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Distance sampling is being extensively used to estimate the abundance of animal populations. Nevertheless, the great variety of ways in which data can be analyzed may limit comparisons due to the lack of standardization of such protocols. In this study, the influence of analytical procedures for distance sampling data on density estimates and their precision was assessed. We have used data from 21 surveys of mountain ungulates in the Iberian Peninsula, France and the Italian Alps. Data from such surveys were analyzed with the program Distance 6.0. Our analyses show that estimated density can be higher for higher levels of data truncation. We also confirm that the estimates tend to be more precise when data are analyzed without binning and without truncating. We found no evidence of size biased sampling as group size and distances were uncorrelated in most of our surveys. Despite distance sampling being a fairly robust methodology, it can be sensitive to some data analysis strategies.

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... Transects were done by walking on the unpaved roads distributed across the study area ( Fig. 1). Transects were carried out at dawn or dusk when ungulates are more active (Pérez et al. 2014). For each transect, we recorded the observed species and number of individuals. ...

In recent decades, intensive techniques of livestock raising have flourished, which has largely replaced traditional farming practices such as transhumance. These changes may have affected scavengers’ behaviour and ecology, as extensive livestock is a key source of carrion. This study evaluates the spatial responses of avian scavengers to the seasonal movements of transhumant herds in south-eastern Spain. We surveyed the abundance of avian scavengers and ungulates, and analysed the factors affecting the space use by 30 GPS-tracked griffon vultures ( Gyps fulvus ). Griffons’ foraging activity increased in the pasturelands occupied by transhumant herds, which implied greater vulture abundance at the landscape level during the livestock season. In contrast, facultative scavengers were more abundant without transhumant livestock herds, and the abundance of wild ungulates did not change in relation to livestock presence. We conclude that fostering transhumance and other traditional farming systems, to the detriment of farming intensification, could favour vulture conservation.

... To date, several methods with different precisions were used to estimate abundance of mountain ungulate populations, including distance sampling, mark-resight, and block counts (e.g. Corlatti et al. 2015;Pérez et al. 2015). However, estimates of densities in inaccessible mountain landscapes with heterogeneous habitats and low detectability rates are very scarce (e.g. ...

Methods for estimating population densities of unmarked species using camera traps
are still under development. One such method is called "Random Encounter Model
(REM)" and, to our knowledge, has never been used to estimate densities of mountaindwelling
ungulates. In this study, we tested the REM method to estimate the density of
Balkan chamois ( Rupicapra r. balcanica ) in a Mediterranean habitat, Mountain
Biokovo. To meet the assumptions of REM, we systematically placed 25 camera traps
throughout the known range of the population (approximately 65 km 2 ) at the
intersections of 2-km grid cells. Prior to data collection, population density was
estimated by visual counts on sample plots in August 2020. Cameras were operational
between July 2020 and September 2020 and active throughout the 24-hour period. All
parameters for REM (i.e., average movement speed, angle and radius) were estimated
using exclusively camera trap data. We obtained 279 independent events of chamois
from 2503 camera trap days. The density estimate obtained by REM resulted to be
20.65 ± 5.27 ind. km -2 , slightly higher than the reference value obtained by visual
counts: 17.33 ± 0.98 ind. km -2 . Other parameters required to calculate density were
speed (1.62 km·day -1 ± 0.21), detection radius (5.56 m ± 0.21) and detection angle
(1.16 + 0.05 radians). Therefore, REM has shown comparable results to visual counts
and may have potential for estimating density of mountain ungulates, especially in
rugged and inaccessible mountainous areas with low detectability where other
approaches are inadequate or impossible.

... In order to be properly managed, aquaculture companies require accurate data of the abundance, biomass and average weight of the fish farmed in their facilities. The more accurate these data are, the better the decision-making for the fish farm will be in terms of feed requirements, growth rate and food conversion calculations, medication administration, early detection of fish losses due to deaths, robberies or escapes, splitting of farming units, economic forecasting, etc. (McCallum, 2005;Soliveres et al., 2014;Pérez et al., 2015;Hofmeester et al., 2016;Føre et al., 2018). ...

The use of hydroacoustics is currently being studied and developed as a promising non-intrusive methodology to monitor and manage fish stocks in aquaculture farms. The main objective of this study was to develop an acoustic method for the estimation of fish density and biomass in inland aquaculture farms and test the accuracy and precision of the estimates with real data provided by the company. The study was conducted in sea bass (Dicentrarchus labrax) production ponds located in Seville (Southern Spain). A Simrad EK60 echosounder with two split-beam circular transducers operating simultaneously at 200 kHz was used for hydroacoustic surveys. Two different hydroacoustic designs were considered: central trajectories and zigzag trajectories. The accuracy and precision of the estimates were examined in order to select the best sampling design. Due to a non-homogeneous fish distribution in the pond caused by the avoidance behaviour, as a response to the sampling disturbance presented by fish, acoustic density and biomass were corrected by applying sampling theory according to the probability of fish detection. When density and biomass were corrected, the estimates became highly accurate and precise with respect to real data, which confirms that the proposed method is adequate. Similarly, acoustic estimates of fish weight were highly in agreement with real data, due to the use of specific equations developed “in situ” for the study. Although no significant differences were recorded in the density and biomass estimates with regard to the trajectory used (central vs. zigzag), it was observed that the most accurate agreement and precision were always obtained in central trajectories. Therefore, central design is proposed as the most appropriate design for hydroacoustic measurements in inland ponds. The results obtained in this study provide estimates of density and biomass that accurately match the real data, supporting the use of hydroacoustics as a potentially valid tool to manage inland aquaculture farms.

... The number and order of adjustment terms required for the analysis were automatically selected by Distance using the databased criteria with three maximum terms, of the AIC selection criterion (Franceschi et al. 2014). The encounter rate was thus obtained for each survey and its variance was estimated empirically with the default estimator (Buckland et al. 2001;Franceschi et al. 214;Pérez et al. 2015). With the Distance program, we performed all the analyses without truncating, nor grouping the data into Fig. 2 Example of a rock ptarmigan count in Andorra. ...

