12 1.The Ecosystem Concept tionary history in shaping ecosystem processes unlike resources,are neither consumed no (Mooney and Dunn 1970). depleted by organisms(Field et al.1992).Mod- Ecosystem processes both respond to and ulators include temperature.pH,redox state of control the factors that directly govern their the soil,pollutants,UV radiation,etc.Modula activity.For example,plants both respond to tors like temperature are constrained by and influence their light.temperature.and climate (a state factor)but are sensitive to moisture environment (Billings 1952).Interae- ecosystem processes,such as shading and evap tive controls are factors that both control and oration.Soil pH likewise depends on parent olled by ecosystem characteristics(Fig. material and time but also responds to vegeta 96).1 the mportant interactive 0 ape-s disturbance by fire wind nd organ- ,and hu es is a cr s that dence the rate ical 1985.Sou 1985. (Pick and Whi Re ources are the ener y and materials in the trols,disturbance regime dep ends on both stat environment that are u and cco support their growth an nance (Field for example,directly affects fire probability et al 1992)The acor isition of resources by and spread but also influences the types and organisms depletes their abundance in the quantity of plants present in an ecosystem environment In terrestrial ecosystems these and therefore the fuel load and flammability resources are spatially separated,being avail of vegetation.Deposition and erosion during able primarily either aboveground (light and floods shape river channels and influenc CO2)or belowground (water r and nutrients). the probability of future floods Change in Resource supply is governed by state factors either the intensity or frequency of disturbance can cause long-term ecosystem ch ge.Woody pro for exampl n invade grassland ecosys availabili fo on clima ents su 0n, es of b s present relative their int vegetation.Similar y.soil fertility de nds on inf ent material and cli ate hut als 0 diff s in sses such as erosional loss of naterial cie soils after overgrazing and inputs of nitro effects can often h from invading nitrogen-fixin species Soil functional types,which are g water availability strongly influences species that are similar in their role in community or composition in dry climates.Soil water avail- ecosystem processes.Most evergreen trees.for ability also depends on other interactive example,produce leaves that have low rates of controls,such as disturbance regime (e.g.com- photosvnthesis and a chemical composition paction by animals)and the types of organisms that deters herbivores.These species s make up a that are present(e.g..the presence or absence of functional type because of their ecological sim- deep-rooted tree such as mesquite that tap the anot wate table ate unctional types xample,t rough intro the activity of organisms 5D0 mportan ang Ox arly gh change of dif th Modulato nd chemical p British min fo ogen-fixi e.substantially erties that affect the ncreases nitrogen supply and
12 1. The Ecosystem Concept tionary history in shaping ecosystem processes (Mooney and Dunn 1970). Ecosystem processes both respond to and control the factors that directly govern their activity. For example, plants both respond to and influence their light, temperature, and moisture environment (Billings 1952). Interactive controls are factors that both control and are controlled by ecosystem characteristics (Fig. 1.3) (Chapin et al. 1996). Important interactive controls include the supply of resources to support the growth and maintenance of organisms, modulators that influence the rates of ecosystem processes, disturbance regime, the biotic community, and human activities. Resources are the energy and materials in the environment that are used by organisms to support their growth and maintenance (Field et al. 1992). The acquisition of resources by organisms depletes their abundance in the environment. In terrestrial ecosystems these resources are spatially separated, being available primarily either aboveground (light and CO2) or belowground (water and nutrients). Resource supply is governed by state factors such as climate, parent material, and topography. It is also sensitive to processes occurring within the ecosystem. Light availability, for example, depends on climatic elements such as cloudiness and on topographic position, but is also sensitive to the quantity of shading by vegetation. Similarly, soil fertility depends on parent material and climate but is also sensitive to ecosystem processes such as erosional loss of soils after overgrazing and inputs of nitrogen from invading nitrogen-fixing species. Soil water availability strongly influences species composition in dry climates. Soil water availability also depends on other interactive controls, such as disturbance regime (e.g., compaction by animals) and the types of organisms that are present (e.g., the presence or absence of deep-rooted trees such as mesquite that tap the water table). In aquatic ecosystems, water seldom directly limits the activity of organisms, but light and nutrients are just as important as on land. Oxygen is a particularly critical resource in aquatic ecosystems because of its slow rate of diffusion through