high-risk ecosystems as foci for considering biodiversity and ecological integrity in ecological...

Environmental Science & Policy 3 (2000) 321–332

High-risk ecosystems as foci for considering biodiversity andecological integrity in ecological risk assessments

Reed F. Noss *Conser6ation Science Incorporated, 7310 NW Acorn Ridge, Cor6allis, OR 97330, USA

Abstract

Ecological risk management historically focused on risks to human health and the immediate human environment. Increasingly,societal interest in biodiversity and ecological integrity demands that risk assessors and managers take a broader view of theenvironment and include non-human species and ecosystems within their realm of concern. The greatest threats to biodiversity inthe USA and much of the world are habitat alteration (including conversion, degradation, and fragmentation) and exotic speciesinvasions. This paper reviews and integrates the results of several recent studies (e.g. by The Nature Conservancy, World WildlifeFund, Defenders of Wildlife, and the National Biological Service) that have identified regions and ecosystems that are highlydistinct biologically, are rare or declining, contain high numbers of imperiled species, face immediate threats, and stand to loseconsiderable biodiversity in the near future. These studies converge on a set of regions that warrant urgent attention from riskmanagers. Focal species and functional groups of species can be selected within these regions to characterize ecological effects andtrack recovery of ecosystems along the same axes (e.g. habitat structure, disturbance frequency) that led to biotic impoverishment.The most fruitful approach to risk management ultimately will be one that addresses the most urgent threats at several levels ofbiological organization, from focal species to communities to ecoregions. © 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Ecological risk assessment; Ecological risk management; Biodiversity; Ecological integrity; Ecoregions

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1. Introduction

My American Heritage Dictionary provides the fol-lowing as the first two definitions of risk: (1) Thepossibility of suffering harm or loss; danger. (2) Afactor, element, or course involving uncertain danger;hazard. Most of us think of risk in a very personal way.What is my risk of getting cancer if I continue smokinga pack of cigarettes daily? What is my child’s risk ofgetting struck by a vehicle while standing at the schoolbus stop? What is the risk that I will burn the roast ifI run to the grocery to get a bag of lettuce? Hence, riskis a familiar and intuitively simple concept to us. A bitless simple are the concepts of uncertainty and proba-bility, which are inextricably connected to risk. Forexample, most people (including lawyers and prosecu-tors in the courtroom) seem to expect statements ofcertainty from scientists, who generally — but notinvariably — know better than to provide suchstatements.

Despite a limited understanding of uncertainty andprobability, the public expects government — to acertain extent — to protect it from harm by minimizingrisks. People do not want the government to go too farin minimizing risks, as they want freedom of choice aswell as safety. Nevertheless, risks to personal health aretaken seriously by most Americans. Even if theirlifestyles are risky, people apparently want to be able tochoose their risks, rather than having risks thrust uponthem. For instance, we feel safer driving our cars thansitting in airplanes, even though the former is actuallyriskier. Increasingly, people are concerned about risksto the environment. For this and other reasons, riskassessment is a socially responsible endeavor of a publicagency.

Ecological risk assessment has been defined by EPAas a process that ‘‘evaluates the likelihood that adverseecological effects may occur or are occurring as a resultof exposure to one or more stressors’’ (US Environ-mental Protection Agency, 1992). In practice, ecologicalrisk assessment has dealt largely with risks to humans(i.e. to the human environment). When the risks to

* Tel.: +1-541-7527639; fax: +1-541-7583453.E-mail address: [email protected] (R.F. Noss).

1462-9011/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.PII: S 1 4 6 2 -9011 (00 )00112 -X

R.F. Noss / En6ironmental Science & Policy 3 (2000) 321–332322

human life and welfare from ecological change aredirect, the case is strongest for interventions or mitiga-tion to reduce these risks. The case has been somewhatweaker, historically, when risks to humans from ecolog-ical change are indirect and diffuse, or when risks areprimarily to non-human species and their habitats.

Most kinds of ecological risks, including loss anddegradation of natural ecosystems and extinctions ofspecies, are in the indirect category, in terms of theireffects on humans. That is, the effects on human societyof losing particular species and habitats are usuallyunknown and can only be conjectured. Considerableredundancy in function exists among the species in anecosystem, such that perhaps 20–50% of species couldbe lost without threatening the major biogeochemicalprocesses of the system. The problem is, we generallydon’t know which species we can ‘afford’ to lose, norhow redundancy might provide insurance against envi-ronmental change (Purvis and Hector, 2000). More-over, a growing segment of the public is concernedabout the status and welfare of native species andecosystems. Many people (according to polls, a major-ity; e.g. Shindler et al. (1993)) believe that non-humanspecies have intrinsic value independent of their useful-ness to humans. Hence, the social-philosophical arenain which environmental policy and management deci-sions are made is changing. As noted by Troyer andBrody (1994), in a review of EPA’s risk assessmentprogram, EPA ‘‘has focused more consistently on pro-tecting human health than ecosystems. In recent years,however, the Agency has worked to address thisimbalance’’.

This paper concentrates on risks to non-human spe-cies and ecosystems; thus, it continues EPA’s effort toredress the current imbalance of risk assessment. Theconcerns addressed here fall under the banner of eco-logical integrity, which I consider an ‘umbrella concept’that incorporates such concepts and assessment criteriaas biodiversity, stability, resilience, sustainability, andnaturalness (Noss, 1995; Noss et al., 1999). Of thesecomponents, I consider biodiversity the most central.My goal is to provide some considerations for identify-ing high-risk ecosystems (i.e. biologically distinctecosystems at high risk of losing biodiversity and eco-logical integrity), selecting appropriate indicators, andsetting objectives for ecological protection and riskmanagement.

