Agriculture,Ecosystems and Environment104(2004)
113–134
Biodiversity and ecosystem rvices in agricultural
landscapes—are we asking the right questions?
M.J.Swift a,A.-M.N.Izac b,M.van Noordwijk b,∗
a Tropical Soil Biology and Fertility Institute of CIAT,Nairobi,Kenya
b International Centre for Rearch in Agroforestry(ICRAF),SE Asia Regional Program,P.O.Box61,Bogor16001,Indonesia
Abstract
The assumed relationship between biodiversity or local richness and the persistence of‘ecosystem rvices’(that can sustain productivity on-site as well as hrough regulation of waterflow and storage)in agricultural landscapes has generated considerable interest and a range of experimental approaches.The abstraction level aimed for,however,may be too high to yield meaningful results.Many of the experiments on which evidence in favour or otherwi are bad are
artificial and do not support the bold generalisations to other spatial and temporal scales that are often made.Future investigations should utili co-evolved communities,be structured to investigate the distinct roles of clearly defined functional groups,parate the effects of between-and within-group diversity and be conducted over a range of stress and disturbance situations.An integral part of agricultural intensification at the plot level is the deliberate reduction of diversity.This does not necessarily result in impairment of ecosystem rvices of direct relevance to the land ur unless the hypothesid diversity–function threshold is breached by elimination of a key functional group or species.Key functions may also be substituted with petro-chemical energy in order to achieve perceived efficiencies in the production of specific goods.This can result in the maintenance of ecosystem rvices of importance to agricultural production at levels of biodiversity below the assumed‘functional threshold’. However,it can also result in impairment of other rvices and under some conditions the de-linking of the diversity–function relationship.Avoidance of the effects or attempts to restore non-esntial ecosystem rvices are only likely to be made by land urs at the plot scale if direct economic benefit can be thereby achieved.At the plot and farm scales biodiversity is unlikely to be maintained for purpos other than tho of direct u or‘utilitarian’benefits and often at levels lower than tho necessary for maintenance of many ecosystem rvices.The exceptions may be traditional systems where intrinsic values (social customs)continue to provide rea
sons for diversity maintenance.High levels of biodiversity in managed landscapes are more likely to be maintained for reasons of intrinsic,rependic(‘option’or‘bequest’)values or utilitarian(‘direct u’)than for functional or ecosystem rvice values.The major opportunity for both maintaining ecosystem rvices and biodiversity outside conrvation areas lies in promoting diversity of land-u at the landscape and farm rather thanfield scale.This requires,however,an economic and policy climate that favours diversification in land us and diversity among land urs.©2004Published by Elvier B.V.
Keywords:Agricultural landscapes;Biodiversity;Ecosystem rvices;Functional groups;Resilience
∗Corresponding author.Tel.:+62-251-315-234; fax:+62-251-315-567.1.Introduction
The role of biological diversity in the provision of ecosystem goods and rvices and the way this
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0167-8809/$–e front matter©2004Published by Elvier B.V. doi:10.1016/j.agee.2004.01.013
114M.J.Swift et al./Agriculture,Ecosystems and Environment104(2004)113–134西组词有哪些
role can be valued and managed during agricultural intensification is much debated but still poorly un-derstood.A key problem in all debates on biologi-cal diversity is that the abstraction‘diversity’has of-ten not been distinguished from the specific attributes of the community of organisms that is under study in any particular location or system.Likewi,eval-uations of diversity have more often than not been asssments of the value of biological resources as such rather than asssments of the value of diver-sity per (Nunes and van der Bergh,2001).For in-stance,if the interest lies in the functional roles of the community the may depend on the‘structure’of the vegetation and the relationships between dif-ferent‘functional groups’,rather than on diversity as such(Woodward,1993).Experiments bad on ran-dom species asmblages may be appropriate tests for hypothes about‘diversity’per ,but tell us very little about the largely lf-lected asmblages that make up natural ecosystems.In the ca of agroe-cosystems,whilst the dominant crops or livestock are human choices,by far the majority of the species(as soon as one takes the below-ground part of the sys-tem into consideration)are lf-lected.So,are we asking the right question about the relations between biodiversity and ecosystem rvices?Does the loss of diversity at plot-to-global scales imply a threat to crit-ical ecosystem functions?Can we identify thresholds in such a process?