water. Modulators are physical and chemical properties that affect the activity of organisms but, unlike resources, are neither consumed nor depleted by organisms (Field et al. 1992). Modulators include temperature, pH, redox state of the soil, pollutants, UV radiation, etc. Modulators like temperature are constrained by climate (a state factor) but are sensitive to ecosystem processes, such as shading and evaporation. Soil pH likewise depends on parent material and time but also responds to vegetation composition. Landscape-scale disturbance by fire, wind, floods, insect outbreaks, and hurricanes is a critical determinant of the natural structure and process rates in ecosystems (Pickett and White 1985, Sousa 1985). Like other interactive controls, disturbance regime depends on both state factors and ecosystem processes. Climate, for example, directly affects fire probability and spread but also influences the types and quantity of plants present in an ecosystem and therefore the fuel load and flammability of vegetation. Deposition and erosion during floods shape river channels and influence the probability of future floods. Change in either the intensity or frequency of disturbance can cause long-term ecosystem change. Woody plants, for example, often invade grasslands when fire suppression reduces fire frequency. The nature of the biotic community (i.e., the types of species present, their relative abundances, and the nature of their interactions) can influence ecosystem processes just as strongly as do large differences in climate or parent material (see Chapter 12). These species effects can often be generalized at the level of functional types, which are groups of species that are similar in their role in community or ecosystem processes. Most evergreen trees, for example, produce leaves that have low rates of photosynthesis and a chemical composition that deters herbivores. These species make up a functional type because of their ecological similarity to one another. A gain or loss of key functional types—for example, through introduction or removal of species with important ecosystem effects—can permanently change the character of an ecosystem through changes in resource supply or disturbance regime. Introduction of nitrogen-fixing trees onto British mine wastes, for example, substantially increases nitrogen supply and productivity
Human-Caused Changes in Earth's Ecosystems 13 and alters patterns of vegetation development vents an uncontrolled growth of a predator's (Brac 1983).Invasion by exotic grasses population,thereby stabilizing the population e rce supply, pr r and prey.Ther phic inte and cy.re in e (D'A positiv ave a p h n the ano utbreak of c eer tha e.pro ide the for nutrients This exehange of wth-limiting controls (see Chapter 12).so functional tyr resources hetw cen plants and fungi promotes respond to and affect most interactive controls the growth of both components of the sym and ecosystem pr biosis until they become constrained by other Human activities have an increasing impact factors on virtually all the processes that govern ecosys- Negative feedbacks are the key to sustaining tem properties(Vitousek 1994a).Our actions ecosystems because strong negative feedbacks infuence interactive controls such as water provide resistance to changes in interactive availability, disturbance regime, and biotic controls and maintain the characteristics of diversity.Humans have been a natural compo- ecosystems in their current state.Ihe acquisi the tion of water,nutrients,and light to suppor ev ever. e plant,for example,re al n roductivity uman (Fig. 1.).Similarly anim ulativ nd an nitely and ual ecosystem and affect state such as edation ce the rate climate,through changes in atmospheric com. lation inc these negative position.and potential biota.through the intro are weak or absent (a low p edation rate du duction and extinction of species The larg magnitude of these effects blurs the distinction cycles can amplify and lead to extinction of one between"independent"state factors and inter- or both of the interacting species.Community active controls at regional and global scales dynamics.which operate within a single eco Human activities are causing major changes in system patch,primarily involve feedbacks the structure and functioning of all ecosystems among soil resources and functional types of resulting in novel conditions that lead to new organisms.Landscape dynamics,which govern types of ecosystem major human effects changes in e cycles th imple phyo mic mate and disturbance system the al dynami regime(see Chapter 14) eco e phy n when a ho cold.The Human-Caused Changes in the Earth's Ecosystems furnace off.Natural ecosystems are c comple networks of interacting feedbacks (DeAnge Human activities transform the land surfac and Post 1991)Negative feedbacks occur when add or remove species,and alter biogeochemi two components of a system have opposite cal cyeles.Some human activities directly affect effects on one another.Consumption of prey by ecosystems through activities such as resource a predator,for example,has a positive effect on harvest,land use change,and management: the consumer but a negative effect on the prey. other effects are indirect,as a result of changes The negative effect of predators on prey pre in atmospheric chemistry,hydrology,and
