Conservation corridors are a promising intervention to preserve biodiversity, yet most research has explored corridors in a landscape context different from their intended purpose. This Community Page asks readers to help identify landscapes that will help answer the question: Do corridors work

Beier, Paul

Gregory, Andrew J.

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Beier Paul

Gregory Andrew J.

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Desperately Seeking Stable 50-Year-Old Landscapes with Patches and Long, Wide Corridors

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Community PageDesperately Seeking Stable 50-Year-Old Landscapes withPatches and Long, Wide CorridorsPaul Beier*, Andrew J. GregorySchool of Forestry and Merriam-Powell Center for Environmental Research, Northern Arizona University, Flagstaff, Arizona, United States of AmericaIntroductionHuman activities such as urbanizationand roads have disrupted movement andgene flow for plants, reptiles, mammals,sedentary birds, and arthropods [1,2].Indeed, human-caused habitat fragmenta-tion is a leading threat to biodiversity [3].As plant and animal populations becomesmaller and more isolated they becomemore susceptible to stochastic events andreduced genetic diversity via drift andinbreeding [4]. The primary conservationinterventions to counteract habitat frag-mentation are conservation corridors andincreased reserve size; conservation corri-dors are also the most frequently citedrecommendation to conserve the ability ofspecies and ecosystems to adapt to climatechange [5].Because corridors are such a promisingconservation intervention, they are beingdesigned and implemented in many parts ofthe world. For example, one of us (PB) hashelped develop high-resolution plans, eachof which is being implemented by govern-ment agencies and nongovernmental orga-nizations, to conserve corridors in Bhutan[6], coastal southern California (http://www.scwildlands.org), Arizona (http://www.corridordesign.org/arizona), southeasternCalifornia, and northern California. Eachcorridor (n = 72) is a swath of natural land500 m to 80 km long identified forconservation so that it can support geneflow and demographic interactions be-tween a pair of natural landscape blocksafter ‘‘build-out’’—i.e., after lands adja-cent to the corridor and the naturallandscape blocks have been converted tourban, agricultural, or industrial uses thatare incompatible with wildlife movement.Despite the large body of researchconfirming that corridors promote wildlifemovement [7,8], there is no strong evi-dence that these 72 corridors will promotegene flow and demographic movementamong plants and animals connected bythe corridors as intended. Nonetheless, it’sclear that swaths of land much smallerthan conservation corridors and embed-ded in a matrix that is not intenselymodified by human activities do supportpresence and movement of plants andanimals [7,9]. These results suggest thatcorridors may be a useful conservationintervention. However, evidence that suchmovements occur often enough to pro-mote genetic connectivity and patchoccupancy across longer distances inhuman-dominated landscapes is generallylacking (but see [10]).Evidence is lacking primarily becausemost corridor studies have been conductedin landscapes that are vastly smaller anddifferent than the landscapes conservationcorridors are designed for and havemeasured species response variables weak-ly related to corridor utility. That’s whywe’re issuing a call for help to identifyappropriate sites to test the effectiveness ofconservation corridors on a global scale.The Gap between CorridorResearch and Corridor DesignFirst, almost all corridor research hasconcerned corridors ,150 m long, butproposed and implemented conservationcorridors are much longer. Second, inmost ecological studies, a corridor is anynarrow swath of land connecting twohabitat patches where the patches andcorridor share a land cover dissimilar fromthe surrounding matrix, e.g., grasslandpatches and corridors in a forest matrix[11]. This definition depends only onstructural layout of the focal land covertype, regardless of adjacent land uses. Incontrast, each conservation corridor is aswath of natural land that is (or eventuallymay be) embedded in urban, agricultural,or industrial landscapes. Although mostconservation corridors are designated forconservation while the matrix is still in arelatively natural state, they are explicitlypredicted to be useful for species conser-vation after build-out.With respect to response variables,almost all studies of corridor utilitydocument only whether focal species werepresent in or moved