Novel habitat associations and seasonality in threatened Mitchell’s water monitors (Varanus mitchelli): Implications for conservation

Abstract

Mitchell's water monitors (Varanus mitchelli) are a small to medium-sized (<1 m TL) Australian varanid lizard endemic to northern Western Australia and the Northern Territory, extending just into north-western Queensland. They are a semi-aquatic, semi-arboreal monitor, typically thought of as being associated with freshwater riparian habitats. Following the arrival of cane toads (Rhinella marina) across their distribution, V. mitchelli were observed to have suffered precipitous population declines and local extinctions due to lethal toxic ingestion of toads. Here, we present observations of V. mitchelli from previously unreported saline and brackish habitats, as well as information on the cryptic behaviour and seasonal activity of this threatened species in Darwin, Northern Territory. Currently, because V. mitchelli are undocumented in mangrove and littoral habitats, ecological consultants in northern Australia are considering such habitats unsuitable for this threatened varanid and are not recommending targeted surveys for this species in these habitats. We argue that the niche occupied by V. mitchelli is broader than currently recognised and they should be considered as potentially occurring in most mangrove habitats across their known range. We also suggest species-appropriate survey techniques to improve detection of this very cryptic threatened species. Concerningly, despite recently being recognised as Critically Endangered by the International Union for Conservation of Nature (IUCN), V. mitchelli are currently not recognised as threatened by the federal Environment Protection and Biodiversity Conservation (EPBC) Act 1999.

An Austral Ecology 5 year retrospective on Natural History Notes: A compelling case for curiosity

“…Sea snake activity recorded at depths of up to 245 m and foraging in the mesopelagic zone highlighted a wider thermal tolerance and significantly extended the known depth range for sea snakes (Crowe‐Riddell et al., 2019). Novel habitat associations such as Mitchell's water monitors ( Varanus mitchelli ) observed in saline and brackish habitats in Darwin, Northern Territory demonstrate the species occupying a broader habitat niche than previously thought (De Laive et al., 2021). Further, Velvet worms (Phylum: Onychophora) found on floating debris in a wetland on Stradbroke Island raised questions about their presence outside the known vegetation and habitat type in southeast Queensland (Marshall & Martin, 2020).…”

Taking the bait: Developing a bait delivery system to target free‐ranging crocodiles and varanid lizards with a novel conservation strategy

“…Keen olfactory senses allow them to locate toads in their daytime shelters (G. Ward-Fear, personal observation). Populations of large-bodied species (yellowspotted monitor, Varanus panoptes; Mertens' water monitor, V. mertensi) have declined by >90% following toad invasion (Brown et al, 2013;Doody et al, 2009;Shine, 2010), and Mitchell's water monitor (Varanus mitchelli) may be endangered due to toads (Laive et al, 2021).…”