insight review articles Systematic conservation planning C.R.Margules*&R.L.Presseyt CSIRO Wildlife and Ecology,Tropical Forest Research Centre,and the Rainforest Cooperative Research Centre,PO Box 780,Atherton, Queensland 4883,Australia NSW National Parks and Wildlife Service,PO Box 402,Armidale,New South Wales 2350,Australia The realization of conservation goals requires strategies for managing whole landscapes including areas allocated to both production and protection.Reserves alone are not adequate for nature conservation but they are the cornerstone on which regional strategies are built.Reserves have two main roles.They should sample or represent the biodiversity of each region and they should separate this biodiversity from processes that threaten its persistence.Existing reserve systems throughout the world contain a biased sample of biodiversity,usually that of remote places and other areas that are unsuitable for commercial activities.A more systematic approach to locating and designing reserves has been evolving and this approach will need to be implemented if a large proportion of today's biodiversity is to exist in a future of increasing numbers of people and their demands on natural resources. t is an ancient and widespread human practice to set have been met in existing reserves.Fourth,it uses simple, aside areas for the preservation of natural values. explicit methods for locating and designing new reserves to The sacred groves of Asia and Africa and royal complement existing ones in achieving goals.Fifth,it hunting forests are historical examples2.Other applies explicit criteria for implementing conservation areas protect ecosystem services such as the delivery action on the ground,especially with respect to theschedul- of clean water or the supply of timber,or mitigate the ing of protective management when not all candidate areas expected adverse effects of over-clearing.Others protect can be secured at once (usually).Sixth and finally,it adopts recreational and scenic values and some have been explicit objectives and mechanisms for maintaining the planned to foster international cooperation'.Many of conditions within reserves that are required to foster the these areas meet the World Conservation Union's persistence of key natural features,together with monitor- definition of a strictly protected area (IUCN categories ing of those features and adaptive management2 as I-IV)s,and hereafter we refer to such protected areas as required.The effectiveness ofsystematic conservation plan- reserves'. These areas are increasingly being ning comes from its efficiency in using limited resources to complemented by reserves established principally for the achieve conservation goals,its defensibility and flexibility in protection of biodiversity,including ecosystems,biological the face of competing land uses,and its accountability in assemblages,species and populations.The basic role of allowing decisions to be critically reviewed.This is an ideal- reserves is to separate elements of biodiversity from ized description of a process that is difficult to achieve in processes that threaten their existence in the wild.They practice.Nevertheless,substantial parts have now been must do this within the constraints imposed by large and implemented around the world and some are used as rapidly increasing numbers of humans in many parts of illustrations below. the world and their attendant requirements for space, The practice of conservation planning has generally not materials and waste disposal. been systematic and new reserves have often been located in The extent to which reserves fulfil this role depends on places that do not contribute to the representation of biodi- how well they meet two objectives.The first is representa- versity.The main reason is that reservation usually stops or tiveness,a long-established goal referring to the need for slows the extraction of natural resources.In some regions reserves to represent,or sample,the full variety ofbiodiver- housing and commercial development compete with sity ideally at all levels of organization.The second is reserves for lands.Theeconomicand political implications persistence.Reserves,onceestablished,should promote the can be serious and reserves can be degraded or even lose long-term survival of the species and other elements of their protected status when they prove to be economically biodiversity they contain by maintaining natural processes valuable.As a result,reserves tend to be concentrated on and viable populations and by excluding threats.To meet land that,at least at the time of establishment,was too these objectives,conservation planning must deal not remote or unproductive to be important economically2 only with the location of reserves in relation to natural This means that many species occurring in productive physical and biological patterns but also with reserve landscapes or landscapes with development potential are design,which includes variables such as size,connectivity, not protected,even though disturbance,transformation to replication,and alignment ofboundaries,for example,with intensive uses,and fragmentation continue21.Another watershedsAstructuredsystematicapproach to conser- reason for the inappropriate location of reserves is the very vation planning provides the foundation needed to meet diversity of reasons for which reserves are established.A these objectives. diversity ofgoals means that different proponents see differ- Systematic conservation planning has several distinctive ent places as important.Because highly valued areas arising characteristics.First,it requires clear choices about the from alternative conservation goals often fail to overlap2, features to be used as surrogates for overall biodiversity in there is competition among proponents for limited funds the planning process.Second,it is based on explicit goals, and the limited attention spans of decision-makers.More- preferably translated into quantitative,operational targets. over,goals such as the protection of grand scenery and Third,it recognizes the extent to which conservation goals wilderness often focus on areas that are remote,rugged and NATURE|MAY 2000 www.nature000 Macmillan Magazines Ltd 243
insight review articles NATURE | VOL 405 | 11 MAY 2000 | www.nature.com 243 I t is an ancient and widespread human practice to set aside areas for the preservation of natural values. The sacred groves of Asia and Africa and royal hunting forests are historical examples1,2. Other areas protect ecosystem services such as the delivery of clean water or the supply of timber, or mitigate the expected adverse effects of over-clearing3 . Others protect recreational and scenic values and some have been planned to foster international cooperation4 . Many of these areas meet the World Conservation Union’s definition of a strictly protected area (IUCN categories I–IV)5 , and hereafter we refer to such protected areas as ‘reserves’. These areas are increasingly being complemented by reserves established principally for the protection of biodiversity, including ecosystems, biological assemblages, species and populations6 . The basic role of reserves is to separate elements of biodiversity from processes that threaten their existence in the wild. They must do this within the constraints imposed by large and rapidly increasing numbers of humans in many parts of the world and their attendant requirements for space, materials and waste disposal7 . The extent to