through the corridor[7,8]. Although presence and movementare necessary for corridor utility, they donot demonstrate that the corridor enhanc-es demographic stability, gene flow, orrecolonization—which are ultimately theintended outcome of conservation corri-dors [12]. Variables that would demon-strate these outcomes include geneticrelatedness, reflecting effective connectivi-ty among patches [13], and long-termpatch occupancy, reflecting demographicrescue and recolonization [14].Help Us Identify Stable, 50-Year-Old Landscapes withPatches and Long, WideCorridorsWe have designed a research project tocollect the evidence to determine ifconservation corridors work and theconditions (such as width, severity ofThe Community Page is a forum for organizationsand societies to highlight their efforts to enhancethe dissemination and value of scientific knowledge.Citation: Beier P, Gregory AJ (2012) Desperately Seeking Stable 50-Year-Old Landscapes with Patches andLong, Wide Corridors. PLoS Biol 10(1): e1001253. doi:10.1371/journal.pbio.1001253Published January 31, 2012Copyright:  2012 Beier, Gregory. This is an open-access article distributed under the terms of the CreativeCommons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium,provided the original author and source are credited.Funding: This research was supported by a grant from the Gordon and Betty Moore Foundation (http://www.moore.org/). The funders had no role in study design, data collection and analysis, decision to publish, orpreparation of the manuscript.Competing Interests: The authors have declared that no competing interests exist.* E-mail: Paul.Beier@nau.eduPLoS Biology | www.plosbiology.org 1 January 2012 | Volume 10 | Issue 1 | e1001253constrictions, or adjacent land uses) asso-ciated with success. Because planned andimplemented conservation corridors de-signed by wildlife planners are too youngfor genetic and demographic effects to beevaluated, we will study landscapes with‘‘de facto’’ conservation corridors (i.e.,corridors .500 m long, in a human-dominated matrix). A de facto corridorcan be thought of as an accidental corridorthat exists as a quirk of the way thelandscape has been developed. In eachlandscape (n . 50), we will collect DNAsamples from focal species in patchesconnected by corridors, isolated patches,and sampling locations within an intactnatural area. A corridor will be deemedsuccessful if genetic distances amongconnected patches are smaller than geneticdistances among isolated patches andsimilar to genetic distances between sam-pling sites in intact habitat. Focal specieswill vary among landscapes and mayinclude any reptile, amphibian, mammal,flightless arthropod, or sedentary birdassociated with the patches and corridors,but not the human-dominated matrix. Ineach landscape, the configuration ofpatches and corridors must have beenstable for at least 20–50 years, so thatgenetic structure is likely reflective oflandscape pattern. (For more informationon the study design, see http://www.docorridorswork.org/study-site-criteria/).Unfortunately, we have not yet been ableto identify the critical number of land-scapes needed to put conservation corri-dors to the test.It is the authors’ good fortune to live inthe southwestern US, an area with manylarge, intact natural landscapes, and wherefragmentation has occurred too recentlyfor a genetic response to manifest. Weneed readers and colleagues to help usidentify appropriate study systems, whereeach study system consists of a landscapeand focal species. We encourage readers tosuggest appropriate sites, or to directothers who might know of appropriatesites, to http://www.docorridorswork.org.Small honoraria are available for infor-mants who provide leads to study systemsthat meet most of the seven criteria andbecome part of our study (see Box 1).Other ConsiderationsA large sample of study systems prob-ably will include some corridors that work,and others that do not. To determine whatfactors are associated with success orfailure of a corridor, we will select studysystems that vary with respect to fivelandscape variables: corridor length, meancorridor width, severity of constrictions inthe corridor, type of matrix (e.g., high-density urban, rural residential, intensiveagricultural, pasture), and degree of hu-man disturbance (including recreation,artificial night lighting, and vehicle traffic)in the corridor. Because traits of the focalspecies can also affect observed geneticpatterns, our analyses will evaluate corri-dor effectiveness for multiple species thatexhibit a range of additional covariates fortraits such as species mobility or edgesensitivity. Strong inferences