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Distance Sampling (distance + sampling)

Distribution by Scientific Domains
Mathematics and Statistics53%
Life Sciences46%

Selected Abstracts

Participatory wildlife surveys in communal lands: a case study from Simanjiro, Tanzania

AFRICAN JOURNAL OF ECOLOGY, Issue 3 2010
Fortunata U. Msoffe
Abstract It is widely accepted that protected areas alone are not sufficient to conserve wildlife populations particularly for migratory or wide-ranging species. In this study, we assess the population density of migratory species in the Tarangire,Simanjiro Ecosystem by conducting a ground census using DISTANCE sampling. We focus on the Simanjiro Plains which are used as a dispersal area by wildebeest (Connochaetes taurinus) and zebra (Equus burchellii). We demonstrate that DISTANCE sampling can provide precise estimates of population density and is an affordable method for monitoring wildlife populations over time. We stress the importance of involving local communities in monitoring programmes across landscapes that incorporate communal lands as well as protected areas. Résumé On reconnaît généralement que les aires protégées ne suffisent pas, seules, à préserver les populations de faune sauvage, particulièrement celles d'espèces migratrices ou très largement distribuées. Dans cette étude, nous évaluons la densité de population d'espèces migratrices de l'Ecosystème Tarangire-Simanjiro en réalisant un recensement au sol recourant à l'échantillonnage par distance. Nous nous concentrons sur les plaines de Simanjiro qui sont utilisées comme aire de dispersion des gnous Connochaetes taurinus et des zèbres Equus burchellii. Nous montrons que l'échantillonnage par distance peut donner des estimations précises de la densité d'une population et que c'est une méthode accessible pour suivre des populations sauvages dans le temps. Nous soulignons l'importance d'impliquer les communautés locales dans les programmes de suivi, dans des paysages qui intègrent des terres publiques aussi bien que des aires protégées. [source]

Estimating the Encounter Rate Variance in Distance Sampling

BIOMETRICS, Issue 1 2009
Rachel M. Fewster
Summary The dominant source of variance in line transect sampling is usually the encounter rate variance. Systematic survey designs are often used to reduce the true variability among different realizations of the design, but estimating the variance is difficult and estimators typically approximate the variance by treating the design as a simple random sample of lines. We explore the properties of different encounter rate variance estimators under random and systematic designs. We show that a design-based variance estimator improves upon the model-based estimator of Buckland et al. (2001, Introduction to Distance Sampling. Oxford: Oxford University Press, p. 79) when transects are positioned at random. However, if populations exhibit strong spatial trends, both estimators can have substantial positive bias under systematic designs. We show that poststratification is effective in reducing this bias. [source]

Advanced Distance Sampling: Estimating Abundance of Biological Populations Edited by S. T. Buckland, D. R. Anderson, K. P. Burnham, J. L. Laake, D. L. Borchers, and L. Thomas

BIOMETRICS, Issue 3 2006
Russell Alpizar-Jara
No abstract is available for this article. [source]

Predicted population trends for Cozumel Curassows (Crax rubra griscomi): empirical evidence and predictive models in the face of climate change

