scholarly journals The influence of animal mobility on the assumption of uniform distances in aerial line-transect surveys

2008 ◽  
Vol 35 (4) ◽  
pp. 275 ◽  
Author(s):  
Rachel M. Fewster ◽  
Colin Southwell ◽  
David L. Borchers ◽  
Stephen T. Buckland ◽  
Anthony R. Pople
Keyword(s):  
Survey Design ◽  
Distance Sampling ◽  
High Mobility ◽  
Line Transect ◽  
Transect Line ◽  
Aerial Surveys ◽  
Pack Ice ◽  
Capture Recapture ◽  
Distance Data ◽  
Crabeater Seals

Line-transect distance sampling is a widely used method for estimating animal density from aerial surveys. Analysis of line-transect distance data usually relies on a requirement that the statistical distribution of distances of animal groups from the transect line is uniform. We show that this requirement is satisfied by the survey design if all other assumptions of distance sampling hold, but it can be violated by consistent survey problems such as responsive movement of the animals towards or away from the observer. We hypothesise that problems with the uniform requirement are unlikely to be encountered for immobile taxa, but might become substantial for species of high mobility. We test evidence for non-uniformity using double-observer distance data from two aerial surveys of five species with a spectrum of mobility capabilities and tendencies. No clear evidence against uniformity was found for crabeater seals or emperor penguins on the pack-ice in East Antarctica, while minor non-uniformity consistent with responsive movement up to 30 m was found for Adelie penguins. Strong evidence of either non-uniformity or a failure of the capture–recapture validating method was found for eastern grey kangaroos and red kangaroos in Queensland.

scholarly journals How many fish? Comparison of two underwater visual sampling methods for monitoring fish communities

PeerJ ◽  
2018 ◽  
Vol 6 ◽  
pp. e5066 ◽  
Author(s):  
Zoi Thanopoulou ◽  
Maria Sini ◽  
Konstantinos Vatikiotis ◽  
Christos Katsoupis ◽  
Panayiotis G. Dimitrakopoulos ◽  
...  
Keyword(s):  
Species Richness ◽  
Large Scale ◽  
Sampling Methods ◽  
Survey Design ◽  
Distance Sampling ◽  
Fish Communities ◽  
High Mobility ◽  
Line Transect ◽  
Rocky Reef ◽  
Line Transects

Background Underwater visual surveys (UVSs) for monitoring fish communities are preferred over fishing surveys in certain habitats, such as rocky or coral reefs and seagrass beds and are the standard monitoring tool in many cases, especially in protected areas. However, despite their wide application there are potential biases, mainly due to imperfect detectability and the behavioral responses of fish to the observers. Methods The performance of two methods of UVSs were compared to test whether they give similar results in terms of fish population density, occupancy, species richness, and community composition. Distance sampling (line transects) and plot sampling (strip transects) were conducted at 31 rocky reef sites in the Aegean Sea (Greece) using SCUBA diving. Results Line transects generated significantly higher values of occupancy, species richness, and total fish density compared to strip transects. For most species, density estimates differed significantly between the two sampling methods. For secretive species and species avoiding the observers, the line transect method yielded higher estimates, as it accounted for imperfect detectability and utilized a larger survey area compared to the strip transect method. On the other hand, large-scale spatial patterns of species composition were similar for both methods. Discussion Overall, both methods presented a number of advantages and limitations, which should be considered in survey design. Line transects appear to be more suitable for surveying secretive species, while strip transects should be preferred at high fish densities and for species of high mobility.

Detectability of penguins in aerial surveys over the pack-ice off Antarctica

2008 ◽  
Vol 35 (4) ◽  
pp. 349 ◽  
Author(s):  
Colin Southwell ◽  
Charles G. M. Paxton ◽  
David L. Borchers
Keyword(s):  
Aerial Survey ◽  
Line Transect ◽  
Mark Recapture ◽  
Transect Line ◽  
Aerial Surveys ◽  
Abundance Estimates ◽  
Transect Sampling ◽  
Pack Ice ◽  
Additional Costs ◽  
Ice Floes

Knowledge of penguin abundance at regional and circumpolar scales across the Southern Ocean is important for the development of ecosystem models and to estimate prey consumption by penguins to assess potential competition with fisheries’ operations. One means of estimating penguin abundance is to undertake aerial surveys across the pack-ice surrounding Antarctica where penguins forage. However, it has long been recognised that aerial counts and resultant abundance estimates are likely to be negatively biased unless detectability is estimated and taken into account. Mark–recapture line-transect methods were used to estimate the detectability of penguin groups resting on ice floes during helicopter surveys over the pack-ice off Antarctica. Group size had the greatest effect of several measured covariates on detectability. Despite a concerted effort to meet the central assumption of conventional line-transect sampling (all objects on the transect line are detected), this was close to being achieved by single observers only in the case of the occasional very large group of >20 penguins. Emperor penguins were more detectable than Adélie penguins. Although observers undertook an extensive simulation training program before the survey, overall they improved in their ability to detect penguin groups throughout the survey. Mark–recapture line-transect methods can provide less biased estimation than conventional line-transect methods in aerial survey applications. This improvement comes with some costs, including the need for more demanding data-recording procedures and the need to use larger, more expensive aircraft. These additional costs will often be small compared with the basic cost, but the gain in terms of improved estimation may be substantial.

