 Ecosystem Concepts in Management 357 Natural variability 357 Resilience and Stability 357 State Factors and Interactive Controls 358 Application of Ecosystem Knowledge in Management. 359 Forest Management 359 Fisheries Management 359 Ecosystem Restoration 36 Management for Endangered Species 36( Integrative Approaches to Ecosystem Management.... 362 Ecosysten 362 Integrated Conservation and Development aion of Ecosystem Goods and Services 36 mmary Additional Reading 369 Abbreviations 371 Glossary..· 375 References .............................. 393 Index 。。 423
xiv Contents The Global Nitrogen Cycle . . . . . . . . . . . . . . . . . . . . . . . 343 Anthropogenic Changes in the Nitrogen Cycle . . . . . . 344 The Global Phosphorus Cycle . . . . . . . . . . . . . . . . . . . . . 347 Anthropogenic Changes in the Phosphorus Cycle . . . . 347 The Global Sulfur Cycle . . . . . . . . . . . . . . . . . . . . . . . . . 348 The Global Water Cycle . . . . . . . . . . . . . . . . . . . . . . . . . 350 Anthropogenic Changes in the Water Cycle . . . . . . . . 351 Consequences of Changes in the Water Cycle . . . . . . . 352 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354 Additional Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 355 Chapter 16 Managing and Sustaining Ecosystems Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 356 Ecosystem Concepts in Management . . . . . . . . . . . . . . . 357 Natural Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 Resilience and Stability . . . . . . . . . . . . . . . . . . . . . . . . 357 State Factors and Interactive Controls . . . . . . . . . . . . . 358 Application of Ecosystem Knowledge in Management . . 359 Forest Management . . . . . . . . . . . . . . . . . . . . . . . . . . . 359 Fisheries Management . . . . . . . . . . . . . . . . . . . . . . . . . 359 Ecosystem Restoration . . . . . . . . . . . . . . . . . . . . . . . . . 360 Management for Endangered Species . . . . . . . . . . . . . 360 Integrative Approaches to Ecosystem Management . . . . 362 Ecosystem Management . . . . . . . . . . . . . . . . . . . . . . . . 362 Integrated Conservation and Development Projects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365 Valuation of Ecosystem Goods and Services . . . . . . . . 366 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 Review Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 Additional Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 369 Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423
The Ecosystem Concept Ecosystem ecology studies the links between organisms and their physical environ- ment within an Earth System context.This chapter provides background on the con- ceptual framework and history of ecosystem ecology. Introduction The supply of fish from the sea is now declin ing because fisheries management depended on Ecosystem ecology addre sses the interactions species-based approaches that did not ade. the quately consider the resources on which com- egrated gar em.The mercial fish depend.A more holistic view of fundamental in managed systems can account for the complex hecause it addresses the interactions that link interactions that prevail in even the simplest biotic systems of which humans are an integra ecosystems.There is also an increasing appreci part.with the physical systems on which they tion that a th orough und ec depend.This applies at the scale of Earth as a whole.a continent.or a farmer's field.An pplies itio ecosystem approach is critical to resource man f the dee agement,as we grapple with the sustainable use of resources in an era of increasing human Overview of Ecosystem Ecology Our nbeedp The Un The flow of en materials through 1992 ergy ns and th pr rather than the ical Why do forests have larg trees but acc appreciation of the role that indivi ual thin layer of dead ea on9menuate2 soil surfac cies,or groups of species.play in the func whereas tundra supp orts small plants but an tioning of ecosystems and how these functions abundance of soil organic matter?Why does provide services that are vital to human the concentration of carbon dioxide in the welfare.An important,and belated,shift in atmosphere decrease in summer and increase thinking has occurred about managing ecosys- in winter?What happens to that portion of the tems on which we depend for food and fiber nitrogen that is added to farmers'fields but is 3