JOURNAL OF FIELD ORNITHOLOGY, Issue 4 2009
Miguel A. Martínez-Morales
ABSTRACT We assessed the current conservation status and population trends of Cozumel Curassows (Crax rubra griscomi), a critically endangered and endemic cracid of Cozumel Island, Mexico. Distance sampling was used to estimate the population size in 2005 and compare it with a previous estimate in 1995. Population viability analysis (PVA) was used to model population trends, considering ecological and environmental conditions prevailing in 1995 and 2005. Additional PVA models were constructed using different hypothetical scenarios to assess the effects of particular factors and management strategies on population trends. We estimated a population size of 372 ± 155 curassows in the island's tropical semideciduous forest, before two hurricanes hit Cozumel Island in 2005. PVA modeling predicted a steep decline in population size in the ensuing decades as a result of an increased frequency and intensity of hurricanes related to global climate change, and an observed female bias in the sex ratio likely caused by a higher mortality rate for adult males. We recommend urgent management actions for the long-term conservation of Cozumel Curassows, including a ban on hunting, eradication of the feral fauna, particularly dogs, and implementation of a captive-breeding program to supplement the wild population. RESUMEN Evaluamos el estado de conservación actual y las tendencias poblacionales del hocofaisán de Cozumel (Crax rubra griscomi), un crácido críticamente amenazado y endémico a la Isla de Cozumel, México. Se empleó el muestreo de distancias para estimar su tamaño poblacional en 2005 y compararlo con la estimación previa de 1995. Se emplearon análisis de viabilidad poblacional (AVP) para modelar tendencias poblacionales considerando las condiciones ecológicas y ambientales prevalecientes en 1995 y 2005. Adicionalmente, se construyeron modelos de AVP bajo diferentes escenarios hipotéticos para evaluar los efectos de factores específicos y estrategias de manejo sobre su tendencia poblacional. Estimamos un tamaño poblacional de 372 ± 155 hocofaisanes en la selva mediana subcaducifolia de la isla, antes del impacto de dos huracanes sobre Cozumel en 2005. El AVP predijo una severa declinación en el tamaño poblacional en las siguientes décadas como resultado de un incremento en la frecuencia e intensidad de huracanes relacionado con el cambio climático global, y de un sesgo en la proporción sexual hacia las hembras, probablemente asociado a una mayor mortalidad de machos adultos. Recomendamos acciones de manejo urgentes para la conservación a largo plazo del hocofaisán de Cozumel, incluyendo una efectiva prohibición de la cacería, la erradicación completa de fauna feral, particularmente perros, y la implementación de un programa de reproducción en cautiverio para suplementar la población silvestre. [source]

Effect of count duration on abundance estimates of Black-capped Vireos

JOURNAL OF FIELD ORNITHOLOGY, Issue 1 2009
David A. Cimprich
ABSTRACT Distance sampling applied to point count surveys (point transects) has become a common method for estimating the absolute abundance of birds. When conducting point transects, detections of focal species are typically recorded during a fixed time interval. However, count duration has varied among studies and the effect of such variation on the resulting abundance estimates is unclear. My objective was to examine the effect of count duration on abundance estimates of male Black-capped Vireos (Vireo atricapilla). The abundance of these vireos in a 349-ha area in central Texas was estimated using 3-, 5-, and 6-min point transects and results were then compared to actual number present as determined by banding and territory mapping. The 3-min counts provided an estimate that was 26% greater than the actual number of male Vireos present (N= 201), but this number was within the corresponding 95% confidence interval (N= 157,413). Confidence intervals for the 5- and 6-min counts did not include the actual number of vireos present. The shortest count duration may have provided the most accurate abundance estimate because male Black-capped Vireos are typically active, sing intermittently, and sometimes move tens of meters between songs. Thus, shorter-duration counts may also yield the most accurate abundance estimates for other species that exhibit similar behavior. However, because behavior varies among species, I recommend that investigators collect preliminary data to establish an appropriate count duration when accurate estimates of absolute, rather than relative, abundance are important. RESUMEN El muestreo a distancia aplicado a conteos de punto (transectos de punto) se ha convertido en un método común para estimar la abundancia absoluta de aves. Cuando se conducen conteos de punto, la detección de especies focales, típicamente, se lleva a cabo durante un periodo de tiempo definido. Sin embargo, la duración de tiempo del conteo, varía en diferentes estudios y el efecto de dicha variación, en los estimados de abundancia, no está definido. Mi objetivo fue examinar el efecto del periodo de tiempo usado en los conteos utilizando como objeto de estudio a Vireo atricapilla. La abundancia de dichas aves en 349 ha en la parte central de Texas, fue estimada utilizando periodos de 3, 5 y 6 minutos en cada punto del transecto. Los resultados fueron comparados a números obtenidos con recobro de animales anillados y mapas de territorios. Los conteos de 3 minutos dieron como resultado un 26% más alto que el número de machos presentes en la localidad (N= 201). Pero dichos resultados estuvieron dentro del intérvalo de confiabilidad de 95% (N= 157,413). Intérvalos de 5 y 6 minutos no estimaron adecuadamente el número de vireos presentes. El conteo más corto (3 minutos) suministró los números más exactos, aparentemente porque el ave estudiada, típicamente está activa, canta de forma intermitente y en ocasiones se mueve de lugar (decenas de metros) entre canciones. Por lo tanto los conteos más cortos deben de proveer los estimados de abundancia más precisos para otras especies de aves con hábitos similares. Sin embargo, dado el caso de que la conducta entre especies varía, recomendamos a otros investigadores, que tomen datos preliminares para establecer la duración más apropiada de conteos, cuando se necesiten estimados de absoluta exactitud, en vez de estimados relativos de abundancia. [source]

