When we assume that contemporary management actions will be effective against the global rise of emerging alien species, we can develop management complacency, which leads to potentially disastrous outcomes for native biodiversity. Here, we propose the use of the probability of detection as a metric to assess the feasibility of management actions for alien species. We explore how detectability can influence the management of alien reptiles, a group of emergent alien vertebrates globally. We use a Rapid Biological Assessment method (time-limited transects) to estimate the probability of detection for alien reptiles present on Christmas Island (Australia). Across the five species studied, we found low probabilities of detection and poor explanatory capacity of the individual covariates included in our models. These findings indicate that management options to deal with alien reptiles are limited due to the potential high cost and low efficacy associated with low probabilities of detection. Strict preventive strategies, firmly espousing the principles of adaptiveness and precautionary policies, combined with early detection and biosecurity response activities are needed to address the emergence of alien reptiles. Our research was focussed on alien reptiles on islands, but the rise of new pools of alien species from all taxonomic realms across the world suggests that our conclusions may be applicable more generally. Further research is called for to explore the applicability of our conclusions and recommendations to other taxonomic groups and regions of the world.

Anticipatory policy-making, Christmas Island, preventive decision-making, probability of detection, Rapid Biological Assessment, uncertainty

The global emergence of a new pool of alien species may render existing management actions, ranging from rapid incursion response to the eradication of self-sustaining populations, ineffective (Hulme 2015; Seebens et al. 2018). For example, while the increasingly successful program of invasive alien rodent eradications is well documented (Howald et al. 2007), large-scale attempts to eradicate invasive alien reptiles, a group of emergent alien vertebrates, have failed (Kraus 2009; García-Díaz et al. 2017a). Additionally, the allocation of resources along the management continuum (from prevention to eradication) is complicated by the uncertainties surrounding the costs and benefits of emerging alien species, and the realised efficacy of practical activities (McIntosh et al. 2009; Rout et al. 2014). Assuming that management actions, which have been demonstrated and validated on historical alien species, will work as effectively with novel pools of alien species is risky and can lead to poor management outcomes (or complete failure) in the face of increasing introductions of new alien species (Gregory et al. 2014; Early et al. 2016).

Successful management of alien species depends on the capacity to anticipate the specific transport pathways, which move alien species, and on adapting management actions to address the associated novel risk. While substantial research effort has been invested in understanding the dynamics of changing pathways (Hulme 2015; Seebens et al. 2018), less has focussed on assessing the efficacy and effectiveness of existing management actions. The probability of detection (or capture) of an individual, provides a simple quantitative measure of the likely effort needed to manage new alien species. In turn, the capacity to assess management efforts needed to contend with new alien species provides an indicator of the estimated efficacy of candidate management actions. Critical alien species management actions such as: (i) quarantine; (ii) early detection and either rapid containment, control, or suppression of recent introductions; and (iii) eradication, cannot proceed unless the individuals of an alien species can be reliably detected (Christy et al. 2010; Holden et al. 2016; García-Díaz et al. 2017b). To complicate matters further, during quarantine and early detection activities the presence of a new alien species and the identity of such are not necessarily known a priori. This undermines the possibility of designing and planning species-specific surveillance approaches and implies a need for a multi-purpose surveying methodology, which is reliable across a broad range of taxa.

Rapid Biological Assessments (RBAs) are a widely used tool for conducting biodiversity inventories and monitoring, balancing reliability in sampling biological communities with time and resource constraints to conduct the surveys (Sutherland 2006; Larsen 2016). These features make RBAs a prime candidate for conducting standardised surveys in a timely and cost-effective manner, fitting well the need for rapid detection of new alien species to trigger management actions. However, the reliability of standardised RBAs to suit the needs of alien species management activities needs to be evaluated before they are adopted. In particular, it is essential to establish the capacity to detect the presence of alien species during RBAs (Catenazzi et al. 2016; García-Díaz et al. 2017a).

Reptiles are a notable group of emergent alien species responsible for serious environmental impacts in recipient regions worldwide, especially on islands (Rodda et al. 1999; Christy et al. 2010; Powell 2011; Smith et al. 2012a; Kraus 2015). Alien reptiles are frequently transported both unintentionally (i.e., as stowaways) and intentionally (e.g., through the pet trade), and the accelerating volume of global traffic has increased their opportunities for introduction into new regions (Kraus 2009; Powell 2011; Helmus et al. 2014; Capinha et al. 2017; Moser et al. 2018). In this work, we use a candidate survey methodology (RBAs) to detect alien reptiles. Given the key importance of detecting alien individuals, we explore how detectability can lead to challenges to the effective management of alien reptiles. Taking our findings into account, we suggest policy directions for the effective management of new and emergent alien species.

