Estimating feral cat (Felis catus) population density in eastern Australia’s subtropical rainforest using spatial capture-recapture incorporating unidentified individuals

Darren McHugh; Matthijs Hollanders; Sarah Legge; Ben C. Augustine

doi:10.1071/WR24102

RESEARCH ARTICLE (Open Access)

Previous Contents Vol 52(2)

Estimating feral cat (Felis catus) population density in eastern Australia’s subtropical rainforest using spatial capture-recapture incorporating unidentified individuals

Darren McHugh

^A ^* , Matthijs Hollanders

^B , Sarah Legge

^C ^D ^E and Ben C. Augustine ^F

+ Author Affiliations

- Author Affiliations

^A Biodiversity and Ecological Health Branch, NSW National Parks and Wildlife Service, Parramatta, NSW 2150, Australia.

^B Quantecol, Ballina, NSW 2478, Australia.

^C Research Institute for the Environment and Livelihoods, Charles Darwin University, Casuarina, NT 0909, Australia.

^D Fenner School of Society and the Environment, The Australian National University, Canberra, ACT 2602, Australia.

^E Centre for Biodiversity and Conservation Research, University of Queensland, St Lucia, Qld, Australia.

^F Department of Natural Resources, Cornell University, Ithaca, NY, USA.

^* Correspondence to: darren.mchugh@environment.nsw.gov.au

Handling Editor: Penny Fisher

Wildlife Research 52, WR24102 https://doi.org/10.1071/WR24102

Submitted: 24 June 2024 Accepted: 11 December 2024 Published: 20 January 2025

© 2025 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

Management of broadly distributed invasive species requires knowledge of population densities across multiple ecosystems. The feral cat (Felis catus) has a continental distribution in Australia and has caused many declines and extinctions; however, density estimates from several ecosystems are few or lacking. Camera trapping data coupled with spatially explicit capture–recapture analysis is a suitable approach for estimating cat densities. However, if a large portion of individuals cannot be identified, density estimation may be difficult and estimates may be too low.

Aims

We aimed to estimate the feral cat density and population size within the subtropical Gondwanan Rainforest of eastern Australia, an area with world heritage status and high biodiversity values, and in which cat density was not known.

Methods

We used feral cat data from a grid of 60 camera traps deployed over 305 days in Border Ranges National Park, totalling 18,300 camera trapping nights. We employed a ‘random thinning’ spatially explicit capture–recapture model that considered detections of both known and unknown feral cat identities. Our modelling included a primary detection history of identified individuals and a secondary detection history that included pelage type, which allowed us to account for individuals with homogeneous pelage types.

Key results

Feral cat density was estimated at 0.858 cats km⁻² (95% HPDI 0.432, 1.385), which is much higher than the average feral cat density across the Australian continent of 0.27 cats km⁻² (95% CI: 0.18–0.45). The probability of identifying unique individuals was low for black and tabby cats compared with other pelage types. Population sizes during the four survey periods were estimated to be similar, with posterior medians ranging from 197 to 202 individuals and 95% highest posterior density intervals ranging from 95 to 329 individuals within a ~234 km² area.

Conclusions

Our study provides the first robust feral cat density estimate from a subtropical rainforest ecosystem.

Implications

Our results add to a growing body of literature that suggests feral cat densities are high in productive mesic reserves of eastern Australia. Management of feral cats in Gondwanan World Heritage Areas should be prioritised to limit impacts on narrow-range endemic species that are likely prey to feral cats.

Keywords: eastern Australia, Felis catus, feral cat, invasive predator, population density, rainforest, spatially explicit capture–recapture.

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