Hierarchical distance sampling reveals increased population size and broader habitat use in the endangered Bahama Oriole

SummaryThe Taita Apalis Apalis fuscigularis (IUCN category: Critically Endangered) is a species endemic to south-eastern Kenya. We assessed population size and habitat use in the three forest sites in which it is known to occur (Ngangao, Chawia and Vuria, totalling 257 ha). The estimate of total population size, derived from distance sampling at 412 sample points, ranged from 310 to 654 individuals, with the northern section of Ngangao fragment having 10-fold higher densities than Chawia (2.47–4.93 versus 0.22–0.41 birds ha−1). Ngangao north alone hosted 50% of the global population of the species. The highly degraded Vuria fragment also had moderately high densities (1.63–3.72 birds ha−1) suggesting that the species tolerates some human disturbance. Taita Apalis prefers vegetation with abundant climbers, but the predictive power of habitat use models was low, suggesting that habitat structure is not a primary cause for the low density of the species in Chawia. Protecting the subpopulation in the northern section of Ngangao is a priority, as is identifying factors responsible of the low abundance in Chawia, because ameliorating conditions in this large fragment could substantially increase the population of Taita Apalis.

Download Full-text

Roost Selection, Population Size and Habitat Use by a Colony of Rafinesque's Big-eared Bats (Corynorhinus rafinesquii)

The American Midland Naturalist ◽

10.1674/0003-0031(1999)142[0363:rspsah]2.0.co;2 ◽

1999 ◽

Vol 142 (2) ◽

pp. 363-371 ◽

Cited By ~ 20

Author(s):

Tracy E. Hurst ◽

Michael J. Lacki

Keyword(s):

Habitat Use ◽

Population Size ◽

Corynorhinus Rafinesquii ◽

Roost Selection

Download Full-text

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.

Download Full-text

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.

Download Full-text

Long-term inference of population size and habitat use in a socially dynamic population of wild western lowland gorillas

Conservation Genetics ◽

10.1007/s10592-019-01209-w ◽

2019 ◽

Vol 20 (6) ◽

pp. 1303-1314 ◽

Cited By ~ 1

Author(s):

Laura Hagemann ◽

Mimi Arandjelovic ◽

Martha M. Robbins ◽

Tobias Deschner ◽

Matthew Lewis ◽

...

Keyword(s):

Habitat Use ◽

Population Size ◽

Western Lowland Gorillas ◽

Dynamic Population ◽

Lowland Gorillas

Download Full-text

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.

Download Full-text

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.

Download Full-text

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.

Download Full-text

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.

Download Full-text

Precision, accuracy and bias of walked line-transect distance sampling to estimate eastern grey kangaroo population size

Wildlife Research ◽

10.1071/wr15029 ◽

2015 ◽

Vol 42 (8) ◽

pp. 633 ◽

Cited By ~ 5

Author(s):

Ruth Glass ◽

David M. Forsyth ◽

Graeme Coulson ◽

Marco Festa-Bianchet

Keyword(s):

Population Size ◽

Distance Sampling ◽

Accurate Method ◽

Line Transect ◽

Good Precision ◽

Design Elements ◽

Abundance Estimates ◽

Grey Kangaroo ◽

Eastern Australia ◽

Free Ranging

Context Distance sampling is widely used to estimate the size of wildlife populations, including kangaroos. However, the performance of distance-sampling abundance estimates has seldom been evaluated for wild mammal populations of known size. Aims We evaluated the precision, accuracy, bias and interval coverage of abundance estimates from walked line-transect sampling, a commonly used distance-sampling method, for a marked free-ranging population of eastern grey kangaroos (Macropus giganteus) at Yanakie Isthmus, Wilsons Promontory National Park, south-eastern Australia. Methods In each of two study periods (November 2012 and May 2013) we first determined the true size of the uniquely marked kangaroo population by conducting 10 intensive searches of the study area. We then conducted distance sampling along six systematically spaced line transects. We walked each transect four times in November 2012 and seven times in May 2013. Data were analysed using Program DISTANCE. Key results Our intensive searches revealed that 141 and 124 collared kangaroos were present in the study area in November 2012 and May 2013, respectively. When transects were walked four or more times (i.e. ≥400 observations), maximum precision (coefficient of variation; CV of ~13%) was achieved in both survey periods. Walking transects twice (i.e. ~200 observations) produced abundance estimates with CVs of <20% in each study period. The accuracy (root mean square error) of abundance estimates varied from 1 to 13 (November 2012) and from 3 to 28 (May 2013). Bias ranged from −9% to +23%, but stabilised at between −1% and −9% when transects were walked four or more times in each study period. The 95% confidence intervals for the abundance estimates always included the true population size. Conclusions Our results indicated that walked line-transect distance sampling is a precise and accurate method for estimating eastern grey kangaroo abundance. The small negative biases that occurred when sample sizes were large were likely to be due to some animals moving outside the study area. Implications Provided that the key design elements and assumptions are met, estimates of kangaroo abundance from walked line-transect distance sampling should have good precision (CV < 20%) and minimal (<10%) bias.

Download Full-text