Global diversity derives from the lack of overlap in species,genetic or agroecosystem composition be
tween geographic or temporal domains.While ‘agricultural development’directly affects plot level)diversity,it probably has even stronger effects by homogenizing at higher scales,facilitating the movement of‘invasive species’and the introduc-tion and spread of‘superior’germplasm of desirable species.Scale is thus of overriding importance in our analysis and we may wellfind that answers may ap-pear contradictory between different ways of defining temporal and spatial boundaries to the system under consideration.In this review we willfirst consider the concepts of‘biodiversity’and‘ecosystem functions’, and then the evidence that links relevant aspects of the two,before we embark on an exploration of how this relationship depends on scale and can be ‘managed’.2.The biological basis of ecosystem goods and rvices
Humans have evolved as part of the world’s ecosys-tems,depending on them for food and other products and for a range of functions that support our exis-tence.Natural ecosystems,as well as tho modified by humans,provide many rvices and goods that are esntial for humankind(Matson et al.,1997).Efforts and interventions to manipulate(agro)ecosystems in order to meet specific production functions reprent costs to the rest of the ecosystem in terms of energy, matter and biological diversity,and often negatively affect goods and rvices that so far were considered to be free and abundant.The are anthropocentrically regarded as rvices becau the
y provide the biophys-ical necessities for human life or otherwi contribute to human welfare(UNEP,1995;Costanza et al., 1997).Most if not all of the rvices are bad on a‘lateralflow’,or movement across the landscape of biomass(such as food,fibre and medicinal prod-ucts derived from the a,inland waters or lands outside of the domesticated‘agricultural’domain), living organisms and their genes,or earth(nutrients), water,fire or air elements.Examples of ecosystem rvices particularly important for agroecosystems and agricultural landscapes are:maintenance of the genetic diversity esntial for successful crop and animal breeding;nutrient cycles;biological control of pests and dias;erosion control and di-ment retention;and water regulation.At a global scale other rvices become important such as the regulation of the gaous composition of the atmo-sphere and thence of the climate.A list of such rvices is given in thefirst column of Table1and Appendix A,and their connection to lateralflows is discusd by Van Noordwijk et al.(this volume, Table1).
The ecosystem goods and rvices are biologi-cally generated.The community of living organisms within any given ecosystem carries out a very di-ver range of biochemical and biophysical process that can also affect neighbouring systems.The can be described at scales ranging from the subcellular through the whole organism and species populations to the aggregative effect of the at the level of the ecosystem(Schulze and Mooney,1993).All ecosys-
M.J.Swift et al./Agriculture,Ecosystems and Environment104(2004)113–134115
tems have permeable boundaries with respect to ma-terial exchanges but the within-systemflows usually dominate tho between systems,such as between land-u or land-cover types within a landscape.For purpo of this paper we define ecosystem functions as the minimum aggregated t of process(includ-ing biochemical,biophysical and biological ones)that ensure the biological productivity,organisational in-tegrity and perpetuation of the ecosystem.There are no agreed criteria for defining a minimum t of such functions but for the purpos of this paper the cond column of Table1lists ecosystem functions alongside the ecosystem rvices they provide.Further explana-tion of the relationships is given below but it is u-ful to note that the functions can be pictured as hav-ing a hierarchical relationship.The energy captured in primary production is utilid in the herbivore and decompor food chains.Interactions between the three subsystems occur through nutrient exchanges and a variety of biotic regulatory mechanisms as well as by energyflow.In particular,the balance between the constituent process of primary production and tho of decomposition determines the amount of en-ergy and carbon maintained within the system and is the major natural regulator of the gaous com-position of the atmosphere at a global scale(Swift, 1999).
3.Biological diversity and its values自律学习
Most discussions and empirical studies on biodiver-sity have focud on issues of a relatively small range of organisms.In contrast,the Convention on Biologi-cal Diversity defines its area of concern as:“...the variability among living organisms from all sources,including,inter alia,terrestrial,marine and other aquatic ecosystems and the ecological com-plexes of which they are part;this includes diversity within species,between species,and of ecosystems”(Heywood and Bates,1995).Diversity within each one of the three fundamental and hierarchically related levels of biological organisation can be fur-ther elaborated as follows:genetic diversity is the variation within and between species populations; species diversity refers to species richness,that is, the number of species in a site,habitat,ecological zone or at global scale;ecosystem diversity means the diversity of asmblages(and their environments) over a defined landscape,ecological zone or at global scale.
