Density and Abundance Estimation of Amazonian River Dolphins: Understanding Population Size Variability

The dolphins Inia geoffrensis—boto and Sotalia fluviatilis—tucuxi are threatened cetaceans inhabiting river ecosystems in South America; population numbers are still lacking for many areas. This paper provides density and abundance estimations of boto and tucuxi in 15 rivers sampled during the past nine years as part of a multinational research alliance. Visual boat-survey data collection protocols and analyses have been developed since 2012 (based on Distance Sampling methods) and recently reviewed (2019) to improve robustness and comparability. Differences across the sampled rivers and the analyzed river basins (Amazon and Orinoco) pointed to a density/population size gradient with lower densities and abundances observed in the Orinoco basin (0.9–1.5 ind./km²), passing through the eastern Amazon basin (2–5 ind./km²), and the largest numbers found at the central Brazilian Amazon (lower Purus River—2012 (14.5 boto/km², N = 7672; 17.1 tucuxi/km², N = 9238)). However, in other parts of the central Amazon, the density of dolphins was smaller than expected for high productive whitewater rivers (1–1.7 ind./km² in the Japurá and Solimões rivers). We attributed these differences to specific features of the basin (e.g., hydro-geomorphology) as well as to the cumulative effects of anthropogenic activities.

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Estimating population size of the cave shrimp Troglocaris anophthalmus (Crustacea, Decapoda, Caridea) using mark–release–recapture data

Animal Biodiversity and Conservation ◽

10.32800/abc.2015.38.0077 ◽

2015 ◽

Vol 38 (1) ◽

pp. 77-86

Author(s):

J. Jugovic ◽

◽

E. Praprotnik ◽

E. V. Buzan ◽

M. Luznik ◽

...

Keyword(s):

Population Size ◽

Aquatic Invertebrates ◽

Anthropogenic Activities ◽

Age Distribution ◽

Mark Release Recapture ◽

Population Size Estimates ◽

Cave Ecosystems ◽

Size Estimates ◽

Abundant Population ◽

Population Numbers

Population size estimates are lacking for many small cave–dwelling aquatic invertebrates that are vulnerable to groundwater contamination from anthropogenic activities. Here we estimated the population size of freshwater shrimp Troglocaris anophthalmus sontica (Crustacea, Decapoda, Caridea) based on mark–release–recapture techniques. The subspecies was investigated in Vipavska jama (Vipava cave), Slovenia, with estimates of sex ratio and age distribution. A high abundance of shrimps was found even after considering the lower limit of the confidence intervals. However, we found no evidence of differences in shrimp abundances between summer and winter. The population was dominated by females. Ease of capture and abundant population numbers indicate that these cave shrimps may be useful as a bioindicator in cave ecosystems.

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Estimated Density, Population Size and Distribution of the Endangered Western Hoolock Gibbon (Hoolock hoolock) in Forest Remnants in Bangladesh

Diversity ◽

10.3390/d13100490 ◽

2021 ◽

Vol 13 (10) ◽

pp. 490

Author(s):

Habibon Naher ◽

Hassan Al-Razi ◽

Tanvir Ahmed ◽

Sabit Hasan ◽

Areej Jaradat ◽

...

Keyword(s):

Population Size ◽

Tropical Forests ◽

Forest Regeneration ◽

Distance Sampling ◽

Suitable Habitat ◽

Careful Evaluation ◽

Maxent Model ◽

Maxent Modeling ◽

Forest Remnants ◽

Density Population

Tropical forests are threatened worldwide due to deforestation. In South and Southeast Asia, gibbons (Hylobatidae) are important to seed dispersal and forest regeneration. Most gibbons are threatened due to deforestation. We studied the western hoolock gibbon (Hoolock hoolock) in Bangladesh to determine population size and extent of suitable habitat. We used distance sampling to estimate density across 22 sites in northeastern and southeastern Bangladesh. We used Maxent modeling to determine areas of highly suitable habitat throughout Bangladesh. Density was estimated to be 0.39 ± 0.09groups/km2, and the total estimated population was 468.96 ± 45.56 individuals in 135.31 ± 2.23 groups. The Maxent model accurately predicted gibbon distribution. Vegetation cover, isothermality, annual precipitation, elevation and mean temperature of the warmest quarter influenced distribution. Two areas in the northeast and two areas in the southeast have high potential for gibbon conservation in Bangladesh. We also found significantly more gibbons in areas that had some level of official protection. Thus, we suggest careful evaluation, comprehensive surveys and restoration of habitats identified as suitable for gibbons. We recommend bringing specific sites in the northeastern and southeastern regions under protection to secure habitat for remaining gibbon populations.