2. Components of ecological integrity to consider inrisk assessment

Ecological integrity has been a policy target in sev-eral national and bi-national laws and agreements, in-cluding the USA Water Quality Amendments of 1972(Clean Water Act), the Great Lakes Water Quality

Agreement, and the Canadian National Parks Act (Bal-lantine and Guarraia, 1977; Woodley, 1993; Davis,1995). Nevertheless, objective measures of ecologicalintegrity, especially for terrestrial ecosystems, have beenslow to emerge. Biological or biotic integrity is thecomponent of ecological integrity that has been bestcharacterized. Defined by Karr and Dudley (1981) as‘‘the ability of an aquatic ecosystem to support andmaintain a balanced, adaptive community of organismshaving a species composition, diversity, and functionalorganization comparable to that of natural habitatswithin a region,’’ biotic integrity has been measured inmany case histories by a multimetric index called theindex of biotic integrity, or IBI (see Karr and Chu(1999)).

A few terrestrial applications of the IBI or similarindices have been attempted, but none is yet widelytested and accepted (Noss et al. 1999). Few studies haveattempted to measure the broader property of ecologi-cal integrity, defined by Karr and Dudley (1981) as ‘‘thesummation of chemical, physical, and biological in-tegrity,’’ but many biologists feel that biotic integrity isthe most sensitive and informative component of eco-logical integrity (e.g. J. Karr, P. Angermeier, personalcommunication). Thus, ecological integrity is closelylinked to the concept of biodiversity and, in particular,to ‘native biodiversity’ (Noss and Cooperrider, 1994;Noss et al., 1999). In any case, integrity corresponds toa condition of completeness for a given biogeographicand temporal setting and is closely associated withnatural, relatively unaltered ecosystems containing afull suite of native species (Karr, 1992; Karr and Chu,1999).

Ecosystems can be characterized in terms of theirstructure, function, and composition (Franklin et al.,1981). Similarly, biodiversity and ecological integritycan be decomposed into structural, functional, andcompositional components, and measurable indicatorscan be selected that correspond to these components(Noss, 1990; Noss et al., 1999). A comprehensive indexor collection of metrics should include measures ofstructure, function, and composition at several levels ofbiological organization (e.g. ecosystem, community,species) and be measurable at a variety of spatial andtemporal scales. For example, composition can be mea-sured by reference to particular taxa or functionalgroups of organisms within a community or in terms ofthe vegetation or plant communities mapped across abroader area. Structure can be measured within habi-tats (e.g. snags, downed logs, and vertical layering inforests) or as landscape patterns characterized by levelsof connectivity, contagion, or other metrics. Measuresof function, such as ecological processes, have been lesswidely applied, though disturbance rate and intensityoften explain compositional and structural patterns inecosystems at several scales. Many applied ecologists

R.F. Noss / En6ironmental Science & Policy 3 (2000) 321–332 323

have noted the need to pay more attention to thestructural and functional components of ecosystems.

I will not discuss indicator selection criteria in thisreport, as this topic was treated at length in otherpapers (e.g. Noss (1990), Noss et al. (1999)) and in themany references therein. EPA’s Environmental Moni-toring and Assessment Program (EMAP) has dealtextensively with this problem. It is important to remem-ber, however, that compositional, structural, and func-tional components of ecosystems are interdependent;we cannot change one of these classes of componentsradically without expecting major changes in the othercomponents. For example, widespread habitat fragmen-tation can be expected to lead to declines and extinc-tions of fragmentation-sensitive species and to changesin the ecological functions these species perform. Aninitial set of indicators for monitoring and assessing theecological integrity of an ecosystem, especially on alandscape scale or larger, should probably include amix of compositional, structural, and functional met-rics. With field validation and statistical analyses of therelationships among indicators, the initial set can benarrowed to a workable list.

In the context of risk assessment, changes in indica-tors of ecological integrity away from the range ofvariation found in reference landscapes are suggestiveof increasing risks to native species and ecosystems.Many deleterious changes in biodiversity caused byhabitat loss and degradation have been documented inthe bountiful literature of conservation biology over thelast three decades (Noss and Cooperrider, 1994; Meffeand Carroll, 1997). Few, if any, ecosystems worldwidehave escaped impacts from human activities, but abroad spectrum of highly degraded to near-pristine sitescan be found in many regions. Hence, ecological riskassessments concerned with ecological integrity shouldvirtually always contain a mix of retrospective andprospective analyses (US Environmental ProtectionAgency, 1998). We need to know the effects of pastactivities in order to forecast future impacts underalternative management scenarios.

I emphasize that despite well-known impacts of pol-lution and toxins on organisms and ecosystems, and thelikely impacts of global climate change and other atmo-spheric alterations, biologists agree that direct habitatalteration (including loss, degradation, and fragmenta-tion of habitats) is the major proximate cause of bioticimpoverishment today, and has been for a long time(Diamond, 1984; Wilson, 1985; Wilcox and Murphy,1985; Ehrlich and Wilson, 1991; Soule, 1991; Noss andCooperrider, 1994; Flather et al., 1994; Meffe andCarroll, 1997; Wilcove et al., 1998; Stein et al., 2000).Introduced species, which generally respond positivelyto habitat alteration, are becoming an increasinglysevere problem, and most authorities agree they ranksecond in importance (Wilcove et al., 1998; Stein et al.,

2000). Therefore, if the EPA is going to address risks tobiodiversity and ecological integrity effectively, it mustexpand beyond traditional concerns and explicitly con-sider land uses and species invasions as the primaryissues in ecological risk assessment and management.