about fivelandscape variables and two species vari-ables will require at least 50—preferably100—replicate study systems.Perhaps the greatest risk is that all oralmost all corridors that meet our criteriawill be narrow (e.g., ,100 to 500 m wide)or will have severe bottlenecks (e.g., smallhighway crossing structures) such that veryfew corridors support gene flow. In otherwords, Earth may not contain enoughsuccessful de facto conservation corridorsto identify minimum width or other traitsof successful corridors. Nonetheless, simplyknowing that the minimum width must begreater than 500 m would conclusivelyrule out such narrow corridors as acredible conservation strategy. If only afew broad corridors are in the sample, andthey all succeeded, the results might not beFigure 1. (B) illustrates an ideal study site; white background indicates natural land cover; stippling indicates land converted tourban or agricultural uses incompatible with movement by the focal species. Reference condition 1 consists of habitat patches/blocksseparated by approximately the same Euclidean distance as the connected patches, but surrounded by human-altered matrix for at least 10–20generations of the focal species (B, M–N). The second reference condition is an intact habitat block large enough to allow researchers to obtaingenetic samples at sampling sites spaced at approximately the same Euclidean distance from each other as the connected patches as in (B, Y–Z).Ideally a landscape would have both types of reference conditions. In addition, the land cover in the corridors must be similar to that of the patchesand large natural landscape blocks.doi:10.1371/journal.pbio.1001253.g001PLoS Biology | www.plosbiology.org 2 January 2012 | Volume 10 | Issue 1 | e1001253statistically significant, but they would bebiologically relevant and persuasive.We anticipate many of our studysystems will be in Europe, because manyEuropean landscapes have been built-outfor decades, in contrast to more rapidlychanging landscapes in much of the restof the world. Nonetheless, if we find thatcorridor utility depends on corridorwidth, species mobility, or an interactionbetween corridor width and type ofmatrix, those results should apply broad-ly. Our focus on the impact of a handfulof high-level, generalizable landscape andspecies variables ensures that the resultswill be globally relevant. This focus alsogives the proposal a strong foundation inbasic science, squarely addressing theo-retical issues related to interactions be-tween species life-history strategies andlandscapes, the degree to which inferenc-es from small corridors scale up to largecorridors and landscapes, and the degreeto which presence and movement arereliable indicators of genetic and demo-graphic connectedness.Conservation scientists need to knowwhat factors are associated with successfulwildlife corridors so that they can designand implement effective conservation cor-ridors. Land managers need to know whatland uses and management practices arecompatible with effective corridors. Ecol-ogists need to know how corridor width,internal characteristics of corridors, char-acteristics of the matrix in which corridorsare embedded, and traits of focal speciesaffect corridor utility. We look forward toengaging and working with many col-leagues in a rigorous, global study toaddress these issues.References1. Riley S, Sauvajot R, Fuller T, York E,Kamradt D, et al. (2003) Effects of urbanizationand habitat fragmentation on bobcats andcoyotes in southern California. Conserv Biol 17:566–576.2. Delaney K, Riley S, Fisher R (2010) A rapid,strong, and convergent genetic response to urbanhabitat fragmentation in four divergent andwidespread vertebrates. PLoS ONE 5: e0012767.doi:10.1371/journal.pone.0012767.3. Hunter M, Gibbs J (2007) Fundamentals ofconservation biology. 3rd edition. Oxford: Black-well.4. Keller L, Waller D (2002) Inbreeding effects inwild populations. Trends Ecol Evol 17: 230–241.5. Heller N, Zavaleta E (2009) Biodiversity man-agement in the face of climate change. BiolConserv 142: 14–32.6. Wildlife Conservation Division (2010) Regula-tory framework for biological corridors inBhutan. Thimphu: Royal Government ofBhutan.7. Beier P, Noss R (1998) Do corridors provideconnectivity? Conserv Biol 12: 1241–1252.8. Gilbert-Norton L, Wilson R, Stevens J, Beard K(2010) A meta-analytic review of corridor effec-tiveness. Conserv Biol 24: 660–668.9. Haddad N (2008) Finding the corridor moretraveled. Proc Natl Acad Sci U S A 105:19569–19570.10. Mansergh I, Scotts D (1989) Habitat continuityand social organization of the mountain pygmy-possum restored by tunnel. Journal of WildlifeManagement 53: 