Confirming the broadscale eradication success of nutria (Myocastor coypus) from the Delmarva Peninsula, USA

Abstract

Nutria (Myocastor coypus) were introduced to the eastern shore of Chesapeake Bay, USA in the 1940s. They reached peak densities in the late 1990s, causing massive wetland loss. Beginning in 2002, a systematic plan to eradicate nutria from the 1.7M ha Delmarva Peninsula was implemented. Since that time the nutria population has been effectively reduced, and no nutria have been detected since May 2015. A lack of detection does not equate with complete absence. We address the following three questions. (1) What is the expected probability of nutria eradication from the Delmarva Peninsula as of the end of 2020? (2) If the probability of eradication is below the management target of 0.95, how much more surveillance is required? (3) How sensitive is the estimated probability of eradication to varying levels of public surveillance and modelled population growth rates? These questions were addressed by employing a stochastic spatially-explicit surveillance model that uses data in which no nutria were detected to quantify the probability of complete absence (PoA) over the entire Delmarva Peninsula. We applied an analytical framework that decomposes the spatial risk of survivors and data into management zones, and took advantage of low-cost public reporting of nutria sightings. Active surveillance by the eradication program included detector dog and tracker surveys, shoreline surveys, detection with ground and water platforms (with hair snares), and camera traps. Results showed that the PoA increased with time and surveillance from a beginning PoA in May 2015 of 0.01 to a mean of 0.75 at the end of 2020. This indicates that the PoA on the Delmarva was well below the target threshold of 0.95 for declaring eradication success. However, given continued surveillance without detection, a PoA of 0.95 would be achieved by June 2022. This analysis provides an objective mechanism to align the expectations of policy makers, managers and the public on when eradication of nutria from the entire Delmarva Peninsula should be declared successful.

Invasive species eradication: How do we declare success?

“…This simple model was first used to model detection in spatially explicit capture-recapture models (Efford 2004;Borchers and Efford 2008). However, this spatial detection function has also proven to be useful in simulation models for designing efficient surveillance for achieving management objectives (Ramsey et al 2005; Warburton 2020; Anderson et al 2022a).…”

“…The spatially explicit surveillance model also incorporates habitat selection by the target species, because the likely location of a limited number of survivors is not expected to be equal across the landscape but concentrated in preferred areas. Resource selection studies (Manly et al 2002) and the results from species distribution models (Elith et al 2006) can inform the relative probabilities of survivors in different locations and assist in the creation of a relative-risk map (Anderson et al 2013;Anderson et al 2022a). The resolution of the grid-cell system superimposed on the eradication area should be finer than the home range size and should also accommodate spatial heterogeneity of the relative-risk map.…”

“…Values for the spatially explicit detection parameters (𝑔 0 and 𝜎) of animals in monitoring devices can be obtained from published reports, experimental or field studies (Efford 2004;Ball et al 2005;Ramsey et al 2015;Anderson et al 2022b) or expert opinion (Anderson et al 2022a). Similarly, detection experiments have typically been used to estimate the probability of weed detection given a certain amount of search effort (Garrard et al 2008;Hauser et al 2022).…”

“…High variances should be used where there is high parameter uncertainty, which is propagated through to estimates of SSe (Anderson et al 2022a). Given high parameter uncertainty, increasing sample size, through increased surveillance effort, will increase the mean and decrease the variance of the SSe.…”

“…New technologies and evidence-based strategies (Nugent et al 2018;Murphy et al 2019) are enabling eradication of pest species from increasingly larger islands and continental areas (Cruz et al 2005;Carrion et al 2011;Anderson et al 2022a). With eradication programs becoming more ambitious and logistically difficult, the need to provide evidenced-based criteria for evaluating the progress and success of eradication programs is becoming more critical.…”