3. Identifying high-risk ecosystems

All ecosystems on earth are at some risk of modifica-tion and degradation by human activities. Efforts tocontrol greenhouse gases, chlorofluorocarbons, excessnitrogen, and toxic chemicals benefit a wide range ofecosystems and, ultimately, the entire biosphere. Landuse and species invasions, however, tend to have moreconcentrated impacts. Hence, a geographic approach isessential to addressing risks to biodiversity and ecologi-cal integrity. As I have discussed elsewhere, focusingdirectly on ecosystems as units of concern, even ifdefined loosely and in an ad hoc fashion, has manyadvantages over a species-by-species or threat-by-threatapproach (Noss, 1996; Noss et al., 1997). Current ad-vances in ecosystem classification will soon make com-parisons between studies that employ differentclassifications more feasible.

The sensitivity of ecosystems to human activitiesappears to vary, but not in entirely predictable ways.The most predictable, and readily explainable, patternin developed regions is that land areas near water (i.e.coastlines, river shores) and bodies of water in devel-oped landscapes are likely to be most heavily affectedby human activities. These areas are essentially theendpoints of human activity across watersheds andoften contain dense human populations. Low-elevationlandscapes with low topographic relief and productivesoils generally tend to be highly modified and/or atgreat risk of modification. These landscapes tend tohave much lower levels of protection in reserves thanlandscapes of higher elevation and poorer soils (Scott etal., 2001). Overwhelmingly, the most threatened regionsof North America and the world are those with highhuman population density and/or high levels of ex-ploitation of natural resources (Noss et al., 1995; Nossand Peters, 1995; Ricketts et al., 1999). The studies justcited, in addition to others (e.g. Grossman et al. (1994),Abell et al. (2000), Stein et al. (2000)) considered high-risk ecosystems and regions in the USA or NorthAmerica from several, related perspectives. Below Isummarize some of the kinds of ecosystems that shouldbe considered high-risk and, therefore, be priorities forrisk assessment and management by EPA and otheragencies. This discussion addresses the ‘Ecosystems Po-tentially at Risk’ section of Text Box 3-4 of the EPA’sGuidelines for Ecological Risk Assessment (US Envi-ronmental Protection Agency, 1998).


3.1. Rare ecosystems (plant communities)

Traditionally, rarity has been the major criterionused by conservationists to prioritize species andecosystems for protection. For example, the Global/State (G/S) ranking system applied by The NatureConservancy and associated state heritage programsconsiders the conservation status of species and plantcommunities primarily in terms of number of occur-rences, number of individuals (for species), and areacovered (for communities). Species and communitieshaving five or fewer occurrences, less than 1000 individ-uals, or less than 2000 acres worldwide are consideredcritically imperiled globally (G1). Species and commu-nities having 6–20 occurrences, 1000–3000 individuals,or 2000–10 000 acres are considered imperiled globally(G2); and so on through G5, which comprises speciesand communities that are demonstrably secure on aglobal scale (Master, 1991; Noss et al., 1995; Andersonet al., 1998). The same numerical criteria are appliedfor determining the conservation status of species orcommunities within state boundaries, as denoted byS-ranks (S1, S2, etc.). Some states (e.g. California) usemodifications of ranks (e.g. S1.1, S1.2) to reflect thelevel of threat. Nevertheless, rarity is the primary crite-rion for priority-setting under this system. The empha-sis on rarity is justifiable to the extent that smallpopulations are generally more vulnerable to extinction,as are species composed of very few populations (Soule,1987). Similarly, plant communities that are very smallin extent (i.e. G1) could be eliminated by even a smalldevelopment, road, mine, or resource managementactivity.

The recently completed National Vegetation Clas-sification System (Grossman et al., 1998; Anderson etal., 1998; http://consci.tnc.org/library/pubs/class/index.html) is useful for considering the status of terrestrialplant communities in the USA. The classification sys-tem is combinational, with the upper levels of thesystem based on the structure of the vegetation and oncharacteristics of the leaves, and the lower levels basedon species composition. More than 4100 types havebeen defined at the lowest level of the system, theassociation, although classification of associations insome categories and regions is incomplete. The conser-vation status of each association is denoted in terms ofThe Nature Conservancy’s Global ranking system de-scribed above. An earlier book (Grossman et al., 1994)concentrated on the rarest plant communities.

Very rare plant communities (i.e. those ranked as G1and G2) obviously are worthy of attention in riskassessment and management. At present, 10% ofclassified associations are ranked as G1 and 18% as G2;an additional 22% of associations are ranked as G3(vulnerable) (Grossman et al., 1998). Unfortunately, acomprehensive mapping of vegetation at the association

level is unavailable nationally; such maps have beenprepared only for a few small areas subject to intensivestudy. Nevertheless, the general distribution of highlyimperiled communities is known. These associationstend to be concentrated in the herbaceous, forest, andwoodland classes of vegetation; deciduous woodlandsare more imperiled than evergreen or mixed woodlands.States with the highest numbers of imperiled associa-tions are Oregon, Washington, Idaho, California,Texas, Florida, North Carolina, and Virginia. Whenevaluated on an ecoregional basis, however, the Mid-west and Appalachian Mountains have the highestnumbers of imperiled associations (Grossman et al.,1998). More informative for risk management arefigures on the percentages of all associations withinregions that are imperiled at the G1 and G2 levels. Bythis measure, the Southeastern Coastal Plain, Florida,Hawaii, the Central Valley of California, and theWillamette Valley of Oregon stand out as most imper-iled (Grossman et al., 1998).

