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RESEARCH ARTICLE
Contents Vol 35(4)

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

Gavin J. Melville A C , John P. Tracey B , Peter J. S. Fleming B and Brian S. Lukins B
+ Author Affiliations
- Author Affiliations

A Biometrics Program, NSW Department of Primary Industries, Trangie Agricultural Research Centre, PMB 19, Trangie, NSW 2823, Australia.

B Vertebrate Pest Research Unit, NSW Department of Primary Industries, Orange Agricultural Institute, Forest Road, Orange, NSW 2800, Australia.

C Corresponding author. Email: gavin.melville@dpi.nsw.gov.au

Wildlife Research 35(4) 310-348 https://doi.org/10.1071/WR07080
Submitted: 2 July 2007  Accepted: 7 May 2008   Published: 27 June 2008

Abstract

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.


Acknowledgements

Thanks to Rob Hurst, Rob Bartell, and Jim Balnaves who assisted with enumeration and discussions on the practicalities of aerial surveys. Funding was from the Invasive Animals Cooperative Research Centre, NSW Department of Environment and Conservation and NSW Department of Primary Industries. We acknowledge the comments of an anonymous referee who provided a useful discussion on various issues raised in the paper.


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