Islands are increasingly used to protect endangered populations from the negative impacts of invasive species. Quarantine efforts on islands are likely to be undervalued in circumstances in which a failure incurs non-economic costs. One approach to ascribe monetary value to such efforts is by modeling the expense of restoring a system to its former state. Using field-based removal experiments on two different islands off northern Australia separated by > 400 km, we estimate cane toad densities, detection probabilities, and the resulting effort needed to eradicate toads from an island. We use these estimates to conservatively evaluate the financial benefit of cane toad quarantine across offshore islands prioritized for conservation management by the Australian federal government. We calculate density as animals per km of freshwater shoreline, and find striking concordance of density estimates across our two island study sites: a mean density of 352 [289, 466] adult toads per kilometre on one island, and a density of 341 [298, 390] on the second. Detection probability differed between our two study islands (Horan Island: 0.1 [0.07, 0.13]; Indian Island: 0.27 [0.22, 0.33]). Using a removal model and the financial costs incurred during toad removal, we estimate that eradicating cane toads would, on average, cost between $22 487 [$14 691, $34 480] (based on Horan Island) and $39 724 [$22 069, $64 001] AUD (Indian Island) per km of available freshwater shoreline. We estimate the remaining value of toad quarantine across islands that have been prioritized for conservation benefit within the toads’ predicted range, and find the net value of quarantine efforts to be $43.4 [28.4–66.6] – $76.7 [42.6–123.6] M depending on which island dataset is used to calibrate the model. We conservatively estimate the potential value of a mainland cane toad containment strategy – to prevent the spread of toads into the Pilbara Bioregion – to be $80 [52.6–123.4] – $142 [79.0–229.0] M. We present a modeling framework that can be used to estimate the value of preventative management, via estimating the length and cost of an eradication program. Our analyses suggest that there is substantial economic value in cane toad quarantine efforts across Australian offshore islands and in a proposed mainland containment strategy.

Cane Toad, density, detection probability, eradication, islands, quarantine

It is a truth universally acknowledged that an ounce of prevention is worth a pound of cure. In alien invasive species management, prevention of impact is achieved by conducting routine surveillance programs aimed at early detection (Holden et al. 2015), and by minimizing human-mediated dispersal of non-indigenous species (Chen et al. 2018). Despite the regular use of such quarantine approaches, conservation managers rarely explicitly value this preventative management. Preventative measures are increasingly being adopted to save imperiled taxa (Burns et al. 2012; Commonwealth of Australia 2015), but without explicitly valuing these efforts, we risk falling prey to cognitive biases (e.g., immediacy bias) and so will tend to commit substantially more money and effort to tactical, “cure” type approaches, than to strategic “prevention”. Indeed, vastly more resources are spent controlling the spread and impact of invaders than are spent on preventing their arrival and establishment (Hoffman and Broadhurst 2016).

Quarantine is particularly likely to be undervalued in circumstances in which a failure incurs non-economic costs (e.g., biodiversity loss) (Leung et al. 2002) or when costs or damages persist over long-time scales (Epanchin-Niell et al. 2015). In cases where restoration is feasible, one way to place monetary value on such quarantine efforts is to calculate the cost of restoring the system to its former state (Kimball et al. 2014; Rohr et al. 2016). In the case of an invasive species with primarily non-economic impacts, where invasion is certain or extremely likely, we can calculate the ongoing benefit of quarantine as this expense, i.e., a subsequent eradication program. Such a valuation is a lower bound on the benefit of quarantine for a number of reasons. First, the same quarantine effort typically protects against many potential invasive species, while eradication costs would apply separately to each species. In addition, any impact that an invasive species has before it is eradicated (e.g., local extinction of a native species) must be added to the cost of restoration (Hoffmann and Broadhurst 2016; Jardine and Sanchirico 2018). Lastly, as more area is invaded the value ascribed to remaining quarantined areas will be of greater value. Thus, the cost of eradicating a single invader is a very conservative estimate of the true value of quarantine efforts. Given the above it is important to note that it is unlikely that all potential islands will be invaded, and as such, the estimated costs of eradication have the potential to be significantly lower than ‘worse case’ cost modeling. Even in the face of reduced costs it is prudent to recognize the likelihood that governments and land managers will respond to the large eradication cost of inaction, or the withstanding preference to attempt eradication when incursions inevitably happen.