Introduction Ecosystem ecology addresses the interactions between organisms and their environment as an integrated system. The ecosystem approach is fundamental in managing Earth’s resources because it addresses the interactions that link biotic systems, of which humans are an integral part, with the physical systems on which they depend. This applies at the scale of Earth as a whole, a continent, or a farmer’s field. An ecosystem approach is critical to resource management, as we grapple with the sustainable use of resources in an era of increasing human population and consumption and large, rapid changes in the global environment. Our growing dependence on ecosystem concepts can be seen in many areas. The United Nations Convention on Biodiversity of 1992, for example, promoted an ecosystem approach, including humans, to conserving biodiversity rather than the more species-based approaches that predominated previously. There is a growing appreciation of the role that individual species, or groups of species, play in the functioning of ecosystems and how these functions provide services that are vital to human welfare. An important, and belated, shift in thinking has occurred about managing ecosystems on which we depend for food and fiber. The supply of fish from the sea is now declining because fisheries management depended on species-based approaches that did not adequately consider the resources on which commercial fish depend. A more holistic view of managed systems can account for the complex interactions that prevail in even the simplest ecosystems. There is also an increasing appreciation that a thorough understanding of ecosystems is critical to managing the quality and quantity of our water supplies and in regulating the composition of the atmosphere that determines Earth’s climate. Overview of Ecosystem Ecology The flow of energy and materials through organisms and the physical environment provides a framework for understanding the diversity of form and functioning of Earth’s physical and biological processes. Why do tropical forests have large trees but accumulate only a thin layer of dead leaves on the soil surface, whereas tundra supports small plants but an abundance of soil organic matter? Why does the concentration of carbon dioxide in the atmosphere decrease in summer and increase in winter? What happens to that portion of the nitrogen that is added to farmers’ fields but is 1 The Ecosystem Concept Ecosystem ecology studies the links between organisms and their physical environment within an Earth System context. This chapter provides background on the conceptual framework and history of ecosystem ecology. 3
4 1.The Ecosystem Concept not harvested with the crop?Why has the intro- tion of plants by herbivores.and the consumn duction of exotic species so strongly affected tion of herbivores by predators.Most of these the productivity and fire frequency of grass- fluxes are sensitive to environmental factors lands and forests?Why does the number of such as temperature and moisture,and to bio people on Earth correlate so strongly with the logical factors that regulate the population concentration of methane in the Antarctic dynamics and species interactions in communi- ice cap or with the quantity of nitrogen enter- ties.The unique contribution of ecosystem ing Earth's oceans?These are representative ecology is its focus on biotic and abiotic factors questions addressed by ecosystem ecology. as interacting components of a single integrated system. environments Spatial les an ecosys organ on d on the ques ns on the al that the m ems studied in the labor ory in small bottles.Othe rather than individual organisms or physical roductiv components Ecosystem analysis seeks to understand the patches of a lake,forest,or agricultural field factors that regulate the pools(quantities)and Still other questions are best addressed at the fluxes(flows)of materials and energy through global scale.The concentration of atmospheric ecological systems.These materials include CO2.for example,depends on global patterns carbon,water,nitrogen,rock-derived minerals of biotic exchanges of CO and the burning o such as phosphorus,and novel chemicals such fossil fuels,which are spatially variable acros the globe.The rapid mixing of CO:in the are atmosphere verages across this variability P0 ng uchand the w ere th as plants,ani atmosphere organis require careful measur nsists of all the ols with which the in which to study ses are the t of fo ts o and materials from on effec and ity another.Ene the water that s ies a enters an ecosystem when light eneroy drive watershed.or catch ent consists of a stream the reduction of carbon dioxide (co.)to form and all the terrestrial surfaces that drain into sugars during photosynthesis.Organic matter it.By studving a watershed we can compare the and energy are tightlv linked as thev move quantities of materials that enter from the through ecosystems.The energy is lost from air and rocks with the amounts that leave in the ecosystem when organic matter is oxidized stream water,just as you balance your check- back to CO:by combustion or by the respira- book.Studies of input-output budgets of water- tion of plants,animals,and microbe Materials sheds have improved our understanding of the move among abiotic components of the system interactions between rock weathering.which through variety inc ing the supples nutrents plant and microbia vap a s nu in ec syster nd the materials d Reiners 1975. Bormann and P n ude ants,th death of 01979 upp and lower boundaries of ar plan als the d also d end on the que matte by soil mic ump asked and the scale that is