As noted, The Nature Conservancy’s rankings arebased primarily on rarity. Rarity, however, is only oneof several legitimate criteria for conservation, and mustbe viewed within a broader context. It is important toconsider whether a plant community is rare naturally oras a result of human activities. Just as many rare plantshave been confined to small areas throughout theirevolutionary histories (Holsinger and Gottlieb, 1991),many rare plant communities presumably have beenlocalized since long before Europeans entered NorthAmerica. Others, such as longleaf pine and tallgrassprairie, were once the dominant vegetation across entireregions when the Europeans arrived, but today persistin their natural condition only as isolated patches.Hence, extent of decline often may be preferable torarity as a conservation criterion and is often closelyassociated with future risk.

3.2. Biologically distinct ecosystems (ecoregions)

Biologically distinct ecoregions are those with highlevels of species richness, endemism, and other out-standing biological qualities. A recent, data-intensivestudy by the World Wildlife Fund (Ricketts et al., 1999)evaluated the terrestrial ecoregions of the USA andCanada according to biological distinctiveness and con-servation status. A companion study, which addedMexico, evaluated freshwater ecoregions (Abell et al.,2000). Similar studies have been or are being under-taken by the World Wildlife Fund on other continents.In the terrestrial study biological distinctiveness wasmeasured by species richness and endemism of seventaxonomic groups (native vascular plants, terrestrialmollusks, butterflies, amphibians, reptiles, birds, andmammals; more than 20 000 species were considered),uniqueness of higher taxa, unusual ecological or evolu-


tionary phenomena, and the global rarity of the majorhabitat type (tundra, boreal forest/taiga, temperateconiferous forests, etc.). Each ecoregion was comparedby these criteria with other ecoregions of the samemajor habitat type continentally and globally (Rickettset al., 1999). The freshwater assessment took a similarapproach. The ecoregions comprised watersheds, andthe biological distinctiveness index was applied usingthe same criteria but with consideration of five differenttaxonomic groups: fishes, crayfishes, unionid mussels,and amphibians and reptiles that depend on aquatichabitats (Abell et al., 2000).

Thirty-two terrestrial ecoregions in the USA andCanada were found to be globally outstanding in termsof biological distinctiveness (Ricketts et al., 1999). Nineof these ecoregions (Table 1) are globally outstandingby two or more criteria. All 32 globally outstandingecoregions deserve emphasis in risk management, be-cause loss of biodiversity in these regions is not onlyregionally and nationally significant, but is a globalconcern. The aquatic assessment (Abell et al., 2000)recognized 16 globally outstanding ecoregions in NorthAmerica, seven of which are in the USA (Table 2) andthe others in Mexico.

Researchers with The Nature Conservancy identifiedhotspots of species rarity and richness, both terrestrialand aquatic, in the USA using a ‘rarity-weighted rich-ness index,’ which highlights regions with large num-bers of limited-range species (Chaplin et al., 2000).Three tiers of hotspots emerge from the analysis (Table3) and generally overlap the globally outstanding ecore-gions identified by the World Wildlife Fund.

3.3. Ecosystems showing major declines

As noted earlier, rarity of species and ecosystems is acommon conservation criterion, but may be less impor-tant ecologically than extent of decline. Some rarespecies and communities have always been rare andmay not be particularly vulnerable to extinction. On the

other hand, a major decline in a once-dominant orwidespread species or ecosystem type may have ecolog-ical consequences far more severe than the loss of thelast few individuals of a chronically rare species or theloss of a plant community that never covered morethan a small area.

In the early 1990s, my colleagues and I undertook acomprehensive review of published and unpublishedliterature on the extent of decline of ecosystems for theNational Biological Service (Noss et al., 1995). Wedefined decline to include significant degradation of thestructure, function, or composition of an ecosystem, aswell as areal losses. We assumed that qualitative lossesof ecosystems (for example, as a result of fire suppres-sion or invasion and domination by exotic species) canbe as important ecologically — and have as great aneffect on biodiversity — as direct, quantitative losses.Because no national classification existed at that time,we used whatever classifications were available andlumped types as appropriate to summarize information.We reported information on changes in ecosystem con-dition (e.g. losses of undammed rivers, old-growth andprimary forests, grasslands ungrazed by livestock) thatwould usually not be reflected in statistics linked toclassified plant communities (e.g. Anderson et al.(1998), Grossman et al. (1998)).

We organized the results of our review by classifyingecosystems as critically endangered (\98% declinesince European settlement), endangered (85–98% de-cline), or threatened (70–84% decline). The lists weredominated by ecosystems in the Northeast, Southeast,Midwest, and California, reflecting the earlier settle-ment by Europeans, more intensive land uses, andperhaps more intensive scientific study in these regions.Forest, grassland, and savanna communities dominatedthe lists, especially in the critically endangered category(Noss et al., 1995). Aquatic ecosystems were under-rep-resented in the data, presumably because losses of thesecommunities have not been as well studied (in mostregions, aquatic habitat and community classifications

Table 1Nine terrestrial ecoregions in the USA and Canada that were found to be globally outstanding by at least two biological distinctiveness indexcriteriaa

Richness EndemismEcoregion name Rare habitat type Rare phenomenon

XSoutheastern conifer forests XXX XCalifornia interior chaparral and woodlands XXCalifornia montane chaparral and woodlands XXCalifornia coastal sage and chaparral X

XHawaiian moist forests XXXHawaiian dry forests

X XAppalachian/Blue Ridge forestsX XSoutheastern mixed forestsX XChihuahuan deserts

a Another 20 ecoregions received globally outstanding scores in only one criterion, and three ecoregions did not receive globally outstandingscores in any single criterion, but accrued enough points from all the criteria to achieve this rank (Ricketts et al. 1999).