A Model-Based Approach for Making Ecological Inference from Distance Sampling Data

BIOMETRICS, Issue 1 2010
Devin S. Johnson
Summary We consider a fully model-based approach for the analysis of distance sampling data. Distance sampling has been widely used to estimate abundance (or density) of animals or plants in a spatially explicit study area. There is, however, no readily available method of making statistical inference on the relationships between abundance and environmental covariates. Spatial Poisson process likelihoods can be used to simultaneously estimate detection and intensity parameters by modeling distance sampling data as a thinned spatial point process. A model-based spatial approach to distance sampling data has three main benefits: it allows complex and opportunistic transect designs to be employed, it allows estimation of abundance in small subregions, and it provides a framework to assess the effects of habitat or experimental manipulation on density. We demonstrate the model-based methodology with a small simulation study and analysis of the Dubbo weed data set. In addition, a simple ad hoc method for handling overdispersion is also proposed. The simulation study showed that the model-based approach compared favorably to conventional distance sampling methods for abundance estimation. In addition, the overdispersion correction performed adequately when the number of transects was high. Analysis of the Dubbo data set indicated a transect effect on abundance via Akaike's information criterion model selection. Further goodness-of-fit analysis, however, indicated some potential confounding of intensity with the detection function. [source]

Optimization of ordered distance sampling,

ENVIRONMETRICS, Issue 2 2004
Ryan M. Nielson
Abstract Ordered distance sampling is a point-to-object sampling method that can be labor-efficient for demanding field situations. An extensive simulation study was conducted to find the optimum number, g, of population members to be encountered from each random starting point in ordered distance sampling. Monte Carlo simulations covered 64 combinations of four spatial patterns, four densities and four sample sizes. Values of g from 1 to 10 were considered for each case. Relative root mean squared error (RRMSE) and relative bias were calculated for each level of g, with RRMSE used as the primary assessment criterion for finding the optimum level of g. A non-parametric confidence interval was derived for the density estimate, and this was included in the simulations to gauge its performance. Superior estimation properties were found for g > 3, but diminishing returns, relative to the potential for increased effort in the field, were found for g > 5. The simulations showed noticeable diminishing returns for more than 20 sampled points. The non-parametric confidence interval performed well for populations with random, aggregate or double-clumped spatial patterns, but rarely came close to target coverage for populations that were regularly distributed. The non-parametric confidence interval presented here is recommended for general use. Copyright © 2004 John Wiley & Sons, Ltd. [source]

How you count counts: the importance of methods research in applied ecology

JOURNAL OF APPLIED ECOLOGY, Issue 5 2008
Chris S. Elphick
Summary 1Methods papers play a crucial role in advancing applied ecology. Counting organisms, in particular, has a rich history of methods development with many key advances both in field sampling and the treatment of resulting data. 2Most counts, however, have associated errors due to portions of the population of interest being unavailable for detection (e.g. target population not fully sampled; individuals present but not detectable), detection mistakes (e.g. detectable individuals missed; non-existent individuals recorded), or erroneous counts (e.g. large groups miscounted; individuals misidentified). 3Developments in field methods focus on reducing biases in the actual counts. Simultaneously, statisticians have developed many methods for improving inference by quantifying and correcting for biases retrospectively. Prominent examples of methods used to account for detection errors include distance sampling and multiple-observer methods. 4Simulations, in which population characteristics are set by the investigator, provide an efficient means of testing methods. With good estimates of sampling biases, computer simulations can be used to evaluate how much a given counting problem affects estimates of parameters such as population size and decline, thereby allowing applied ecologists to test the efficacy of sampling designs. Combined with cost estimates for each field method, such models would allow the cost-effectiveness of alternative protocols to be assessed. 5Synthesis and applications. Major advances are likely to come from research that looks for systematic patterns, across studies, in the effects of different types of bias and assumption violation on the ecological conclusions drawn. Specifically, determining how often, and under what circumstances, errors contribute to poor management and policy would greatly enhance future application of ecological knowledge. [source]

