J. Field Ornithol. 81(4):325–339, 2010 DOI: 10.1111/j.1557-9263.2010.00289.x Predicting potential distributions of two rare allopatric sister species, the globally threatened Doliornis cotingas in the Andes Fr´ ed´ eric Jiguet, 1,3 Morgane Barbet-Massin, 1 and Pierre-Yves Henry 2 1 UMR 7204 MNHN-CNRS-UPMC, Mus´ eum National d’Histoire Naturelle, 55 rue Buffon, CP 51, 75005 Paris, France 2 UMR 7179 MNHN-CNRS, Mus´ eum National d’Histoire Naturelle, 1 avenue du Petit Chˆ ateau, 91800 Brunoy, France Received 28 October 2009; accepted 8 July 2010 ABSTRACT. Conservation of rare, elusive species is difficult because of limited knowledge of their biology and distribution. The two species of Doliornis cotingas are known from 7 (Chestnut-bellied Cotinga, D. remseni) and 13 (Bay-vented Cotinga, Doliornis sclateri) locations, respectively. Their limited ranges in combination with habitat loss make them vulnerable to extinction. We modeled the potential distribution of these two rare, allopatric sister species separated by an orographic barrier using species distribution modeling with an ensemble forecast approach using eight modeling techniques. Predicted distributions (with conservative thresholds of probability of presence, i.e., lowest presence thresholds) for these sister species showed virtually no overlap, although their respective niches were statistically not dissimilar. Hence, the existence of the recently discovered Chestnut-bellied Cotinga could not have been predicted from the range of its sister species, unless using very crude distribution models (with extensive extrapolation). New areas of likely occurrences were identified for both species, and the genus, and will be useful for directing future field searches. The estimates of potentially suitable range for both species still qualify both species as “vulnerable to extinction.” Our study illustrates how opportunistic records collected by field ornithologists can be objectively transformed, with the help of existing software, into information potentially useful in the conservation of rare species. RESUMEN. Predicci ´ on de las distribuciones potenciales de dos especies hermanas alop´ atricas, los globalmente amenazados cotingas Doliornis en los Andes Conservaci´ on de especies escasas es dif´ ıcil por falta de documentaci´ on de su biolog´ ıa y su distribuci´ on. Las dos especies de cotingas Doliornis son conocidas de solo 7 (Cotinga de Remsen, D. remseni) y 13 (Cotinga de Sclater, D. sclateri) registros localizados precisamente. Su peque˜ na ´ area de distribuci´ on y desaparici´ on de su h´ abitat hace que son vulnerables a la extinci´ on. Modelamos la distribuci´ on potencial de esas dos especies hermanas alop´ atricas, separadas por una barrera orogr´ afica, utilizando ocho t´ ecnicas de modelo de distribuci´ on con un m´ etodo de pron´ ostico de ensamble. Las distribuciones predichas (con niveles umbrales de probabilidad de presencia conservativos, el nivel umbral m´ ınimo de presencia) para cada especie hermana eran casi completamente distintas. As´ ı, el rango de distribuci´ on de la especie descubierta recientemente, D. remseni, no hubiera podido ser predicho con los registros de su especie hermana, D. sclateri, aunque hayamos utilizado modelos muy b´ asicos (con extensiva sobre-predicci´ on). Nuevas ´ areas de alta probabilidad de ocurrencia fueron identificadas para cada especie, y el g´ enero, y podr´ an servir para orientar futuros inventarios de campo. Las estimaciones de los rangos potencialmente favorables para cada especie siguen calific´ andolas como ‘vulnerables a la extinci´ on’. Este estudio ilustra como registros masivamente colectados de manera oportunista por ornit´ ologos de campo pueden ser transformados objetivamente, con programas existentes, en conocimientos ´ utiles para la investigaci´ on y la conservaci´ on de especies escasas. Key words: allopatric sister taxa, alloprediction, BIOMOD, distribution range, species distribution modeling, tropical montane forest Field ornithologists are the very first providers of distributional data for rare or elusive species. Following the discovery of new sites of occur- rence, revisions of species distribution ranges are regularly published. Often, only a descriptive or mapping interpretation of these records is 3 Corresponding author. Email: [email protected]provided. Conclusions are highly sensitive to reporting biases, such as stochastic nondetection or inaccessibility of potentially suitable areas (a common problem in the tropics). One method for making a parsimonious use of these scat- tered, anecdotal data is to identify probabilistic species distributions with models. These models combine occurrence data with environmental data to statistically identify areas resembling the C 2010 The Authors. Journal of Field Ornithology C 2010 Association of Field Ornithologists 325 Journal of Field Ornithology
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J. Field Ornithol. 81(4):325–339, 2010 DOI: 10.1111/j.1557-9263.2010.00289.x
Predicting potential distributions of two rare allopatricsister species, the globally threatened Doliornis cotingas
in the Andes
Frederic Jiguet,1,3 Morgane Barbet-Massin,1 and Pierre-Yves Henry2
1UMR 7204 MNHN-CNRS-UPMC, Museum National d’Histoire Naturelle, 55 rue Buffon, CP 51,75005 Paris, France
2UMR 7179 MNHN-CNRS, Museum National d’Histoire Naturelle, 1 avenue du Petit Chateau,91800 Brunoy, France
Received 28 October 2009; accepted 8 July 2010
ABSTRACT. Conservation of rare, elusive species is difficult because of limited knowledge of their biology anddistribution. The two species of Doliornis cotingas are known from 7 (Chestnut-bellied Cotinga, D. remseni) and13 (Bay-vented Cotinga, Doliornis sclateri) locations, respectively. Their limited ranges in combination with habitatloss make them vulnerable to extinction. We modeled the potential distribution of these two rare, allopatric sisterspecies separated by an orographic barrier using species distribution modeling with an ensemble forecast approachusing eight modeling techniques. Predicted distributions (with conservative thresholds of probability of presence,i.e., lowest presence thresholds) for these sister species showed virtually no overlap, although their respective nicheswere statistically not dissimilar. Hence, the existence of the recently discovered Chestnut-bellied Cotinga could nothave been predicted from the range of its sister species, unless using very crude distribution models (with extensiveextrapolation). New areas of likely occurrences were identified for both species, and the genus, and will be useful fordirecting future field searches. The estimates of potentially suitable range for both species still qualify both species as“vulnerable to extinction.” Our study illustrates how opportunistic records collected by field ornithologists can beobjectively transformed, with the help of existing software, into information potentially useful in the conservationof rare species.
