|Scientific Name:||Varanus mitchelli Mertens, 1958|
|Taxonomic Source(s):||Uetz, P. and Hallermann, J. 2015. The Reptile Database. Available at: http://reptile-database.reptarium.cz/.|
|Red List Category & Criteria:||Critically Endangered A4be ver 3.1|
|Assessor(s):||Shea, G., Woinarski, J. & Cogger, H.|
|Reviewer(s):||Pianka, E.R., Sweet, S. & Doody, J.S.|
|Contributor(s):||Woods, C. & Macdonald, S.M|
Listed as Critically Endangered on the basis that this species has undergone a decline estimated to be approximately 80% over a three generation period (considered here to cover 18 years, with this species' generation length estimated at six years) inferred from the past and ongoing impacts of invasive Cane Toads (Rhinella marina), which have invaded almost the entirety of this species' range since the year 2000. There is no compelling evidence of recovery once Cane Toads have become established in an area, although monitor subpopulations are thought to stabilize at low levels after the initial population crash, and Cane Toad invasion is not reversible.
|Range Description:||This species is found in aquatic habitats throughout the northern parts of Western Australia and the Northern Territory (Cogger 2014). Its range may extend into far northwest Queensland (Macdonald 2016). It is not known to occur on any offshore islands: surveys of more than 66 islands in total across the Kimberley region and the Wessel, English and Tiwi island groups in the Northern Territory did not record the presence of Varanus mitchelli (Woinarski et al. 1999, 2003; Palmer et al. 2013).|
Native:Australia (Northern Territory, Queensland, Western Australia)
|Range Map:||Click here to open the map viewer and explore range.|
|Population:||This species was previously reasonably common, however, populations have rapidly declined as a result of their predation on the highly toxic and invasive Cane Toad (Ward 2012). Declines of 71 and 97% were reported within three years of the Cane Toad's arrival in two sites in the Daly River system (Doody et al. 2009, 2013), and of nearly 50% over a 3-year period following invasion along the M1 Channel Road, compared with numbers over the two preceding years (Doody et al. 2015). Rates of decline are probably similar to better-documented rates for Varanus mertensi, but a lower natural detectability of V. mitchelli makes it more difficult to quantify declines (C. Moritz and J. Woinarski pers. comm. 2017). As with V. mertensi, it is likely that declines are ongoing over a longer period than these studies cover, as suggested by Doody et al. (2015), and as such the reported rates of decline may be underestimates (J. Woinarski pers. comm. 2017). Doody et al. (2015) report that there is no clear evidence in the literature for recovery of V. mertensi subpopulations in areas where toads have been long-established, and the same is likely true of this species. While a large amount of unpublished and anecdotal evidence suggests that V. mertensi has recovered in a number of areas (G. Gillespie, H. Cogger and M. Greenlees pers. comm. 2017) there is no compelling evidence of recovery in V. mitchelli although the species remains present at low densities throughout its range (G. Gillespie pers. comm. 2017).|
|Current Population Trend:||Decreasing|
|Habitat and Ecology:||This semi-aquatic species inhabits margins of watercourses, lagoons and swamps, basking on rocks and limbs. It feeds on aquatic insects, fish, small lizards and frogs (Ward 2012, Cogger 2014). Its generation length is not known with certainty, but it is thought likely that animals reach sexual maturity at 3-4 years and live for 10 years (J. Woinarski pers. comm. 2017). For the purposes of this assessment, generation length is assumed to be around 6 years (J. Woinarski pers. comm. 2017).|
|Generation Length (years):||6|
|Use and Trade:||This species can be kept as a pet in some states of Australia with the appropriate licenses.|
|Major Threat(s):||The invasive Cane Toad is the largest current threatening process for this species, with recorded declines between 49 and 97% following Cane Toad invasions (Doody et al. 2009, 2013, 2015; Ward 2012). Although quantified data on rates of decline exists for only three sites, they are expected to be similar to declines recorded for the more conspicuous Varanus mertensi (J. Woinarski pers. comm. 2017). Cane Toads are now found across most of this species' range and are projected to invade most of the remainder. While the toads may not reach some offshore islands, no island subpopulations of V. mitchelli are known to exist although surveys of islands off the coast of the Northern Territory have not been comprehensive (J. Woinarski pers. comm. 2017).|
|Conservation Actions:||The species is listed as vulnerable in the Northern Territory, and is known to occur within multiple conservation reserves (Atlas of Living Australia 2015). The species is listed on CITES Appendix II.|
Atlas of Living Australia (ALA). 2015. Atlas of Living Australia. Available at: http://www.ala.org.au/. (Accessed: 24 January 2017).
Cogger, H.G. 2014. Reptiles and Amphibians of Australia: Seventh Edition. CSIRO Publishing, Collingwood, Victoria.
Doody, J.S., Castellano, C.M. and Rhind, D. 2015. Indirect facilitation of a native mesopredator by an invasive species: Are cane toads re-shaping tropical riparian communities? Ecology 96(9): 2544-2544.
Doody, J.S., Castellano, C.M., Rhind, D. and Green, B. 2013. Indirect facilitation of a native mesopredator by an invasive species: are cane toads re-shaping tropical riparian communities? Biological Invasions 15: 559-568.
Doody, J.S., Green, B., Rhind, D., Castellano, C.M., Sims, R. and Robinson, T. 2009. Population-level declines in Australian predators caused by an invasive species. Animal Conservation 12: 46-53.
IUCN. 2018. The IUCN Red List of Threatened Species. Version 2018-1. Available at: www.iucnredlist.org. (Accessed: 28 June 2018).
Macdonald, S. 2016. Mitchell's water monitor (Varanus mitchelli). Available at: http://www.arod.com.au/arod/reptilia/Squamata/Varanidae/Varanus/mitchelli. (Accessed: 23 January 2016).
Palmer, R. Pearson, D.J. Cowan, M.A. and Doughty, P. 2013. Islands and scales: a biogeographic survey of reptiles on Kimberley islands, Western Australia. Records of the Western Australian Museum, Supplement 81: 183-204.
Ward, S. 2012. Threatened Species of the Northern Territory; Mitchell's Water Monitor (Varanus mitchelli).
Woinarski, J., Brennan, K., Hempel, C., Armstrong, M., Milne, D. and Chatto, R. 2003. Biodiversity conservation on the Tiwi islands, Northern Territory. Part 2. Fauna. Department of Infrastructure Planning and Environment, Darwin.
Woinarski, J.C.Z., Horner, P., Fisher, A, Brennan, K., Lindner, N.G., Chatto, R. and Morris I. 1999. Distributional patterning of terrestrial herpetofauna on the Wessel and English Company Island groups, northeastern Arnhem Land, Northern Territory, Australia. Australian Journal of Ecology 24: 60-79.
|Citation:||Shea, G., Woinarski, J. & Cogger, H. 2018. Varanus mitchelli. The IUCN Red List of Threatened Species 2018: e.T83778268A101752345.Downloaded on 18 September 2018.|
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