which reserves fulfil this role depends on how well they meet two objectives. The first is representativeness, a long-established goal referring to the need for reserves to represent, or sample, the full variety of biodiversity8 , ideally at all levels of organization. The second is persistence. Reserves, once established, should promote the long-term survival of the species and other elements of biodiversity they contain by maintaining natural processes and viable populations and by excluding threats9 . To meet these objectives, conservation planning must deal not only with the location of reserves in relation to natural physical and biological patterns but also with reserve design, which includes variables such as size, connectivity, replication, and alignment of boundaries, for example, with watersheds10,11. A structured systematic approach to conservation planning provides the foundation needed to meet these objectives. Systematic conservation planning has several distinctive characteristics. First, it requires clear choices about the features to be used as surrogates for overall biodiversity in the planning process. Second, it is based on explicit goals, preferably translated into quantitative, operational targets. Third, it recognizes the extent to which conservation goals have been met in existing reserves. Fourth, it uses simple, explicit methods for locating and designing new reserves to complement existing ones in achieving goals. Fifth, it applies explicit criteria for implementing conservation action on the ground, especially with respect to the scheduling of protective management when not all candidate areas can be secured at once (usually). Sixth and finally, it adopts explicit objectives and mechanisms for maintaining the conditions within reserves that are required to foster the persistence of key natural features, together with monitoring of those features and adaptive management12 as required. The effectiveness of systematic conservation planning comes from its efficiency in using limited resources to achieve conservation goals, its defensibility and flexibility in the face of competing land uses, and its accountability in allowing decisions to be critically reviewed. This is an idealized description of a process that is difficult to achieve in practice. Nevertheless, substantial parts have now been implemented around the world13–17 and some are used as illustrations below. The practice of conservation planning has generally not been systematic and new reserves have often been located in places that do not contribute to the representation of biodiversity. The main reason is that reservation usually stops or slows the extraction of natural resources. In some regions, housing and commercial development compete with reserves for land18. The economic and political implications can be serious and reserves can be degraded or even lose their protected status when they prove to be economically valuable19. As a result, reserves tend to be concentrated on land that, at least at the time of establishment, was too remote or unproductive to be important economically20. This means that many species occurring in productive landscapes or landscapes with development potential are not protected, even though disturbance, transformation to intensive uses, and fragmentation continue21. Another reason for the inappropriate location of reserves is the very diversity of reasons for which reserves are established. A diversity of goals means that different proponents see different places as important. Because highly valued areas arising from alternative conservation goals often fail to overlap22, there is competition among proponents for limited funds and the limited attention spans of decision-makers. Moreover, goals such as the protection of grand scenery and wilderness often focus on areas that are remote, rugged and Systematic conservation planning C. R. Margules* & R. L. Pressey† *CSIRO Wildlife and Ecology, Tropical Forest Research Centre, and the Rainforest Cooperative Research Centre, PO Box 780, Atherton, Queensland 4883, Australia †NSW National Parks and Wildlife Service, PO Box 402, Armidale, New South Wales 2350, Australia The realization of conservation goals requires strategies for managing whole landscapes including areas allocated to both production and protection. Reserves alone are not adequate for nature conservation but they are the cornerstone on which regional strategies are built. Reserves have two main roles. They should sample or represent the biodiversity of each region and they should separate this biodiversity from processes that threaten its persistence. Existing reserve systems throughout the world contain a biased sample of biodiversity, usually that of remote places and other areas that are unsuitable for commercial activities. A more systematic approach to locating and designing reserves has been evolving and this approach will need to be implemented if a large proportion of today’s biodiversity is to exist in a future of increasing numbers of people and their demands on natural resources. © 2000 Macmillan Magazines Ltd
insight review articles Figure 1 Social,economic and political factors often compete with reserves for land.a,Kings Canyon,Watarrka National Park,Northern Territory.Australia.This is a spectacular landscape,worthy of protection both for its outstanding natural beauty and for its biodiversity.But it is a remote and rugged area, valuable for tourism but not for extractive uses so it was easier to protect than more productive and economically valuable landscapes.b.An agricultural landscape in the Adelaide Hills,South Australia,with remnant woodland in the background.Remnants such as these contain species that are not represented in more remote and inaccessible areas,so their contribution to the overall goal of maintaining biodiversity is just as great.Despite their natural values it is always a difficult social and political decision to protect them because they have economic value as well as biodiversity value.Photographs by Liz Poon. residual from intensive uses,giving them a political advantage over planning will enhance the effectiveness with which science can do goals such as representativeness,which focus also on disturbed, these three things. economically productive landscapes(Fig.1). Conservation planning is therefore an activity in which social, A framework for systematic conservation planning economic and political imperatives modify,sometimes drastically, Systematic conservation planning can be seen as a process in six scientific prescriptions.This interaction need not be all one way. stages2(Box 1),each of which is discussed below with examples of Science has at least three means of influencing the practice of the tasks and decisions required.The process is not unidirectional- nature conservation.First,an available body of scientific theory and there will be many feedbacks and reasons for revised decisions about application can provide some of the raw material for constructing priority areas.For example,it will be necessaryto re-examine conser- policies2.Second,science can offer solutions when called upon vation goals as knowledge accumulates,and replacement candidate to assist in the implementation of policies and conventions,while reserves will have to be identified when unforeseen difficulties arise also clarifying the social and economic implications of alternative in implementation.Although our discussion focuses on reserves,the methods