How many are there? The use and misuse of continental-scale wildlife abundance estimates

Abstract

The number of individuals in a wildlife population is often estimated and the estimates used for wildlife management. The scientific basis of published continental-scale estimates of individuals in Australia of feral cats and feral pigs is reviewed and contrasted with estimation of red kangaroo abundance and the usage of the estimates. We reviewed all papers on feral cats, feral pigs and red kangaroos found in a Web of Science search and in Australian Wildlife Research and Wildlife Research, and related Australian and overseas scientific and ‘grey’ literature. The estimated number of feral cats in Australia has often been repeated without rigorous evaluation of the origin of the estimate. We propose an origin. The number of feral pigs in Australia was estimated and since then has sometimes been quoted correctly and sometimes misquoted. In contrast, red kangaroo numbers in Australia have been estimated by more rigorous methods and the relevant literature demonstrates active refining and reviewing of estimation procedures and management usage. We propose four criteria for acceptable use of wildlife abundance estimates in wildlife management. The criteria are: use of appropriate statistical or mathematical analysis; precision estimated; original source cited; and age (current or out-of-date) of an estimate evaluated. The criteria are then used here to assess the strength of evidence of the abundance estimates and each has at least one deficiency (being out-of-date). We do know feral cats, feral pigs and red kangaroos occur in Australia but we do not know currently how many feral cats or feral pigs are in Australia. Our knowledge of red kangaroo abundance is stronger at the state than the continental scale, and is also out-of-date at the continental scale. We recommend greater consideration be given to whether abundance estimates at the continental scale are needed and to their use, and not misuse, in wildlife management.

Conservation or politics? Australia's target to kill 2 million cats

“…Unverified continental-scale estimates reported in the literature ranged from 3.8 to 18.4 million cats (Hone & Buckmaster, 2014). Based on the land area of Australia, Hone and Buckmaster (2014) suggested that the commonly quoted estimate of ∼5-18 million cats may have been extrapolated using density estimates from a single location. In 2017, empirical modeling using 91 density estimates from across the continent estimated that the true number of feral cats in Australia is likely considerably lower, ranging from ∼2.1 to 6.3 million (Legge et al, 2017).…”

“…When the Threatened Species Strategy targets were developed, there was no reliable estimate of the number of feral cats in Australia. Unverified continental‐scale estimates reported in the literature ranged from 3.8 to 18.4 million cats (Hone & Buckmaster, ). Based on the land area of Australia, Hone and Buckmaster () suggested that the commonly quoted estimate of ∼5–18 million cats may have been extrapolated using density estimates from a single location.…”

Estimating density of forest bats and their long‐term trends in a climate refuge

“…Nonetheless, there have been pleas for more rigorous attempts at estimating rather than guessing density (Hone & Buckmaster, 2015).…”

“…Yet, density and other attributes are especially difficult or expensive to estimate for mobile or cryptic species. Nonetheless, there have been pleas for more rigorous attempts at estimating rather than guessing density (Hone & Buckmaster, 2015 ).…”