Biodiversity in this paper refers to the totality of the species(including the genetic variation reprented in the species populations)across the full range of ter-restrial invertebrate animals,protists, bacteria and fungi,above-and below-ground,as well as the vertebrates and plants which often constitute the main concerns of biodiversity conrvation.With a definition as broad and inclusive as this,it is highly unlikely that any clear and preci statements about relationships betwee
二维码怎么扫描n‘biodiversity’and functions can be formulated and tested that can be helpful in guiding human activity.Similar to the situation with‘water-shed functions’,which are considered in the next part of this volume,we mayfind that discussions on com-ponents of the overall biodiversity concept in relation to land-u are more productive and open to progress than tho that stay at the aggregate level.In the c-tion immediately following we shall refer to the diver-sity within ecosystems(often termed alpha diversity) and in later ctions to that at the broader scale of the landscape(which embraces concepts of both beta and gamma diversity).
The analysis of biodiversity and its management are highly influenced by the perspective ud.In particu-lar,different ctors of society attribute different val-ues to biodiversity.Since biological diversity concerns different levels,from genes to species and ecosystems, the value of diversity can likewi be defined in a num-ber of different ways.Broadly speaking,four different types of value can be ufully recognid,although different terminology is often ud by different au-thors(e Nunes and van der Bergh,2001for further details).
First is the intrinsic(sometimes called‘non-u’) value of diversity to humans,or the value that biodi-versity has on its own.This value compris cultural, social,aesthetic,and ethical benefits.Some groups in society attribute high social and religious values to in-dividual species or communities of or
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ganisms;others derive value from the simple fact of high diversity per in such systems as tropical rainforests or coral reefs. Second is the utilitarian(also called direct u,con-tributory,primary or infrastructure)value of compo-nents of biodiversity.The are the subsistence and commercial benefits of species or their genes derived
116M.J.Swift et al./Agriculture,Ecosystems and Environment104(2004)113–134
by one or other ctors in society.The utilitarian value may be private and accrue to the land managers(farm-ers,local community,government).This is most ob-vious with respect to high value agricultural crops but also applies to the other types of good listed in Table1. Utilitarian value may also accrue to other ctors in society,in addition to private land managers.For in-stance,the pharmaceutical industry values the tropi-cal forest tree Prunus africana very highly becau its bark contains chemicals ud for manufacturing a drug.Another example is that in Africa,many farm-ers living near natural(and protected)forests with-draw substantial monetary benefits from their hunting and from collecting plants and tree products in the forests(Pottinger and Burley,1992).Utilitarian value thus refers to the u of organisms that are part of the local diversity as inputs into consumption and produc-tion process.
Thirdly,biodiversity can be said to have rependic (‘option’,or bequest)value.This is the belief in future but yet unknown value of biodiversity to future gener-ations,for example,the prence of a microorganism with an as-yet undiscovered genetic potential for in-dustrial products.The three types of value of biodi-versity are ethnocentric and depend very much upon the cultural values and preferences of different c-tors of society.This is why some authors,interested in such values,stress that‘the conrvation of biological diversity depends as much on society’s ethical views as on facts’(Barrett,1993).
Finally,biodiversity contributes to ecosystem life support functions and the prervation of ecological structure and integrity.We refer to the functions as the functional value of diversity.This category of value has only been relatively recently recognid in the eco-nomic literature as an important category per which overlaps partially with concepts such as that of‘indi-rect u’value(e Kerry-Turner,1999).Part of this functional significance may result in direct utilitarian value for Homo sapiens in the production of goods and rvices that can be priced.Beyond this lie a range of ecosystem rvices that are of acknowledged benefit to humans but which generally lie outside the bound-aries of recognid direct utilitarian benefit.The pur-po of this paper is to analy the functional values of biodiversity with particular reference to the diversity in agricultural landscapes.4.What is the relationship between diversity and function?