RESUMEN. Prediccion de las distribuciones potenciales de dos especies hermanasalopatricas, los globalmente amenazados cotingas Doliornis en los Andes
Conservacion de especies escasas es difıcil por falta de documentacion de su biologıa y su distribucion. Las dosespecies de cotingas Doliornis son conocidas de solo 7 (Cotinga de Remsen, D. remseni) y 13 (Cotinga de Sclater,D. sclateri) registros localizados precisamente. Su pequena area de distribucion y desaparicion de su habitathace que son vulnerables a la extincion. Modelamos la distribucion potencial de esas dos especies hermanasalopatricas, separadas por una barrera orografica, utilizando ocho tecnicas de modelo de distribucion con unmetodo de pronostico de ensamble. Las distribuciones predichas (con niveles umbrales de probabilidad de presenciaconservativos, el nivel umbral mınimo de presencia) para cada especie hermana eran casi completamente distintas.Ası, el rango de distribucion de la especie descubierta recientemente, D. remseni, no hubiera podido ser predichocon los registros de su especie hermana, D. sclateri, aunque hayamos utilizado modelos muy basicos (con extensivasobre-prediccion). Nuevas areas de alta probabilidad de ocurrencia fueron identificadas para cada especie, y elgenero, y podran servir para orientar futuros inventarios de campo. Las estimaciones de los rangos potencialmentefavorables para cada especie siguen calificandolas como ‘vulnerables a la extincion’. Este estudio ilustra como registrosmasivamente colectados de manera oportunista por ornitologos de campo pueden ser transformados objetivamente,con programas existentes, en conocimientos utiles para la investigacion y la conservacion de especies escasas.
Key words: allopatric sister taxa, alloprediction, BIOMOD, distribution range, species distribution modeling,tropical montane forest
Field ornithologists are the very first providersof distributional data for rare or elusive species.Following the discovery of new sites of occur-rence, revisions of species distribution ranges areregularly published. Often, only a descriptiveor mapping interpretation of these records is
provided. Conclusions are highly sensitive toreporting biases, such as stochastic nondetectionor inaccessibility of potentially suitable areas (acommon problem in the tropics). One methodfor making a parsimonious use of these scat-tered, anecdotal data is to identify probabilisticspecies distributions with models. These modelscombine occurrence data with environmentaldata to statistically identify areas resembling the
locations where the species is known to occur(Araujo and New 2007, Pearson et al. 2007,Thuiller et al. 2009). Such niche modeling isbecoming an important tool in conservationand biogeography, providing spatially explicit,predicted distribution ranges that are robustto the usual lack of previous ecological anddistributional knowledge of poorly known taxa(Guisan et al. 2006). This feature is particularlyuseful for species known from a limited num-ber of locations, including poorly detectable,inconspicuous or low-density species, speciesin remote, poorly investigated habitats, or re-cently discovered species (Raxworthy et al. 2003,Engler et al. 2004, Franklin et al. 2009, Mariniet al., 2010). These models are increasingly usedby field ornithologists to exploit bird distribu-tional data (e.g., Rojas-Soto et al. 2008, Henryand Aznar 2009).
From a conservation perspective, niche mod-eling provides a methodological framework foridentifying spatially optimal conservation strate-gies, such as designing networks of priorityareas (Marini et al. 2009a) or targeting con-servation efforts for threatened species (Petersonand Robins 2003, Thorn et al. 2009, Mariniet al. 2009b) and regional diversity hotspots(Myers et al. 2000, Garcıa 2005). From a biogeo-graphical perspective, one achievement of nichemodeling is the ability to predict the distribu-tion of unknown, still to be discovered, speciesfrom modeled distributions of closely relatedknown species in cases of niche conservatismin evolutionary time (Raxworthy et al. 2003) orniche similarity between phylogenetically closelyrelated species (Warren et al. 2008). Theorysupports this assumption of niche conservatism,or similarity between closely related species,predicting low niche differentiation betweenvicariant species over evolutionary time scales(Ricklefs and Latham 1992). Further em-pirical work documented the conservativenature of ecological niche evolution by suc-cessfully extrapolating distributions of closelyrelated species using data from the other species(Peterson et al. 1999), and phylogenetic infor-mation has been combined with species distribu-tion modeling to explore speciation mechanisms(Graham et al. 2004). Indeed, in practice, forclosely related allopatric species pairs, nicheconservatism can be deduced from areas ofoverprediction (i.e., areas where presence is pre-dicted outside of the actual range) in one-species
models that actually fit the known descriptionrange of the other species (Peterson et al. 1999,Raxworthy et al. 2003, Pearson et al. 2007).
