Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes
Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes I Published by the Ecological Society of America Number 4, Fall 1999 ssues in Ecology
Issues in Ecology Number 4 al1999 Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes Shahid Naeem,Chair,F.S.Chapin lll,Robert Costanza,Paul R.Ehrlich,Frank B.Golley,David U.Hooper, J.H.Lawton.Robert V.O'Neill.Harold A.Mooney,Osvaldo E.Sala,Amy J.Symstad,and David Tilman affect human welfare.These ecosystem processes are controlled by both the diversity and identity of the plant,animal,and microbial species living within a community.Human modifications to the living community in an ecosystem-as well as to the collective biodiversity of the earth-can therefore alter ecological functions and life support services that are vital to the well-being of human societies. h网woT rred,especially local and global losses of biodiversity.The widesprea n of on ce highly dive Recent studies suest that biodiversity can alter both the magnitude and the stability of ecosystem processes,especially when biodiversity is reduced to the low levels typical of many managed systems. Our review of the available evidence has identified the following certainties concerning biodiversity and ecosystem functioning: Human impactson global biodiversity have been dramatic,resulting in unprecedented osses in global biodiversity at all levels.from genes and species to entire ecosystems: .Local declines in biodiversity are even more dramatic than global declines.and the beneficial effects of many organisms on local processes are lost long before the species become globally extinct: Many ecosystem pr ocesses are sensitive to declines in biodiversity dance of species in an ecosystem can be as important a From current research,we have identified the following impacts on ecosystem functioning that often result from loss of biodiversity: .Plant production may decline as regi ional and local dive sity declines ,such as drought,may be lessened as Ecosystem processes such as soil nitrogen levels,water use,plant productivity,and pest and disease cycles may become more variable as diversity declines. Given its importance to human welfare,the maintenance of ecosystem functioning should be included as n integra part of national and international policies designed toconserve local and global biodiversity. Cover photo credits clockwise from top left:Jack Dykinga USDA/ARS.Scott Bauer USDA/ARS,USDA,Kevin Fitzsimmons/University of Arizona Aquaculture,stock photo.Nadine Lymn. 2
Issues in Ecology Number 4 Fall 1999 2 by Shahid Naeem, Chair, F.S. Chapin III, Robert Costanza, Paul R. Ehrlich, Frank B. Golley, David U. Hooper, J.H. Lawton, Robert V. ONeill, Harold A. Mooney, Osvaldo E. Sala, Amy J. Symstad, and David Tilman Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes Critical processes at the ecosystem level influence plant productivity, soil fertility, water quality, atmospheric chemistry, and many other local and global environmental conditions that ultimately affect human welfare. These ecosystem processes are controlled by both the diversity and identity of the plant, animal, and microbial species living within a community. Human modifications to the living community in an ecosystem as well as to the collective biodiversity of the earth can therefore alter ecological functions and life support services that are vital to the well-being of human societies. Substantial changes have already occurred, especially local and global losses of biodiversity. The primary cause has been widespread human transformation of once highly diverse natural ecosystems into relatively species-poor managed ecosystems. Recent studies suggest that such reductions in biodiversity can alter both the magnitude and the stability of ecosystem processes, especially when biodiversity is reduced to the low levels typical of many managed systems. Our review of the available evidence has identified the following certainties concerning biodiversity and ecosystem functioning: • Human impacts on global biodiversity have been dramatic, resulting in unprecedented losses in global biodiversity at all levels, from genes and species to entire ecosystems; • Local declines in biodiversity are even more dramatic than global declines, and the beneficial effects of many organisms on local processes are lost long before the species become globally extinct; • Many ecosystem processes are sensitive to declines in biodiversity; • Changes in the identity and abundance of species in an ecosystem can be as important as changes in biodiversity in influencing ecosystem processes. From current research, we have identified the following impacts on ecosystem functioning that often result from loss of biodiversity: • Plant production may decline as regional and local diversity declines; • Ecosystem resistance to environmental perturbations, such as drought, may be lessened as biodiversity is reduced; • Ecosystem processes such as soil nitrogen levels, water use, plant productivity, and pest and disease cycles may become more variable as diversity declines. Given its importance to human welfare, the maintenance of ecosystem functioning should be included as an integral part of national and international policies designed to conserve local and global biodiversity. Cover photo credits clockwise from top left: Jack Dykinga USDA/ARS, Scott Bauer USDA/ARS, USDA, Kevin Fitzsimmons/University of Arizona Aquaculture , stock photo, Nadine Lymn.