The rock ptarmigan (Lagopus muta) has a wide Holarctic geographical range, within which its status changes locally from threatened to abundant. For the correct management of populations under differing scenarios, accurate and precise estimates of bird abundances and/or densities are required. We used geolocated datasets from 56 counts of singing males carried out in 2003–2017 to estimate the density of Pyrenean rock ptarmigans (Lagopus muta pyrenaica). We analysed these datasets using two methodologies: (1) a plot sampling (PS) approach with two effective detection ranges (EDR) of 250 m and 350 m and (2) a conventional distance sampling (DS) approach. Our results showed that the density estimates obtained by DS were more precise and 30–87% higher than those obtained by PS using 250 m and 350 m EDR. Monitoring with PS underestimates populations and is less precise than DS; this bias is magnified when high EDR values are used. The monitoring of the Pyrenean rock ptarmigan could be significantly improved with greater use of the DS method.

... These field data have been used totally or partially in some publications, most of these being technical documents [41][42][43][44][45][46] . ...

this dataset provides long-term information on the presence of the Iberian ibex (Capra pyrenaica hispanica Schimper, 1848) in Sierra Nevada (SE Iberian Peninsula). Data on the abundance and demographic structure of the Iberian ibex population were compiled over the last three decades. Transects were laid out to record different variables such as the number of individuals sighted, the perpendicular distance of each group of Iberian ibex to the transect line and sex as well as age of individuals in the case of males. these data enabled the calculation of population parameters such as density, sex ratio, birth rate, and age structure. These parameters are key for Iberian ibex conservation and management, given that Sierra Nevada harbours the largest population of this species in the Iberian Peninsula. The data set we present is structured using the Darwin Core biological standard, which contains 3,091 events (582 transect walk events and 2,509 group sighting events), 5,396 occurrences, and 2,502 measurements. The occurrences include the sightings of 11,436 individuals (grouped by sex and age) from 1993 to 2018 in a total of 88 transects distributed along Sierra Nevada, of which 33 have been continuously sampled since 2008.

... Only two adjustment terms were used to balance between bias and precision, and the lowest AIC value was used for model selection. Data were neither binned, truncated nor grouped into distance intervals for density estimation (Pérez et al., 2015). Variance was estimated empirically and the coefficient of variation (CV) of the density estimates and their associated 95% confidence interval were also obtained. ...

Estimating animal abundances in small areas is a difficult task and because a limited number of observations often results in low-precision estimates whose inaccuracies may even be exacerbated if surveys are focussed on clustered populations and/or are only carried out once a year. In an attempt to overcome this problem, we used point transects to monthly survey two small areas of a game reserve to assess the density of Pyrenean chamois (Rupicapra p. pyrenaica). The coefficient of variation associated with the density estimates after pooling observations by season was still high but decreased to reasonable values (<20%) when observations were over 29 chamois groups (clusters). Our results suggest that Distance Sampling may be a useful way of estimating the population density of mountain ungulates such as Pyrenean chamois in small rugged areas where only a small or moderate number of observations are to be expected.

... However, DS has also produced values lower than block counts, with block count value falling out of the 95% DS confidence interval, when used to estimate chamois numbers in complex and rough areas (Corlatti et al. 2015). In DS applied to mountain ungulates, such as the chamois, the use of different data analysis strategies and new approaches considering the density gradient from linear structures are recommended to improve the precision of density estimation (Marques et al. 2013;Pérez et al. 2014). As compared to BC, CMR, IPS and DS have the advantage of providing the accuracy of the estimates computed instead of a single value and to provide estimates of detection probabilities (for IPS see recent advances in N-mixture models; e.g. ...

The Iberian wild goat (Capra pyrenaica) is an endemic species of the Iberian Peninsula. Of the four generally accepted subspecies (Capra pyrenaica victoriae, C.p. hispanica, C.p. pyrenaica, C.p. lusitanica) only two subsist (C.p. victoriae, C p. hispanica). The subspecies once found in Portugal, C. p. lusitanica, became extinct in the 19th century. However, the reintroduction of the C. p. victoriae in north-west Spain, led to the natural recolonization of this subspecies into Portugal. Knowledge of the Iberian wild goat in Portugal is still limited, making it difficult to evaluate the major conservation/management needs. To fill this gap, here we describe the historical distribution of the Iberian wild goat in Portugal and summarize the available information on the reintroduction and recolonization process of this species. Additionally, we used line itinerary survey (2011–2012), coupled with Distance Sampling, to estimate current densities, range and population structure of the Iberian wild goat distribution in Portugal. The Iberian wild goat density is 2.78/100 ha (95% CI: 1,72–4,50; CV: 18,36%) and the abundance is of 576 (CI 95%: 356–930; CV: 18,36%). The distribution data shows that the population is divided in three nuclei: 1- Serra do Gerês (13,840 ha); 2–Serra Amarela (1235 ha); 3–Castro Laboreiro (343 ha), in a total area of 15,418 ha. Iberian wild goat population in Portugal has greatly increased in the last years, both in number and distribution range. The demographic data shows a potential for increase in the next years but our current ecological background knowledge still remains limited. Iberian wild goat management will benefit from a long-term project including public awareness, scientific research and management solutions.

Wildlife monitoring and the identification of factors associated with disease outbreaks are major goals in wildlife conservation. We reviewed demographic and epidemiological data for the Alpine ibex Capra ibex from 1975–2013 to characterize the species’ abundance and distribution dynamics on a large scale. We also explored methodological bias in monitoring and analyzed the factors potentially associated with the risk of disease outbreaks. Our results revealed that the overall abundance and distribution of Alpine ibex appeared to be increasing at both national and international scales, in agreement with the IUCN’s ‘Least Concern’ conservation status on the international scale and on the national scale for Italy, Switzerland and France. Our comparative analysis of common monitoring methods highlights the fact that abundance values from counts are underestimated and suggests that the Alpine ibex is more abundant than is usually reported. The appearance and persistence of disease outbreaks (e.g. sarcoptic mange, keratoconjunctivitis or brucellosis) are related to local ibex density and abundance. The observed correlation between the demographic growth of ibex populations and disease outbreaks suggests that the risk of epizooties may be increasing or might already be high in several populations of Capra ibex.