Table 2Aquatic ecoregions of the USA recognized as globally outstanding in biological distinctiveness (Abell et al., 2000)

Teays–Old Ohio High species richness and endemism across taxa, second only to Tennessee-Cumberland ecoregion in total numberof species; most northerly globally outstanding ecoregion; named for the historic Teays River, which flowed beforethe last glaciation

Tennessee Probably the most diverse temperate freshwater ecoregion in the world; most species-rich in fishes (231 species, 29%endemic), mussels (125 species, 16% endemis), and crayfishes (65 species, 62% endemic) and has highest freshwater

–Cumberlandendemism of all North American ecoregions

Mississippi High species richness, especially of fishes (206 species, second only to Tennessee-Cumberland and tied withTeays-Old Ohio); served as a glacial refugiumembaymentHighest aquatic diversity and endemism of freshwater taxa in the eastern Gulf coast; once supported the mostMobile Baydiverse assemblage of aquatic snails in the world (120 species), but many are now extinctRanging from eastern Georgia to southern Virginia, has high richness and endemism of fishes, mussels, andSouth AtlanticcrayfishesAlthough possessing only a modestly rich fish and mussel fauna, has high habitat diversity and very rich andFloridaendemic crayfish fauna (36 species, 81% endemic); many cave-dwelling aquatic invertebrates

Pacific Central Valley Sacramento-San Joaquin river system is a center of evolution, making this one of the richest ecoregions in NorthAmerica west of the Rocky Mountains; 49 native freshwater fish (29% endemic)

are lacking). Subsequent analysis suggested that dis-ruption of ecological processes was perhaps as impor-tant as outright conversion in causing ecosystemdeclines. Some 80% of all critically endangered, 39%of endangered, and 36% of the threatened ecosystemsare plant communities that depend on recurring firefor their perpetuation (Noss, 1999). Introduced speciesare known to be problematic in most of the imperiledecosystems (Noss et al., 1995; Noss and Peters, 1995).

Because many environmental programs are carriedout by or in cooperation with state agencies, we un-dertook further analysis of endangered ecosystems bystate (Noss and Peters, 1995). States were ranked ac-cording to how many endangered ecosystems theycontain, how many imperiled species they have, andhow much development is occurring (based on humanpopulation growth rates, change in population den-sity, amount of land developed, and rate of increasein developed land, all measured over the precedingdecade). The ten states with the greatest overall riskto ecosystems are Florida, followed by California andHawaii (tied), Georgia, North Carolina and Texas(tied), South Carolina and Virginia (tied), and Ala-bama and Tennessee (tied). Several states, particularlyin the Midwest, scored low in present risk to ecosys-tems because so little natural area remains. In Iowa,for example, agriculture has already destroyed 95% ofthe native prairies, forests, and wetlands (Noss andPeters 1995).

The report by Noss and Peters (1995) also summa-rized the data on endangered ecosystems from Noss etal. (1995) by lumping ecosystem types into broad cat-egories and scoring them according to extent of de-cline since European settlement, present area,imminence of threat, and number of associated threat-ened and endangered species. Twenty-one broadecosystem types were identified as most endangeredby these criteria; a twenty-second type, shrublands

and grasslands of the Intermountain West, was addedlater when more data became available (Table 4).These types are not organized by any particular clas-sification scheme, because some of them (e.g. largestreams and rivers, caves and karst systems) spanmany geographical regions.

3.4. Ecosystems with high numbers of imperiled species

It stands to reason that, as an ecosystem declines inarea or quality, the species associated with that

Table 3The hot spots of the USA, as identified by a rarity-weighted richnessindex employed by The Nature Conservancy (Chaplin et al., 2000).Three tiers of priority were recognized.

Tier 1 (the hottest spots)Hawaii (as a whole)San Francisco Bay AreaCoastal and Interior Southern CaliforniaDeath Valley RegionSouthern Appalachians (where Tennessee, Kentucky, and Virginia

meet)Florida Panhandle (Apalachicola Lowlands)

Tier 2South FloridaCentral AlabamaBig Bend Region of TexasSouthern Channel Islands and Central Coast of CaliforniaSierra Nevada Mountains

Tier 3Klamath-Siskiyou Ecoregion of Northern California and

Southwestern OregonColorado Plateau of Utah and ArizonaEdwards Plateau of TexasSky Islands of Southern Arizona and New MexicoCentral Ridge of FloridaBlue Ridge Escarpment Gorges of Southwestern North CarolinaLimestone Region of Northern Alabama and South-Central

Tennessee


Table 4The most highly endangered major ecosystems of the USA, as determined by a coarse analysis of extent of areal decline since European settlement,current rarity (areal extent), number of endangered and threatened species associated with each type, and level and urgency of continuing threats(each criterion ranked on a scale of 1–5)a

Imminence ofPresent areaExtent ofEcosystem T and E species Total scoredecline threat

5 5 3 18South Florida landscape 54 55 2Southern Appalachian spruce–fir forests 16

5Longleaf pine forests and savannas 2 4 5 163Eastern grasslands, savannas, and barrens 3 4 5 15

3 45 3Northwestern native grasslands and savannas 15California native grasslands 5 1 4 5 15

4 1 5 5Coastal communities (terrestrial, marine, estuarine) in 15the lower 48 States and Hawaii