4 1. The Ecosystem Concept not harvested with the crop? Why has the introduction of exotic species so strongly affected the productivity and fire frequency of grasslands and forests? Why does the number of people on Earth correlate so strongly with the concentration of methane in the Antarctic ice cap or with the quantity of nitrogen entering Earth’s oceans? These are representative questions addressed by ecosystem ecology. Answers to these questions require an understanding of the interactions between organisms and their physical environments—both the response of organisms to environment and the effects of organisms on their environment. Addressing these questions also requires that we think of integrated ecological systems rather than individual organisms or physical components. Ecosystem analysis seeks to understand the factors that regulate the pools (quantities) and fluxes (flows) of materials and energy through ecological systems. These materials include carbon, water, nitrogen, rock-derived minerals such as phosphorus, and novel chemicals such as pesticides or radionuclides that people have added to the environment. These materials are found in abiotic (nonbiological) pools such as soils, rocks, water, and the atmosphere and in biotic pools such as plants, animals, and soil microorganisms. An ecosystem consists of all the organisms and the abiotic pools with which they interact. Ecosystem processes are the transfers of energy and materials from one pool to another. Energy enters an ecosystem when light energy drives the reduction of carbon dioxide (CO2) to form sugars during photosynthesis. Organic matter and energy are tightly linked as they move through ecosystems. The energy is lost from the ecosystem when organic matter is oxidized back to CO2 by combustion or by the respiration of plants, animals, and microbes. Materials move among abiotic components of the system through a variety of processes, including the weathering of rocks, the evaporation of water, and the dissolution of materials in water. Fluxes involving biotic components include the absorption of minerals by plants, the death of plants and animals, the decomposition of dead organic matter by soil microbes, the consumption of plants by herbivores, and the consumption of herbivores by predators. Most of these fluxes are sensitive to environmental factors, such as temperature and moisture, and to biological factors that regulate the population dynamics and species interactions in communities. The unique contribution of ecosystem ecology is its focus on biotic and abiotic factors as interacting components of a single integrated system. Ecosystem processes can be studied at many spatial scales. How big is an ecosystem? The appropriate scale of study depends on the question being asked (Fig. 1.1). The impact of zooplankton on the algae that they eat might be studied in the laboratory in small bottles. Other questions such as the controls over productivity might be studied in relatively homogeneous patches of a lake, forest, or agricultural field. Still other questions are best addressed at the global scale. The concentration of atmospheric CO2, for example, depends on global patterns of biotic exchanges of CO2 and the burning of fossil fuels, which are spatially variable across the globe. The rapid mixing of CO2 in the atmosphere averages across this variability, facilitating estimates of long-term changes in the total global flux of carbon between Earth and the atmosphere. Some questions require careful measurements of lateral transfers of materials. A watershed is a logical unit in which to study the effects of forests on the quantity and quality of the water that supplies a town reservoir. A watershed, or catchment, consists of a stream and all the terrestrial surfaces that drain into it. By studying a watershed we can compare the quantities of materials that enter from the air and rocks with the amounts that leave in stream water, just as you balance your checkbook. Studies of input–output budgets of watersheds have improved our understanding of the interactions between rock weathering, which supplies nutrients, and plant and microbial growth, which retains nutrients in ecosystems (Vitousek and Reiners 1975, Bormann and Likens 1979). The upper and lower boundaries of an ecosystem also depend on the question being asked and the scale that is appropriate to the