Our objective was to improve our knowl-edge of two rare sister, tropical humid mon-tane forest bird species, Bay-vented Cotingas(Doliornis sclateri) and Chestnut-bellied Coting-as (D. remseni), by analyzing previously reported(Appendix 1) and new (Henry 2008) occur-rence data. These species are restricted to hu-mid montane forests at elevations ranging from2740—to 3800 m in the central and northernAndes (Ridgely and Tudor 1994, Ridgely andGreenfield 2001, BirdLife International 2010).Bay-vented Cotingas were discovered in 1871at Maraynioc, Peru (Taczanowski 1874; seeAppendix 1) and subsequently reported at 13other locations, all on the east slope of theeastern Peruvian Andes (Appendix 1). Chestnut-bellied Cotingas were first observed in the fieldin 1989 and described as recently as 1994(Robbins et al. 1994), and are known fromonly 10 locations on the eastern slope of theAndes in Ecuador and from the central Andes ofColombia (Appendix 1). These two species areseparated by an orographic barrier, that is, anarea of low elevation (2000–3000 m) and aridhabitats delimiting the northern and central An-des called the North Peru Low (NPL). The NPLis regarded as a major biogeographic barrier, anda transitional area for many sister-species pairsof the high-elevation montane forest (Cracraft1985, Stotz et al. 1996; but see Parker et al.1985, Garcıa-Moreno and Fjeldsa 1999). Wefirst modeled the distribution range of the twosister species, then tested for evolutionary nichesimilarity (Warren et al. 2008) between thetwo species on both sides of the biogeographicbarrier. We also tested the hypothesis that theexistence and distribution of the most recentlydiscovered species (Chestnut-bellied Cotinga;first described by Robbins et al. 1994) couldhave been predicted with location data fromits sister species described 120 yr earlier (Bay-vented Cotinga). We further used all 20 re-ported locations for both species to model thepotential distribution of the genus Doliornis.If a more robust prediction of the overall lin-eage’s potential distribution could be achieved(Graham et al. 2004), it may be possible toidentify areas where one of the two species,or a still unknown congeneric species, mayremain to be discovered. Identifying such gaps
Vol. 81, No. 4 Distributions of Doliornis Cotingas 327
should ensure that most of the remnant patchesof humid montane forest that potentially stillhost a Doliornis cotinga can be identified andconsidered in conservation planning. Our studyillustrates how species distribution modelingcould be used for assessing the distributionand conservation status of rare species knownonly from scattered, opportunistic records col-lected by field ornithologists (Bourg et al. 2005,Peterson and Papes 2007, Rojas-Soto et al. 2008,Marini et al., 2010).
METHODS
Study species and occurrence localities.Both Doliornis species are rare to locally un-common, with population sizes in the range of2500–9999 individuals (BirdLife International2010). They are confined to dense thicketsin the paramo-cloud forest ecotone at eleva-tions of 2875–3650 m for Chestnut-belliedCotingas (Appendix 1; contra Ridgely andGreenfield 2001) and 2740–3800 m for Bay-vented Cotingas. The low number of knownlocalities for Doliornis cotingas is likely due totheir low density and their inconspicuous, low-volume calls, lethargic perching behavior, andremote habitats. The biology of both Doliornisspecies is poorly known (Henry 2008, BirdLifeInternational 2010).
Data for presence records (Appendix 1) wereextracted from original publications, comple-mented with direct contact with observers andcollection curators, to assess the precision ofthe published coordinates. For niche modelingto be as robust as possible when the numberof occurrence records is very low, geographicalcoordinates of sites need to be precise andspecies identification verified. Three Chestnut-bellied Cotinga records were therefore discarded:locations 3 and 9 (Appendix 1) because ofimprecise locations, and location 4 because oflack of appropriate habitat in the area and aposteriori identification ( J.F. Freile in Henry2008). Two Bay-vented Cotinga records wereobtained in the same pixel of the study area(locations 2 and 3; Appendix 1) so were notreplicated in the data we used for modeling. Wethus used 13 locations for Bay-vented Cotingasand seven for Chestnut-bellied Cotingas.