Human-Caused Changes in Earth’s Ecosystems 13 and alters patterns of vegetation development (Bradshaw 1983). Invasion by exotic grasses can alter fire frequency, resource supply, trophic interactions, and rates of most ecosystem processes (D’Antonio and Vitousek 1992). Elimination of predators by hunting can cause an outbreak of deer that overbrowse their food supply. The types of species present in an ecosystem depend strongly on other interactive controls (see Chapter 12), so functional types respond to and affect most interactive controls and ecosystem processes. Human activities have an increasing impact on virtually all the processes that govern ecosystem properties (Vitousek 1994a). Our actions influence interactive controls such as water availability, disturbance regime, and biotic diversity. Humans have been a natural component of many ecosystems for thousands of years. Since the Industrial Revolution, however, the magnitude of human impact has been so great and so distinct from that of other organisms that the modern effects of human activities warrant particular attention. The cumulative impact of human activities extend well beyond an individual ecosystem and affect state factors such as climate, through changes in atmospheric composition, and potential biota, through the introduction and extinction of species. The large magnitude of these effects blurs the distinction between “independent” state factors and interactive controls at regional and global scales. Human activities are causing major changes in the structure and functioning of all ecosystems, resulting in novel conditions that lead to new types of ecosystems. The major human effects are summarized in the next section. Feedbacks analogous to those in simple physical systems regulate the internal dynamics of ecosystems. A thermostat is an example of a simple physical feedback. It causes a furnace to switch on when a house gets cold. The house then warms until the thermostat switches the furnace off. Natural ecosystems are complex networks of interacting feedbacks (DeAngelis and Post 1991). Negative feedbacks occur when two components of a system have opposite effects on one another. Consumption of prey by a predator, for example, has a positive effect on the consumer but a negative effect on the prey. The negative effect of predators on prey prevents an uncontrolled growth of a predator’s population, thereby stabilizing the population sizes of both predator and prey. There are also positive feedbacks in ecosystems in which both components of a system have a positive effect on the other, or both have a negative effect on one another. Plants, for example, provide their mycorrhizal fungi with carbohydrates in return for nutrients. This exchange of growth-limiting resources between plants and fungi promotes the growth of both components of the symbiosis until they become constrained by other factors. Negative feedbacks are the key to sustaining ecosystems because strong negative feedbacks provide resistance to changes in interactive controls and maintain the characteristics of ecosystems in their current state. The acquisition of water, nutrients, and light to support growth of one plant, for example, reduces availability of these resources to other plants, thereby constraining community productivity (Fig. 1.4). Similarly, animal populations cannot sustain exponential population growth indefi- nitely, because declining food supply and increasing predation reduce the rate of population increase. If these negative feedbacks are weak or absent (a low predation rate due to predator control, for example), population cycles can amplify and lead to extinction of one or both of the interacting species. Community dynamics, which operate within a single ecosystem patch, primarily involve feedbacks among soil resources and functional types of organisms. Landscape dynamics, which govern changes in ecosystems through cycles of disturbance and recovery, involve additional feedbacks with microclimate and disturbance regime (see Chapter 14). Human-Caused Changes in Earth’s Ecosystems Human activities transform the land surface, add or remove species, and alter biogeochemical cycles. Some human activities directly affect ecosystems through activities such as resource harvest, land use change, and management; other effects are indirect, as a result of changes in atmospheric chemistry, hydrology, and
1.The Ecosystem Concept half of the world's accessible runoff (see Process Nature of Chapter 15).and humans use about 8%of the primary production of the oceans(Pauly and Christensen 1995).Commercial fishing reduces Predato A+B + the size and abundance of target species and a w u alters the population characteristics of species Predatio that are incidentally caught in the nshery.In the mid-1990s.about 22% E Population growth of marine at then Mycorrhizal n(Vitous et a 7c.A1 ut 6 fungus nan es withi + 0 margin d by 8 ant是 richment of r example,has incr eased al A -B eated anaerobic c elv to tr Shared resources port of nutri. ents derived from agricultural fertilizers and from human and livestock sewage cks in ecosysten Land use change and the resulting loss of habitat.is the primary driving force causing when the reciprocal effects of each organism (or specie s extinctions and loss of biological diver resource)have the same sign(both positive or both sity (Sala et al.2000a)(see Chapter 12).The time lag between ecosystem change and species are negat tendenc vstems to change.whereas posi be n where rate s of tive feedbacks tend to push ecosystems toward anew nave 。 