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Estimating feral cat (Felis catus) population density in eastern Australia’s subtropical rainforest using spatial capture-recapture and random thinning to account for unmarked pelage types

Darren McHugh , Matthijs Hollanders , Sarah Legge , Ben Augustine

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 in sub-tropical Gondwanan rainforests of eastern Australia, a region with world heritage status and high biodiversity values, and in which cat density was unknown. 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 0.86 cats km-2 (95% HPDI: 0.53–1.09) 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 to other pelage types. Population sizes during the four survey periods were estimated to be similar, with posterior medians ranging from 197–202 individuals and 95% HPDIs ranging from 95–329 individuals within a ~234 km2 area. Conclusions. Our study provides the first robust feral cat density estimate from a sub-tropical 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.

WR24102  Accepted 11 December 2024

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