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Evaluating Alternative Flight Plans in Thermal Drone Wildlife Surveys—Simulation Study

Remote Sensing ◽

10.3390/rs13061102 ◽

2021 ◽

Vol 13 (6) ◽

pp. 1102

Author(s):

Julia Witczuk ◽

Stanisław Pagacz

Keyword(s):

Computer Simulations ◽

Survey Design ◽

Distance Sampling ◽

Line Of Sight ◽

Density Estimates ◽

Accuracy And Precision ◽

Survey Costs ◽

Random Survey ◽

Density Population

The rapidly developing technology of unmanned aerial vehicles (drones) extends to the availability of aerial surveys for wildlife research and management. However, regulations limiting drone operations to visual line of sight (VLOS) seriously affect the design of surveys, as flight paths must be concentrated within small sampling blocks. Such a design is inferior to spatially unrestricted randomized designs available if operations beyond visual line of sight (BVLOS) are allowed. We used computer simulations to assess whether the VLOS rule affects the accuracy and precision of wildlife density estimates derived from drone collected data. We tested two alternative flight plans (VLOS vs. BVLOS) in simulated surveys of low-, medium- and high-density populations of a hypothetical ungulate species with three levels of effort (one to three repetitions). The population density was estimated using the ratio estimate and distance sampling method. The observed differences in the accuracy and precision of estimates from the VLOS and BVLOS surveys were relatively small and negligible. Only in the case of the low-density population (2 ind./100 ha) surveyed once was the VLOS design inferior to BVLOS, delivering biased and less precise estimates. These results show that while the VLOS regulations complicate survey logistics and interfere with random survey design, the quality of derived estimates does not have to be compromised. We advise testing alternative survey variants with the aid of computer simulations to achieve reliable estimates while minimizing survey costs.

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The importance of survey design in distance sampling: field evaluation using domestic sheep

Wildlife Research ◽

10.1071/wr10234 ◽

2011 ◽

Vol 38 (3) ◽

pp. 221 ◽

Cited By ~ 6

Author(s):

Tom A. Porteus ◽

Suzanne M. Richardson ◽

Jonathan C. Reynolds

Keyword(s):

Population Size ◽

Survey Design ◽

Distance Sampling ◽

Field Evaluation ◽

Stage I ◽

Stage Ii ◽

Line Transect ◽

Domestic Sheep ◽

Management Decisions ◽

Mobile Species

Context Sampling methods to estimate animal density require good survey design to ensure assumptions are met and sampling is representative of the survey area. Management decisions are often made based on these estimates. However, without knowledge of true population size it is not possible for wildlife biologists to evaluate how biased the estimates can be if survey design is compromised. Aims Our aims were to use distance sampling to estimate population size for domestic sheep free-ranging within large enclosed areas of hill country and, by comparing estimates against actual numbers, examine how bias and precision are impaired when survey design is compromised. Methods We used both line and point transect sampling to derive estimates of density for sheep on four farms in upland England. In Stage I we used limited effort and different transect types to compromise survey design. In Stage II we increased effort in an attempt to improve on the Stage I estimates. We also examined the influence of a walking observer on sheep behaviour to assess compliance with distance sampling assumptions and to improve the fit of models to the data. Key results Our results show that distance sampling can lead to biased and imprecise density estimates if survey design is poor, particularly when sampling high density and mobile species that respond to observer presence. In Stage I, walked line transects were least biased; point transects were most biased. Increased effort in Stage II reduced the bias in walked line transect estimates. For all estimates, the actual density was within the derived 95% confidence intervals, but some of these spanned a range of over 100 sheep per km2. Conclusions Using a population of known size, we showed that survey design is vitally important in achieving unbiased and precise density estimation using distance sampling. Adequate transect replication reduced the bias considerably within a compromised survey design. Implications Management decisions based on poorly designed surveys must be made with an appropriate understanding of estimate uncertainty. Failure to do this may lead to ineffective management.

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Flood disturbance and shade stress shape the population structure of açaí palm Euterpe precatoria, the most abundant Amazon species

Botany ◽

10.1139/cjb-2019-0090 ◽

2020 ◽

Vol 98 (3) ◽

pp. 147-160

Author(s):

Heloisa Dantas Brum ◽

Alexandre F. Souza

Keyword(s):

Population Structure ◽

Population Size ◽

Size Distribution ◽

Amazon Basin ◽

Habitat Type ◽

Forest Products ◽

Life Stages ◽

Disturbance Regime ◽

Human Settlements ◽

Human Management

Euterpe precatoria Mart. is the most abundant plant species in the Amazon basin, and one of the main non-timber forest products on the continent. A thorough understanding of the ecology of this species is needed to support sustainable management initiatives. Resource availability, disturbance regime, and human management are some of the main factors influencing population structure. We described the species’ life stages, evaluated its allometric relationships, and assessed the effects of habitat type (floodplain and upland) and proximity to human settlements on population size distribution in the Central Amazon near the Purus River. The height:diameter ratio increased from Seedlings to Juvenile 2, but decreased from Juvenile 2 to Reproductive 2, indicating changing height investment for any given diameter along these life stages. There was a marked habitat dependency in both the density and population size distribution, with populations in upland forests dominated by juveniles, whereas populations in the floodplains were dominated by reproductive palms. Proximity to human settlements was not related to population structure parameters. Our results suggest that the disturbance regime may have opposite meanings in várzea forests, where it limits recruitment under increased light levels, and in terra firme forests, where it may stimulate recruitment under limited light conditions.

