Projecting invasion risk of non-native watersnakes (Nerodia fasciata and Nerodia sipedon) in the western United States.

Jonathan P Rose,Brian D Todd

doi:10.1371/journal.pone.0100277

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.

Highlights

Invasive species are a leading threat to native biodiversity [1,2]
We identified native fish and amphibian species expected to serve as prey and be negatively affected by the establishment of N. fasciata and N. sipedon based on the following criteria: 1) The species is of conservation concern at the state or federal level
The distribution of N. fasciata was most influenced by the mean temperature of the warmest (Bio 10) and coldest (Bio 11) quarters according to the Boosted Regression Trees (BRT) and Maxent models

Summary

Introduction

Invasive species are a leading threat to native biodiversity [1,2]. Their impacts include extinction of native species [3], shifts in trophic dynamics [4], and alteration of ecosystem processes [5]. An analysis of the extinctions caused by invasive species suggests that it is their role as predators through which they most often pose the greatest risk to native fauna [6] In this regard, snakes, because they are universally predatory carnivores, are capable of having extensive impacts on ecological communities through predation on native species. A recent study has shown a remarkable decline in the abundance of native mammals in Everglades National Park that coincided with the introduction and wider establishment of the Burmese Python, Python molurus bivittatus [10]. These cautionary examples suggest that snakes as a group deserve special attention when screening potential invaders and monitoring recently established non-native species. This is especially true for habitats already subject to degradation from other anthropogenic activities because the density of invasive species is often elevated in altered communities [11]

Methods

Results

Conclusion