Environmental variables. To model thepotential distribution of the two species, weconsidered a geographic area defined by country
borders, including Peru, Ecuador, Colombia,and Venezuela. Within this area, we obtaineddata concerning 11 environmental variablesobtained at a resolution of 2.5 s (i.e., abouta 4.5 km × 4.5 km pixel). We restricted thestudy area to pixels with an average altitude over2000 m to consider only the altitudinal rangewhere both species may occur. We extractedthree topographical variables from the Hydro-1K dataset of the USGS (http://edc.usgs.gov/products/elevation/gtopo30/hydro/index.html),namely slope, aspect, and the compoundtopographic index. The slope describes themaximum change in elevation between each celland its eight neighbors. The slope is expressedin integer degrees between 0 and 90. Theaspect describes the direction of maximum rateof change in the elevations between each celland its eight neighbors and gives the slopedirection, measured in positive integer degreesfrom 0 to 360 (clockwise from north). Thecompound topographic index, commonlyreferred to as the wetness index, is a function ofthe upstream contributing area and the slope ofthe landscape. We further used eight climaticvariables from the BioClim database (http://cres.anu.edu.au/outputs/anuclim/doc/bioclim.html)that are integrative annual or seasonal climaticvariables: annual mean temperature (BIO1),temperature seasonality (BIO4), maximumtemperature of the warmest month (BIO5),minimum temperature of the coldest month(BIO6), annual precipitation (BIO12),precipitation seasonality (BIO15), and amountof precipitation during the wettest (BIO13) anddriest (BIO14) months. We did not considervegetation layers in the modeling runs becauseof a large temporal mismatch between the birdoccurrence data (mostly collected in the 1970s,but one in the 19th century) and availablevegetation layers (like the Leaf Area Index orthe NDVI, for the 2000s; see Myneni et al.2002, Parra et al. 2004). Massive deforestationoccurred between these two periods (Stotzet al. 1996, Renjifo et al. 2002, BirdLifeInternational 2010). As a result, this temporalmismatch could lead to, at best, a weak and, atworst, a false modeled effect of vegetation onbird occurrence. We wanted, first, to objectivelyidentify the potential distribution areas withrelatively stationary ecoclimatic variables and,second, to include a posteriori the relationshipto current, suitable habitat availability (cf.
328 F. Jiguet et al. J. Field Ornithol.
section “Extents of occurrence and vegetationcoverage”), hence identifying areas where thespecies may have disappeared due to habitatloss.
Species distribution modeling. We usedeight different niche-based modeling techniqueswith the BIOMOD computational framework(detailed by Thuiller et al. 2009): (1) generalizedlinear model (GLM), a regression method withpolynomial terms for which a stepwise proce-dure is used to select the most significant vari-ables, (2) generalized additive model (GAM),another regression method with four degrees offreedom and a stepwise procedure to select themost parsimonious model, (3) classification treeanalysis (CTA), a classification method runninga 50-fold cross-validation to select the best trade-off between the number of leaves of the treeand the explained deviance, (4) artificial neuralnetworks (ANN), a machine-learning method,with the mean of three runs used to providepredictions and projections, as each simula-tion gives slightly different results, (5) mixturediscriminant analysis (MDA), a classificationmethod based on mixture models, (6) mul-tivariate adaptive regression splines (MARS),a regression method, (7) generalized boostingmodel (GBM), a machine-learning method thatcombines a boosting algorithm and a regressiontree algorithm to construct an “ensemble” oftrees, and (8) Random Forest (RF), a machine-learning method that is a combination of treepredictors such that each tree depends on the val-ues of a random vector sampled independentlyand with the same distribution for all trees inthe forest. All models require information aboutpresence and absence to be able to determinesuitable conditions for a given species, so a set of10,000 pseudo-absences was selected at randomfrom the overall area to calibrate the models.The models compute a probability distributionbased on environmental variables spread overthe entire study area and assign a probability ofsuitability to each cell in the study area.
Presence threshold and Jackknifetesting. To validate and interpret models,distinguishing suitable from unsuitable areas isnecessary and is achieved by setting a decisionthreshold above which the model output isinterpreted as predicting presence (Pearsonet al. 2004). Different approaches have beenemployed for setting thresholds (Liu et al.2005), and they must be chosen carefully
because receiver operating characteristic (ROC)tests can be weak at discerning models ableto predict species’ distributions in broad,unsampled areas from those that cannot(Peterson et al. 2007), and are difficult toimplement when presence data are scarce. Thethreshold we used was the lowest predictedvalue associated with any one of the observedpresence records, called the “lowest presencethreshold” (LPT). This approach can beinterpreted ecologically as identifying pixelspredicted to be at least as suitable as thosewhere a species has been recorded as present; itis thus conservative, identifying the minimumpredicted area possible while maintainingzero omission error in the data set of knownoccurrences (Pearson et al. 2007). Such athreshold may be too conservative for speciesknown from very few locations, that is, whereavailable records may not represent the wholerange of ecoclimatic conditions where thespecies might occur. In that case, the variabilityof the output among the different modelingtechniques can be used to further assess therobustness of the predictions of species presence.
Because of the importance of model assess-ment and small numbers of locations for bothspecies, the predictive performances of our mod-els were tested with a jackknife (or “leave-one-out”) procedure (Pearson et al. 2007). Eachobserved location was removed from the dataonce, and a model was built using the remainingn − 1 localities. Predictive performance wasthen assessed based on the ability of each modelto predict the single location excluded fromthe data set. We tested the statistical predictiveperformance of a model with the program ofPearson et al. (2007). For each modeling tech-nique and each species, this program returns aP value that is the probability of predicting ran-domly the known locations, according to the to-tal area predicted as suitable by the model usinga given threshold and the success in predictingthe single excluded location. This procedure wasconducted for all eight modeling techniques foreach data set (Bay-vented Cotinga, Chestnut-bellied Cotinga, and Doliornis sp.) using theLPT as threshold.
