Comparison of thermal cameras and human observers to estimate population density of fallow deer (Dama dama) from aerial surveys in Tasmania, Australia

Mark R. Lethbridge; Andy Sharp; Elen Shute; Ellen Freeman

doi:10.1071/WR24056

RESEARCH ARTICLE (Open Access)

Previous Next Contents Vol 51(10)

Comparison of thermal cameras and human observers to estimate population density of fallow deer (Dama dama) from aerial surveys in Tasmania, Australia

Mark R. Lethbridge ^A ^* , Andy Sharp ^B ^D , Elen Shute

^C and Ellen Freeman ^B

+ Author Affiliations

- Author Affiliations

^A University of Southern Queensland, Centre for Sustainable Agricultural Systems, Toowoomba, Qld, Australia.

^B Game Services Tasmania, Department of Natural Resources and Environment, Hobart, Tas, Australia.

^C Flinders University, College of Science and Engineering, Bedford Park, SA, Australia.

^D Present address: Parks and Wildlife Service Tasmania, Department of Natural Resources and Environment, Hobart, Tas, Australia.

^* Correspondence to: Mark.Lethbridge@unisq.edu.au

Handling Editor: Penny Fisher

Wildlife Research 51, WR24056 https://doi.org/10.1071/WR24056

Submitted: 16 April 2024 Accepted: 4 September 2024 Published: 1 October 2024

© 2024 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND)

Abstract

Context

The population of introduced fallow deer (Dama dama) is thought to have increased exponentially across much of the island of Tasmania, Australia, since 2000. Historically, deer management decisions have relied on population trend data from vehicular spotlight surveys. Renewed focus on the contemporary management of the species requires development of more robust and precise population estimation methodology.

Aims

This study demonstrates two aerial survey methods – conventional counts by trained human observers, and thermal imaging footage recorded during the same flights – to inform future survey practices.

Methods

Conventional counts were carried out by three observers, two seated on the left side of the helicopter, and one on the right. A high-resolution thermal camera was fitted to the helicopter and was orientated to meet the assumptions of distance sampling methodologies. Both survey methods were used to generate deer population density estimates. Spatial distribution of deer was also analysed in relation to patches of remnant native vegetation across an agricultural landscape. Mark–recapture distance sampling was used to estimate density from human observer counts and provide a comparison to the distance sampling estimates derived from the thermal camera.

Key results

Human observer counts gave a density estimate of 2.7 deer per km², while thermal camera counts provided an estimate of 2.8 deer per km². Deer population density estimates calculated via both methods were similar, but variability of the thermal camera estimate (coefficient of variation (CV) of 36%) was unacceptably high. Human observer data was within acceptable bounds of variability (CV, 19%). The estimated population size in central and north-eastern Tasmania for 2019 approximated 53,000 deer. Deer were primarily congregated within 200 metres of the interface between canopy cover and open pasture.

Conclusions

The population density estimate provides a baseline for monitoring and managing the Tasmanian deer population. Human observer data was more precise than thermal camera data in this study, but thermal counts could be improved by reducing sources of variability.

Implications

Improvements for the collection of thermal imagery are recommended. Future control efforts may be more efficient if they preferentially target habitat edges at this time of year, paired with random or grid-based searches where population density is lower.

Keywords: abundance, geographical range, habitat use, invasive species, mark–recapture, modelling, population management, population density, vertebrates.

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