Projecting Invasion Risk of Non-Native Watersnakes (Nerodia fasciata and Nerodia sipedon) in the Western United States

Abstract

Species distribution models (SDMs) are increasingly used to project the potential distribution of introduced species outside their native range. Such studies rarely explicitly evaluate potential conflicts with native species should the range of introduced species expand. Two snake species native to eastern North America, Nerodia fasciata and Nerodia sipedon, have been introduced to California where they represent a new stressor to declining native amphibians, fish, and reptiles. To project the potential distributions of these non-native watersnakes in western North America, we built ensemble SDMs using MaxEnt, Boosted Regression Trees, and Random Forests and habitat and climatic variables. We then compared the overlap between the projected distribution of invasive watersnakes and the distributions of imperiled native amphibians, fish, and reptiles that can serve as prey or competitors for the invaders, to estimate the risk to native species posed by non-native watersnakes. Large areas of western North America were projected to be climatically suitable for both species of Nerodia according to our ensemble SDMs, including much of central California. The potential distributions of both N. fasciata and N. sipedon overlap extensively with the federally threatened Giant Gartersnake, Thamnophis gigas, which inhabits a similar ecological niche. N. fasciata also poses risk to the federally threatened California Tiger Salamander, Ambystoma californiense, whereas N. sipedon poses risk to some amphibians of conservation concern, including the Foothill Yellow-legged Frog, Rana boylii. We conclude that non-native watersnakes in California can likely inhabit ranges of several native species of conservation concern that are expected to suffer as prey or competing species for these invaders. Action should be taken now to eradicate or control these invasions before detrimental impacts on native species are widespread. Our methods can be applied broadly to quantify the risk posed by incipient invasions to native biodiversity.

Literature review of giant gartersnake (<em>Thamnophis gigas</em>) biology and conservation

“…Introduced populations of Nerodia sipedon (northern watersnake) and Nerodia fasciata near Sacramento pose a potential incipient threat if these populations spread to the Central Valley (U.S. Fish and Wildlife Service, 2006;Rose and others, 2013;Rose and Todd 2014). Habitat conversion could favor other Thamnophis species and bring them into contact with T. gigas, promoting competition where it previously did not exist (Hansen, 1986).…”

Using citizen science data to identify the sensitivity of species to human land use

“…), to examine trends in species occupancy over time (Kery et al. ), and to predict invasions by non‐native species to anticipate management problems (Rose & Todd ). One primary benefit of atlas data collected by citizen scientists is the immediacy with which data can be accumulated online, which facilitates addressing contemporary conservation challenges (Sullivan et al.…”

“…However, data from atlas projects are now being used to provide key insights into how contemporary threats shape biodiversity. For example, atlas data are used to project changes in species distributions related to climate change (Maggini et al 2014), to examine trends in species occupancy over time (Kery et al 2009), and to predict invasions by non-native species to anticipate management problems (Rose & Todd 2014). One primary benefit of atlas data collected by citizen scientists is the immediacy with which data can be accumulated online, which facilitates addressing contemporary conservation challenges (Sullivan et al 2009;Hobern et al 2014).…”

SDMdata: A Web-Based Software Tool for Collecting Species Occurrence Records

“…Species occurrence records are very important for building species distribution models (SDMs) for use in assessing the potential niches of species [ 1 ], testing conservation applications, evolutionary and biogeographical hypotheses [ 2 – 4 ], projecting species invasion risk and proliferation [ 5 ], assessing the impact of climate change, land use, urbanization and other environmental changes [ 6 – 8 ], suggesting potential suitable sites for rare species [ 9 ], and supporting appropriate conservation planning and reserve selection [ 10 ].…”