or scenarios(this role is best filled when science is integral framework applies equally well to many problems in 'off-reserve' to the process,notsimply called in for peer reviewor when technical conservation,including habitat restoration2.26.Decisions about or political problems emerge).Third,science can and should be used both on-and off-reserve conservation,ifthey are not to be ad hocand to review the effectiveness of political processes for achieving stated uncoordinated,should be guided by explicit goals,identification of biodiversity goals.A structured framework for conservation priorities in regional or broader contexts,and clear choices between Figure 2A map of biodiversity prioriy areas in Papua New Guinea The targets that are met by this set of areas are the representation of 608 environmental domains,564 vegetation types,10 species assemblages and 12 rare and threatened species.For the derivation of these targets,see refs 16,92.In meeting targets,the set of areas also minimizes foregone opportunities for timber extraction, represents all existing reserves,minimizes the number of areas currently used for intensive agriculture,gives preference to areas with low human population density and gives preference to areas identified previously by experts as biodiversity priority areas.The selected areas occupy 16.8%of the country and are inhabited by 210,000 people out of a population of approximately 4 million.A total of 398 areas were selected from 4,470 candidate areas or planning units.These units were aerial photograph pattems that were previously mapped for a database on agricultural and forestry suitability.The trade-off between biodiversity gain and opportunity costs,and the application of the other spatial constraints,was achieved with the TARGET software.The colours represent different index classes of timber volume.Yellow is highest,red next highest,purple next and green lowest. 244 2000 Macmillan Magazines Ltd NATURE VOL 40511 MAY 2000 www.nature.com
residual from intensive uses, giving them a political advantage over goals such as representativeness, which focus also on disturbed, economically productive landscapes (Fig. 1). Conservation planning is therefore an activity in which social, economic and political imperatives modify, sometimes drastically, scientific prescriptions. This interaction need not be all one way. Science has at least three means of influencing the practice of nature conservation. First, an available body of scientific theory and application can provide some of the raw material for constructing policies23. Second, science can offer solutions when called upon to assist in the implementation of policies and conventions, while also clarifying the social and economic implications of alternative methods or scenarios (this role is best filled when science is integral to the process, not simply called in for peer review24or when technical or political problems emerge). Third, science can and should be used to review the effectiveness of political processes for achieving stated biodiversity goals. A structured framework for conservation planning will enhance the effectiveness with which science can do these three things. A framework for systematic conservation planning Systematic conservation planning can be seen as a process in six stages25 (Box 1), each of which is discussed below with examples of the tasks and decisions required. The process is not unidirectional — there will be many feedbacks and reasons for revised decisions about priority areas. For example, it will be necessary to re-examine conservation goals as knowledge accumulates, and replacement candidate reserves will have to be identified when unforeseen difficulties arise in implementation. Although our discussion focuses on reserves, the framework applies equally well to many problems in ‘off-reserve’ conservation, including habitat restoration25,26. Decisions about both on- and off-reserve conservation, if they are not to be ad hoc and uncoordinated, should be guided by explicit goals, identification of priorities in regional or broader contexts, and clear choices between insight review articles 244 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com Figure 1 Social, economic and political factors often compete with reserves for land. a, Kings Canyon, Watarrka National Park, Northern Territory, Australia. This is a spectacular landscape, worthy of protection both for its outstanding natural beauty and for its biodiversity. But it is a remote and rugged area, valuable for tourism but not for extractive uses so it was easier to protect than more productive and economically valuable landscapes. b, An agricultural landscape in the Adelaide Hills, South Australia, with remnant woodland in the background. Remnants such as these contain species that are not represented in more remote and inaccessible areas, so their contribution to the overall goal of maintaining biodiversity is just as great. Despite their natural values it is always a difficult social and political decision to protect them because they have economic value as well as biodiversity value. Photographs by Liz Poon. Figure 2 A map of biodiversity priority areas in Papua New Guinea16. The targets that are met by this set of areas are the representation of 608 environmental domains37, 564 vegetation types, 10 species assemblages and 12 rare and threatened species. For the derivation of these targets, see refs 16, 92. In meeting targets, the set of areas also minimizes foregone opportunities for timber extraction, represents all existing reserves, minimizes the number of areas currently used for intensive agriculture, gives preference to areas with low human population density and gives preference to areas identified previously by experts as biodiversity priority areas92. The selected areas occupy 16.8% of the country and are inhabited by 210,000 people out of a population of approximately 4 million. A total of 398 areas were selected from 4,470 candidate areas or planning units. These units were aerial photograph patterns that were previously mapped for a database on agricultural and forestry suitability. The trade-off between biodiversity gain and opportunity costs, and the application of the other spatial constraints, was achieved with the TARGET software94,109. The colours represent different index classes of timber volume. Yellow is highest, red next highest, purple next and green lowest. © 2000 Macmillan Magazines Ltd