This study compared the efficacies of total block counts (BC) and distance sampling (DS) procedures to estimate the abundance of chamois populations in two mountain massifs, Posets and Maladeta, Spain. In 2002, 2006, 2007, and 2008, chamois populations were surveyed along itineraries above the timberline, twice per year: in July, after the parturition period, and in November, during the rut. The latter survey was used to estimate the sex ratio and to correct the proportion of males present in July. In 2008, poor weather prevented surveys in November. In the DS procedure, we used the data collected using BC and calculated the distances of the mapped groups to the itinerary, using a Geographical Information System. In Posets, estimates of population densities derived using BC ranged from 5.5 to 9.1 chamois km<sup>-2</sup>, while those derived using DS ranged from 7.5 to 9.7 chamois km<sup>-2</sup>. In Maladeta, the estimates ranged from 3.4 to 5.4 chamois km<sup>-2</sup> (BC) and from 4.6 to 8.5 chamois km<sup>-2</sup> (DS). Coefficients of variation of DS ranged between 14% and 26%. In five of eight cases the counts of population size derived from BC were within the 95% confidence interval of the estimate derived from DS. In two of the other three cases, weather conditions created poor visibility during the rut, and few chamois were seen and, consequently, the rut sex ratio could not be estimated. BC provided objective, high-quality counts of chamois populations and it is easy to obtain, even if its efficacy can be constrained by the need of simultaneous itineraries and an underestimation of unknown magnitude. DS does not require sampling throughout the entire area above the timberline, and generates an estimate and a confidence interval; however, calculations require some skill and sample size must be high (n > 300 groups observed to produce a CV < 15%), which represents a sampling effort at least as large as the one derived from BC. BC represents a valuable tool and, it should continue to be used to estimate minimum population size and the derived hunting quotas.
Se comparan los resultados de estimar la abundancia de sarrio aplicando el conteo simultáneo supraforestal (BC) y el muestreo de distancias (DS) a lo largo de itinerarios, durante cuatro años (2002, 2006, 2007 y 2008) en dos macizos pirenaicos, Posets y Maladeta. Se realizaron dos operaciones anuales, una en julio (tras la paridera) y otra en noviembre (en el celo), esta última para conocer la proporción de sexos que se utiliza para corregir el número de sarrios adultos de julio. En 2008 la operación de noviembre no se pudo realizar por inclemencias meteorológicas. Para el DS se utilizan los datos del BC, calculando las distancias perpendiculares al recorrido de los grupos observados y cartografiados, con la ayuda de un Sistema de Información Geográfica. Las densidades de julio de Posets oscilaron entre 5,5 y 9,1 sarrios km<sup>-2</sup> (BC) y entre 7,5 y 9,7 sarrios km<sup>-2</sup> (DS) y en Maladeta entre 3,4 y 5,4 sarrios km<sup>-2</sup> (BC) y entre 4,6 y 8,5 sarrios km<sup>-2</sup> (DS), con coeficientes de variación (CV) del DS del 14 al 26%. El número de animales mínimo estimado con el BC está dentro del intervalo de confianza del 95% de la densidad calculada por el DS en 5 de los 8 casos. En los otros tres casos, las condiciones meteorológicas adversas durante el celo no hicieron posible el cálculo de la sex ratio. El BC aporta un número objetivo de indudable calidad y su cálculo es sencillo aunque cuenta con el inconveniente de la necesaria simultaneidad en la realización de los recorridos, la coincidencia para ello de buenas condiciones de visibilidad y la existencia de una subestima de magnitud desconocida. El DS, sin embargo, no requiere batir todo el territorio supraforestal y produce una estima de la densidad con su intervalo de confianza. No obstante, el cálculo numérico requiere cierta destreza y dedicación y el tamaño muestral debe ser elevado (n > 300 grupos avistados para un CV < 15%) que supone un esfuerzo de muestreo al menos igual al que se realiza en el BC. El BC constituye una herramienta valiosa y su uso debería continuar para estimar el tamaño mínimo poblacional y los cupos de caza derivados.

We used line transect (distance) methodology to estimate the population density of fallow deer (Dama dama), roe deer (Capreolus capreolus), and wild boars (Sus scrofa) in a dense Mediterranean forest. Three different surveys (1 per year from 1995 to 1997) were analyzed. Density estimates, pooled among years, for fallow deer (9,9 deer/km(2), %CV = 24) and roe deer (8.5 deer/km(2). %CV = 21) were more precise than the estimate for wild boars (10.6 boars/km(2), %CV = 55). Fallow deer density, was significantly higher during 1997 than during 1995 or 1996. For wild boars. we found differences among years (1995: 23.6 boars/km(2), %CV = 23; 1996; 10.9 boars/km(2), %CV = 24; 1997: 3.9 boars/km(2), %CV = 33). We attempted a confirmatory analysis using an independent survey method for fallow deer and total Counts at supplementary feeding sites for wild boars. These comparisons showed that line transect estimates were negatively biased for fallow deer during 1996-1997 and for wild boars during 1997. For adults, the composition by sex front line transects was compared with information collected from fixed observation points. Differences in sex composition for fallow deer and wild boars were found when these 2 methods were compared. The structure of the toe deer population was similar when line transect information was compared with independent survey data. A spike in the detection probability, function for wild boars may have been caused by the difficulty of detecting boars >20 in from the transect. The careful usc of line transect sampling may prove useful for both research and management of fallow deer, roe deer, and wild boars.