Overview of Ecosystem Ecology 5 FIGURE 1.1.Examples a)Global ecosystem by i0 ord of magnitude an endolithic ecosystem in 5.000k How does carbon ss (e):a watershed,1x 10m in influence global climate? length(b);and Earth,4x 10'm (a b)Watershed 10 km do c)Forest ecosystem r does id rai productivity d)Endolithic ecosysten rock surface lichen zone What are the biological question.The atm questions that addres plant effects on water from the gases b and nutr cycling,th tem mig depth the ecohich b 01 th be m ethis height tion.Studie soil.which constitute the long r of regional impact of grasslands on the moisture many nutrients that gradually become incorpo- content of the atmosphere might,however,be rated into surface soils (see chanter 3) measured at a height of several kilometers Ecosystem dynamics are a product of many above the ground surface,where the moisture temporal seales.The rates of ecosystem pro- released by the ecosystem condenses and cesses are constantly changing due to fluctua- returns as precipitation (see Chapter 2).For tions in environment and activities of organisms
Overview of Ecosystem Ecology 5 question.The atmosphere, for example, extends from the gases between soil particles all the way to outer space. The exchange of CO2 between a forest and the atmosphere might be measured a few meters above the top of the canopy because, above this height, variations in CO2 content of the atmosphere are also strongly influenced by other upwind ecosystems. The regional impact of grasslands on the moisture content of the atmosphere might, however, be measured at a height of several kilometers above the ground surface, where the moisture released by the ecosystem condenses and returns as precipitation (see Chapter 2). For questions that address plant effects on water and nutrient cycling, the bottom of the ecosystem might be the maximum depth to which roots extend because soil water or nutrients below this depth are inaccessible to the vegetation. Studies of long-term soil development, in contrast, must also consider rocks deep in the soil, which constitute the long-term reservoir of many nutrients that gradually become incorporated into surface soils (see Chapter 3). Ecosystem dynamics are a product of many temporal scales. The rates of ecosystem processes are constantly changing due to fluctuations in environment and activities of organisms c) Forest ecosystem 1 km How does acid rain influence forest productivity? a) Global ecosystem 5,000 km How does carbon loss from plowed soils influence global climate? b) Watershed 10 km How does deforestation influence the water supply to neighboring towns? d) Endolithic ecosystem 1 mm rock surface What are the biological controls over rock algal zone weathering? lichen zone Figure 1.1. Examples of ecosystems that range in size by 10 orders of magnitude: an endolithic ecosystem in the surface layers of rocks, 1 ¥ 10-3m in height (d); a forest, 1 ¥ 103m in diameter (c); a watershed, 1 ¥ 105m in length (b); and Earth, 4 ¥ 107m in circumference (a). Also shown are examples of questions appropriate to each scale
6 1.The Ecosystem Concept on time scales ranging from microseconds to have a pervasive influence.The complications millions of years(see Chapter 13).Light capture associated with the current nonequilibrium during photosynthesis responds almost instan- view require a more dynamic and stochastic taneously to fluctuations in light availability view of controls over ecosystem processes. to a leaf At the opposite extreme,the evolution Ecosystems are considered to be at steady of photosynthesis 2 billion years ago added state if the balance between inputs and outputs oxygen to the atmosphere over millions of to the system shows no trend with time years,causing the prevailing geochemistry (Johnson 1971,Bormann and Likens 1979) of Earth's surface Steady state assumptions differ from equilib nge sumptions ecause they acc orga ral and spa in th the group Archaea Earth.These amic en eady state. tha organ nt growth c om sum 6).At a stand scale dry and the interi ors of soil or animal die from old age intestines Episodes of mo erosion stro ngly influence the availability of d by y rindividuals At a landscan scale,so atches may be altered by fire or minerals to support plant growth.Vegetation is other disturbances.and other patches will be still migrating in response to the retreat of Pleis- in various stages of recovery.These ecosystems tocene glaciers 10,000 to 20,000 years ago.After or landscapes are in steady state if there is disturbances such as fire or tree fall.there are no long-term directional trend in their pro gradual changes in plant,animal,and microbial perties or in the balance between inputs and communities over years to centuries.Rates of outputs. carbon input to an ecosystem through photo- Not all ecosystems and landscapes are in es of seconds to in light,temperature. .In fact.directional changes in at cause by humar e quite likely to caus udies in ecosystem ecology stem proper som often easier to und nd the rela vith th undis. nt i atio in which the h Once we the heh of we can elements,(2)self-regulation and deterministic add the complexities associated with time lags dynamics,(3)stable end points or cycles,and and rates of ecosystem change. (4)absence of disturbance and human influ- Ecosystem ecology uses concepts developed ence (Pickett et al.1994,Turner et al.2001). at finer levels of resolution to build an under One of the most important conceptual standing of the mechanisms that govern the advances in ecosystem ecology has been the entire Earth System.The biologically mediated increasing recognition of the importance ol movement of carb on and trogen througl events and orc depends on e functioning of ecosystems n non or plan anim organ inp oducts dyna of t ary int where dynan and hur Gould 1986). also