insight review articles potential conservation areas and alternative forms ofmanagement. vertebrates or butterflies.We may know that the presence ofa butter- fly indicates the presence ofits food plant somewhere nearby.Thereal Stage 1.Measure and map biodiversity question,however,is whether the presence of that butterfly,or any Because ofthe complexity of biodiversity,surrogates such as sub-sets other taxon,indicates the presence of other taxa to the extent that it of species,species assemblages and habitat types have to be used as can beconsideredasuitablesurrogate for overallbiodiversity.Testsof measures of biodiversity,and the locations ofthese surrogates within taxonomic surrogacy in Britain"and South Africa?are not encour- areas have to be plotted so that similarities or differences among areas aging,but more promising results have been obtained in Uganda can be estimated. Divergent results are attributable to differences in analytical meth- Biological systems are organized hierarchically from the molecular ods,geographical scales and biogeographical histories of the study to the ecosystem level.Logical classes such as individuals,populations, areas.Reliable generalizations and an understanding of how such species,communities and ecosystems are heterogeneous.Each mem- factors affect taxonomic surrogacy are still developing.Higher levels ber ofeach class can be distinguished from everyother member.Itisnot in the biological hierarchy,such as species assemblages,habitat types even possible to enumerate all of the species of any one area,let alone and ecosystems lose biological precision,but have other advantages. the members of logical classes at lower levels such as populations and They can integrate more ofthe ecological processes that contribute to individuals.Yet this is biodiversity,and maintaining that complexity is the maintenance of ecosystem function3(although there is active the goal of conservation planning.For the foreseeable future it will be debate on this issue)and the relevant data are more widely and necessary to accept this incomplete knowledge and adopt methods for consistently available.In addition,there are sound theoretical rea- makingthe most ofwhat we do know or candiscover from new surveys. sons why environmental variables should be good estimators of the Thus,surrogate or partial measures of biodiversity must be used to spatial distribution patterns of species and there are now some estimate similarity or difference among areas within planning regions. empiricalstudies that add supportNew statistical techniques are The choice of surrogate measures is not trivial.The strong also being developed to compare how well different environmental temptation is to use a group of species:for example,vascular plants, surrogates reflect the distribution patterns ofspecies Box 1 Stages in systematic conservation planning Systematic conservation planning can be separated into six stages,and some examples of tasks and decisions in each are presented belows Note that the process is not unidirectional;there will be many feedbacks and reasons for altering decisions(see text for examples). 1.Compile data on the biodiversity of the planning region Review existing data and decide on which data sets are sufficiently consistent to serve as surrogates for biodiversity across the planning region .If time allows,collect new data to augment or replace some existing data sets. Collect information on the localities of species considered to be rare and/or threatened in the region (these are likely to be missed or under-represented in conservation areas selected only on the basis of land classes such as vegetation types). 2.Identify conservation goals for the planning region Set quantitative conservation targets for species,vegetation types or other features(for example,at least three occurrences of each species, 1,500 ha of each vegetation type,or specific targets tailored to the conservation needs of individual features).Despite inevitable subjectivity in their formulation,the value of such goals is their explicitness. Set quantitative targets for minimum size,connectivity or other design criteria. Identify qualitative targets or preferences(for example,as far as possible,new conservation areas should have minimal previous disturbance from grazing or logging). 3.Review existing conservation areas Measure the extent to which quantitative targets for representation and design have been achieved by existing conservation areas. Identify the imminence of threat to under-represented features such as species or vegetation types,and the threats posed to areas that will be important in securing satisfactory design targets. 4.Select additional conservation areas Regard established conservation areas as 'constraints'or focal points for the design of an expanded system. Identify preliminary sets of new conservation areas for consideration as additions to established areas.Options for doing this include reserve selection algorithms or decision-support software to allow stakeholders to design expanded systems that achieve regional conservation goals subject to constraints such as existing reserves,acquisition budgets,or limits on feasible opportunity costs for other land uses. 5.Implement conservation actions Decide on the most appropriate or feasible form of management to be applied to individual areas(some management approaches will be fallbacks from the preferred option) If one or more selected areas prove to be unexpectedly degraded or difficult to protect,return to stage 4 and look for alternatives. Decide on the relative timing of conservation management when resources are insufficient to implement the whole system in the short term (usually). 6.Maintain the required values of conservation areas Set conservation goals at the level of individual conservation areas(for example,maintain seral habitats for one or more species for which the area is important).Ideally,these goals will acknowledge the particular values of the area in the context of the whole system. Implement management actions and zonings in and around each area to achieve the goals. Monitor key indicators that will reflect the success of management actions or zonings in achieving goals.Modify management as required. NATURE|VOL 40511 MAY 2000www.nature.com 2000 Macmillan Magazines Ltd 245
potential conservation areas and alternative forms of management. Stage 1. Measure and map biodiversity Because of the complexity of biodiversity, surrogates such as sub-sets of species, species assemblages and habitat types have to be used as measures of biodiversity, and the locations of these surrogates within areas have to be plotted so that similarities or differences among areas can be estimated. Biological systems are organized hierarchically from the molecular to the ecosystem level. Logical classes such as individuals, populations, species, communities and ecosystems are heterogeneous. Each member of each class can be distinguished from every other member. It is not even possible to enumerate all of the species of any one area, let alone the members of logical classes at lower levels such as populations and individuals. Yet this is biodiversity, and maintaining that complexity is the goal of conservation planning. For the foreseeable future it will be necessary to accept this incomplete knowledge and adopt methods for making the most of what we do know or can discover from new surveys. Thus, surrogate or partial measures of biodiversity must be used to estimate similarity or difference among areas within planning regions. The choice of surrogate measures is not trivial. The strong temptation is to use a group of species: for example, vascular plants, vertebrates or butterflies. We may know that the presence of a butterfly indicates the presence of its food plant somewhere nearby. The real question, however, is whether the presence of that butterfly, or any other taxon, indicates the presence of other taxa to the extent that it can be considered a suitable surrogate for overall biodiversity. Tests of taxonomic surrogacy in Britain27 and South Africa28 are not encouraging, but more promising results have been obtained in Uganda29. Divergent results are attributable to differences in analytical methods, geographical scales and biogeographical histories of the study areas. Reliable generalizations and an understanding of how such factors affect taxonomic surrogacy are still developing. Higher levels in the biological hierarchy, such as species assemblages, habitat types and ecosystems lose biological precision, but have other advantages. They can integrate more of the ecological processes that contribute to the maintenance of ecosystem function30 (although there is active debate on this issue31) and the relevant data are more widely and consistently available. In addition, there are sound theoretical reasons why environmental variables should be good estimators of the spatial distribution patterns of species32–34 and there are now some empirical studies that add support35–37. New statistical techniques are also being developed to compare how well different environmental surrogates reflect the distribution patterns of species38. insight review articles NATURE | VOL 405 | 11 MAY 2000 | www.nature.com 245 Systematic conservation planning can be separated into six stages, and some examples of tasks and decisions in each are presented below25. Note that the process is not unidirectional; there will be many feedbacks and reasons for altering decisions (see text for examples). 