buoand world in at al 1006) ca/ the ed i climate (Fig.1.5)(Vitousek et al.1997c).At of ational transpor Nonind sp least some of thes anth enic Gie hu count for 20%or of the ant sD cie caused)effects influence all ecosystems on in many continental are eas and 50%or more of Farth the plant species on many islands (Vitousek The most direct and substantial human alter- et al 1997c).International commerce breaks ation of ecosvstems is through the transforma- down biogeographic barriers,through both tion of land for production of food.fiber.and purposeful trade in live organisms and inad- other goods used by people.About 50%of vertent introductions. Purposeful introduc Earth's ice-free land surface has been directly tions deliberately select species that are likely altered by h 990 to grow and reproduce er 10 to ne envi ronme Many biologic nvasions of the land are bec difficul 0 Even mor stry and grazing bitively remove invas specie use ge impa rs alte the and tivities have also altered fresh y lea ding to furthe r lo ss of spec ne ns.We
14 1. The Ecosystem Concept climate (Fig. 1.5) (Vitousek et al. 1997c). At least some of these anthropogenic (i.e., humancaused) effects influence all ecosystems on Earth. The most direct and substantial human alteration of ecosystems is through the transformation of land for production of food, fiber, and other goods used by people. About 50% of Earth’s ice-free land surface has been directly altered by human activities (Kates et al. 1990). Agricultural fields and urban areas cover 10 to 15%, and pastures cover 6 to 8% of the land. Even more land is used for forestry and grazing systems. All except the most extreme environments of Earth experience some form of direct human impact. Human activities have also altered freshwater and marine ecosystems. We use about half of the world’s accessible runoff (see Chapter 15), and humans use about 8% of the primary production of the oceans (Pauly and Christensen 1995). Commercial fishing reduces the size and abundance of target species and alters the population characteristics of species that are incidentally caught in the fishery. In the mid-1990s, about 22% of marine fisheries were overexploited or already depleted, and an additional 44% were at their limit of exploitation (Vitousek et al. 1997c). About 60% of the human population resides within 100 km of a coast, so the coastal margins of oceans are strongly influenced by many human activities. Nutrient enrichment of many coastal waters, for example, has increased algal production and created anaerobic conditions that kill fish and other animals, due largely to transport of nutrients derived from agricultural fertilizers and from human and livestock sewage. Land use change, and the resulting loss of habitat, is the primary driving force causing species extinctions and loss of biological diversity (Sala et al. 2000a) (see Chapter 12). The time lag between ecosystem change and species loss makes it likely that species will continue to be driven to extinction even where rates of land use change have stabilized. Transport of species around the world is homogenizing Earth’s biota. The frequency of biological invasions is increasing, due to the globalization of the economy and increased international transport of products. Nonindigenous species now account for 20% or more of the plant species in many continental areas and 50% or more of the plant species on many islands (Vitousek et al. 1997c). International commerce breaks down biogeographic barriers, through both purposeful trade in live organisms and inadvertent introductions. Purposeful introductions deliberately select species that are likely to grow and reproduce effectively in their new environment. Many biological invasions are irreversible because it is difficult or prohibitively expensive to remove invasive species. Some species invasions degrade human health or cause large economic losses. Others alter the structure and functioning of ecosystems, leading to further loss of species diversity. - + Predator Herbivore Plant A Plant B Shared resources Mycorrhizal fungus A B D C E F + + + + + - Resource uptake Competition Mutualism Herbivory Predation Population growth Process Nature of feedback A A+B C D E F - - + - - + + - - Figure 1.4. Examples of linked positive and negative feedbacks in ecosystems. The effect of each organism (or resource) on other organisms can be positive (+) or negative (-). Feedbacks are positive when the reciprocal effects of each organism (or resource) have the same sign (both positive or both negative). Feedbacks are negative when reciprocal effects differ in sign. Negative feedbacks resist the tendencies for ecosystems to change, whereas positive feedbacks tend to push ecosystems toward a new state. (Modified with permission from American Naturalist, Vol. 148 © 1996 University of Chicago Press, Chapin et al. 1996.)