Despite a [greater-than-or-equal]75% reduction in the geographic range of Mongolian gazelles Procapragutturosa over the past 50 years, the species is still recognized as the most numerous large grassland herbivore in Asia. Its actual population size, however, is still disputed, and we therefore estimated its numbers in an 80,000 km2 area in the eastern steppe of Mongolia by driving long-distance (1,2002002. Quantitative estimates of gazelle numbers are essential for understanding the causes of changes in the population, and thus devising conservation strategies to assure its long-term viability. Observed herds ranged in size from 142. Density estimates varied from 10.7 gazelles km2 in autumn, with total population estimates of 803,820 (483,7901,491,278 95% confidence interval), respectively. Confidence limits were wide, and to obtain a coefficient of variation of 20%, transect lengths would need to be extended three- to four-fold. Until more efficient means for conducting population surveys can be implemented, driving long-distance transects, combined with distance analysis, seem to provide the best quantitative estimate of Mongolian gazelle populations.

1994. Population dynamic of the Spanish ibex Capra pyrenaica in Sierra Nevada Natural Park (southern Spain). Acta theriol. 39: 289-294. The whole are a of distribution of the Spanish ibex Capra pyrenaica Schinz, 1838 population of Sierra Nevada Natural Park was surveyed for the first time during July-August 1993 and the densities were estimated by using the line transects method. A IDean density of 7.69 :t 0.50 ibexes/km2 was obtained, with sex ratio (males/ /females) of 0.80 and kids/adult females rate of 0.39. Data Oil size and composition of groups and on altitudinal distribution are alBo included. The demographic trend of ibexes in Sierra Nevada during last 30 years is reconstructed on the basis of data referred to the National Game Reserve, located within OUTstudy aTea,

Capsule Field methods used by the UK's Breeding Bird Survey (BBS) provide a practical approach to estimating breeding bird abundance and this paper discusses how they might be adapted to increase accuracy.Aims Using Salisbury Plain as a case study, examine the use of distance sampling to produce estimates of breeding bird abundance.Methods During 2005, 157 1 km squares were surveyed on Salisbury Plain using the UK’s BBS methods, with the exception that all birds were sexed whenever possible and allocated to one of five distance bands (0–10 m/10–25 m/25–100 m/100–250 m/ >250 m). Data were modelled to investigate the effects of truncated, pooled and sexed bird data in estimating breeding populations.Results Pooling of the inner distance bands made little difference to estimates, while truncation over 100 m affected them by up to 25%, generally leading to an increase in density and the width of the confidence limits. The national BBS does not distinguish between bird sexes and therefore any density estimate produced relates to individuals. Our analysis suggests that halving the number of individuals (assuming an equal sex ratio) could lead to significant underestimation of population. This is particularly the case for species where there are differences in detectability between sexes or skewed sex ratios. In such cases, the density of males may represent a more accurate assessment of the population.Conclusions Bird surveys incorporate distance sampling because it assesses changes in detection probability and this paper suggests how these methods might be adapted to increase accuracy. Changes include increasing both survey effort (e.g. number of visits or distance covered), the number of distance bands, and recording sexed bird data. Numbers of pairs or territories could then be estimated by either halving the density of individuals, or using the density of males, whichever is highest. These suggestions have not been tested against a known population and further work is desirable.

Estimating the density or abundance of mountain ungulates is difficult and rarely conducted in a statistically valid manner. The rough terrain they inhabit, their group-living habits, their relatively low density, and the difficulty of marking individuals all contribute to making rigorous estimates of abundance logistically difficult. Raw (uncalibrated) counts are usually reported, and although their drawbacks are often acknowledged, biases are rarely quantified. In September 2009, we took advantage of the presence of a radio-marked sample of argali Ovis ammon in the Ikh Nart Nature Reserve in south-central Mongolia, as well as the area's comparatively forgiving topography to estimate abundance simultaneously using two independent methods: distance sampling and mark-resight sampling. Distance sampling produced an abundance estimate of 539 (95% CI: 196-1,081) argali within a 330 km2 study area on the same day that we visually tallied 189 animals. Mark-resight sampling using the Poisson log-normal model yielded an estimate of 747 (95% CI: 484-1,009) argali when we observed, at most, 223 animals in any given day. Although both were imprecise, their similarity increases our confidence that neither estimator was highly biased. Because of budget or logistical restrictions, uncalibrated counts of mountain ungulates are often the only alternative. They should be viewed cautiously, however, and when possible, more rigorous approaches to estimating abundance should be taken.

Censusing wild populations and detecting trends in population size over time is an important task in the conservation and management of wildlife. We compared two methods used to monitor numbers of chamois Rupicapra rupicapra and R. pyrenaica in two contrasting populations, and explored the relationship between the sampling effort and the repeatability of the results using resampling methods. One population in the Alps had been stable at a high density for several years, whereas the other population, located in the Pyrénées, was increasing exponentially, following a reintroduction. In both sites, a long-term monitoring programme based on individually marked chamois, allowed us to estimate population size using capture-mark-recapture methods (CMR). In addition, we calculated an index of population size as the mean number of animals observed on a foot transect surveyed repeatedly. We then compared whether trends estimated by each method were consistent. In the increasing population, both the index and the CMR estimates revealed an exponential increase in population size. In the stable population, neither the index nor the CMR estimate revealed any trend in size. Consistent results between the index and the CMR suggest that the index could be used to monitor trends in population size. Resampling techniques, however, pointed out that the index is only reliable when calculated over a sufficient number of surveys per year (10 in the Pyrenees, three in the Alps) and over a sufficient number of years of monitoring (about five years).

