Abundance and population growth estimates for bare-nosed wombats

Wildlife managers often rely on population estimates, but estimates can be
challenging to obtain for geographically widespread species. Spotlight
surveys provide abundance data for many species and, when conducted over
wide spatial scales, have the potential to provide population estimates of
geographically widespread species. The bare-nosed wombat (Vombatus
ursinus) has a broad geographic range and is subject to spotlight surveys.
We used 19 years (2002–2020) of annual spotlight surveys to provide the
first estimates of population abundance for two of the three extant
bare-nosed wombat subspecies: V. u. ursinus on Flinders Island; and V. u.
tasmaniensis on the Tasmanian mainland. Using distance sampling methods,
we estimated annual rates of change and 2020 population sizes for both
sub-species. Tasmanian mainland surveys included habitat data, which
allowed us to also look for evidence of habitat associations for V. u.
tasmaniensis. The average wombat density estimate was higher on Flinders
Island (0.42 ha-1, 95% CrI = 0.25 – 0.79) than on the Tasmanian mainland
(0.11 ha-1, CrI = 0.07 – 0.19) and both wombat subspecies increased over
the 19-year survey period with an estimated annual growth rate of 2.90%
(CrI = -1.7 – 7.3) on Flinders Island and 1.20% (CrI = -1.1 – 2.9) on
mainland Tasmania. Habitat associations for V. u. tasmaniensis
were weak, possibly owing to survey design; however, we detected regional
variation in density for this subspecies. We estimated the population size
of V. u. ursinus to be 71,826 (CrI = 43,913 – 136,761) on Flinders Island,
which when combined with a previously published estimate of 2,599 (CI =
2,254 – 2,858) from Maria Island, where the subspecies was introduced,
provides a total population estimate. We also estimated 840,665 (CrI =
531,104 – 1,201,547) V. u. tasmaniensis on mainland Tasmania. These
estimates may be conservative, owing to individual heterogeneity in when
wombats emerge from burrows. Although these two sub-species are not
currently threatened, our population estimates provide an important
reference when assessing their population status in the future, and
demonstrate how spotlight surveys can be valuable to inform management of
geographically widespread species.