Human-Caused Changes in Earth's Ecosystems 15 FIGURE 1.5.Direct and indi th' Human population s (Red Size Resource use American Human enterprises ndustry ational commer Land Biotic additions transformation and losses Global biochemistry Syn YYY Climate change Loss of biological diversity Human activities have influenced biogeo- less anthropogenic gases have had drastic chemical cycles in many ways.Use of fossil fuels effects on the atmosphere and ecosystems. and the expansion and intensification of agri- Chlorofluorocarbons (CFCs).for example culture have altered the cycles of carbon,nitro- were first produced in the 1950s as refrigerants. gen,phosphorus,sulfur,and water on a global propellants,and solvents.They were heralded scale (see Chapter 15).These hanges in bio- for their nonreactivity in the lower atmosphere geochemic the ecosys In the upper atmosphere however where there they occ on,CFCs reac unmanaged ecosyste through changes ulting o struction,w of nut other mater primarily ove crea 14) a apte and int the South Po and irrigation.have increased the concen has expanded to lower latitudes in the South tions of atmospheric gases that influence ern Hemispher and now also occurs at higl climate (see chanter 2)I and transformations northern latitudes.As a result of the Montreal also cause runoff and erosion of sediments and Protocol the production of many CECs has nutrients that lead to substantial changes in ceased.Due to their low reactivity,however, lakes.rivers.and coastal oceans. their concentrations in the atmosphere are only Human activities introduce novel chemicals now beginning to decline,so their ecological into the environment.Some apparently harm- effects will persist for decades.Persistent novel
Human-Caused Changes in Earth’s Ecosystems 15 Human activities have influenced biogeochemical cycles in many ways. Use of fossil fuels and the expansion and intensification of agriculture have altered the cycles of carbon, nitrogen, phosphorus, sulfur, and water on a global scale (see Chapter 15). These changes in biogeochemical cycles not only alter the ecosystems in which they occur but also influence unmanaged ecosystems through changes in lateral fluxes of nutrients and other materials through the atmosphere and surface waters (see Chapter 14). Land use changes, including deforestation and intensive use of fertilizers and irrigation, have increased the concentrations of atmospheric gases that influence climate (see Chapter 2). Land transformations also cause runoff and erosion of sediments and nutrients that lead to substantial changes in lakes, rivers, and coastal oceans. Human activities introduce novel chemicals into the environment. Some apparently harmless anthropogenic gases have had drastic effects on the atmosphere and ecosystems. Chlorofluorocarbons (CFCs), for example, were first produced in the 1950s as refrigerants, propellants, and solvents. They were heralded for their nonreactivity in the lower atmosphere. In the upper atmosphere, however, where there is greater UV radiation, CFCs react with ozone. The resulting ozone destruction, which occurs primarily over the poles, creates a hole in the protective blanket of ozone that shields Earth’s surface from UV radiation. This ozone hole was initially observed near the South Pole. It has expanded to lower latitudes in the Southern Hemisphere and now also occurs at high northern latitudes. As a result of the Montreal Protocol, the production of many CFCs has ceased. Due to their low reactivity, however, their concentrations in the atmosphere are only now beginning to decline, so their ecological effects will persist for decades. Persistent novel Human population Size Resource use Human enterprises Agriculture Industry Recreation International commerce Land transformation Land clearing Intensification Forestry Grazing Biotic additions and losses Invasion Hunting Fishing Global biochemistry Water Carbon Nitrogen Other elements Synthetic chemicals Radionuclides Climate change Enhanced greenhouse effect Aerosols Land cover Loss of biological diversity Extinction of species and populations Loss of ecosystems Figure 1.5. Direct and indirect effects of human activities on Earth’s ecosystems. (Redrawn with permission from Science, Vol. 277 © 1997 American Association for the Advancement of Science; Vitousek et al. 1997c.)