This book is available for purchase, The 1993 version is available on the page at
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Summary • High-density populations of large herbivores are now widespread. Wildlife managers commonly attempt to control large herbivores through hunting to meet specific management objectives, considering population density as the minimal key source of information. Here, we review the problems of censusing populations of large herbivores and describe an alternative approach, employing indicators of ecological change. • Estimating density of large herbivores with high precision and accuracy is difficult, especially over large areas, and requires considerable investment of time, people and money. Management decisions are often made on an annual basis, informed by population changes over the previous year. However, estimating year-to-year changes in density is not a realistic goal for most large herbivores. Furthermore, population density per se provides no information on the relationship between the population and its habitat. • For successful management of large herbivores, we need to consider not only the fate of the population, but rather changes in both population and habitat features, as well as their interaction. Managers require information on trends in both the animal population and habitat quality in order to interpret changes in the interaction between these two compartments. • We propose that a set of indicators of animal performance, population abundance, habitat quality and/or herbivore habitat impact provides relevant information on the population–habitat system. Monitoring temporal changes in these indicators provides a new basis for setting hunting quotas to achieve specific management objectives. This sort of adaptive management is employed widely in France for managing roe deer Capreolus capreolus. • Synthesis and applications. The management of large herbivores would be improved by investing fewer resources in trying to estimate the absolute abundance of ungulates, and more resources in collecting additional data to inform understanding of the ecological status of the ungulate–habitat system being managed. This paper presents a set of indicators of ecological change for monitoring the interaction between a population and its habitat as a basis for adaptive management to attain explicit goals and to improve knowledge of the system. This approach could improve management for a variety of large herbivores, by harmonizing actions at wide spatial scales. Journal of Applied Ecology (2007) 44, 634–643 doi:10.1111/j.1365-2664.2007.01307.x

1.Distance sampling is a widely used technique for estimating the size or density of biological populations. Many distance sampling designs and most analyses use the software Distance.
2.We briefly review distance sampling and its assumptions, outline the history, structure and capabilities of Distance, and provide hints on its use.
3.Good survey design is a crucial prerequisite for obtaining reliable results. Distance has a survey design engine, with a built-in geographic information system, that allows properties of different proposed designs to be examined via simulation, and survey plans to be generated.
4.A first step in analysis of distance sampling data is modelling the probability of detection. Distance contains three increasingly sophisticated analysis engines for this: conventional distance sampling, which models detection probability as a function of distance from the transect and assumes all objects at zero distance are detected; multiple-covariate distance sampling, which allows covariates in addition to distance; and mark–recapture distance sampling, which relaxes the assumption of certain detection at zero distance.
5.All three engines allow estimation of density or abundance, stratified if required, with associated measures of precision calculated either analytically or via the bootstrap.
6.Advanced analysis topics covered include the use of multipliers to allow analysis of indirect surveys (such as dung or nest surveys), the density surface modelling analysis engine for spatial and habitat modelling, and information about accessing the analysis engines directly from other software.
7.Synthesis and applications. Distance sampling is a key method for producing abundance and density estimates in challenging field conditions. The theory underlying the methods continues to expand to cope with realistic estimation situations. In step with theoretical developments, state-of-the-art software that implements these methods is described that makes the methods accessible to practising ecologists.

Accurate assessment of the populations of mountain ungulates is difficult. Topography and behaviour of animals are important factors influencing detectability, on which direct methods are based. Sympathry with other wild and domestic ungulates increases error of dung identification. Other factors, such as the size of the area to be surveyed, or unmarked populations of the target species, suggests the choice of curvilinear transects. If the basic assumptions of Distance Sampling methodology are met, then estimates of population density and accuracy can be obtained. Current technology (GPS, laser rangefinders, and G.I.S.) can be used to achieve accurate measurements of distances and angles. A crucial problem is to estimate the effective area sampled around each travelling path. Therefore, researchers and managers of mountain ungulates need an adaptation of Distance Sampling methodology to account for a tridimensional scenario imposed by the slope of mountains. There is also a requirement to standardize protocols for collecting data. Alternatives for design surveys and collection data when working with populations of mountain ungulates are discussed.
[fr]
L'estimation précise de l'abondance des ongulés de montagne devient difficile. La détectabilité est conditionnée par la topographie et le comportement des animaux; c'est sur ce dernier cas que les méthodes directes s'appuient. La sympatrie entre les ongulés sauvages et domestiques augmente l'erreur d'identification des excréments. D'autres facteurs, comme par exemple les grandes surfaces à surveiller ou les populations non marquées des espèces-cibles, peuvent nous conduire à des comptages sur transect curvilignes ou sur des points. Si l'on tient compte des idées basiques concernant la méthodologie appelée «Échantillonage à distance», alors les estimations correspondantes de densité avec leurs mesures de précision peuvent être obtenues. La technologie à notre portée (GPS, laser «rangefinders» et GIS) peut aider de manière non négligeable l'obtention de valeurs précises de distances et d'angles. Il devient alors fondamental d'estimer la surface d'échantillonage autour de chacun de nos parcours. Par conséquence, tant les chercheurs que les gestionnaires des ongulés de montagne ont besoin d'une adaptation de l'échantillonage à distance de manière à construire un scénario tridimensionel imposé par les versants des montagnes ; aussi faudrait-t-il développer des protocoles standardisés pour la collecte de données. Dans ce travail, les différentes alternatives pour la mise en place des études et pour la collecte de données concernant les populations des ongulés de montagne sont discutées.
[es]
Valorar con exactitud la abundancia de ungulados de montaña es difícil. La detectabilidad es la base de los métodos directos, y ésta se ve influida por factores como la topografía y él comportamiento de los animales. La simpatría con otros ungulados, salvajes o domésticos, aumenta el error en la identificación de los excrementos. Otros factores, como la extensión del área a prospectar o la ausencia de poblaciones no marcadas de la especie objetivo, aconsejan el censado a partir de transectos o de puntos fijos. Si los supuestos básicos de la metodología basada en la distancia de detección se respetan, se pueden obtener estimas de densidad y evaluar su precisión. La tecnología actual (GPS, telémetros laser y SIG) permite un cálculo exacto de distancias y ángulos. Un problema crucial es estimar el área muestreada alrededor de cada transecto. Por ello, investigadores y gestores de ungulados de montaña necesitan una adaptación de la metodología basada en la distancia de detección, para reconstruir el área tridimensional impuesta por las montañas, además de estandarizar los protocolos de toma de datos. Se discuten varias alternativas para diseñar el estudio y recogida de datos en poblaciones de ungulados de montaña.