We investigated a standardised RBA using the case study of the detection of alien reptiles on Christmas Island (Fig. 1). The introduction of five species of invasive alien reptiles on Christmas Island provides the opportunity to investigate how the probabilities of detection could influence the feasibility of management actions for four of the most commonly transported and introduced families of alien reptiles worldwide, including on islands (Kraus 2009; Helmus et al. 2014; Capinha et al. 2017). To date, Christmas Island has been invaded by two species of geckos (family Gekkonidae; common house gecko, Hemidactylus frenatus, and stump-toed gecko, Gehyra mutilata), a skink (Scincidae: grass skink, Lygosoma bowringii), and two snakes (family Typhlopidae: flowerpot snake, Indotyphlops braminus: and family Colubridae: wolf snake, Lycodon capucinus).

Figure 1.

Major land uses on Christmas Island and the geographical distribution of the 34 survey sites (red dots), left panel, and geographical location of Christmas Island (box in right panel map) illustrating its remoteness and position with respect to Australia and Indonesia.

The probabilities of detecting individuals using a standardised RBA were consistently low across all the alien reptiles found on Christmas Island. Both the detectability and the abundance of those alien reptiles were difficult to explain given the rather uncertain effects of the covariates tested in our models, even when our surveys explored a representative and variable sample of environmental conditions (Table 1). Only common house geckos were more detectable with increasing temperature 120 cm above ground, which is concurrent with the species habit of occupying buildings and being found active on the walls of those buildings during our surveys. Low detection probabilities, and the difficulty in predicting both abundance and detection, indicate that managing novel pools of alien reptiles, of yet unknown taxonomic identity, will prove a challenge on both Christmas Island and, by extension, in other places subject to increasing pressure of novel alien reptiles.

It is particularly important to protect island ecosystems, where alien reptiles have produced substantial negative impacts and whose native biodiversity is highly exposed to the threat of alien species (Rodda et al. 1999; Powell 2011; Smith et al. 2012b; Silva-Rocha et al. 2015; Bellard et al. 2017; Moser et al. 2018; van Wilgen et al. 2018). Management activities on islands, excluding targeted eradication post-introduction, will need to be able to detect any potential alien reptiles that arrive on the island, to provide effective protection against these novel alien species (García-Díaz et al. 2017b; Scott et al. 2017). However, even well-resourced management programs cannot detect and intercept every single alien individual arriving at a new location (Jarrad et al. 2011; Scott et al. 2017). We found important differences in the probability of individual detection of different species of alien reptiles, which implies that one survey method and surveillance effort cannot fit all. It is of particular concern that the posterior estimates of the probability of individual detection of wolf snakes were very low (mean < 0.02; Table 1). This is unfortunate, because alien snakes represent, amongst all the alien reptiles, some of the worst threats to native species in the recipient communities, including islands (Dorcas et al. 2012; Kraus 2015; Silva-Rocha et al. 2015). Our posterior estimate of the detection probability of wolf snakes was very similar to that reported for other snakes in places around the world and surveyed using different techniques (Kéry 2002; Christy et al. 2010; Durso et al. 2011; Sewell et al. 2012; Durso and Seigel 2015), suggesting that regardless of the location, species and surveying method, snakes are very difficult to detect in the field. In order to properly address the differences between species, and in anticipation of future changes in pathways and species transported, we recommend that management activities base their surveying efforts on species with the smallest detection probability. This approach will maximise the number of new alien reptiles detected and intercepted, even when accepting that not all individuals can possibly be detected.

The compounded effects of low detection probabilities and uncertain effects of covariates will hinder effective measures to manage the emergence of alien reptiles, a conclusion reinforced by previous species-specific research into the management of invasive brown snakes (Boiga irregularis) in Guam (Christy et al. 2010), invasive Burmese pythons (Python bivittatus) in Florida (Reed et al. 2011), and invasive red-eared slider turtles (Trachemys scripta elegans) in the Iberian peninsula (García-Díaz et al. 2017a). Unfortunately, the detectability of alien reptiles remains a challenge to both management and research activities even when sufficient resources are available, as shown by long-term detectability research conducted in Guam (Rodda et al. 2015). In conclusion, managing alien reptiles already established on islands, and future pools of new invasive alien species, will be costly and with no guarantee of success due to both the low detectability of individuals and the difficulty to predict the likely abundance and detectability of alien reptiles.

Framed in this context of overarching uncertainties and high costs, strong preventive policies should be a priority to address the emergent threat of alien reptiles on islands and elsewhere (Jarrad et al. 2011; Rout et al. 2014; Lodge et al. 2016; Scott et al. 2017). As a first step, public agencies tasked with preventive policies for managing alien species (e.g., environmental protection and primary industries agencies) should implement strict quarantine measures and biosecurity regulations (Meyerson and Reaser 2002; Brenton-Rule et al. 2016; Early et al. 2016). These measures and regulations should be aimed at detecting incoming alien reptiles at quarantine border controls before those individuals can escape containment into the wild (Meyerson and Reaser 2002; Jarrad et al. 2015; Brenton-Rule et al. 2016; Chapple et al. 2016; Scott et al. 2017). Australia and New Zealand have biosecurity arrangements in place, even if imperfect, to achieve these two management goals and islands elsewhere could draw lessons from these two countries to implement their own preventive strategies (Jarrad et al. 2011; Brenton-Rule et al. 2016; Chapple et al. 2016; García-Díaz et al. 2017b; Scott et al. 2017).