4.1.Concepts
Biologists have for many decades speculated on the question of why there are so many species of living organisms.As explored in the theory of island bio-geography,the diversity within any ecosystem at any point in time is the result of a‘lf-lection’process that involves co-evolution of the species comprising the biological community within a given ecosystem by interactions among them and with the abiotic envi-ronment through time.This is not an isolated process. New species may enter an ecosystem from neighbour-ing areas,some establishing themlves and others failing to do so.Partly as a result of successful new-comers or new adaptations emerging in existing ones (be they competitors,predators,pests or dias),and partly as a result offluctuations in abiotic environ-mental conditions,some of the existing species may become(locally)extinct over any period of time.The species richness of any given ecosystem or land unit is therefore a dynamic property.Recently,the con-ventional explanation of local diversity as well as its ‘functionality’embodied in the niche concept has been challenged by theories that derive patterns clo to the obrved ones from‘random walks’in abundance of species without any a priori prediction of the direction of lection pressures and bad on an equivalence of intra-and interspecific competition(Hubbell,2001). In agroecosystems farmers take a dominant role in this dyna
mic by the lection of which organisms are prent,by modifying the abiotic environment and by interventions aimed at regulating the populations of specific organisms(‘weeds’,‘pests’,‘dias’and their vectors,alternate hosts and antagonists).The dy-namic nature of the(local,patch level)diversity of any system,whether natural or agricultural,is often under-rated,as is the importance of the lection pressure and process.The diversity of any system is not adequately reprented simply by the number of species(or geno-types)prent,but by the relationships between them in space and time.Attempts to asmble combina-tions of the same number of species under slightly different conditions and in particular without the his-tory of interaction often fail(Ewel,1986,1999).But
M.J.Swift et al./Agriculture,Ecosystems and Environment 104(2004)113–134117
what makes any existing species combination into a ‘system’is still largely elusive.Some insights obtained in analysing food webs may help.For example,Neutel (2001)showed that the majority of below-ground food webs constructed from random combinations of or-ganisms did not meet dynamic stability criteria,even though all parameters such as abundance of groups and dynamic properties were chon in a ‘normal’range when considered one-by-one.Yet,systems with the actual parameter combinations that are attained in the field did meet stability criteria,suggesting that partly uncovered rules about the proportionalities and co-variance within the normal range are crucial.安全启动
Debate on the relationship between biological diver-sity and ecosystem function has a long history which has taken on new vigour (and sometimes even rancour)since the advent of the Convention on Biological Di-versity (e Woodwell and Smith (1969)for the older literature and Schulze and Mooney (1993),Mooney et al.(1995,1996),and many of the citations below for more recent discussion).Vitouk and Hooper (1993)contributed a major focus to this debate through hypothesising three different possible relationships between plant diversity and broad-bad ecosystem functions such as the rate of primary production (Fig.1).Their analysis of current evidence led them to propo that the asymptotic relationship shown as Curve 2in Fig.1was the correct one.This suggests that whilst the esntial functions of an ecosystem,such as primary production,require a minimal
level
Fig.1.Possible relationships between biological diversity and ecosystem functions for the plant subsystem (from Vitouk and Hooper (1993)).The authors hypothesid that Curve 2was the most probable of the three propositions.
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of diversity to maximi efficiency this effect is satu-rated at a relatively low number.Swift and Anderson (1993)propod that this relationship could also ap-ply to the decompor system.Examples of esntial functions in this ca are the basic suite of catabolic enzymes (e.g.for cellulolysis,lignin degradation,etc.),the facilitation role that invertebrates play by reducing particle size by their feeding activity,and biophysical process of pore formation and particle aggregation.It is interesting to note,however,that the communities of organisms contributing to the ecosys-tem function of decomposition are taxonomically much more diver than tho of primary production.4.2.Experimental approaches
Over recent years a number of authors have reported on experiments investigating the links between diver-sity and specific functions (e.g.e Ewel et al.,1991;Naeem et al.,1994;Naeem and Li,1997;Tilman and Downing,1994;Tilman et al.,1996,1997;Hooper and Vitouk,1997)that appear to broadly corrobo-rate the predictions of the Vitouk–Hooper hypoth-esis for primary production.This has however gener-ated an equal amount of discussion in refutation and the issue remains significantly a matter of interpre-tation and opinion (e Grime,1997;Hodgson et al.,1998;Lawton et al.,1998;Wardle et al.,2000;Naeem,2000).There is no space here to review the studies in detail,but refer to Kinzig and Pacala (2001)and
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