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Effectiveness of unmanned aerial vehicles to detect Amazon dolphins

Oryx ◽

10.1017/s0030605319000279 ◽

2019 ◽

Vol 54 (5) ◽

pp. 696-698 ◽

Cited By ~ 1

Author(s):

Marcelo Oliveira-da-Costa ◽

Miriam Marmontel ◽

Daiane S. X. da-Rosa ◽

André Coelho ◽

Serge Wich ◽

...

Keyword(s):

Unmanned Aerial Vehicles ◽

Amazon Basin ◽

Accurate Estimate ◽

Amazon River ◽

Tropical Rivers ◽

River Dolphin ◽

Aerial Vehicles ◽

Aerial Surveys ◽

Inia Geoffrensis ◽

Management And Conservation

AbstractQuantifying the abundance of species is essential for their management and conservation. Much effort has been invested in surveys of freshwater dolphins in the Amazon basin but river dimensions and complex logistics limit replication of such studies across the region. We evaluated the effectiveness of using unmanned aerial vehicles (UAVs) for surveying two Amazon dolphin species, the tucuxi Sotalia fluviatilis and pink river dolphin Inia geoffrensis, in tropical rivers. In 2016 we conducted drone and visual surveys over 80 km of the Juruá River in Brazil. The aerial surveys provided higher accuracy than human observers in counting individuals detected in groups. Compared to estimates derived from visual surveys, the use of UAVs could provide a more feasible, economical and accurate estimate of Amazon river dolphin populations. The method could potentially be replicated in other important areas for the conservation of these species, to generate an improved index of river dolphin populations in the Amazon.

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Protected by dragons: Density surface modeling confirms large population of the critically endangered Yellow-crested Cockatoo on Komodo Island

The Condor ◽

10.1093/condor/duaa042 ◽

2020 ◽

Author(s):

Anna Reuleaux ◽

Benny A Siregar ◽

Nigel J Collar ◽

Maria R Panggur ◽

Ani Mardiastuti ◽

...

Keyword(s):

Population Size ◽

Local Density ◽

Large Population ◽

Distance Sampling ◽

Surface Modeling ◽

Habitat Associations ◽

Critically Endangered ◽

Topographic Wetness Index ◽

Monsoon Forest ◽

Density Surface

Abstract Intense trapping of the critically endangered Yellow-crested Cockatoo (Cacatua sulphurea) for the international pet trade has devastated its populations across Indonesia such that populations of >100 individuals remain at only a handful of sites. We combined distance sampling with density surface modeling (DSM) to predict local densities and estimate total population size for one of these areas, Komodo Island, part of Komodo National Park (KNP) in Indonesia. We modeled local density based on topography (topographic wetness index) and habitat types (percentage of palm savanna and deciduous monsoon forest). Our population estimate of 1,113 (95% CI: 587–2,109) individuals on Komodo Island was considerably larger than previous conservative estimates. Our density surface maps showed cockatoos to be absent over much of the island, but present at high densities in wooded valleys. Coincidence between our DSM and a set of independent cockatoo observations was high (93%). Standardized annual counts by KNP staff in selected areas of the island showed increases in cockatoo records from <400 in 2011 to ~650 in 2017. Taken together, our results indicate that KNP, alongside and indeed because of preserving its iconic Komodo dragons (Varanus komodoensis), is succeeding in protecting a significant population of Indonesia’s rarest cockatoo species. To our knowledge this is the first time DSM has been applied to a critically endangered species. Our findings highlight the potential of DSM for locating abundance hotspots, identifying habitat associations, and estimating global population size in a range of threatened taxa, especially if independent datasets can be used to validate model predictions.

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Current status and conservation of the Endangered Przewalski's gazelle Procapra przewalskii, endemic to the Qinghai–Tibetan Plateau, China

Oryx ◽

10.1017/s0030605310001134 ◽

2012 ◽

Vol 46 (1) ◽

pp. 145-153 ◽

Cited By ~ 13

Author(s):

Chunlin Li ◽

Zhigang Jiang ◽

Xiaoge Ping ◽

Jing Cai ◽

Zhangqiang You ◽

...