Surveys were conducted from aircraft over waters off the coasts of Louisiana, Mississippi, and Alabama during the summers of 1974 and 1975 to evaluate field procedures and analytical techniques for estimating population densities of bottlenosed dolphins (Tursiops truncatus). Samples using a strip proved more reliable than those based on a quadrat. Line-transect estimates were generally not feasible because the observed distribution of distances from the flight line was different from the theoretical distribution required. Differences in counts between observer teams were significant. Based on strip censuses, the following population estimates were obtained: western Louisiana (1975), 897 ± 461; Mississippi, Chandeleur and Breton sounds, marshlands habitats (1974) 438 ± 294; and Mississippi Sound 1,342 ± 847 (1974) and 879 ± 368 (1975). The differences in the last 2 estimates reflect the large variability inherent in the population and in the estimation procedure and are not indicative of population fluctuation. Suggestions are made for future censuses of dolphins inhabiting inshore waters.

An important problem in line transect sampling is that objects or point clusters of objects of different sizes have different sighting probabilities. In a recent paper Drummer and McDonald (1987, Biometrics 43, 13-21) develop a bivariate sighting function. Their function is dependent on perpendicular distance and object size. One important special case is an extension of the exponential power series sighting function first proposed by Pollock (1978, Biometrics 34, 475-478). In this note empirical evidence is given for this model based on a field test of line transect sampling theory. Beer cans were used to simulate point clusters of objects with cluster sizes 1, 2, 4, and 8. To achieve approximately equal precision of parameter estimates, equal numbers of each cluster size were taken.

Line transect sampling is used to estimate the number of individuals in a study area. It is usually assumed that the detections of individuals constitute independent events, and that the probability of detecting an item is a function of its perpendicular distance from the transect. However, certain species of plants/animals tend to aggregate, or cluster, thus possibly violating the assumption of independence. Also, larger clusters may tend to have a higher probability of detection, thus distorting the observed probability distribution of cluster sizes. This can cause overestimation of the true population size. A similar problem occurs if the experimenter is using line transect data to estimate the mean and total amount of some attribute of the items, and this attribute influences the item's probability of detection. Bivariate detection functions that incorporate the cluster size, or another attribute, as a covariate are proposed. Estimators of total population size and of the moments of the additional variate are derived using the theory of weighted distributions. An application of the method to a shipboard survey of minke whales is provided.

In some line transect surveys, the population of interest consists of groups or clusters of individuals rather than single items. If group size influences the probability of detecting a group, then the observed group size data are size-biased. This can lead to overestimation of the true mean group size and hence could lead to overestimation of the density of individuals. The method of line transect sampling used here incorporates group size as a covariate in line transect detection functions, and is applied to shipboard surveys of sea otter Enhydra lutris and to surveys of pellet groups from barren ground caribou Rangifer tarandus granti. Group size influence varied in the sea otter surveys. The method of estimation adequately adjusted for size bias when it was present. Estimates of pellet group density were unaffected by significant size bias. -Authors

Abstract When human interventions interfere with the natural regulation of wildlife populations by favouring some species, overabundance can emerge. We evaluated different methods of estimating red deer abundance in a wide range of population densities from southern Spain. Distance sampling estimates were used as the reference method across 22 localities and were compared with two kilometric abundance indices (KAIs), four indices based on pellet group counts and two browsing indices (BWIs). The average red deer density estimated by distance sampling was 19.51±3.19 deer per 100 ha, showing a wide range across the study area (0.04–66.77). Distance sampling estimates correlated with the KAIs, pellet group-based index and the BWI. The agreement with distance sampling improved when groups were used instead of individuals in the KAIs, when the minimum pellet group size was fixed at 20 pellets in the dropping counts, and when only highly palatable species were used in the BWI. Thus, several direct and indirect methods can estimate red deer abundance in Mediterranean habitats from Southern Spain with appropriate modifications.

We examined three aspects of line-transect analytical procedures: data grouping, data truncation and
the use of individuals or clusters as the analytical unit. Bias and precision of density estimation in
relation to various levels of these factors were assessed for 4 types of line-transect estimator (simple
parametric, generalised parametric, non-parametric and quasi-strip) using line-transect survey data from
macropod populations of known density. The effect of data grouping on bias and precision varied
between estimators. Bias was stable across all grouping levels tested for the simple parametric estimator,
and stable across aU but the coarsest grouping level for the generalised parametric and non-parametric
estimators, but varied substantially across the range of levels tested for the quasi-strip estimator.
Precision improved as the number of grouping levels increased for all estimators tested, but the extent
of improvement varied between estimators, and for the estimator most affected, improvement was
marginal beyond intermediate grouping levels. Density estimates were generally more accurate and
precise when analysed in ungrouped form than in grouped form. No effects of truncation on bias or
precision were detected. Varying the analytical unit did not affect bias, but precision was significantly
lower for cluster analysis than individual analysis for all estimators.

Aerial line transect methods have been successfully used to count animals in open country; we describe their application for assessing red deer (Cervus elaphus) numbers in Scotland. The results of a pilot study in the north of Scotland are compared to a census count carried out in the same area in the previous year. The aerial line transect method gave an estimate of 2240 red deer in May 1995. This agreed well with the census count giving 2270 in April 1994. The line transect estimate had a coefficient of variation of 25%, whereas no measure of precision is available for the census count. Separate estimates for the number of stags and the number of hinds and calves were obtained with both methods. We suggest improvements in the practical application of the aerial line transect method to increase precision of the abundance estimates. Aerial line transect surveys require fewer person-days of effort than census counts, and are appreciably more cost effective for assessing deer numbers over a large area. We present three case studies for applying aerial line transect methods in Scotland. The case studies compare the cost and effort for an aerial line transect estimate with those of a conventional ground-based census count in West Sutherland and North Ross (Red Deer Commission's counting blocks). Obtaining an estimate of red deer numbers in North Ross with a coefficient of variation of 10% would take 33% of the time of a census count, require 50% of the people and take about 60% of the resources required for a ground-based census count. A joint estimate for North Ross and West Sutherland would reduce the time demand for the line transect estimate (10% coefficient of variation) to about 18% of that required by a census and the resources to 33%.

