Leptotila Wellsi) Author(S): Frank F
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Distance sampling survey and abundance estimation of the critically endangered Grenada Dove (Leptotila wellsi) Author(s): Frank F. Rivera-Milán, Paulo Bertuol, Fernando Simal, and Bonnie L. Rusk Source: The Condor, 117(1):87-93. 2015. Published By: Cooper Ornithological Society DOI: http://dx.doi.org/10.1650/CONDOR-14-131.1 URL: http://www.bioone.org/doi/full/10.1650/CONDOR-14-131.1 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non-commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. Volume 117, 2015, pp. 87–93 DOI: 10.1650/CONDOR-14-131.1 RESEARCH ARTICLE Distance sampling survey and abundance estimation of the critically endangered Grenada Dove (Leptotila wellsi) Frank F. Rivera-Mila´n,1* Paulo Bertuol,2 Fernando Simal,2,3 and Bonnie L. Rusk4 1 U.S. Fish and Wildlife Service, Division of Migratory Bird Management, Laurel, Maryland, USA 2 STINAPA Bonaire, Bonaire, Dutch Caribbean 3 WILDCONSCIENCE, Nawati, Bonaire, Dutch Caribbean 4 Grenada Dove Conservation Programme, c/o Forestry and National Parks Department, Queen’s Park, Grenada, West Indies * Corresponding author: [email protected] Submitted August 13, 2014; Accepted November 11, 2014; Published January 21, 2015 ABSTRACT The Grenada Dove (Leptotila wellsi) is critically endangered; its abundance, as estimated by territory mapping, ranges from 68 to 91 calling males (or 136–182 individuals, assuming a census of paired males). However, an accurate census is unlikely in dry and moist forests, unpaired males may be more detectable than paired males, and sex ratio may be male biased. Because methodology can limit the value of monitoring, we used a systematic grid of survey points and distance sampling to estimate abundance (density and population size), accounting for covariates that may influence detection. Time of day was the most important covariate (e.g., individuals were detected at larger distances early than late in the morning). Density was negatively influenced by disturbance level (deforestation) and positively influenced by food abundance and vegetation cover (leguminous trees). None of the covariates caused extreme heterogeneity; and conventional and multiple-covariate analyses generated similar detection and density estimates, which suggests that model selection was of secondary importance for abundance inferences. Detection probability (mean 6 SE) was 0.166 6 0.031 (95% confidence interval [CI]: 0.114–0.242) within 340 m, density was 0.021 6 0.004 individuals haÀ1 (95% CI: 0.014–0.030), and population size was 160 6 30 individuals (95% CI: 107–229) in 7,621 ha. Although spatial distribution was slightly clumped (dispersion parameter: bˆ~1.31), we recommend surveying 150 points twice between late July and early August for abundance coefficient of variation (CV) 0.15, even if spatial distribution becomes more clumped (e.g., b ¼ 2.5). More survey data are needed to better understand spatial and temporal density variation, test hypotheses about survey design (e.g., road bias in density estimation) and Grenada Dove ecology (rainfall, food, cover, and density correlations), and evaluate management actions (predator removal in nesting areas). With ,250 Grenada Doves in the survey region, our data highlight the precarious conservation status of this island endemic, and the urgent need for effective management and targeted monitoring. Keywords: abundance, distance sampling, Grenada Dove, Leptotila wellsi Muestreo de distancia y estimacion´ de abundancia de la Paloma de Granada (Leptotila wellsi) en peligro de extincion´ cr´ıtico RESUMEN La Paloma de Granada (Leptotila wellsi) esta´ en peligro de extincion´ cr´ıtico; su abundancia, estimada usando el metodo´ de mapeo de territorios, alcanzando de 68 a 91 machos cantando (o 136–182 palomas, asumiendo un censo de machos apareados). Sin embargo, un censo es poco probable en bosques secos y humedos;´ los machos no apareados pueden ser ma´s detectables que los machos apareados; y la proporcion´ sexual puede estar sesgada en favor de los machos. Porque la metodolog´ıa puede limitar el valor del monitoreo, usamos una cuadr´ıcula sistema´tica de puntos y el metodo´ de muestreo de distancia para estimar abundancia (densidad y tamano˜ poblacional), tomando en cuenta covariables que pueden influenciar la deteccion.