3 4Southwestern riparian communities 53 153 2 4 5Southern California coastal sage scrub (and 14

associated communities)1 5Hawaiian dry forest (and associated communities) 53 14

4 1 4 5Large streams and rivers in the lower 48 States and 14Hawaii

1 5Cave and karst systems 53 141 44 4Tallgrass prairie 13

California riparian communities and wetlands 3 1 4 5 133 41 5Florida scrub 131 2 4Ancient eastern deciduous forest 125

3 1 3 5Ancient forests of the Pacific Northwest (including 12redwoods)

5 3Ancient red and white pine forests of the Great 2 1 11Lakes States

1 4Ancient ponderosa pine forests 33 111 4Shrublands and grasslands of the intermountain west 41 101 33 2Midwestern wetlands 91 3 3Southern forested wetlands 81

a Adapted from Noss and Peters (1995).

ecosystem also decline. As noted earlier, habitat destruc-tion is the chief threat to biodiversity in the USA andworldwide. The most thorough review of threats toimperiled species in the USA (Wilcove et al., 1998)found that 85% of all imperiled species are threatenedby habitat degradation or loss (including 92% of verte-brates, 87% of invertebrates, and 81% of plants; otherpercentages were reported for taxonomic subcategories).Alien species were second in importance, threatening49% of all imperiled species, 47% of vertebrates, 27% ofinvertebrates, and 57% of plants. Pollution threatened17% of all imperiled species. However, because pollu-tion was defined to include siltation, aquatic specieswere heavily affected — for example, 66% of fishes and90% of mussels. Siltation could have been just as cor-rectly placed in the habitat degradation category, whichwould have elevated the importance of that threatconsiderably. Overexploitation affected 17% of all im-periled species (highest for reptiles, at 66%), whereasdisease threatened only 3% (Wilcove et al., 1998).

Noss et al. (1995) tallied federally listed threatened,endangered, and candidate species for three endangeredecosystems: late successional forests in western Oregon,

Washington, and northwestern California; coastal sagescrub in southern California; and longleaf pine andwiregrass communities in the southeastern coastal plain.The longleaf pine/wiregrass ecosystem contained, as of1993, an astounding 27 federally listed species and 99species that were candidates or proposed for listing. Ofthe 22 most endangered, major ecosystems in the USA(Table 4), the longleaf pine; eastern grasslands, savan-nas, and barrens; California native grasslands; coastalcommunities; southwestern riparian communities;southern California coastal sage scrub; Hawaiian dryforests; large streams and rivers; cave and karst systems;California riparian and wetland communities; Floridascrub; and ancient forests of the Pacific Northwest havelarge numbers of threatened and endangered speciesassociated with them (Noss and Peters, 1995).

The outstanding lesson from these examples is that,instead of addressing the loss of biodiversity species-by-species, site-by-site, or threat-by-threat, it makes muchmore sense to focus on entire ecosystems. Preparingrecovery plans, habitat conservation plans, and riskassessment and management plans at the ecosystemlevel would be much more efficient than preparing


individual recovery plans for each of the thousands ofimperiled species in this country.

3.5. Ecosystems with the greatest threats

As shown in Table 4, many major ecosystems faceimminent threats to their biodiversity and ecologicalintegrity (Noss and Peters, 1995). A more comprehen-sive approach was taken by Ricketts et al. (1999), whoassessed the conservation status of 116 terrestrial ecore-gions in the USA and Canada, and by Abell et al.(2000), who similarly assessed the status of 76 freshwa-ter ecoregion in the USA, Canada, and Mexico. Thecriteria used by Ricketts et al. (1999) to determineconservation status were landscape-scale measures: ex-tent of habitat loss and degradation, size and numberof remaining habitat blocks, and amount of protectedarea, whereas Abell et al. (2000) used degree of landcover (catchment) alteration, water quality degradation,alteration of hydrographic integrity, degree of habitatfragmentation, additional losses of original intact habi-tat, effects of introduced species, and direct speciesexploitation. In both studies, a consideration of futurethreat was added to a ‘snapshot’ assessment of currentstatus. In the terrestrial assessment, the final conserva-tion status assessment (including threat) revealed nearlyhalf of all ecoregions to be critical (28%) or endangered(18%); these ecoregions are concentrated in the Mid-west and Great Plains, the Eastern Seaboard, theSoutheast, the Willamette Valley and Puget Lowlandsof the Northwest, and the Central Valley, North Coast,and Coastal Sage and Chapparal of California. Only14% of all ecoregions, all in the far north of Canada,were found to be relatively intact (Ricketts et al., 1999).Similarly, the assessment of freshwater ecoregions, aftermodification by threat, classified 28% of the ecoregionsas critical and 32% as endangered. These ecoregionswere concentrated in the western USA, Baja California,north-central to south-central Mexico, the Great Lakesregion, and the southeastern coastal plain (Abell et al.,2000). Hence, analyses of conservation status andthreats converge on a similar set of regions and ecosys-tems as those that have shown major degradation anddecline since European settlement and those with highnumbers of imperiled species (Noss and Peters, 1995).