1. Compile data on the biodiversity of the planning region • Review existing data and decide on which data sets are sufficiently consistent to serve as surrogates for biodiversity across the planning region. • If time allows, collect new data to augment or replace some existing data sets. • Collect information on the localities of species considered to be rare and/or threatened in the region (these are likely to be missed or under-represented in conservation areas selected only on the basis of land classes such as vegetation types). 2. Identify conservation goals for the planning region • Set quantitative conservation targets for species, vegetation types or other features (for example, at least three occurrences of each species, 1,500 ha of each vegetation type, or specific targets tailored to the conservation needs of individual features). Despite inevitable subjectivity in their formulation, the value of such goals is their explicitness. • Set quantitative targets for minimum size, connectivity or other design criteria. • Identify qualitative targets or preferences (for example, as far as possible, new conservation areas should have minimal previous disturbance from grazing or logging). 3. Review existing conservation areas • Measure the extent to which quantitative targets for representation and design have been achieved by existing conservation areas. • Identify the imminence of threat to under-represented features such as species or vegetation types, and the threats posed to areas that will be important in securing satisfactory design targets. 4. Select additional conservation areas • Regard established conservation areas as ‘constraints’ or focal points for the design of an expanded system. • Identify preliminary sets of new conservation areas for consideration as additions to established areas. Options for doing this include reserve selection algorithms or decision-support software to allow stakeholders to design expanded systems that achieve regional conservation goals subject to constraints such as existing reserves, acquisition budgets, or limits on feasible opportunity costs for other land uses. 5. Implement conservation actions • Decide on the most appropriate or feasible form of management to be applied to individual areas (some management approaches will be fallbacks from the preferred option). • If one or more selected areas prove to be unexpectedly degraded or difficult to protect, return to stage 4 and look for alternatives. • Decide on the relative timing of conservation management when resources are insufficient to implement the whole system in the short term (usually). 6. Maintain the required values of conservation areas • Set conservation goals at the level of individual conservation areas (for example, maintain seral habitats for one or more species for which the area is important). Ideally, these goals will acknowledge the particular values of the area in the context of the whole system. • Implement management actions and zonings in and around each area to achieve the goals. • Monitor key indicators that will reflect the success of management actions or zonings in achieving goals. Modify management as required. Box 1 Stages in systematic conservation planning © 2000 Macmillan Magazines Ltd
insight review articles Figure 3 White Rhinos cumrently persist in relatively small intensively managed populations in game reserves.Off-reserve management in suitable habitat would probably be necessary if populations were to return to self-sustaining levels,although conflict with human populations makes it extremely unlikely that this would ever happen Photograph by Liz Poon. Planning is essentially a matter of comparison so it is preferable to necessary to compile data on biodiversity surrogates for each of the compare two or more areas with the same kind of information at the planning units in the region.Data on tenure(for stages 3,4 and 5, samelevel of detail.A map of vegetation types(communities or habi- below)and other contextual data that might influence selection and tat types)and/or environmental classes provides spatial consistency implementation (for example,roads,rivers,terrain,timber across wide areas.On the other hand,museum and herbarium data resources and threats)should also be compiled at this stage. on the locations of taxa are notoriously biased,having been collected for a different purpose(systematics),and often in an opportunistic Stage 2.Identify conservation goals for the planning region manner,from the places that collectors expected to find what they The overall goals ofsystematic conservation planning-representa- were looking for or that were conveniently accessible3940.Plots of the tiveness and persistence-have to be translated into more specific, field records from many collections therefore map road networks. preferably quantitative,targets for operational use.Targets allow Various methods-empirical,statistical and computational- are clear identification of the contributions ofexisting reserves to region- now available for modelling wider spatial distribution patterns from al goals and provide the means for measuring the conservation value thepoint records that field samples represent,but their reliability of different areas during the area selection process in stage 4 below. is also at least partly a function of the degree of spatial bias.New Targets such as 10 or 12%of the areas ofcountries or vegetation types systematic field surveys to fill gaps are the best solution but they can have been criticized because they are too small to prevent the extinc- beexpensive and time consuming. tion of many species,can be subverted by reserving the least produc- There is no best surrogate.The decision on which to use will tive and least threatened landscapes,and can mislead the public into depend on many factors including what data are available and what believing that limited conservation action is adequate.A focus on resources there are for data analysis(for example,spatial modelling) targets for reserves may also remove incentives to implement other and the collection of new data.In most parts of the world,the only conservation actions such as off-reserve management'.These criti- spatially consistent information available is on higher-order surro- cisms are valid,but are aimed at how targets are set rather than expos- gates such as vegetationtypesandenvironmental classes.Collections ing reasons for not setting targets at all.Planners need to know what