Ensemble forecast and contribution ofvariables. Ensemble forecast techniques at-tempt to account for variability among speciesdistribution models to determine the centraltendency (Araujo and New 2007). For each
Vol. 81, No. 4 Distributions of Doliornis Cotingas 329
taxon, the consensus distribution was obtainedby selecting the outputs of those modeling tech-niques validated by the jackknife tests (leave-one-out procedure), that is, those modelingtechniques with P-values < 0.05. We then pro-duced maps reporting the unweighted averagesuitability of the selected models (Marmion et al.2009). Contributions of the variables to themodels were obtained with the BIOMOD com-puter platform. Independently, we characterizedthe contributions of the variables to the defini-tion of the bioclimatic niche of each species andthe genus by running a principal componentanalysis (PCA) and plotting coordinates of the20 independent Doliornis locations along thefirst and second principal components.
Interpredictivity and genus distributionmodel. To assess ecological similarity betweenthe two sister species, we used one of the statis-tical tests of niche conservatism described byWarren et al. (2008). We tested the equivalencyof the niches of the two species by looking at thesimilarity between the modeled niches. We cal-culated two similarity indices, D and I (rangingfrom 0 when there is no overlap to 1 when nichemodels are identical), that allow the plausibilityof niche equivalency between the two species(i.e., no statistical difference between alternativeniche models) to be assessed. To know if thesesimilarity measures were different from whatcould be obtained randomly, we created 100null distributions, as proposed by Warren et al.(2008), by considering 100 randomly partitionsof the 20 locations in groups of 13 and sevenlocations for which the same similarity indiceswere estimated. We then compared the D and Ivalues obtained for the species-specific data andthe distributions of those obtained for the 100null distributions.
Finally, we used a data set containing all20 locations from both species to model thepotential distribution of the genus Doliornis as awhole, potentially helping to identify new areasof occurrence for both species or undescribedrelated taxa. This model was evaluated using thesame leave-one-out procedure, and we producedthe same ensemble forecast maps.
Extents of occurrence and vegetationcoverage. The extent of occurrence is de-fined as the area within the shortest continuousimaginary boundary that can be drawn to en-compass all known, inferred, or projected sitesof present occurrence of a species, excluding
cases of vagrancy (IUCN 2001). We estimatedthe potential extent of occurrence using theensemble forecast predictions for each speciesin the study area. We summed the suitabilityvalues of all pixels where a given species waspredicted as present by models retained in theensemble forecast prediction, considering onlysuitability values above the LPT. By multiplyingthese sums by the pixel area (ca. 4.6 km ×4.6 km, or 21.16 km2, in the latitudinal range10◦N to 10◦S), we obtained an estimate of thepotential extent of occurrence of each species inkm2. These estimates were compared to previ-ous estimates of extent of occurrence (BirdLifeInternational 2010).
To account for actual habitat suitability, weused land-cover data from the vegetation mapfor South America published by Eva et al.(2002). This cover is a layer of 73 vegetationtypes at a 1 km × 1 km scale. In a con-servative approach, we retained as suitable allvegetation types that could include some of thehabitat of Doliornis cotingas at the pixel level,including mosaic agriculture/degraded forests,closed shrublands, closed and open montanegrasslands, and all montane forests (>1000 m,the only available elevation threshold) exceptclosed and open deciduous forests. Several ofthese habitats are not suitable for Doliorniscotingas, but, given the square kilometer scaleused, their habitat (paramo-cloud forest eco-tone) might occur within squares dominated bythese vegetation types. To obtain estimates ofpotential distribution sizes corrected for habitatsuitability, we first redefined pixel size of thedistribution output with a bilinear interpolationto fit that of the vegetation map (so a 1 km ×1 km scale). We then recalculated the estimateof extent of occurrence for each species for pixelsthat (1) had a suitable habitat (from the vegeta-tion map), and (2) were considered suitable for aspecies given the threshold (LPT).
RESULTS
Jackknife model testing. The jackknifetests of the ability of the different models topredict excluded localities were implementedwith prediction thresholds set at the LPT. Theresults of the tests are presented in Table 1,with the success rate and associated P-valuesfor each modeling technique per species andfor the genus. To produce the ensemble forecast
330 F. Jiguet et al. J. Field Ornithol.
Table 1. Jackknife tests of the ability of the different models to predict excluded localities, and performedto validate or not the modeling techniques for each data set. Models in bold are those used in the ensembleforecasts. See the “Methods” section for descriptions of the different models.
predictions, we first retained the modeling tech-niques with significant P-values (P < 0.05),then restrained the subset to the four modelswith the best success rates: GLM, GAM, ANNand MARS for Bay-vented Cotingas, GLM andCTA for Chestnut-bellied Cotingas, and GLM,GAM, CTA and MARS for Doliornis sp.