3.6. Ecosystems that stand to lose the most in the nearfuture

Considering all of the criteria just reviewed, theecosystems that arguably warrant the most immediateand concerted attention are those that are biologicallyrich and distinctive, still have considerable biodiversityremaining, but are at high risk of losing it soon. Theecosystems listed in Table 4 mostly all stand to losesignificant biodiversity in the near future unless current

threats are ameliorated. Ricketts et al. (1999) integratedtheir biological distinctiveness and conservation statusrankings for ecoregions and grouped ecoregions intofive classes of conservation priority and action:

3.6.1. Class IGlobally outstanding ecoregions requiring immediate

protection of remaining habitat and extensive restora-tion. These are the highest-risk ecoregions, where hu-man population growth, urban development, orexcessive resource exploitation threaten globally out-standing concentrations of biodiversity. Twenty-one

Table 5Terrestrial ecoregions of the USA and Canada ranked as Class I(globally outstanding ecoregions requiring immediate protection andrestoration of remaining habitat and extensive restoration) and ClassII (regionally outstanding ecoregions requiring immediate protectionof remaining habitat and extensive restoration) by World WildlifeFund (Ricketts et al. 1999)

Class I ecoregionsHawaiian moist forestsHawaiian dry forestsAppalachian/Blue Ridge forestsAppalachian mixed mesophytic forestsSoutheastern mixed forestsSoutheastern conifer forestsFlorida sand pine scrubEvergladesNorthern cordillera forestsQueen Charlotte IslandsBritish Columbia mainland coastal forestsCentral Pacific coastal forestsKlamath-Siskiyou forestsNorthern California coastal forestsSierra Nevada forestsCalifornia interior chaparral and woodlandsCalifornia montane chaparral and woodlandsCalifornia coastal sage and chaparralCentral tall grasslandsFlint Hills tall grasslandsChihuahuan deserts

Class II ecoregionsSouth Florida rocklandsCentral USA hardwood forestsOzark mountain forestsPiney woods forestsMiddle Atlantic coastal forestsPalouse grasslandsCalifornia Central Valley grasslandsWestern short grasslandsCentral forest/grassland transition zoneEdwards plateau savannasTexas blackland prairiesWestern Gulf coastal grasslandsTamaulipan mezquitalEastern Canadian forests


terrestrial ecoregions in Canada and the USA fell intothis class (Table 5). These ecoregions form two majorgeographic aggregations: the Southeastern USA andthe North American Pacific Rim. The Tallgrass Prairieand Chihuahuan Desert areas are outliers.

3.6.2. Class IIRegionally outstanding ecoregions requiring immedi-

ate protection of remaining habitat and extensiverestoration. These ecoregions are almost as high-risk asClass I, and include 14 ecoregions concentrated in themidwest and central states, with a few outliers (Table5).

3.6.3. Class IIIRare opportunities to conserve large blocks of intact

habitat in globally or regionally outstanding ecore-gions. These ecoregions, though biologically distinct,are not at as high risk of impoverishment. This classincludes 19 ecoregions concentrated in the arctic andboreal region of Canada and Alaska and in the south-western states.

3.6.4. Class IVBioregionally and nationally important ecoregions

requiring protection of remaining habitat and extensiverestoration. Twenty-four ecoregions fall into this mod-erate-risk group, including much of the northeasternUSA, Great Lakes states, and Great Plains.

3.6.5. Class VBioregionally and nationally important ecoregions

requiring protection of representative habitat blocksand proper management elsewhere for biodiversity con-servation. This collection of ecoregions includes muchof Canada (including the Great Lakes, Hudson Bay,and High Arctic regions), as well as much of Alaska,the Rocky Mountains (USA and Canada), and theGreat Basin.

Within each of the high-risk ecoregions (i.e. Classes Iand II; Table 5) identified by Ricketts et al. (1999) areparticular plant communities and other ecosystems thatare especially rich in biodiversity and/or vulnerable tohuman activities (e.g. see Noss et al. (1995), Noss andPeters (1995), Anderson et al. (1998), Grossman et al.(1998)). These highly distinct, highly threatened ecosys-tems are arguably the topmost priorities for detailedrisk assessment and aggressive risk management in thenear term.

In contrast to the terrestrial assessment (Ricketts etal., 1999), the experts involved in the aquatic ecoregionassessment (Abell et al., 2000) agreed that the ecore-gions rated as ‘critically imperiled’ are likely beyondrepair and that, therefore, conservation actions should

be focused on the ‘endangered’ and ‘vulnerable’ cate-gories with globally outstanding biodiversity, whereconservation actions would have a greater chance ofsuccess. Considering future threats, the final assessmentselected two freshwater ecoregions — the Teays–OldOhio and the Tennessee–Cumberland (see Table 2) —as the highest priorities for conservation in the USA

3.7. Identifying focal species

Risk assessors and managers sometimes assume thatan ecosystem or landscape focus obviates the need toconsider individual species. In many cases, however, theresponses of species or functional groups of species arehighly informative of the status and condition of anecosystem. Indices of landscape pattern, for instance,are meaningless unless they are tied to the requirementsof species. As noted by Lambeck (1997), ‘‘althoughapproaches that consider pattern and processes at alandscape scale help to identify the elements that needto be present in a landscape, they are unable to definethe appropriate quantity and distribution of those ele-ments…Therefore, we cannot ignore the requirementsof species if we wish to define the characteristics of alandscape that will ensure their retention.’’ Informationon the responses of focal species to changes in habitatstructure and landscape pattern is crucial for interpret-ing such changes and for validating landscape-levelmetrics and indices.

The problem of selecting appropriate focal speciesfor conservation has been investigated by several au-thors (e.g. Hunter (1990), Noss (1990), Lambeck (1997),Noss et al. (1997), Miller et al. (1998/99)). Focal speciesoften are selected to aid in reserve selection and designdecisions and are discussed in the context of monitoringand adaptive management. I believe they also holdmerit for use in risk assessment and management. In-deed, the EPA’s risk management guidelines provideseveral examples of demographic parameters (e.g.breeding success, survivorship) of particular species be-ing used as assessment endpoints (US EnvironmentalProtection Agency, 1998). I suggest that focal specieswill be especially useful in characterizations of ecologi-cal effects, including measures of effect, ecological re-sponse analysis, development of stressor-responseprofiles, and risk estimation and description (see USEnvironmental Protection Agency, 1998, Figs. 1–2).