´ La hora del d´ıa fue la covariable ma´s importante (p.ej., las palomas fueron detectadas a mayor distancia ma´s temprano que tarde en la manana).˜ La densidad fue influenciada negativamente por el nivel de disturbio (deforestacion),´ y positivamente por la abundancia de comida y la cobertura vegetacional (a´rboles leguminosos). Ninguna de las covariables causo´ heterogeneidad extrema; y los ana´lisis convencionales y multiple-covariados generaron estimados similares de deteccion´ y densidad, sugiriendo que la seleccion´ de modelos tuvo una importancia secundaria sobre las inferencias de abundancia. La probabilidad deteccion´ (median 6 ES) fue 0.166 6 0.031 (intervalo de confianza de 95% [CI]: 0.114–0.242) en 340 m la densidad fue 0.021 6 0.004 palomas haÀ1 (0.014, 0.030), y el tamano˜ poblacional fue 160 6 30 palomas (107, 229) en 7,621 ha. Aunque la distribucion´ estuvo ligeramente aglomerada (para´metro de dispersion:´ bˆ~1.31), recomendamos muestrear 150 puntos dos veces entre finales de julio y principios de agosto para un CV de abundancia 0.15, aunque la distribucion´ espacial sea ma´s aglomerada (p.ej., b ¼ 2.5). Ma´s datos de muestreo son necesarios para entender mejor la variacion´ Q 2015 Cooper Ornithological Society. ISSN 0004-8038, electronic ISSN 1938-5129 Direct all requests to reproduce journal content to the Central Ornithology Publication Office at [email protected] 88 Grenada Dove survey and abundance F. F. Rivera-Mila´n, P. Bertuol, F. Simal, and B. L. Rusk espacial y temporal de densidad, examinar hipotesis´ de diseno˜ de muestreo (p.ej., sesgo de camino en la estimacion´ de densidad) y de ecolog´ıa de la paloma (correlaciones entre lluvia, comida, cobertura, y densidad), y la evaluacion´ de acciones de manejo (la remocion´ de depredadores en a´reas de nidificacion).´ Con menos de 250 palomas, nuestros datos ponen en relieve el estado precario de conservacion´ de esta especie endemica´ islena,˜ y la urgente necesidad de un manejo efectivo .y un monitoreo orientado. Palabras clave: Abundancia, Leptotila wellsi, muestreo de distancia, Paloma de Granada INTRODUCTION wide population survey of the Grenada Dove using a systematic grid of points and distance sampling to The Grenada Dove (Leptotila wellsi;Figure1)was estimate abundance (density and population size), ac- thoughttobenearlyextinctinthe1960s(Devas1970). counting for survey and site-specific covariates that may At present, this island endemic is listed as critically influence detection probability. We use the data to test endangered by the International Union for Conservation hypotheses about survey design (e.g., density differs along of Nature (Rusk 2008). Habitat loss and degradation due and away from roads; Marques et al. 2010) and ecological to forest clearing for agriculture, cattle grazing, and urban factors (density differs at points with low and high food development; catastrophic natural events, such as Hur- abundance; Rivera-Mila´n et al. 2003a); and provide ricane Ivan in 2004; and predation by introduced recommendations to standardize data collection, meet mammals are major conservation threats (Rusk 2008). method assumptions, and estimate abundance with However, survey data are needed to estimate population precision (desired coefficient of variation [CV] 0.15) abundance, establish population-based conservation ob- for management evaluation and targeted monitoring jectives to increase or maintain demographic sustainabil- (Nichols and Williams 2006). ity and ecological viability (Tear et al. 2005, Sanderson 2006), and assess population response to environmental Study Area disturbances and management actions (B. L. Rusk The survey region covered 7,621 ha (Figure 2). We personal observation). established a systematic grid of 180 points separated by Territory mapping was used to estimate Grenada Dove 400 m in primary habitat and 1,000 m in potential habitat (hereafter ‘‘dove’’) abundance intermittently between 1987 (P. J. Rivera-Lugo personal observation). Logwood (Hae- and 2007; the most recent abundance estimates range from matoxylum campechianum), gumbo limbo (Bursera si- 68 to 91 calling males (or 136–182 individuals, assuming a maruba), black loblolly (Pisonia fragrans), strongbark census of paired males in dry and moist forests; B. L. Rusk (Bourreria succulenta), and wild tamarind (Leucaena personal observation). However, detection is likely imper- leucocephala) were among the most common