Predicted distribution ranges. For bothspecies, variables that contributed most to themodels were maximum temperature of the
Fig. 1. Graphic output of a principal component analysis (the two axes are the first two principal components)based on 11 environmental variables (see Methods for details) for the 20 locations where D. remseni andD. sclateri have been observed (7 for D. remseni—DR, in red; 13 for D. sclateri—DS, in blue). Ellipses of95% confidence interval are shown for each species.
hottest month (BIO5) and minimum tem-perature of the coldest month (BIO6). ForBay-vented Cotingas, precipitation seasonality(BIO15) contributed to the same extent. Thegraphic output of the PCA conducted on thevalues of the 11 environmental variables for the20 locations (7 for Chestnut-bellied Cotinga—DR, and 13 for Bay-vented Cotinga—DS) isprovided in Figure 1, and reveals that locationsof the two species already segregate along the first
Vol. 81, No. 4 Distributions of Doliornis Cotingas 331
Fig. 2. Predicted distribution ranges of (A) Chestnut-bellied Cotingas (Doliornis remseni), (B) Bay-ventedCotingas (Doliornis sclateri), and (C) the genus Doliornis sp. (combining the data from the two species) inthe Andes as the average suitability across models contributing to the ensemble forecast predictions and forthose areas where the suitability values overpass the mean lowest presence threshold (LPT). The color gradient(light, medium, and dark) represents the increasing average niche suitability (ranging from 0 to 1). Alsodepicted are record locations used in modeling (dots and squares) and country and province borders.
axis (51.5% of explained variance) characterizedby a positive association with precipitation(BIO12-13-14) and a negative associationwith temperature (BIO1-4-5). Chestnut-belliedCotingas is positively associated with this axis,whereas Bay-vented Cotingas loads negatively.The distribution maps obtained by the ensembleforecast predictions are presented in Figure 2A(Chestnut-bellied Cotinga) and Figure 2B (Bay-vented Cotinga). The ensemble forecast predic-tions were also obtained for the complete setof all Doliornis locations (N = 20) to modelthe potential distribution range of the genus(Fig. 2C).
Niche similarity between sister species.The PCA already revealed that locations ofboth species largely diverged for at least fourbioclimatic variables. With the conservativeLPT threshold, species-specific models couldnot predict any occurrence of the other species.When considering the ensemble forecast modelcalibrated with the data for Chestnut-bellied
Cotingas, the average suitability of Chestnut-bellied Cotinga locations was 0.806, whereas theaverage suitability of the 13 Bay-vented Cotingalocations was 0.102. Similarly, when consideringthe ensemble forecast model calibrated withthe data for Bay-vented Cotingas, the averagesuitability of Bay-vented Cotinga locations was0.873 and the average suitability of the sevenChestnut-bellied Cotinga locations was 0.051.Finally, the two indices of niche similarity hadlow values (D = 0.241 and I = 0.526), smallerthan any of the values obtained for the 100null distributions. Hence, the niches of the twospecies can be considered dissimilar.
Estimated extents of occurrence. Esti-mated extents of occurrence—the sum of suit-ability values for pixels with values above theLPT as predicted by the ensemble forecast—were 22 498 km2 for Bay-vented Cotingas and62 616 km2 for Chestnut-bellied Cotingas. Afteraccounting for habitat suitability within theseprobabilistic ranges (i.e., discarding pixels with
332 F. Jiguet et al. J. Field Ornithol.
Fig. 3. Predicted distribution of D. remseni restrictedto suitable vegetation cover, overlaid on the BirdLifeInternational (2010) range map (in red). The bluecolor gradient (light, medium, and dark) indicatesthe increasing average niche suitability (ranging from0 to 1) within the ensemble forecast predictions.Also shown are country (black) and province (gray)borders, and species records, including those usedin modeling (gray dots), imprecise locations (blackovals), and a doubtful record (black circle, in Henry2008). The gray star outside the BirdLife Interna-tional’s range is a new location (Henry 2008).
predicted presence, but unsuitable habitat andfurther restricting the range to the north orsouth of the NPL according to the species),predicted extents of occurrence were downto 37 672 km2 for Chestnut-bellied Cotingas(Fig. 3) and 14 790 km2 for Bay-vented Cotingas(Fig. 4).
Fig. 4. Predicted distribution of D. sclateri restrictedto suitable vegetation cover, overlaid on the Nature-Serve (Young et al. 2009) range map (in red). The bluecolor gradient (light, medium, and dark) indicatesthe increasing average niche suitability (ranging from0 to 1) within the ensemble forecast predictions.Also shown are country (black) and province (gray)borders, and species records used in modeling (graysquares).
DISCUSSION
Modeling performance and predicteddistributions. When trying to predict thedistribution ranges of rare species, having asmall number of occurrence locations avail-able for modeling is a problem. However, forthe rare cotingas in our study, the predicteddistributions were statistically powerful, thatis, they predicted distributions better than ex-pected even with few locations. This couldbe due to the highly specific habitat used byDoliornis cotingas, that is, mainly treeline ofAndean humid montane forest (Henry 2008).This habitat is confined to a very narrow eco-climatic zone, making delimitation with cli-matic variables easy and robust (Parra et al.2004). Robustness of models is illustrated whenprojecting known localities that were excludedfrom the analysis onto the predicted distribu-tion range of Chestnut-bellied Cotingas. Thisspecies was known to occur at two addi-tional locations (3 and 9 in Appendix 1),
Vol. 81, No. 4 Distributions of Doliornis Cotingas 333
but precise locations for these records werelacking. Approximate locations (Fig. 3), owingto location names, indeed point to large areas ofcontinuous suitable areas (over 0.80 probabilityof suitability for the species in darker blue). Therecord from location 4 (see Appendix 1) was con-sidered doubtful because of a lack of appropriatehabitat in surrounding areas ( J.F. Freile in Henry2008). This is supported by the intermediate(0.5) probability of bioclimatic suitability inthis area, mostly below the LPT (Fig. 2A), andthe actual lack of suitable habitat (Fig. 3). ForBay-vented Cotingas, we compared our modeloutput with those of the NatureServe project(Young et al. 2009; Fig. 4). Again, the core areasof the distribution range of Bay-vented Cotin-gas are congruent among both distribution-modeling projects. The comparison also revealsmajor western overpredictions by our model,even with the LPT. Note that thresholds forthe NatureServe project were defined a poste-riori, according to expert judgment. Reviewerswere asked to choose the predicted distributionmaps that looked the most reasonable. Thus,by construction, overpredictions are stronglyminimized in such predicted distribution rangescompared to our models’ output.