Among the first steps in characterizing the specificrisks present in ecosystems is to identify the trends ofimpoverishment — those trajectories of change that,either in the past or today, threaten the ecosystem interms of contributing to population declines of nativespecies and other losses of biodiversity. For example,for forests these trajectories include changes in agedistributions from old growth to young forest, simplifi-


cation of forest structure, loss of overall forest area,reduction in size and increasing isolation of forestpatches, disruption of natural fire cycles, increases inroad density, and so on (Noss, 1993). Indicators can beselected to track recovery along the same axes that ledto declines. Recovery seems an appropriate objectivefor risk management. Among the many potential indi-cators are species that are especially sensitive to trendsof impoverishment. Groups of species can be identifiedwhose vulnerability is attributable to a common cause,and species in each group can be ranked in terms oftheir vulnerability to those threats. Finally, focal speciescan be identified whose requirements for protection ormanagement encompass all others in their group, i.e. an‘umbrella’ function (Lambeck, 1997).

Lambeck (1997) identified area-limited species, dis-persal-limited species, resource- limited species, andprocess-limited species as vulnerability groups. Foreach group the focal species are the ones most demand-ing for the attribute used to define the group. Forexample, the species with the largest home ranges orlowest population densities will define the needs of thearea-limited group; the species most limited in mobilitydefine the dispersal-limited group; the species mostdependent on resources that are occasionally in criti-cally short supply define the resource-limited group;and the species most dependent on frequent fire, peri-odic flooding, or other processes define the process-lim-ited group. More than one species might be selectedwithin a group, and a single species may occur in morethan one group. The needs of the selected focal speciesdefine the thresholds — in terms of patch size, isola-tion, disturbance frequency, etc. — that must be ex-ceeded if the native biota in the region is to bemaintained (Lambeck, 1997).

To Lambeck’s four groups of focal species, Noss etal. (1997) added three categories: keystone species, nar-row endemics, and special cases. Of these, keystonespecies, ‘‘whose impact on its community or ecosystemis large, and disproportionately large relative to itsabundance’’ (Power et al. 1996), are perhaps mostimportant in the context of risk assessment and man-agement. These species, which are often the sole mem-bers of their functional groups (Walker, 1995) includepredators that enhance species diversity; engineer spe-cies, such as beavers (Castor canadensis), gopher tor-toises (Gopherus polyphemus), alligators (Alligatormississippiensis) and cavity-excavating birds, which cre-ate and maintain habitats used by many other species;key pollinators and seed dispersers; plants that provideresources utilized by many animal species at criticaltimes of the year; and large herbivores that controlvegetation structure through their feeding, trampling,and other activities (Noss, 1991; Power et al., 1996;Simberloff, 1998). Maintaining populations of keystonespecies that are ecologically optimal, not just minimally

viable, is advisable because so many other species de-pend on them. Risks to ecological integrity can behypothesized to increase with the reduction in popula-tions of keystone species.

4. Conclusions

Ultimately, attaining the broad goals of conservingbiodiversity and ecological integrity requires attentionto all ecoregions, states, and ecosystems. When re-sources are limited, however, ecosystems that stand tolose the most biodiversity in the near future are clearpriorities for conservation actions. In this paper, I havereviewed the results of several studies that identifyecoregions, states, and particular ecosystem types thatwarrant greatest attention from EPA and other agen-cies that engage in one essential type of conservationaction: risk assessment and management. These agen-cies should avail themselves of these data, which repre-sent the most comprehensive information yet collectedon risks to biodiversity, to set priorities for risk man-agement. Agencies and organizations that engage inother kinds of conservation action — land acquisition,ecosystem management, restoration, and so on — canuse these same data to determine priorities in theirareas of emphasis. For example, the Conservation Re-serve Program, established in the 1985 Farm Bill to setaside highly erodible or otherwise environmentally sen-sitive land from agricultural production, uses the list ofendangered ecosystems from Noss et al. (1995) to iden-tify priority lands to set aside as reserves.

A first step in implementing risk management inhigh-risk ecosystems would be to address the questionsin Text Box 3-4 of US Environmental ProtectionAgency, (1998) concerning source and stressor charac-teristics, exposure characteristics, and ecological effects.I have suggested that carefully selected focal species willbe useful in answering many of these questions. Thequestions under the category of ecosystems potentiallyat risk have been addressed in a general way by thestudies reviewed in this paper, but intensive studieswithin high-risk ecoregions and other ecosystems areneeded to add detail to the general patterns that areemerging. The most fruitful approach to risk manage-ment ultimately will be one that addresses the mosturgent threats — particularly habitat alteration andexotic species invasions — at several levels of biologicalorganization, from focal species to communities toecoregions.

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Reed Noss is Chief Scientist for Conservation Science, Incorpo-rated, an international consultant and lecturer, President of theSociety for Conservation Biology (1999–2001), and Science Editorfor Wild Earth magazine. He is an Adjunct Professor of Biology atthe University of Oregon, where he teaches conservation biology,and has courtesy appointments in Forest Science and Fisheries andWildlife at Oregon State University. He has a MS in Ecology fromthe University of Tennessee and a PhD in Wildlife Ecology fromthe University of Florida. His present research involves the applica-tion of science to conservation planning at regional to global scales.

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