Issues in Ecology Number 4 a1999 Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes SUMMARY INTRODUCTION scales.biodiversity declines are already pronounced in many areas.especially where natural ecosystems have One of the most striking features of the earth's biota been converted to croplands.timber plantations. is its extraordinary diversity,estimated to include about aquaculture and other managed ecosyste ms 10 million different species.One of the ost conspicuous ersity of these managed ecosy stems is often low.an species composition very diff erent.comp red with those of this diversity in many ecosystems (Figure 1).The of the natural systems they have replaced(Figure 3). decline is not limited to increased rates of species What are the consequences of such declines in extinction,but includes losses in genetic and functional biodiversity and how might they affect human welfare? diversity across population.community.ecosystem. The earth's living organisms contribute to human welfare landscape,and global scales (Figure 2).The term refers collectively to all the du ssential to life. ng food biotic diversity. The and industrial or cro breeding.and natural pest control service increased rates of invasions by deliberately or accidentally can be viewed as the market values of biodiversity because introduced non-native species,over-exploitation and other they are are readily tied to our economy and often can human-caused imnacts be assigned a dollar value in the marketplace.Second Ona global scale,even at the lowest estimated current biodiversity has nonmarket values that can be expressed ho0aeoanoap could be ext in te s such as knov aesthetic exis nd othe Such an would be e alues of biodive ersity are istory of life on earth.On caand rei to quantify,but are,for many,sufficient justification for preserving biodiversity independent of market values. Figure I-The predicted decline of biodiversity in association with increases in human populations.Estimates for global biodiversity loss are between 50 and 75%by 2100,but in many transformed habitats,such as crop farms,local declines of similar magnitude have already occurred.(From Soule 1991 Science.)
1 Issues in Ecology Number 4 Fall 1999 Biodiversity and Ecosystem Functioning: Maintaining Natural Life Support Processes SUMMARY INTRODUCTION One of the most striking features of the earths biota is its extraordinary diversity, estimated to include about 10 million different species. One of the most conspicuous aspects of contemporary global change is the rapid decline of this diversity in many ecosystems (Figure 1). The decline is not limited to increased rates of species extinction, but includes losses in genetic and functional diversity across population, community, ecosystem, landscape, and global scales (Figure 2). The term biodiversity refers collectively to all these aspects of biotic diversity. The wide-ranging decline in biodiversity results largely from habitat modifications and destruction, increased rates of invasions by deliberately or accidentally introduced non-native species, over-exploitation and other human-caused impacts. On a global scale, even at the lowest estimated current extinction rate, about half of all species could be extinct within 100 years. Such an event would be similar in magnitude to the five mass extinction events in the 3.5 billion year history of life on earth. On local and regional scales, biodiversity declines are already pronounced in many areas, especially where natural ecosystems have been converted to croplands, timber plantations, aquaculture and other managed ecosystems. The diversity of these managed ecosystems is often low, and species composition very different, compared with those of the natural systems they have replaced (Figure 3). What are the consequences of such declines in biodiversity and how might they affect human welfare? The earths living organisms contribute to human welfare in a variety of ways. First, humans derive from them goods and products essential to life, including food, medicine, and industrial products, genetic resources for crop breeding, and natural pest control services. Such benefits can be viewed as the market values of biodiversity because they are are readily tied to our economy and often can be assigned a dollar value in the marketplace. Second, biodiversity has nonmarket values that can be expressed in terms such as knowledge, aesthetic, existence and other values. These non-market values of biodiversity are difficult to quantify, but are, for many, sufficient justification for preserving biodiversity independent of market values. Figure 1 - The predicted decline of biodiversity in association with increases in human populations. Estimates for global biodiversity loss are between 50 and 75% by 2100, but in many transformed habitats, such as crop farms, local declines of similar magnitude have already occurred. (From Soulé 1991 Science.)