701–707.11. HaddadN, BowneD,CunninghamA,DanielsonB,Levey D, Sargent S, Spira T (2003) Corridor use bydiverse taxa. Ecology 84: 609–615.12. Haddad NM, Tewksbury JJ (2006) Impacts ofcorridors on populations and communities.Crooks K, Sanjayan M, eds. Connectivity con-servation. Cambridge: Cambridge UniversityPress. pp 390–415.13. Slatkin M (1993) Isolation by distance in equilib-rium and non-equilibrium populations. Evolution47: 264–279.Box 1. Criteria for Potential Conservation Corridor Study SitesHistorically continuous habitat. Prior to human alteration of the landscape,the natural cover types used by the focal species must have been widespread andrelatively continuous, as in the ‘‘Before’’ panel of Figure 1A. In other words, we willnot study landscapes in which the patches are naturally isolated, such as a groupof naturally disconnected marshes.Focal species restricted to natural matrix and dependent on corridorsfor connectivity. There must be at least one mammal, reptile, amphibian,sedentary bird, or flightless arthropod that is expected to occur in natural patches,but probably cannot disperse through habitat in the matrix. Although bats andflying birds have been shown to travel along linear habitat features, most of themare not suitable focal species because they can maintain demographic and geneticflows without corridors. Focal species will differ among landscapes, and weanticipate that each landscape will have only one or two focal species. Estimatedeffective population sizes (typically 10% to 20% of census population sizes [15]) inthe isolated patches must be low enough for genetic divergence to have occurredduring the period of landscape stability since build-out.At least one corridor and one reference condition in the landscape. Thelandscape must contain at least one corridor and at least one type of referencecondition (Figure 1).Corridor . 0.5 km long. Pairs of patches must be separated by distances. 0.5km, with similar distance between pairs of isolated patches, pairs connected bycorridors, and pairs of sampling locales within intact habitat. Each of thepreviously mentioned 72 conservation corridors is longer than 500 m.Corridor . 100 m wide. The corridor must be .100 m wide (except for shortconstrictions such as a highway crossing structure). Conservation practitionersnever recommend narrower corridors as a conservation intervention [16].Landscape stable for .20–50 years. The configuration of patches, corridors,and matrix must have been stable for at least 20 to 50 years, except for naturaldisturbances. This duration makes it likely that genetic pattern reflects landscapepattern. Although genetic equilibrium is probably never reached for anypopulation in a human-altered landscape, genetic distances approach equilibriumvalues quickly after perturbations [17,18]. More specifically, genetic divergenceshould be evident after ten generations of the focal species for effectivepopulation sizes ,50 per patch and after 20 generations for larger effectivepopulation sizes [13,15,19].Matrix dominated by urban, agricultural, or industrial forestry uses. Thematrix should be dominated by urban, industrial, agricultural, industrial forestry, orany other intense land uses. Secondarily, we will consider landscapes in which thematrix is dominated by semi-natural pasture or forests where logging is constrainedby ecological goals, but only if there is strong evidence that the human-causedalteration presents a strong barrier to movement of the focal species.PLoS Biology | www.plosbiology.org 3 January 2012 | Volume 10 | Issue 1 | e100125314. Hanski I, Gaggiotti O (2004) Ecology, genetics,and evolution of metapopulations. Burlington(Massachusetts) Elsevier Academic Press Inc.15. Waples R (2007) LDNE: a program for estimatingeffective population size form data on linkagedisequilibrium. Mol Ecol Notes 7: 161–164.16. Harrison RL (1992) Toward a theory of inter-refuge corridor design. Conserv Biol 6: 293–295.17. Crow J, Aoki K (1984) Group selection for apolygenic behavioral trait: estimating the degreeof population subdivision. Proc Natl AcadSci U S A 81: 6073–6077.18. Slatkin M (1985) Gene flow in natural popula-tions. Annu Rev Ecol Syst 16: 393–430.19. Wright S (1943) Isolation by distance. Genetics28: 114–138.PLoS Biology | www.plosbiology.org 4 January 2012 | Volume 10 | Issue 1 | e1001253

Desperately seeking stable 50-Year-Old landscapes with patches and long, wide corridors

Paul Beier

Andrew J Gregory

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Desperately Seeking Stable 50-Year-Old Landscapes with Patches and Long, Wide Corridors

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