of taxa might form an accurate representation of some biological they are aiming for.Moreequals better'is good in principle,but does distributions in some countries where well designed and well little to resolve choices between areas with different biotas when resourced surveys have been used to collect the data.Taxa collections other demands narrow the geographical scope for reservation. may also be used with some reliability at coarse scales(for example, Accordingly,planners need targets that do several things:focus on grid cells of 50 km x 50 km),but usually become less reliable at the scales that are much finer than whole countries or regions;deal with scale ofindividual reserves4.If taxa sub-sets are used without spatial natural processes as well as biodiversity pattern;reflect the relative modelling,it is usually with the understanding that the disadvantage needs of species and landscapes for protection;recognize that ofspatial bias is offset by the advantage of having at least some direct reserves must be complemented by off-reserve management,prefer- biological information to complement higher-order surrogates. ably also with targets;and leave options open for revision as social Combinations of surrogates will be most practicable in most and economic conditions change.Ideally,reservation targets will be situations.In a recent study in Papua New Guinea,environmental an integral part of policies and government processes".Failure to domains classified from climate,landform and geology",vegetation achieve targets for economically valuable landscapes is likely,so types mapped from aerial photographs,and the known locations of periodic reviews(stage 3,below)are necessary. rare and threatened species were all used as biodiversity surrogates Most exercises in systematic conservation planning have chosen (Fig.2) areas on the basis of the occurrences of species.Some have used A decision is also needed at this stage on how to define planning predicted probabilities of occurrenceRecent applications have set units,the building blocks ofthe reserve system.Planning units can be targets for the spatial extent of communities,habitat types or envi- regular(for example,grids or hexagons)or irregular (for example, ronmental classes,sometimes with explicit formulae for adjusting tenure parcels,watersheds or habitat remnants).A mix of planning targets according to factors such as natural rarity and vulnerability to units might be appropriate in regions that contain both fragmented threats.These are all targets for representing a biodiversity pattern. landscapes and extensive tracts of uncleared vegetation.The choice Targets for ecological processes can be more problematic.Because has implications for the efficiency with which representation goals conservation planning is a spatial exercise,protection of natural can be achieved as well as for the design and management of processes must be based on their spatial surrogates rather than the reserves.For the reserve selection process described in stage 4,it is processes themselves (for example,size,lack of roads,watershed 246 2000 Macmillan Magazines Ltd 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Planning is essentially a matter of comparison so it is preferable to compare two or more areas with the same kind of information at the same level of detail. A map of vegetation types (communities or habitat types) and/or environmental classes provides spatial consistency across wide areas. On the other hand, museum and herbarium data on the locations of taxa are notoriously biased, having been collected for a different purpose (systematics), and often in an opportunistic manner, from the places that collectors expected to find what they were looking for or that were conveniently accessible39,40. Plots of the field records from many collections therefore map road networks. Various methods — empirical, statistical and computational — are now available for modelling wider spatial distribution patterns from the point records that field samples represent41–43, but their reliability is also at least partly a function of the degree of spatial bias. New systematic field surveys to fill gaps are the best solution but they can be expensive and time consuming. There is no best surrogate. The decision on which to use will depend on many factors including what data are available and what resources there are for data analysis (for example, spatial modelling) and the collection of new data. In most parts of the world, the only spatially consistent information available is on higher-order surrogates such as vegetation types and environmental classes. Collections of taxa might form an accurate representation of some biological distributions in some countries where well designed and well resourced surveys have been used to collect the data. Taxa collections may also be used with some reliability at coarse scales (for example, grid cells of 50 km 2 50 km), but usually become less reliable at the scale of individual reserves44. If taxa sub-sets are used without spatial modelling, it is usually with the understanding that the disadvantage of spatial bias is offset by the advantage of having at least some direct biological information to complement higher-order surrogates. Combinations of surrogates will be most practicable in most situations. In a recent study in Papua New Guinea, environmental domains classified from climate, landform and geology37, vegetation types mapped from aerial photographs, and the known locations of rare and threatened species were all used as biodiversity surrogates (Fig. 2)16. A decision is also needed at this stage on how to define planning units, the building blocks of the reserve system. Planning units can be regular (for example, grids or hexagons) or irregular (for example, tenure parcels, watersheds or habitat remnants). A mix of planning units might be appropriate in regions that contain both fragmented landscapes and extensive tracts of uncleared vegetation. The choice has implications for the efficiency with which representation goals can be achieved as well as for the design and management of reserves45. For the reserve selection process described in stage 4, it is necessary to compile data on biodiversity surrogates for each of the planning units in the region. Data on tenure (for stages 3, 4 and 5, below) and other contextual data that might influence selection and implementation (for example, roads, rivers, terrain, timber resources and threats) should also be compiled at this stage. Stage 2. Identify conservation goals for the planning region The overall goals of systematic conservation planning — representativeness and persistence — have to be translated into more specific, preferably quantitative, targets for operational use. Targets allow clear identification of the contributions of existing reserves to regional goals and provide the means for measuring the conservation value of different areas during the area selection process in stage 4 below. Targets such as 10 or 12% of the areas of countries or vegetation types have been criticized because they are too small to prevent the extinction of many species, can be subverted by reserving the least productive and least threatened landscapes, and can mislead the public into believing that limited conservation action is adequate46. A focus on targets for reserves may also remove incentives to implement other conservation actions such as off-reserve management1 . These criticisms are