6 1. The Ecosystem Concept on time scales ranging from microseconds to millions of years (see Chapter 13). Light capture during photosynthesis responds almost instantaneously to fluctuations in light availability to a leaf.At the opposite extreme, the evolution of photosynthesis 2 billion years ago added oxygen to the atmosphere over millions of years, causing the prevailing geochemistry of Earth’s surface to change from chemical reduction to chemical oxidation (Schlesinger 1997). Microorganisms in the group Archaea evolved in the early reducing atmosphere of Earth. These microbes are still the only organisms that produce methane. They now function in anaerobic environments such as wetland soils and the interiors of soil aggregates or animal intestines. Episodes of mountain building and erosion strongly influence the availability of minerals to support plant growth. Vegetation is still migrating in response to the retreat of Pleistocene glaciers 10,000 to 20,000 years ago.After disturbances such as fire or tree fall, there are gradual changes in plant, animal, and microbial communities over years to centuries. Rates of carbon input to an ecosystem through photosynthesis change over time scales of seconds to decades due to variations in light, temperature, and leaf area. Many early studies in ecosystem ecology made the simplifying assumption that some ecosystems are in equilibrium with their environment. In this perspective, relatively undisturbed ecosystems were thought to have properties that reflected (1) largely closed systems dominated by internal recycling of elements, (2) self-regulation and deterministic dynamics, (3) stable end points or cycles, and (4) absence of disturbance and human influence (Pickett et al. 1994, Turner et al. 2001). One of the most important conceptual advances in ecosystem ecology has been the increasing recognition of the importance of past events and external forces in shaping the functioning of ecosystems. In this nonequilibrium perspective, we recognize that most ecosystems exhibit inputs and losses, their dynamics are influenced by both external and internal factors, they exhibit no single stable equilibrium, disturbance is a natural component of their dynamics, and human activities have a pervasive influence. The complications associated with the current nonequilibrium view require a more dynamic and stochastic view of controls over ecosystem processes. Ecosystems are considered to be at steady state if the balance between inputs and outputs to the system shows no trend with time (Johnson 1971, Bormann and Likens 1979). Steady state assumptions differ from equilibrium assumptions because they accept temporal and spatial variation as a normal aspect of ecosystem dynamics. Even at steady state, for example, plant growth changes from summer to winter and between wet and dry years (see Chapter 6). At a stand scale, some plants may die from old age or pathogen attack and be replaced by younger individuals.At a landscape scale, some patches may be altered by fire or other disturbances, and other patches will be in various stages of recovery. These ecosystems or landscapes are in steady state if there is no long-term directional trend in their properties or in the balance between inputs and outputs. Not all ecosystems and landscapes are in steady state. In fact, directional changes in climate and environment caused by human activities are quite likely to cause directional changes in ecosystem properties. Nonetheless, it is often easier to understand the relationship of ecosystem processes to the current environment in situations in which they are not also recovering from large recent perturbations. Once we understand the behavior of a system in the absence of recent disturbances, we can add the complexities associated with time lags and rates of ecosystem change. Ecosystem ecology uses concepts developed at finer levels of resolution to build an understanding of the mechanisms that govern the entire Earth System. The biologically mediated movement of carbon and nitrogen through ecosystems depends on the physiological properties of plants, animals, and soil microorganisms. The traits of these organisms are the products of their evolutionary histories and the competitive interactions that sort species into communities where they successfully grow, survive, and reproduce (Vrba and Gould 1986). Ecosystem fluxes also depend