Issues in Ecology Number 4 a1999 Figure 2-Biodiversity loss and the earth's changing biomes.The top map shows the biomes.such as graslands in orange and forests in blue,prior to the introduction of agri- culture.The bottom map shows the spread of agronomic and other man- aged ecosystems where red represents (After Sisk et al A third category of value.ecosystem services,is the to shed considerable light on the issue.These studies have focus of this report.The organisms that live.grow. shown that ecosystems are indeed sensitive to changes in the numbers and kinds of species found in their communities. 4).Energy flow this report.we provide an overview of ccosistn ctioning.review gren plantr algal photosynthesis and its dispersal as chemical energy throughout the food web to plant-or and evaluate the current status of research concerning algal-feeding animals,predators,and eventually ecosystem responses to changes in biodiversity. decomposers.The flow of materials involves the recycling of carbon.nitrogen.phosphorus and other elements ECOSYSTEM FUNCTIONING betw. and the and soil nute to mn eic flows contri benefit human welfare such as greenhouse gas regulation. these activities-feeding,growing,moving,excreting water treatment.erosion control.soil quality control.and waste,etc.-have on the physical and chemical conditions plant growth.Ecosystem services can also include cultural of their environment.(Note that "functioning"means benefits.such as religious,aesthetic,recreational, 0 "showing activity"and does not imply that organisms inspirational values that humans derive fro perform p purposeful roles in ecosystem-ve esses. Determin ng whether biodiversity pe r se is i A functio cosy hatewhibitsbiolbgica to ecosystem functioning ha een difficult,partly because 】chemic& s chara cteristic for its many of the factors such as habitat conversion that reduce local biodiversity also directly affect many ecological of plant production,carbon storage,and nutrient cycling processes.masking the more subtle impacts of species that are characteristic of most forests.If the forest is loss on functioning.Recent studies,however,have begun converted to an agroecosystem.its functioning changes
Issues in Ecology Number 4 Fall 1999 4 A third category of value, ecosystem services, is the focus of this report. The organisms that live, grow, reproduce, and interact within ecosystems help to mediate local and regional flows of energy and materials (Figure 4). Energy flow refers to the capture of light energy by green plant or algal photosynthesis and its dispersal as chemical energy throughout the food web to plant- or algal-feeding animals, predators, and eventually decomposers. The flow of materials involves the recycling of carbon, nitrogen, phosphorus and other elements between living organisms and the air, water, and soil. These biologically mediated energy and materials flows contribute to many ecological or life support services that benefit human welfare such as greenhouse gas regulation, water treatment, erosion control, soil quality control, and plant growth. Ecosystem services can also include cultural benefits, such as religious, aesthetic, recreational, or inspirational values that humans derive from ecosystems. Determining whether biodiversity per se is important to ecosystem functioning has been difficult, partly because many of the factors such as habitat conversion that reduce local biodiversity also directly affect many ecological processes, masking the more subtle impacts of species loss on functioning. Recent studies, however, have begun to shed considerable light on the issue. These studies have shown that ecosystems are indeed sensitive to changes in the numbers and kinds of species found in their communities. In this report, we provide an overview of ecosystem functioning, review the distinction between taxonomic biodiversity (i.e., species numbers) and functional biodiversity, and evaluate the current status of research concerning ecosystem responses to changes in biodiversity. ECOSYSTEM FUNCTIONING Ecosystem functioning reflects the collective life activities of plants, animals, and microbes and the effects these activities feeding, growing, moving, excreting waste, etc. have on the physical and chemical conditions of their environment. (Note that functioning means showing activity and does not imply that organisms perform purposeful roles in ecosystem-level processes.) A functioning ecosystem is one that exhibits biological and chemical activities characteristic for its type. A functioning forest ecosystem, for example, exhibits rates of plant production, carbon storage, and nutrient cycling that are characteristic of most forests. If the forest is converted to an agroecosystem, its functioning changes. Figure 2 - Biodiversity loss and the earths changing biomes. The top map shows the earths major biomes, such as graslands in orange and forests in blue, prior to the introduction of agriculture. The bottom map shows the spread of agronomic and other managed ecosystems where red represents a region that is greater than 50% managed ecosystems. (After Sisk et al. 1994, BioScience)