valid, but are aimed at how targets are set rather than exposing reasons for not setting targets at all. Planners need to know what they are aiming for. ‘More equals better’ is good in principle, but does little to resolve choices between areas with different biotas when other demands narrow the geographical scope for reservation. Accordingly, planners need targets that do several things: focus on scales that are much finer than whole countries or regions; deal with natural processes as well as biodiversity pattern; reflect the relative needs of species and landscapes for protection; recognize that reserves must be complemented by off-reserve management, preferably also with targets; and leave options open for revision as social and economic conditions change. Ideally, reservation targets will be an integral part of policies and government processes47. Failure to achieve targets for economically valuable landscapes is likely, so periodic reviews (stage 3, below) are necessary. Most exercises in systematic conservation planning have chosen areas on the basis of the occurrences of species. Some have used predicted probabilities of occurrence48. Recent applications have set targets for the spatial extent of communities, habitat types or environmental classes, sometimes with explicit formulae for adjusting targets according to factors such as natural rarity and vulnerability to threats13. These are all targets for representing a biodiversity pattern. Targets for ecological processes can be more problematic. Because conservation planning is a spatial exercise, protection of natural processes must be based on their spatial surrogates rather than the processes themselves (for example, size, lack of roads, watershed insight review articles 246 NATURE | VOL 405 | 11 MAY 2000 | www.nature.com Figure 3 White Rhinos currently persist in relatively small intensively managed populations in game reserves. Off-reserve management in suitable habitat would probably be necessary if populations were to return to self-sustaining levels, although conflict with human populations makes it extremely unlikely that this would ever happen. Photograph by Liz Poon. © 2000 Macmillan Magazines Ltd
insight review articles boundaries,and migration routes).Setting process targets can be the retention of patches of suitable,but currently unoccupied, difficult in practice because the environment is heterogeneous in habitato space and time and different species function at different spatial and Source-pool effects and successional pathways temporal scales49.Nevertheless,seven aspects of theory on ecological The species composition of an area changes over time in a process and evolutionary processes,now supported by some empirical usually called ecological succession.Some of these changes will be evidence,can provide guidelines. due to dispersal but others will be the products of initial conditions. Biogeographical theory There is a mix of starting propagules available in an area and Traditionally,the equilibrium theory of island biogeographyand subsequent changes reflect a sorting of this mix according to life- associated biogeographical theory has been used to help set targets history traits and interspecific interactions Because of periodic, for size,shape and distance between reserves(although usually such patchy disturbances,most regions contain areas at various stages targets were not quantitative).This body of theory tells us that bigger along these pathways and many species exploit the temporal and reserves are better,the closer they are the better,the more circular the spatial variation of natural disturbance regimes.The implications better,and that reserves should be linked by habitat corridors2 In for target setting are that all successional stages might need to be rep- the real world of conservation planning,the opportunity to apply resented,replication of reserves to sample different successional such guidelines is constrained by costs and patterns ofland-use histo- stages might be desirable,and large reserves are better because they ry.These design principles also introduced an important trade-off can better accommodate natural patch dynamics without succession into planning that is seldom acknowledged.If the area available for being reset throughout by asingleevent such as a wildfire reservation is limited,a choice might have to be made between a few Spatial autecological requirements large reserves that favour the persistence of some species or more Different species require different amounts ofspace to complete their smaller reserves that together are more representative of the region's life cycles"?(Fig.3).Most reserves contain one or more species that biodiversity but individually are less effective for the persistence of would not persist as residentseven for one generation ifthey became some species,for example,large,wide-ranging species5 An early isolated.Many other reserves,without supplementation by and widely ignored criticism of the equilibrium theory was that it unreserved habitat,would be likely to lose species in the long term treated islands as featureless plains with no internal habitat diversity througha variety ofchance events.Thus,the long-term persistence of and species as characterless features with no genetic or geographical some taxa requires sustainable populations across entire landscapes variations4.There is now some experimental support for the or regions as predicted,for example,for the northern spotted owl prediction that increased isolation reduces the likelihood of persis- (Strix occidentalis caurina)in the Pacific northwest United States tence of certain speciess,supporting targets for connectivity. Thereisa vast literature on population viability analysisReserva- However,attention has rightly shifted to the roles of environmental tion targets should include viable population sizes and structures(for heterogeneity,species interactions,local-and regional-scale popula- example,age classes and sex ratios)when these are known.Many tion dynamics,and the effects of habitat modification in reserve species exploit temporal variation by moving between different planning. habitats,requiring targets to recognize key habitat combinations Metapopulation dynamics where these can be identified.The focal species approachattempts In general,a metapopulations is a network of local populations to integrate patterns and processes by identifying those species in a linked by dispersal.More narrowly,the term is used to describe landscape that are most demanding of resources and then targeting systems in which local populations periodically go extinct with them for management.The kinds ofresources needed by focal species recolonization occurring by migration from other local popula- may be,for example,large areas,connectivity between habitat patch- tions37.Metapopulations go extinct when the rate of extinction of es and complex heterogeneous habitats.The argument is that if local populations exceeds the rate of migration and recolonization. management can maintain these species in a landscape,then most Confining a species to a reserve may disrupt metapopulation other species will be maintained as well. dynamics,increasing the risk oflocal extinction due,for example,to a Source-sink population structures catastrophic event such as wildfire,and decreasing the chances of If,in some high-quality habitats(sources)a species'reproduction recolonization.Metapopulation theory calls for targets that consider rate exceeds mortality,but in low-quality habitats(sinks)its repro- reservation across species'natural ranges so that some populations duction rate is lower than mortality,then a net dispersal away from might escape the impact of unpredictable events,thereby spreading sources may sustain populations in sinks37.In southeastern Aus- the risk of extinction.It also calls for the retention of landscape tralia,63%ofthe arboreal marsupial population is found in only 9% linkages to promote dispersal and the exchange of individuals of the forest with high foliar nutrients.Dispersal throughout the between geographically separate sub-populations39 and for remainder of the forest occurs from these areas of high population Figure 4 Isolated habitat remnants in the wheat belt of Western Australia. Isolation causes physical changes to habitat remnants,which in tum can lead to changes in species composition and population sizes.Photograph courtesy of CSIRO,Wildlife&cy NATURE|VOL 40511 MAY 2000www.nature.com 2000 Macmillan Magazines Ltd 247