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

2011 ◽  
Vol 38 (3) ◽  
pp. 221 ◽  
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.

Aerial surveys of multiple species: critical assumptions and sources of bias in distance and mark - recapture estimators

2008 ◽  
Vol 35 (4) ◽  
pp. 310 ◽  
Author(s):  
Gavin J. Melville ◽  
John P. Tracey ◽  
Peter J. S. Fleming ◽  
Brian S. Lukins
Keyword(s):  
Line Transect ◽  
Implicit Assumption ◽  
Distance Method ◽  
Mark Recapture ◽  
Transect Line ◽  
Aerial Surveys ◽  
Transect Sampling ◽  
Multiple Covariates ◽  
Recent Developments ◽  
Multiple Species

Recent developments in the application of line-transect models to aerial surveys have used double-observer sampling to account for undercounting on the transect line, a crucial step in obtaining correct population estimates. This method is commonly called the mark–recapture line-transect sampling method and estimates the detection probability at zero distance to correct line-transect estimates of abundance. An alternative approach, which uses the same methodology during data collection, is to use a range of covariates, including distance from the transect, in a mark–recapture model. This approach overcomes the implicit assumption of uniform distribution of distances in line-transect estimators. In this paper, we use three alternative approaches (a multiple-covariates distance method, a distance method incorporating adjustment for incomplete detection on the transect line using mark–recapture sampling, and a mark–recapture method with distance as a covariate) to estimate the abundance of several medium-sized mammals in semiarid ecosystems. Densities determined with the three estimators varied considerably within species and sites. In some cases distance estimates were larger than mark–recapture estimates and vice versa. Despite large numbers of observations, distance uniformity was not observed for any species at any site, nor for any species where sites were combined. Possible reasons, which include sampling variability, movement in response to the aircraft and failure of the mark–recapture independence assumption, are discussed in detail.

scholarly journals Using encounter rates as surrogates for density estimates makes monitoring of heavily-traded grey parrots achievable across Africa

Oryx ◽  
2015 ◽  
Vol 50 (4) ◽  
pp. 617-625 ◽  
Author(s):  
Stuart J. Marsden ◽  
Emmanuel Loqueh ◽  
Jean Michel Takuo ◽  
John A. Hart ◽  
Robert Abani ◽  
...  
Keyword(s):  
Local Density ◽  
Conservation Status ◽  
Distance Sampling ◽  
Central Africa ◽  
High Mobility ◽  
Encounter Rate ◽  
Line Transect ◽  
African Countries ◽  
Density Estimates ◽  
Encounter Rates

AbstractEstimating population sizes in the heavily traded grey parrots of West and Central Africa would provide insights into conservation status and sustainability of harvests. Ideally, density estimates would be derived from a standardized method such as distance sampling, but survey efforts are hampered by the extensive ranges, patchy distribution, variable abundance, cryptic habits and high mobility of the parrots as well as by logistical difficulties and limited resources. We carried out line transect distance sampling alongside a simpler encounter rate method at 10 sites across five West and Central African countries. Density estimates were variable across sites, from 0–0.5 individuals km−2 in Côte d'Ivoire and central Democratic Republic of the Congo to c. 30 km−2 in Cameroon and > 70 km−2 on the island of Príncipe. Most significantly, we identified the relationship between densities estimated from distance sampling and simple encounter rates, which has important applications in monitoring grey parrots: (1) to convert records of parrot groups encountered in a day's activities by anti-poaching patrols within protected areas into indicative density estimates, (2) to confirm low density in areas where parrots are so rare that distance sampling is not feasible, and (3) to provide a link between anecdotal records and local density estimates. Encounter rates of less than one parrot group per day of walking are a reality in most forests within the species’ ranges. Densities in these areas are expected to be one individual km−2 or lower, and local harvest should be disallowed on this basis.