Issues in Ecology Number 4 Fall 1999 Ecologists abstract the essential features of an BIODIVERSITY:SPECIES.FUNCTIONAL ecosystem into two compartments.the biotic and the TYPES.AND COMPOSITION abiotic.The biotic compartment consists of the commu nity of species, which can be divided Although every organism contributes to ecosystem functionally into plant producers.the consumers that processes,the nature and magnitude of individua feed on producers and on each other.and the contributions vary considerably.Research in biodiversity decomposers (Figure 5).The abiotic compartment places much emphasis on the uniqueness of individual consists of organic and inorganic nutrient pools.Energy species and their singular contributions to ecosystem and materials move between these two co mpartment services.Yet most ecosystem processes are driven by as well as into and out of the system Ecosystem f many species andi processes are quant ified by measuring rates of these ive con movements (e.g.,plant production,decomposition. Species nutrient leaching or other measures of material within groups such as grazing mammals.large predators production.transport or loss).Ecosystem functioning. perennial grasses,or nitrogen-fixing microbes may in turn,is quantified by measuring the magnitudes and therefore be functionally similar despite their uniqueness dynamics of ecosystem processes. esults fr of species that peric similar roles in an within rent leve ecosystem process are known as functional types or ecologists describe as a"nested"hierarchy.For example functional groups.Species may also be divided into functional green plant production on land is the end product of types based on what they consume or by trophic status (e.g. interactions of individual plants nested within populations: their place in the food web as producers.decomposers interactions among populations nested within a single predators).Within trophic groups.species may be further species:interactions among a variety of species nes ed divided according to life history climatic or nutrient needs within a of functionally simila ies:and s aits Rese ons between different typeso a species into several different functional categorie ecosystems nested within landscapes. depending on the ecosystem process they are studying. Figure 3-In the tropics,highly biologically diverse rainforests are replaced by monocultures of bananas
3 Issues in Ecology Number 4 Fall 1999 Ecologists abstract the essential features of an ecosystem into two compartments, the biotic and the abiotic. The biotic compartment consists of the community of species, which can be divided functionally into plant producers, the consumers that feed on producers and on each other, and the decomposers (Figure 5). The abiotic compartment consists of organic and inorganic nutrient pools. Energy and materials move between these two compartments, as well as into and out of the system. Ecosystem processes are quantified by measuring rates of these movements (e.g., plant production, decomposition, nutrient leaching or other measures of material production, transport or loss). Ecosystem functioning, in turn, is quantified by measuring the magnitudes and dynamics of ecosystem processes. Ecosystem functioning results from interactions among and within different levels of the biota, which ecologists describe as a nested hierarchy. For example, green plant production on land is the end product of interactions of individual plants nested within populations; interactions among populations nested within a single species; interactions among a variety of species nested within a group of functionally similar species; and so on up to the level of interactions between different types of ecosystems nested within landscapes. BIODIVERSITY: SPECIES, FUNCTIONAL TYPES, AND COMPOSITION Although every organism contributes to ecosystem processes, the nature and magnitude of individual contributions vary considerably. Research in biodiversity places much emphasis on the uniqueness of individual species and their singular contributions to ecosystem services. Yet most ecosystem processes are driven by the combined biological activities of many species, and it is often not possible to determine the relative contributions of individual species to ecosystem processes. Species within groups such as grazing mammals, large predators, perennial grasses, or nitrogen-fixing microbes may therefore be functionally similar despite their uniqueness in genes, life history, and other traits. Groups of species that perform similar roles in an ecosystem process are known as functional types or functional groups. Species may also be divided into functional types based on what they consume or by trophic status (e.g., their place in the food web as producers, decomposers, predators). Within trophic groups, species may be further divided according to life history, climatic or nutrient needs, physiology or other biological traits. Researchers may place a species into several different functional categories depending on the ecosystem process they are studying. Figure 3 - In the tropics, highly biologically diverse rainforests are replaced by monocultures of bananas.