boundaries, and migration routes). Setting process targets can be difficult in practice because the environment is heterogeneous in space and time and different species function at different spatial and temporal scales49. Nevertheless, seven aspects of theory on ecological and evolutionary processes, now supported by some empirical evidence, can provide guidelines. Biogeographical theory Traditionally, the equilibrium theory of island biogeography50 and associated biogeographical theory has been used to help set targets for size, shape and distance between reserves (although usually such targets were not quantitative). This body of theory tells us that bigger reserves are better, the closer they are the better, the more circular the better, and that reserves should be linked by habitat corridors51,52. In the real world of conservation planning, the opportunity to apply such guidelines is constrained by costs and patterns of land-use history. These design principles also introduced an important trade-off into planning that is seldom acknowledged. If the area available for reservation is limited, a choice might have to be made between a few large reserves that favour the persistence of some species or more smaller reserves that together are more representative of the region’s biodiversity but individually are less effective for the persistence of some species, for example, large, wide-ranging species17,53. An early and widely ignored criticism of the equilibrium theory was that it treated islands as featureless plains with no internal habitat diversity and species as characterless features with no genetic or geographical variation54. There is now some experimental support for the prediction that increased isolation reduces the likelihood of persistence of certain species55, supporting targets for connectivity. However, attention has rightly shifted to the roles of environmental heterogeneity, species interactions, local- and regional-scale population dynamics, and the effects of habitat modification in reserve planning. Metapopulation dynamics In general, a metapopulation56 is a network of local populations linked by dispersal. More narrowly, the term is used to describe systems in which local populations periodically go extinct with recolonization occurring by migration from other local populations57. Metapopulations go extinct when the rate of extinction of local populations exceeds the rate of migration and recolonization. Confining a species to a reserve may disrupt metapopulation dynamics, increasing the risk of local extinction due, for example, to a catastrophic event such as wildfire, and decreasing the chances of recolonization. Metapopulation theory calls for targets that consider reservation across species’ natural ranges so that some populations might escape the impact of unpredictable events, thereby spreading the risk of extinction58. It also calls for the retention of landscape linkages to promote dispersal and the exchange of individuals between geographically separate sub-populations59 and for the retention of patches of suitable, but currently unoccupied, habitat60. Source-pool effects and successional pathways The species composition of an area changes over time in a process usually called ecological succession. Some of these changes will be due to dispersal but others will be the products of initial conditions. There is a mix of starting propagules available in an area and subsequent changes reflect a sorting of this mix according to lifehistory traits and interspecific interactions61. Because of periodic, patchy disturbances, most regions contain areas at various stages along these pathways and many species exploit the temporal and spatial variation of natural disturbance regimes62. The implications for target setting are that all successional stages might need to be represented, replication of reserves to sample different successional stages might be desirable, and large reserves are better because they can better accommodate natural patch dynamics without succession being reset throughout by a single event such as a wildfire63. Spatial autecological requirements Different species require different amounts of space to complete their life cycles57 (Fig. 3). Most reserves contain one or more species that would not persist as residents even for one generation if they became isolated. Many other reserves, without supplementation by unreserved habitat, would be likely to lose species in the long term through a variety of chance events. Thus, the long-term persistence of some taxa requires sustainable populations across entire landscapes or regions as predicted, for example, for the northern spotted owl (Strix occidentalis caurina) in the Pacific northwest United States64. There is a vast literature on population viability analysis65,66. Reservation targets should include viable population sizes and structures (for example, age classes and sex ratios) when these are known. Many species exploit temporal variation by moving between different habitats, requiring targets to recognize key habitat combinations where these can be identified. The focal species approach67 attempts to integrate patterns and processes by identifying those species in a landscape that are most demanding of resources and then targeting them for management. The kinds of resources needed by focal species may be, for example, large areas, connectivity between habitat patches and complex heterogeneous habitats17. The argument is that if management can maintain these species in a landscape, then most other species will be maintained as well. Source–sink population structures If, in some high-quality habitats (sources) a species’ reproduction rate exceeds mortality, but in low-quality habitats (sinks) its reproduction rate is lower than mortality, then a net dispersal away from sources may sustain populations in sinks57,68. In southeastern Australia, 63% of the arboreal marsupial population is found in only 9% of the forest with high foliar nutrients69. Dispersal throughout the remainder of the forest occurs from these areas of high population insight review articles NATURE | VOL 405 | 11 MAY 2000 | www.nature.com 247 Figure 4 Isolated habitat remnants in the wheat belt of Western Australia. Isolation causes physical changes to habitat remnants, which in turn can lead to changes in species composition and population sizes. Photograph courtesy of CSIRO, Wildlife & Ecology. © 2000 Macmillan Magazines Ltd