This monograph is out of print and not available. Much of the material is covered in the books on Distance Sampling. One of the early versions is available at
http://www.colostate.edu/depts/coopunit/download.html
or
http://www.ruwpa.st-and.ac.uk/distance.book/download.html

Post-release monitoring, including abundance estimation, is an important part of reintroductions, providing a basis for management intervention designed to achieve long-term persistence. The Arabian oryx Oryx
leucoryx became extinct in the wild in 1972, surviving as captive populations. Since 1982 reintroductions of Arabian oryx have taken place in Oman and Saudi Arabia. Modelling of oryx population dynamics has highlighted the importance of precise estimation of population size (N). Between 1990 and 2000 three methods of estimating N have been applied in Mahazat as-Sayd protected area in Saudi Arabia: derived population estimates (DPE) based on known births and deaths, distance sampling, and mark-resighting (MR). This study assesses the feasibility and precision of these methods. Inability to assess precision, interdependence of consecutive estimates, and the assumption that all gains and losses are recorded, make DPE of limited value. At current densities, distance sampling along 455 km of driven transects yields too few detections to derive precise estimates of N. To achieve a coefficient of variation of 20% it would be necessary to drive up to c. 2,900 km of transect; this amount of survey effort could be achieved through pooling of data across repeat surveys of established transects. MR estimates, based on re-sighting of collared oryx, have the potential to yield the most precise estimates of N when the proportion of marked animals reaches 30% of the total population. The most reliable MR estimates available indicate the Mahazat as-Sayd Arabian oryx population had grown to >400 animals by 2000.

1. Roe deer Capreolus capreolus were surveyed at night along tracks and roads in plantation forests in North Yorkshire, UK. Distance sampling was used to estimate their density. This method has been proposed for monitoring deer populations in state-owned forests throughout the UK.
2. Most deer were stationary on detection and the vast majority did not change their behaviour during observation. Few deer were observed on the transects, suggesting that they were avoiding tracks and roads at night as opposed to moving in response to the observer.
3. This has implications for data analysis and hence the results of surveys. Left-truncation or grouping of data close to the transect line to cope with road avoidance decreased the precision of the population estimate and may have impacted upon its accuracy.
4. Use of roads and tracks in forests is the only realistic option for transect surveys of deer at night but the influence of these features on deer distribution and hence density estimates must be taken into consideration when conducting such surveys.

Summary • Accurate and precise estimates of abundance are required for the development of management regimes for deer populations. In woodland areas, indirect dung count methods, such as the clearance plot and standing crop methods, are currently the preferred procedures to estimate deer abundance. The use of line transect methodology is likely to provide a cost-effective alternative to these methods. • We outline a methodology based on line transect surveys of deer dung that can be used to obtain deer abundance estimates by geographical block and habitat type. Variance estimation procedures are also described. • As an example, we applied the method to estimate sika deer Cervus nippon abundance in south Scotland. Estimates of deer defecation and length of time to dung decay were used to convert pellet group density to deer density by geographical block and habitat type. The results obtained agreed with knowledge from cull and sightings data, and the precision of the estimates was generally high. • Relatively high sika deer densities observed in moorland areas up to 300 m from the forest edge indicated the need to encompass those areas in future surveys to avoid an underestimate of deer abundance in the region of interest. • It is unlikely that a single method for estimating deer abundance will prove to be better under all circumstances. Direct comparisons between methods are required to evaluate thoroughly the relative merits of each of them. • Line transect surveys of dung are becoming a widely used tool to aid management and conservation of a wide range of species. The survey methodology we outline is readily adaptable to other vertebrates that are amenable to dung survey methodology.

Many ecotourism reserves in the Eastern Cape Province of South Africa have introduced large carnivores which prey on ungulates such as kudu. Kudu occupy thick vegetation and helicopter-based counts are not appropriate. In this study we tested line transect counts of faecal pellets in conjunction with DISTANCE analysis as a method for assessing the abundance of kudu. Transects were undertaken in three regions of two game reserves and the results compared with helicopter based game counts (for two regions) and a known kudu population in the third region (an enclosed breeding centre). Estimates of kudu abundance were two to three times greater than the helicopter counts, but within the known number of kudu in the breeding centre. The increased accuracy in the breeding centre was probably because the transects covered a greater percentage of the available habitat (0.07%) than in the reserves (approximately 0.007%). We suggest that DISTANCE software and line transects counts of pellet groups can be used for kudu and probably for other ungulates in thicket-type vegetations. Accuracy will depend on the percentage of the available habitat that is surveyed and on the accuracy of the defecation and decay rates.

Effective management and conservation of wildlife populations require reliable estimates of population size, which can be difficult and costly to obtain. We evaluated how precision in estimates of herd size and abundance varies with sample size and strip width using two field surveys and bootstrap resampling of the field data. We also examined precision under distance sampling and evaluated the cost-effectiveness of both survey techniques. Precision in estimates of abundance increased with increasing sample size and varied with strip width independently of sample size. The hazard rate key function was best for five species in two surveys with contrasting visibility conditions. Precision in density was more sensitive to the number of herds sighted than to variation in herd size and effective strip width for distance sampling. Strip counts produced lower abundance estimates but higher precision than distance sampling. We estimated that distance sampling would cost about US$3.1 km−1 of transect. Strip counts deserve serious consideration for surveys of species that occur at high densities and form large, loose agglomerations but distance methods are suitable for species occurring at moderate to low densities in areas where visibility varies substantially. Distance sampling may thus need to be supplemented by strip counts to efficiently estimate densities of rare, abundant and highly clustered multi-species assemblages of African savanna mammals. In small areas, it may often prove necessary to conduct several surveys to obtain adequate sample sizes for distance models.

We present a robust sampling methodology to estimate population size using line transect and capture-recapture procedures for aerial surveys. Aerial surveys usually underestimate population density due to animals being missed. A combination of capture-recapture and line transect sampling methods with multiple observers allows violation of the assumption that all animals on the centreline are sighted from the air. We illustrate our method with an example of inanimate objects which shows evidence of failure of the assumption that all objects on the centreline have probability 1 of being detected. A simulation study is implemented to evaluate the performance of three variations of the Lincoln-Petersen estimator: the overall estimator, the stratified estimator, and the general stratified estimator based on the combined likelihood proposed in this paper. The stratified Lincoln-Petersen estimator based on the combined likelihood is found to be generally superior to the other estimators.

