Free Standard AU & NZ Shipping For All Book Orders Over $80!
Register      Login
Wildlife Research Wildlife Research Society
Ecology, management and conservation in natural and modified habitats
Shopping Cart: ( empty )
You are here: Home > Journals > WR > WR07105
RESEARCH ARTICLE
Contents Vol 35(4)

Accuracy of some aerial survey estimators: contrasts with known numbers

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

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

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

C Corresponding author. Email: john.tracey@dpi.nsw.gov.au

Wildlife Research 35(4) 377-384 https://doi.org/10.1071/WR07105
Submitted: 27 July 2007  Accepted: 15 April 2008   Published: 27 June 2008

Abstract

Density estimates are seldom examined against actual population size, hence the ability of estimators to correct for bias is unknown. Studies that compare techniques are difficult to interpret because of the uncertainty of adherence to their respective assumptions. Factors influencing detection probability, estimators that correct for bias, the validity of their assumptions and how these relate to true density are important considerations for selecting suitable methods. Here we contrasted five estimates of feral goat (Capra hircus) densities obtained from aerial surveys (strip counts, Petersen, stratified Petersen, Chao, Alho) against known densities derived from total counts. After correcting for recounting, the Alho and stratified Petersen estimators applied to helicopter surveys were the most accurate (bias = 0.08 and –0.09 respectively), which suggests that estimates were improved by correcting individual observations according to the characteristics of each observation. An approach using modified Horvitz–Thompson equations for unequal-sized units is described and is recommended to allow for this. Both the Chao (bias = 0.35) and Petersen (bias = 0.22) estimators were positively biased, which is likely to be a consequence of averaging detection probability across all observations. Helicopter survey using capture–recapture with multiple observers is recommended for estimating the density of wildlife populations. However, adjustment for the factors that influence detection probability is required.


Acknowledgements

Thank you to Glen Saunders for his ongoing support. Funding provided by the Wildlife and Exotic Diseases Preparedness Program and the National Feral Animal Control Program through the Bureau of Rural Sciences and the Invasive Animals Cooperative Research Centre is appreciated. Particular thanks to Commercial Helicopters Australia P/L, Matt Hollingdale and Ken England for aerial survey and mustering assistance. We also thank Mike Martin, Doug and Richard Arnott and their staff, and Greg Jones, Matt Gentle, Ryan Breen, Glen Walker, Craig Faulkner, and Peter West for assistance on the ground, and Remy Van de Ven for assistance with data analysis and interpretation.


References

Alho, J. M. (1990). Logistic regression in capture–recapture models. Biometrics 46, 623–635.
| Crossref | GoogleScholarGoogle Scholar | PubMed | Borchers D. L. , Buckland S. T. , and Zucchini W. (2002). ‘Estimating Animal Abundance: Closed Populations.’ (Springer-Verlag: London.)

Buckland, S. T. , Goudie, I. B. J. , and Borchers, D. L. (2000). Wildlife population assessment: past developments and future directions. Biometrics 56, 1–12.
| Crossref | GoogleScholarGoogle Scholar | PubMed | Caughley G. (1980). ‘Analysis of Vertebrate Populations.’ Reprinted. (John Wiley and Sons: London.)

Caughley, G. , and Grice, D. (1982). A correction factor for counting emus from the air, and its application to counts in Western Australia. Australian Wildlife Research 9, 253–259.
| Crossref | GoogleScholarGoogle Scholar | Druhan J. (1993). Evaluation of the frequency-of-capture, Petersen and three non-parametric techniques for estimating population size from capture–recapture data. B.App.Sci. Thesis, University of Canberra.

Eberhardt, L. L. (1969). Population estimates from recapture frequencies. Journal of Wildlife Management 33, 28–39.
| Crossref | GoogleScholarGoogle Scholar | Fleming P. J. S. (2004). Relationships between feral goats (Capra hircus) and domestic sheep (Ovis aries) with reference to exotic disease transmission. Ph.D. Thesis, University of Canberra.