Preventive management activities should be complemented with early detection surveys aimed at detecting new populations of alien reptiles promptly (Vander-Zanden et al. 2010; Jarrad et al. 2015; Scott et al. 2017). Again, the costs incurred by early detection surveys will be high due to the need for large survey efforts to overcome low probabilities of detection of alien reptiles. A combination of citizen-science surveys, public education programs, and strategic surveys by biosecurity agencies is an avenue worth continuing to invest in to implement effective early detection surveys within environmental budgets, which are heavily constrained by limited funding (Gallo and Waitt 2011; Lawson et al. 2015; Scott et al. 2017). Our results and methodology can be extended by incorporating additional quantitative tools, such as Baye’s rule, to estimate the surveying effort needed to confidently declare the absence of new alien reptiles given no individuals are detected (Ramsey et al. 2009; Anderson et al. 2013; García-Díaz et al. 2017a). Additionally, the adequacy—particularly, the rates of false positives and negatives—of novel detection methods, such as environmental trace DNA, should be thoroughly explored; although indirect surveillance techniques do not replace the need for capturing alien individuals implicit to many management actions (Hunter et al. 2015; Guillera-Arroita et al. 2017).

More broadly, biosecurity regulations, strict quarantine, and early detection activities should be considered within the framework of robust anticipatory policy-making (Boston 2016, 2017). Anticipatory policy-making for emerging alien species is to be guided by the philosophies of adaptiveness and the precautionary principle (Conroy and Peterson 2013; Boston 2016, 2017). Adaptiveness will require frequently evaluating the performance of preventive tools and policies, such as risk assessments and quarantine, against indicators of risk reduction, and updating the management actions as necessary (Rout et al. 2014; Lodge et al. 2016). The precautionary principle should be central to policy and decision-making to prevent the establishment and spread of emergent alien vertebrates. This is particularly important given the scant knowledge about the impacts of most new alien reptiles and the low likelihood of succeeding in eradicating established, self-sustaining, alien reptile populations (Kraus 2015; García-Díaz et al. 2017a; van Wilgen et al. 2018).

The implementation of stringent preventive policies and early detection activities might be more straightforward on oceanic islands, where their remoteness commonly requires all goods and commodities to be imported via shipping and air traffic routes arriving in a small number (usually one) of ports and airports, limiting the number of potential pathways of transport and points of entry into the island (Kraus 2009; Helmus et al. 2014; Moser et al. 2018). The situation is different on continents and mainland contexts, where there are multiple pathways of transport and many points of entry (Hulme et al. 2008; Leung et al. 2014; Hulme 2015; Yemshanov et al. 2015; García-Díaz et al. 2017b). Further research could explore how well our findings and conclusions transfer to mainland scenarios. Research into the detectability of invasive snakes in Florida suggests that they are similarly difficult to detect in mainland settings (Reed et al. 2011).

We have focussed on alien reptiles on islands in this research, but our conclusions are likely applicable to other groups of emergent alien species globally; such as vertebrates in the pet trade (Hulme 2015; Seebens et al. 2018). Building on the realisation of the cornerstone importance of detectability to manage alien species, future work should investigate the extent to which our findings and recommendations hold for different groups of emergent alien species and under other conditions such as mainland environments. In any case, the lack of research and the abundant uncertainties do not preclude the implementation of good preventive policies to manage alien species, an urgent task considering the lack of preventive and early detection capacities in most countries of the world (Early et al. 2016).

This work would not have been possible without the support provided by the all the staff of the Christmas Island National Park, the Christmas Island Natural Resource Management Board, and the Department of Agriculture and Water Resources (Indian Ocean). This research was conducted under permit number CIN_2015_5 (Christmas Island National Park, Director of National Parks, Australian Government) and a Licence to undertake research activities on regulated crown land. The research was approved by the University of Adelaide Animal Ethics Committee (S-2014-155). Andrew Robinson, John Measey, and an anonymous reviewer provided constructive feedback that helped improve this manuscript. PG-D was supported by an IPRS/APA scholarship (DET), an Invasive Animals CRC PhD scholarship, and the IA CRC Student Grant. This work was supported by an ARC Discovery Grant (DP140102319) to JVR and PC and ARC Future Fellowships to PC and JVR (FT0991420, FT130100254).