The condition of the Spanish ibex population of Sierra Magina Natural Park (southern Spain) was studied during the rut period 1991, after the appearance of the first signs of sarcoptic mange in this population. A total of 151 individuals in 53 different groups were observed during the study. The population was characterized by a sex-ratio close to 1, a reproductive index of 0.57, and a reproductive potential of 1.5. Males were mainly 2–3 (40.4%) and 4–5 (38.3%) years old. Density was estimated as 2.6 ind./km2 (SE=0.27), which represented about 406 individuals for the study area. Spanish ibex were present all over the study area, although the preferred elevations were between 1200 and 1600 m. The sarcoptic mange prevalence in the Spanish ibex population was estimated as 25%, and it was spread throughout the study area. Results are discussed in relation to conservation perspectives for this Spanish ibex population and five basic measures are recommended.

The monitoring of animal populations is necessary to conserve and manage the rare or harvest species and to understand the
population change over several years. We used distance sampling methods to estimate seasonal density of blue sheep in a 2,740km2 area of Helan Mountain region by walking along 32 transect lines from winter 2003 to autumn 2005. In all, 367–780 blue sheep
were observed in 91–143 groups in the surveys during the seasons. Observed mean group size ranged from 3.42 to 8.35 individuals;
encounter rate, the number of groups detected per kilometre, varied from 0.19 to 0.30 during the seasons. A hazard rate key
function with cosine series expansion and a half-normal key function with either cosine or simple polynomial series expansion
were the best fitting models based on the lowest value of Akaike’s information criterion (AIC). Density estimates varied between
3.627 sheep per square kilometre in spring 2004 and 4.635 per square kilometre in summer 2005. There were no detectable differences
in estimated density among seasons (P=0.887). The estimated density of blue sheep was negatively correlated with the total number of deaths (P<0.05), number of sub-adult males’ deaths (P<0.05), number of sub-adult females’ deaths (P<0.05), and number of male lambs’ deaths (P<0.05). We concluded that distance sampling surveys should be used to monitor long-term population trends to provide the
best quantitative estimates of blue sheep populations in the Helan Mountains region.

When conducting line transect sampling to estimate the abundance of a clustered wildlife population, detection of a school depends not only on the perpendicular distance of the school to the transect line, but also on the size of school. Larger size schools are easier to detect than smaller schools. Thus, a bivariate detection function with distance and size as covariates should be considered. This paper considers using the kernel smoothing method to fit the bivariate line transect data in order to estimate both abundance and the mean school size. Two kernel estimators are studied: the fixed kernel estimator, which uses the same smoothing bandwidth for all data points, and the adaptive kernel estimator, which allows the bandwidth to vary across the data points.

When using bivariate line transect methods to estimate the biomass density of a tightly clustered biological population, it is generally assumed that both the perpendicular distance from the trackline to the cluster and the cluster size, or biomass, are measured without error. This is unlikely to be the case in practice. In this article, assuming additive mean zero errors in distance and multiplicative errors in size, we develop an estimator of density that corrects for these errors. We use the method of moments for the case of gamma cluster size, randomly placed transect lines, and the generalized exponential detection function. We derive results that show that it may not be necessary to correct for errors in distance or size when the distance and size estimates are not biased. When the size estimates are biased, the biomass density estimate has approximately the same bias as the size estimates. The work is illustrated in the context of annual aerial surveys for juvenile southern bluefin tuna in the Great Australian Bight.

Melville and Welsh (2001, Biometrics 57, 1130-1137) consider an approach to line transect sampling using a separate calibration study to estimate the detection function g. They present a simulation study contrasting their results with poor results from a traditional estimator, labeled the "Buckland" estimator and referenced to Buckland et al. (1993, Distance Sampling: Estimating Abundance of Biological populations). The poor results from the "Buckland" estimator can be explained by the following observations: (i) the estimator is designated for untruncated distance data, but was applied by Melville and Welsh to truncated distance data; (ii) distance data were not pooled across transects, contrary to standard practice; and (iii) bias of the estimator was evaluated with respect to a fixed rather than a randomized grid of transect lines. We elaborate on the points above and show that the traditional methods perform to expectation when applied correctly. We also emphasize that the estimator labeled the "Buckland" estimator by Melville and Welsh is not an estimator recommended by Buckland et al. for practical survey applications.

We consider the method of distance sampling described by Buckland, Anderson, Burnham and Laake in 1993. We explore the properties of the methodology in simple cases chosen to allow direct and accessible comparisons of distance sampling in the design- and model-based frameworks. In particular, we obtain expressions for the bias and variance of the distance sampling estimator of object density and for the expected value of the recommended analytic variance estimator within each framework. These results enable us to clarify aspects of the performance of the methodology which may be of interest to users and potential users of distance sampling.

Modelo combinado captura-recaptura e transectos lineares: uma abordagem bayesiana

- J F Monteiro

Monteiro JF. 2010. Modelo combinado captura-recaptura e transectos lineares: uma abordagem bayesiana. Ph.D. Thesis.
University of Évora, Évora. [In Portuguese].

SPSS 17.0 for windows

- Spss Inc

SPSS Inc. 2009. SPSS 17.0 for windows. Chicago: SPSS.

La stima di popolazione degli ungulate mediante Distance Sampling e termocamera infrarossi

- B Franzetti
- S Focardi

Franzetti B, Focardi S. 2006. La stima di popolazione degli ungulate mediante Distance Sampling e termocamera infrarossi.

Sizetran: Analysis of size-biased line transect data

- T D Drummer

Drummer TD. 1991. Sizetran: Analysis of size-biased line transect
data. Wildlife Society Bulletin 19:117-118.

Analysis and management of animal populations: Modelling, estimation, and decision making

- Bk Williams
- Jd Nichols
- Mj Conroy

Williams BK, Nichols JD, Conroy MJ. 2002. Analysis and management of animal populations: Modelling, estimation, and
decision making. San Diego: Academic Press.