Based on the current state of knowledge of Do-liornis distributions (Ridgely and Tudor 1994,Ridgely and Greenfield 2001, Henry 2008),these species avoid the western slopes of theAndes (except at locations 2 and 5; see Ap-pendix 1). However, our models predict theiroccurrence further west than actually known(e.g., western Colombia, west side of the cen-tral inter-Andean valley in Ecuador, and westof the Rıo Maranon valley in Peru). Severalfactors suggest that these western extrapola-tions are unwarranted (unless future inven-tories provide supportive evidence for them).First, according to field visits by J. Fjeldsa(pers. comm.), climatic conditions seem inap-propriate for Doliornis cotingas in these west-ern areas. At similar elevation, the habitat ismuch drier on the western slope than theeastern slope. In addition, no Doliornis wasrecorded in these western areas despite sev-eral intensive investigations (e.g., Imbabura,Pichincha, and Azuay provinces in Ecuador;Krabbe et al. 1998, Ridgely and Greenfield2001; several departments in Peru, J. Fjeldsa,pers. comm.). Also, if we consider the occurrenceof vegetation types (compare Figs. 2A—and B
and 3–4), the western part of the predicted rangeturns out to be largely unsuitable for Doliornis.And, finally, for Chestnut-bellied Cotingas, thewestern part of the predicted distribution ap-pears highly fragmented, with few continuousareas of high occurrence probability. This is par-ticularly evident when comparing fragmentationof the eastern versus western parts of the range.This suggests that the western overpredictionareas are unlikely to host Doliornis populations.Even though these areas may have hosted suit-able humid montane forest in the past, humansmay have largely modified these habitats, ren-dering them unsuitable for Doliornis (Stotz et al.1996, Renjifo et al. 2002, BirdLife International2010). Actually, the western overprediction areasfit well with zones of dense, traditional, ruralhuman populations (cf. Fig. 7b in Fjeldsa et al.1999).
Environmental niches of allopatric sisterspecies. We found a common ecologicalinfluence of environmental components whendefining the ecological niches of the two sis-ter species, corresponding to the hottest andthe coldest temperatures of the year. Overall,observed locations of one species could not bepredicted by the niche model developed for thesister species. Hence, the existence of the recentlydescribed Chestnut-bellied Cotinga (Robbinset al. 1994) could not have been predicted byextrapolation from a species distribution modelcalibrated with occurrence data for its previouslydescribed sister species, the Bay-vented Cotinga.This is also supported by the nonequivalence ofthe niches of the two species.
Peterson et al. (1999) showed conservatism inecological niches evolution for 37 pairs of sisterbird, mammal, and butterfly species isolatedon either side of a lowland barrier in south-ern Mexico. In contrast, niche divergence wasrevealed in some sympatric sister species (Sattleret al. 2007). Indeed, conservatism or divergenceof ecological niches depends on the speciationmechanisms involved. Strict vicariant speciationdepends simply on geographic isolation, andniche conservation is expected (e.g., Petersonet al. 1999). Other scenarios, such as the pe-ripheral isolates model of speciation and manymodels of sympatric speciation (Futuyma 1997),invoke invasion of novel ecological situationsas part of the speciation process. In the caseof Doliornis, sister taxa are found in similarhabitats (east slope, humid montane forest) on
334 F. Jiguet et al. J. Field Ornithol.
both sides of a low-elevation orographic barrier,the NPL, proposed as a major biogeographicbarrier for high-elevation montane forest species(Cracraft 1985, Parker et al. 1985, Garcıa-Moreno and Fjeldsa 1999). Their phenotypesare similar, with limited differentiation (Rob-bins et al. 1994). Along with simple vicariantisolation and low phenotypic differentiation,niche conservatism was expected. Moleculardata suggest that the Doliornis clade (Ohlsonet al. 2007) dates back to the mid-Miocene(Garcıa-Moreno and Fjeldsa 1999), and vi-cariance of the Doliornis species would datefrom the mid-Pleistocene (Garcıa-Moreno andFjeldsa 1999). Thus, vicariance would be justposterior to the late Miocene uplift of thenorthern Andes and to the formation of theNPL (Garcıa-Moreno and Fjeldsa 1999). Toresolve the incongruence between vicariance andobserved niche divergence, we hypothesize thatniches differentiated after the isolation event,while species adapted to north–south variationsin bioclimatic conditions of their habitat, theparamo-cloud forest ecotone. To test the gen-erality of niche conservatism/divergence in theAndes, similar patterns of ecological divergencecould be tested among other sister allopatric taxaoccurring north and south of the NPL,for exam-ple, with the 21 species pairs identified by Parkeret al. (1985) and Garcıa-Moreno and Fjeldsa(1999).