1.The Ecosystem Concept chemicals such as cecs often have long-lasting centrate cesium and strontium.as do people ecological effects than cannot be predicted at who feed on reindeer for this reason the inpu the time they are first produced and which of radioisotopes into the atmosphere or water extend far beyond their region and duration of from nuclear power plants,submarines.and use. weapons has had impacts that extend far Other synthetic organic chemicals include beyond the regions where they were used. DDT (an insecticide)and polychlorinated The growing scale and extent of human activ- biphenyls(PCBs;industrial compounds).which ities suggest that all ecosystems are being influ were use extensively in the developed world enced,directly or indirectly,by in the 96 re thei ec o Many of conse ecosyst ns quences were w videly recogn ence by humar activi e P unds cor ue to be t dev an uma ies are lead ange n rted to all ec vste pe of th Ma of these compounds are fat soluble, so thev gen deposit osion.di s)dis accumulate in ome increa urhan me(land u se chan e.fire control) ingly concentrated as they move through food and fun chains (see Chapter 11).When these introductions and extinctions).Many of these pounds reach critical concentrations,they can global changes interact with each other at cause reproductive failure.This occurs most regional and local scales.Therefore,all eco frequently in higher trophic levels and in systems are experiencing directional changes animals that feed on fat-rich species. in ecosystem controls,creating novel condi processes,such as eggshell formation in birds tions and,in many cases,positive feedbacks are particularly sensitive to pesticide accumu- hat lead to new types of ecosystems.These populatio in interactive control will inev e the perigrine declines in predatory en note ems and far removed from the locations of may unpre ble losses of ed pes hich nitie eric testing of atomic weapons in depenc the foll g chapters w s of ma any ecosystem processes d Explosions and leaks in enerate electricity ntinue to he re nal or global sources of radioactivity.The explosion Summarv of a power-generating plant in 1986 at Chernobyl in Ukraine,for example,released Ecosystem ecology addresses the interactions substantial radioactivity that directly affected among organisms and their environment as an human health in the region and increased the integrated system through study of the factors atmospheric deposition of radioactive mate- that regulate the pools and fuxes of materials rials over eastern Europe and Scandinavia. and energy through ecological systems.The Some isotopes of atoms,such as spatial scale at which we study ecosystems is strontium chen o chosen to fa of impor calcium)and cesium (which nd out of the ecosys lar to po m)are a depen % the a acqu organisn athe d the t and nd s d t ogy is hi nts.The Sy. yierdiscipli udy of Reindeer which feed ary a
16 1. The Ecosystem Concept chemicals, such as CFCs, often have long-lasting ecological effects than cannot be predicted at the time they are first produced and which extend far beyond their region and duration of use. Other synthetic organic chemicals include DDT (an insecticide) and polychlorinated biphenyls (PCBs; industrial compounds), which were used extensively in the developed world in the 1960s before their ecological consequences were widely recognized. Many of these compounds continue to be used in some developing nations. They are mobile and degrade slowly, causing them to persist and to be transported to all ecosystems of the globe. Many of these compounds are fat soluble, so they accumulate in organisms and become increasingly concentrated as they move through food chains (see Chapter 11). When these compounds reach critical concentrations, they can cause reproductive failure. This occurs most frequently in higher trophic levels and in animals that feed on fat-rich species. Some processes, such as eggshell formation in birds, are particularly sensitive to pesticide accumulations, and population declines in predatory birds like the perigrine falcon have been noted in regions far removed from the locations of pesticide use. Atmospheric testing of atomic weapons in the 1950s and 1960s increased the concentrations of radioactive forms of many elements. Explosions and leaks in nuclear reactors used to generate electricity continue to be regional or global sources of radioactivity.The explosion of a power-generating plant in 1986 at Chernobyl in Ukraine, for example, released substantial radioactivity that directly affected human health in the region and increased the atmospheric deposition of radioactive materials over eastern Europe and Scandinavia. Some radioactive isotopes of atoms, such as strontium (which is chemically similar to calcium) and cesium (which is chemically similar to potassium) are actively accumulated and retained by organisms. Lichens, for example, acquire their minerals primarily from the atmosphere rather than from the soil and actively accumulate cesium and strontium. Reindeer, which feed on lichens, further concentrate cesium and strontium, as do people who feed on reindeer. For this reason, the input of radioisotopes into the atmosphere or water from nuclear power plants, submarines, and weapons has had impacts that extend far beyond the regions where they were used. The growing scale and extent of human activities suggest that all ecosystems are being influenced, directly or indirectly, by our activities. No ecosystem functions in isolation, and all are influenced by human activities that take place in adjacent communities and around the world. Human activities are leading to global changes in most major ecosystem controls: climate (global warming), soil and water resources (nitrogen deposition, erosion, diversions), disturbance regime (land use change, fire control), and functional types of organisms (species introductions and extinctions). Many of these global changes interact with each other at regional and local scales. Therefore, all ecosystems are experiencing directional changes in ecosystem controls, creating novel conditions and, in many cases, positive feedbacks that lead to new types of ecosystems. These changes in interactive controls will inevitably change the properties of ecosystems and may lead to unpredictable losses of ecosystem functions on which human communities depend. In the following chapters we point out many of the ecosystem processes that have been affected. Summary Ecosystem ecology addresses the interactions among organisms and their environment as an integrated system through study of the factors that regulate the pools and fluxes of materials and energy through ecological systems. The spatial scale at which we study ecosystems is chosen to facilitate the measurement of important fluxes into, within, and out of the ecosystem. The functioning of ecosystems depends not only on their current structure and environment but also on past events and disturbances and the rate at which ecosystems respond to past events. The study of ecosystem ecology is highly interdisciplinary and builds on