Relationship between road dust and ungulate density in Serengeti National Park, Tanzania

AFRICAN JOURNAL OF ECOLOGY, Issue 4 2008
Vedasto G. Ndibalema
Abstract We measured the responses of ungulate grazers to roads by recording the density of dust on grasses and compared distance sampling of ungulates in Serengeti National Park (SNP), Tanzania. Data were collected on the east and west side of the Ngorongoro,Seronera main gravel road to test if road traffic and dust were important factors determining distribution patterns amongst grazers. Results indicate that dust increased progressively with traffic speed and volume during dry season. More dust was intercepted on the west than on the east side of the road mainly because of wind effects. Dust deposition (measured as density g grass g,1 dust) was higher on short grasses than on long grasses during the dry and late-dry seasons than during the wet season, when paired perpendicular distances up to 300 m were compared. Mean number of observed grazer species indicated that most fed further from the west side of the road than from the east perhaps to minimize higher density of dust commonly spread on foliage up to 200 m away from the road. Despite that most grazers avoided road side grass shoulders, supporting the ,dust aversion hypothesis', the test predictions from the ,road disturbance' and the ,road attraction' hypotheses did not support the responsive behaviours of grazers toward roads. Résumé Nous avons mesuré la réponse d'ongulés brouteurs à la présence de routes en enregistrant la densité de poussière sur les herbes et en comparant la distance des ongulés dans le Parc National du Serengeti, en Tanzanie. Les données ont été récoltées à l'est et à l'ouest de la principale route en gravier du Ngorongoro Seronera pour voir si le trafic routier et la poussière étaient des facteurs importants qui déterminaient les schémas de distribution des animaux brouteurs. Les résultats indiquent que la poussière augmente progressivement avec la vitesse et le volume du trafic pendant la saison sèche. On a recueilli plus de poussière du côté ouest de la route, surtout à cause des effets du vent. Les dépôts de poussière (mesurés comme la densité g d'herbe/g de poussière) étaient plus abondants sur les herbes courtes que sur les longues pendant la saison sèche et en fin de saison sèche qu'en saison des pluies lorsqu'on comparait deux à deux des distances perpendiculaires allant jusqu'à 300 mètres. Le nombre moyen d'espèces d'animaux brouteurs observées indiquait que la plupart se nourrissaient plus loin du côté ouest de la route que du côté est, peut-être pour minimiser la forte densité de poussière qui se dépose régulièrement sur le feuillage jusqu'à 200 mètres de la route. Bien que la plupart des brouteurs évitent les accotements herbeux de la route, ce qui vient soutenir l'hypothèse de l'aversion pour la poussière, les tests de prédiction concernant les hypothèses de « perturbations par la route » ou d'« attraction par la route » ne supportent pas le comportement réactif des ongulés brouteurs vis-à-vis de la route. [source]

Habitat use, abundance, and persistence of Neotropical migrant birds in a habitat matrix in northeast Belize