Population densities of pack ice seals in the Lazarev Sea, Antarctica

2002 ◽  
Vol 14 (2) ◽  
pp. 123-127 ◽  
Author(s):  
M.N. Bester ◽  
J.W.H. Ferguson ◽  
F.C. Jonker
Keyword(s):  
Species Composition ◽  
Outer Edge ◽  
Population Densities ◽  
Significant Preference ◽  
Ground Speed ◽  
Weddell Seals ◽  
Aerial Surveys ◽  
Pack Ice ◽  
Dronning Maud Land ◽  
Crabeater Seals

From 15 December 1992–4 January 1993 aerial surveys of ice seals were undertaken by Puma helicopter across the pack ice off Dronning Maud Land. The 31 transects of 1/2 nautical mile (n.m.) width were flown at an elevation of 200 ft and a ground speed of 60 knots and covered 805.6 n.m.2 of pack ice. The overall species composition of the seals was 97.8% crabeater seals, 1.67% Ross seals, 0.34% leopard seals and 0.15% Weddell seals. The density abundance of seals was 2.47 n.m.−2 for crabeater, 0.01 n.m.−2 for leopard, 0.004 n.m.−2 for Weddell and 0.04 n.m.−2 for Ross seals. Leopard seals were largely found near the outer edge of the pack, Ross seals were absent only in the outer pack, whilst Weddell seals were virtually absent in the pack ice. Present throughout, crabeater seals and Ross seals showed a statistically significant preference for the inner pack, the reasons for this being unclear.

Measuring Abundance

2020 ◽  
Author(s):  
Graham Upton
Keyword(s):  
Prince Edward Island ◽  
Survey Design ◽  
Plant Cover ◽  
Distance Sampling ◽  
Species Abundance ◽  
Ecological Research ◽  
Species Abundance Distributions ◽  
Capture Recapture ◽  
Using Data ◽  
Removal Sampling

Measuring the abundance of individuals and the diversity of species are core components of most ecological research projects and conservation monitoring. This book brings together in one place, for the first time, the methods used to estimate the abundance of individuals in nature. The statistical basis of each method is detailed along with practical considerations for survey design and data collection. Methods are illustrated using data ranging from Alaskan shrubs to Yellowstone grizzly bears, not forgetting Costa Rican ants and Prince Edward Island lobsters. Where necessary, example code for use with the open source software R is supplied. When appropriate, reference is made to other widely used programs. After opening with a brief synopsis of relevant statistical methods, the first section deals with the abundance of stationary items such as trees, shrubs, coral, etc. Following a discussion of the use of quadrats and transects in the contexts of forestry sampling and the assessment of plant cover, there are chapters addressing line-intercept sampling, the use of nearest-neighbour distances, and variable sized plots. The second section deals with individuals that move, such as birds, mammals, reptiles, fish, etc. Approaches discussed include double-observer sampling, removal sampling, capture-recapture methods and distance sampling. The final section deals with the measurement of species richness; species diversity; species-abundance distributions; and other aspects of diversity such as evenness, similarity, turnover and rarity. This is an essential reference for anyone involved in advanced undergraduate or postgraduate ecological research and teaching, or those planning and carrying out data analysis as part of conservation survey and monitoring programmes.

scholarly journals Monitoring an Endangered savannah ungulate, Grevy's zebra Equus grevyi: choosing a method for estimating population densities

Oryx ◽  
2013 ◽  
Vol 47 (3) ◽  
pp. 410-419 ◽  
Author(s):  
Victoria H. Zero ◽  
Siva R. Sundaresan ◽  
Timothy G. O'Brien ◽  
Margaret F. Kinnaird
Keyword(s):  
Distance Sampling ◽  
Cost Effective ◽  
Single Species ◽  
Line Transect ◽  
Encounter Rates ◽  
Equus Grevyi ◽  
Capture Recapture ◽  
A New Technique ◽  
Grevy’S Zebra

AbstractMethods that accurately estimate animal abundance or density are crucial for wildlife management. Although numerous techniques are available, there have been few comparisons of the precision and cost-effectiveness of different approaches. We assess the precision and cost of three methods for estimating densities of the Endangered Grevy's zebra Equus grevyi. We compare distance sampling and photographic capture–recapture, and a new technique, the random encounter model (REM) that uses camera-trap encounter rates to estimate density. All three methods provide comparable density estimates for Grevy's zebra and are preferable to the common practice of raw counts. Photographic capture–recapture is the most precise and line-transect distance sampling the least precise. Line transects and photographic capture–recapture surveys are cost-effective in the first year and REM is most cost-effective in the long-term. The methods used here for Grevy's zebra may be applied to other rangeland ungulates. We suggest that for single species monitoring programmes in which individuals can be identified, photographic capture–recapture surveys may be the preferred method for estimating wildlife abundances. When encounter rates are low, distance sampling lacks the precision of the other methods but its cost advantage may make it appropriate for long-term or multi-species monitoring programmes. The REM is an efficient and precise method of estimating densities but has high initial equipment costs. We believe REM has the potential to work well for many species but it requires independent estimates of animal movements and group size.

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