Fleming, P. J. S. , and Tracey, J. P. (2008). Some human, aircraft and animal factors affecting aerial surveys: how to enumerate animals from the air. Wildlife Research 35, 258–267.
| Crossref | GoogleScholarGoogle Scholar | Maas S. (1997). Population dynamics and control of feral goats in a semi-arid environment. M.App.Sci. Thesis, University of Canberra.

Marques, F. F. C. , and Buckland, S. T. (2003). Incorporating covariates into standard line transect analyses. Biometrics 59, 924–935.
| Crossref | GoogleScholarGoogle Scholar | PubMed | Melville G. J. (1999). Sampling ecological populations. Ph.D. Thesis, Australian National University, Canberra.

Melville, G. J. , and Welsh, A. H. (2001). Line transect sampling in small regions. Biometrics 57, 1130–1137.
| Crossref | GoogleScholarGoogle Scholar | PubMed | Oh H. L. , and Scheuren F. J. (1983). Weighting adjustment for unit non-response. In ‘Incomplete Data in Sample Surveys’. (Eds W. G. Madow, I. Olkin and D. B. Rubin.) pp. 143–184. (Academic Press: New York.)

Parkes J. , Henzell R. , and Pickles G. (1996). ‘Managing Vertebrate Pests: Feral Goats.’ (Australian Government Publishing Service: Canberra.)

Pollock, K. H. (1982). A capture–recapture design robust to unequal probability of capture. Journal of Wildlife Management 46, 752–757.
| Crossref | GoogleScholarGoogle Scholar | Seber G. A. F. (1982). ‘The Estimation of Animal Abundance and Related Parameters.’ 2nd edn. (MacMillan: New York.)

Short, J. , and Bayliss, P. (1985). Bias in aerial survey estimates of kangaroo density. Journal of Applied Ecology 22, 415–422.
| Crossref | GoogleScholarGoogle Scholar | Southwell C. (1989). Techniques for monitoring the abundance of kangaroo and wallaby populations. In ‘Kangaroos, Wallabies and Rat-Kangaroos’. (Eds G. Grigg, P. Jarman and I. Hume.) pp. 659–693. (Surrey Beatty: Sydney.)

Southwell, C. (1994). Evaluation of walked line transect counts for estimating macropod density. Journal of Wildlife Management 58, 348–356.
| Crossref | GoogleScholarGoogle Scholar | Specht R. L. (1970). Vegetation. In ‘The Australian Environment’. (Ed. G. W. Leeper.) pp. 44–67. (CSIRO & Melbourne University Press: Melbourne.)

Steinhorst, R. K. , and Samuel, M. D. (1989). Sightability adjustment methods for aerial surveys of wildlife populations. Biometrics 45, 415–425.
| Crossref | GoogleScholarGoogle Scholar | Tracey J. P. (2004). Assessing estimators of feral goat (Capra hircus) abundance. M.App.Sci. Thesis, University of Canberra.

Tracey, J. P. , and Fleming, P. J. S. (2007). Behavioural responses of feral goats (Capra hircus) to helicopters Applied Animal Behaviour Science 108, 114–128.
| Crossref | GoogleScholarGoogle Scholar |

Tracey, J. P. , Fleming, P. J. S. , and Melville, G. J. (2005). Does variable probability of detection compromise the use of indices in aerial surveys of medium-sized mammals? Wildlife Research 32, 245–252.
| Crossref | GoogleScholarGoogle Scholar |

Trick, L. M. , and Pylyshyn, Z. W. (1994). Why are small and large numbers enumerated differently? A limited-capacity preattentive stage in vision. Psychological Review 101, 80–102.
| Crossref | GoogleScholarGoogle Scholar | PubMed |

Walter, M. J. , and Hone, J. (2003). A comparison of 3 aerial survey techniques to estimate wild horse abundance in the Australian Alps. Wildlife Society Bulletin 31, 1138–1149.


White, G. C. , Bartmann, R. M. , Carpenter, L. H. , and Garrott, R. A. (1989). Evaluation of aerial line transects for estimating mule deer densities. Journal of Wildlife Management 53, 625–635.
| Crossref | GoogleScholarGoogle Scholar |





Appendix 1.  Horvitz–Thompson-like estimator and variance for unequal sized units
Click to zoom