JOURNAL OF FIELD ORNITHOLOGY, Issue 3 2010
Camila Gómez-Montes
ABSTRACT To ensure adequate protection of nonbreeding habitats used by Neotropical migratory landbirds, we must first address questions about habitat use and quality. On the Yucatan peninsula, migrants use many habitats, several of which remain unstudied, and methodological differences preclude interhabitat comparisons based on studies to date. We used distance sampling along line transects in six habitats in northeast Belize to examine use of previously unstudied habitats (e.g., salt marsh) by Neotropical migrants and to permit comparison across habitats. We calculated unadjusted and adjusted (for detectability) density estimates for individual migrant species and for all species combined to generate hypotheses about habitat quality based on the assumption that density and quality are positively correlated. Adjusted density estimates for all migrants were highest in black mangrove habitat (1799 ± 110 ind/km2), intermediate in three forest types and milpa (range 598,802 ind/km2), and lowest in salt marsh (207 ± 32.3 ind/km2). By combining density estimates with habitat availability in our study region, we estimated that evergreen forest and black mangrove supported 70% and 9% of the region's migrant population, respectively. At the species level, five of the 10 most common species had habitat preferences (>50% detections in one habitat). Given the diversity of habitat preferences among species and apparent seasonal movements, our results indicate that Neotropical migrants in northeast Belize are dependent on a matrix of interconnected habitats. RESUMEN Para asegurar la protección adecuada del hábitat no- reproductivo utilizado por aves migratorias Neotropicales terrestres, debemos responder preguntas sobre el uso y la calidad del hábitat. En la península de Yucatán, lasaves migratorias utilizan muchos hábitats, varios de los cuales aún continúan sin ser estudiados. Además las diferencias metodológicas evitan hacer comparaciones inter-hábitat basadas en los estudios que se han hecho hasta ahora. Utilizamos unmuestreo a lo largo de transectos de distancia variable, en seis hábitats, algunos previamente no estudiados (ej. ciénagas salobres), en el noreste de Belize para examinar y comparar el uso por parte de las migratorias Neotropicales, Calculamos estimados de densidad, ajustados y no ajustados (para detectabilidad), para especies particulares de migratorias y para todas las especies combinadas para generar una hipótesis sobre la calidad del hábitat basándonos en la presunción que la densidad estaría positivamente correlacionada con la calidad del hábitat. Los estimados de densidad ajustados para todos los migratorios fueron más altos en mangle negro (1799 ± 100 ind/km2), intermedios en tres tipos de bosque y milpa rango 598,802 ind/km2) y menores en ciénagas salobres (207 ± 32.3 ind/km2). Combinando los estimados de densidad con la disponibilidad de hábitats en nuestra región de estudio, estimamos que el bosque siempreverde y el mangle negro sostienen el 70% y 9% de los migratorios en la región, respectivamente. A nivel de especies, cinco de las 10 especies más comunes tienen preferencias de hábitat (>50% de las detecciones en un hábitat). Dada la diversidad de preferencias de hábitat entre especies y el aparente movimiento estacional, nuestros resultados indican que las aves migratorias Neotropicales en el noreste de Belize dependen de una matriz de hábitats interconectados. [source]

Estimates of population size of white-chinned petrels and grey petrels at Kerguelen Islands and sensitivity to fisheries

ANIMAL CONSERVATION, Issue 3 2009
C. Barbraud
Abstract White-chinned petrels Procellaria aequinoctialis and grey petrels Procellaria cinerea are among the most frequently killed seabird species by accidental bycatch, and both species have received strong conservation concern. Data on population size are required to evaluate the impact of bycatch and to establish management plans. We estimated the population size of both species at Kerguelen, Southern Indian Ocean, from 2004 to 2006 by explicitly taking into account detection probability of burrows using distance sampling and burrow occupancy. A total of 31 line-transects were distributed across the eastern part of Kerguelen, representing a total length of 566 km. Detectability was low (from 0.19 to 0.54 for white-chinned petrels, 0.58 for grey petrels). Burrow densities varied from 1.37±0.67 to 25.77±5.23 burrows ha,1 for white-chinned petrels and was 2.78±0.79 burrows ha,1 for grey petrels. For white-chinned petrels, these densities were extrapolated to the entire surface area of vegetation and there were 234 000 (186 000,297 000) active burrows on Kerguelen. For grey petrels, the number of active burrows for the eastern part of Kerguelen was 3400 (1900,5600). Based on these estimates, the potential biological removal method suggests that the additional mortality on birds caused by the fisheries operating around Kerguelen can be considered a serious threat for the species at least at the regional scale of the Southern Indian Ocean, especially for grey petrels. [source]

Wildlife Population Assessment: Past Developments and Future Directions

BIOMETRICS, Issue 1 2000
S. T. Buckland
Summary. We review the major developments in wildlife population assessment in the past century. Three major areas are considered: mark-recapture, distance sampling, and harvest models. We speculate on how these fields will develop in the next century. Topics for which we expect to see methodological advances include integration of modeling with Geographic Information Systems, automated survey design algorithms, advances in model-based inference from sample survey data, a common inferential framework for wildlife population assessment methods, improved methods for estimating population trends, the embedding of biological process models into inference, substantially improved models for conservation management, advanced spatiotemporal models of ecosystems, and greater emphasis on incorporating model selection uncertainty into inference. We discuss the kind of developments that might be anticipated in these topics. [source]