Keyword(s):

Tibetan Plateau ◽

Population Size ◽

Conservation Status ◽

Distance Sampling ◽

Current Status ◽

Qinghai Lake ◽

Conservation Strategies ◽

Lake Basin ◽

Domestic Livestock ◽

Procapra Przewalskii

AbstractIn 2008 Przewalski's gazelle Procapra przewalskii, endemic to the Qinghai–Tibetan Plateau, China, was recategorized from Critically Endangered to Endangered on the IUCN Red List. The species is still, however, threatened, and the human population and accompanying domestic livestock are increasing around Qinghai Lake. Here we provide up-to-date information on the distribution and population size of the species, evaluate its current conservation status and discuss the difficulties of protecting this gazelle species. We used both distance sampling and total counts to survey 16 sites where the species has been reported and found it at 13 of these, occupying a total area of c. 250 km2. Population size estimated from distance sampling (1,635) and total counts (1,544) was similar. About 20% of the gazelles located were in newly discovered areas for the species. The results indicate an overall growth in the population of Przewalski's gazelle since 2003, although some subpopulations have declined or been extirpated. In spite of conservation efforts Przewalski's gazelle is still threatened by habitat degradation and loss, habitat fragmentation, fencing, intensified competition with domestic livestock and predation. Further growth of this gazelle population is constrained by limited habitat availability and human–gazelle conflict. We recommend that management and conservation strategies need to incorporate comprehensive knowledge of the gazelle, long-term monitoring, and captive breeding of injured and orphaned gazelles to form a potential pool of individuals for future reintroductions to the historical range of the species outside the Qinghai Lake basin.

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Estimating western ringtail possum (Pseudocheirus occidentalis) density using distance sampling

Australian Mammalogy ◽

10.1071/am09037 ◽

2010 ◽

Vol 32 (2) ◽

pp. 197 ◽

Cited By ~ 1

Author(s):

G. R. Finlayson ◽

A. N. Diment ◽

P. Mitrovski ◽

G. G. Thompson ◽

S. A. Thompson

Keyword(s):

Urban Development ◽

Population Size ◽

Western Australia ◽

Distance Sampling ◽

Reliable Estimate ◽

Population Estimates ◽

Density Estimates ◽

The Subject ◽

Ringtail Possum

A reliable estimate of population size is of paramount importance for making management decisions on species of conservation significance that may be impacted during development. The western ringtail possum (Pseudocheirus occidentalis) is regularly encountered during urban development and is the subject of numerous surveys to estimate its abundance. A variety of techniques have been used for this species with mixed results. This paper reports on a case study using distance sampling to estimate density of P. occidentalis in a small habitat remnant near Busselton, Western Australia. Density estimates obtained were within the range of previous studies of this species and we suggest that this technique should be employed in future surveys to improve the accuracy of population estimates for this species before development.

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Assessing the status of a threatened island endemic, Ctenosaura oedirhina, on Roatán, Honduras

Wildlife Research ◽

10.1071/wr18195 ◽

2020 ◽

Vol 47 (2) ◽

pp. 137

Author(s):

A. B. C. Goode ◽

S. A. Pasachnik ◽

T. L. Maple

Keyword(s):

Population Size ◽

Distance Sampling ◽

Adult Mortality ◽

Isolated Populations ◽

Cultural Traditions ◽

Island Endemic ◽

Wild Fauna ◽

The Status ◽

In The Wild ◽

Study Sites

Abstract Context Organisms living in small, isolated populations with very restricted ranges are at high risk of extirpation due to various direct and indirect forces than mainland populations. Roatán spiny-tailed iguanas (Ctenosaura oedirhina) are endemic to the 146-km2 island of Roatán, Honduras. Harvesting for consumption, fragmentation of habitat and predation by domestic animals threaten the existence of this lizard. This species is federally protected in Honduras; however, enforcement is rare. These iguanas are also listed as Endangered by the International Union for Conservation of Nature (IUCN) and are on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). This species is geographically and genetically isolated into small subpopulations that are declining in density. Aims To estimate the population size of Roatán spiny-tailed iguanas (Ctenosaura oedirhina) on the island of Roatán, Honduras. Methods Distance sampling surveys have been used to monitor this species since 2012, and have been used to determine population density at five study sites. Estimates of density at those sites and across the island were used to calculate the population size of this species. Key results The present study elucidates that the high-density populations remaining are declining. The current population size is estimated to be 3759 (95% CI=1406–12616) individuals within the study sites, with 730 additional iguanas potentially outside of the study sites. Conclusions If the current level of decline continues, this species may become extirpated from some locations on Roatán, and go extinct in the wild. Although Honduras does have laws protecting this species and other wildlife, enforcement must be enhanced. Implications Lack of enforced protection for this species allows poaching for consumption to continue, which has been shown to alter its distribution and cause increased adult mortality. Local customs value the consumption of this species, creating a delicate management situation. Recommendations include strategies that mitigate the threat posed by consumption and increase enforcement of the current laws, while acknowledging cultural traditions.

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