Distributions and conservation. Ac-cording to IUCN (2001) Red List criteria,Chestnut-bellied Cotingas is classified as vul-nerable because of its small, decreasing popula-tion, and restricted, declining extent of occur-rence, area of occupancy, and area/extent/qualityof habitat (BirdLife International 2010). Bay-vented Cotingas also qualified as vulnerable forthe same reasons, but less evidence of criticalhabitat loss is available for this species (BirdLifeInternational 2010). The potential extent of oc-currences that we estimated (about 37 000 km2
for Chestnut-bellied Cotingas and 15 000 km2
for Bay-vented Cotingas), with the LPT thresh-old and after accounting for habitat availabil-ity, are different from the estimated Extentsof Occurrence provided by BirdLife Interna-tional (2010; 10 900 km2 for Chestnut-belliedCotingas, 13 100 km2 for Bay-vented Cotingas).For Chestnut-bellied Cotingas, the new recordlocations (Henry 2008) explain this differenceto some extent. However, our estimate includes
large, northern areas of predicted occurrence(in Colombia) that are supported by only onelocation and, therefore, may be unwarranted.Hence, the actual extent of occurrence maystill be close to the 20 000-km2 threshold usedwhen evaluating extinction risk (IUCN 2001).For Bay-vented Cotingas, our estimated valueis similar to the formerly available estimate.Thus, our estimates support a Red List statusas “vulnerable to extinction” for both species(BirdLife International 2010, IUCN 2010).
Our models suggest a relatively continuousdistribution of Chestnut-bellied Cotingas northof the NPL where habitat is suitable. The south-ern part of the range is highly fragmented. Fitbetween the predicted distribution range and therange provided by BirdLife experts is relativelygood in the highly fragmented, outer parts ofthe range (cf. northern and southern margins).The major discrepancy is the large, continuousarea of high probability of occurrence in centralEcuador. This omission by Birdlife Internationalis due to the lack of precise occurrence data inthe intervening area, data first documented afterBirdLife International had produced their map(Henry 2008). Our maps of potential distribu-tion will be important for guiding searches foreach species in areas where they are currently notknown to occur.
Species distribution modeling, using locationrecords and readily available environmental vari-ables, provides objective biogeographic infor-mation for poorly known tropical landscapes,and offers an innovative tool for the refinementof species distribution ranges and the searchof unknown species (Raxworthy et al. 2003,Engler et al. 2004, Bourg et al. 2005, Guisanet al. 2006, Peterson and Papes 2007). Modelscalibrated for the two Doliornis species, in ourstudy, revealed areas of likely occurrence thatremain to be validated in the field. The rangeof Bay-vented Cotingas may actually extendfurther to the southeast, reaching the PeruvianDepartment of Cusco, and to the northwest,with an area of high probability of occurrencein the Department of Cajamarca. Chestnut-bellied Cotingas is likely to be found betweenthe Colombian provinces of Cauca and Huila,as well as in several parts of Ecuador and north-ernmost Peru. Two areas of overprediction inthe northern Andes deserve specific comments.First, a Doliornis cotinga is predicted to occur,with an extended area of high probability, in the
Vol. 81, No. 4 Distributions of Doliornis Cotingas 335
eastern Andes of Columbia, south of Bogota.High Andean bird species are poorly differenti-ated among the three Colombian Andean Ridges(Cracraft 1985, Ridgely and Tudor 1994) and, ifa Doliornis sp. was to be found there, we suggestit would be Chestnut-bellied Cotingas and not anew species. Second, the models of Bay-ventedCotingas predict the occurrence of the speciesin the Andes of Venezuela (La Culata or NevadaSierras), whereas models of the closest species(Chestnut-bellied Cotinga) do not predict thispopulation. It suggests that, if a Doliornis sp.was to be discovered in this area, it may bea third species, with bioclimatic requirementscloser to Bay-vented Cotingas than Chestnut-bellied Cotingas.
To conclude, by modeling the ecological nicheof two allopatric, rare Andean endemics, weidentified the most suitable areas where thesespecies should occur based on climatic andtopographic modeling, some of which have beenconfirmed as occupied by one species. Suchspecies distribution modeling can play a majorrole in identifying the probable distributions ofrare species and helping to focus searches forthem and inform conservation planning. Ourstudy illustrates how the observations of fieldornithologists can be used objectively, with thehelp of existing statistical software, to trans-form anecdotal records into knowledge of thedistribution, ecology, and conservation of rarespecies.
ACKNOWLEDGMENTS
We are grateful to collection curators N. Rice, S.Cardiff, P. Sweet, and D. Willard for specimen labelsinformation, J. F. Freile, T. Santander, G. Engblom, R.Ahlman, F. Schmitt, and T. Davis for precise informa-tion about localities and geographical coordinates, M.Balman for providing shapefiles for BirdLife Interna-tional’s distribution maps, and B. Young for the Nature-Serve’s maps. A.T. Peterson kindly provided the USGSHydro-1K dataset. S. Butchart, J. Fjeldsa, and J. F. Freileprovided invaluable comments on an earlier draft of thispaper.
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Vol. 81, No. 4 Distributions of Doliornis Cotingas 339A