Islands are important resources for conservation quarantine because they offer a natural barrier to the spread of invasive species. Conservation biologists routinely exploit this property of islands, not only to protect species that naturally occur on islands, but also to provide refuge for species under threat on the mainland (Thomas 2011; Tershy et al. 2015; Legge et al. 2018). In Australia alone, a minimum of 47 conservation translocations to islands have been carried out to date (Department of the Environment, Water, Heritage and the Arts 2009). In these circumstances – where the conservation value of an island has been artificially bolstered – the subsequent arrival of invasive species can have a larger impact than they otherwise would. Typically, island quarantine is used by conservation managers to protect native species from wildlife disease (e.g, Tasmanian devil facial tumor disease; McCallum et al. 2009) or invasive predators (e.g., foxes, cats, weasels, rats). In Australia, however, islands are also used to mitigate the impact of cane toads (Rhinella marina) on native predators (Moro et al. 2018; Ringma et al. 2018). Cane toads were introduced to northeastern Australia in the 1930s and, in northern Australia, continue to spread westerly at a rate of ~50 km per year (Phillips et al. 2010). This invasion has had major impacts on populations of native predators, many of which have no resistance to the toad’s toxin (Greenlees et al. 2010; Nelson et al. 2010; Llewelyn et al. 2014). In response to declines of multiple predator species (e.g., dasyurids, monitors, snakes) the Australian government implemented the Cane Toad Threat Abatement Plan (2011), which aimed to identify, and where possible reduce, the impact of cane toads on native species (Shanmuganathan et al. 2010). A lack of viable methods for broad-scale control, however, has since led the Australian government to place an increased emphasis on containment (on the mainland) and on quarantine (on offshore islands) to mitigate the biodiversity impacts of cane toads (Tingley et al. 2017).

While quarantine is currently the best available strategy, it is not a panacea: cane toads have already established themselves on at least 48 islands across northern Australia (McKinney et al. 2018 unpub data), with potential for further self and anthropogenic introductions. In addition, whilst many methods are being proposed to combat the spread of toads, the most likely control method is quarantine (Tingley et al. 2017), possibly aided by targeted gene flow. Thus, execution of the strategy outlined in the Cane Toad Threat Abatement Plan requires ongoing quarantine, eradication, and containment efforts. Here we estimate the lower bound of the monetary value of these ongoing efforts by estimating the effort required to eradicate cane toads from two islands in northern Australia and generalizing this cost to islands and areas that are currently free of toads. We approach this problem by estimating the density and detection probability of toads on each island and use these estimates to calculate the amount of time and money it would take to remove toads across a subset of islands prioritized for conservation in Australia.

The number of cane toads removed from both Horan and Indian Island, c_t, declined over time (Figure 1). Across the duration of our surveys, we captured and removed a total of 1550 cane toads (1251 on Horan Island, 299 on Indian Island). The estimated posterior probability of detecting an individual toad on a given night differed between our two study sites (Horan Island: mean p [95% credible interval] = 0.10 [0.07, 0.13]; Indian Island: 0.27 [0.22, 0.33]) (Suppl. material 4: Figure S3). Given site-specific detection probabilities, the estimated number of toads present at the initiation of our surveys (N₀) was much higher on Horan Island (2696 [2183, 3549]) than on Indian Island (353 [308, 407]) (Suppl. material 5: Figure S4).

Figure 1.

Numbers of individual cane toads captured per night on Horan (gray) and Indian (black) Islands.

Horan Island – situated in a freshwater lake – has a circumference of 7.63 km, which translates to a cane toad density of 352 [287, 466] individuals per kilometer of freshwater shoreline. The freshwater source on Indian Island has a circumference of 1.04 km, translating to a density of 341 [298, 391] individuals per kilometer of freshwater shoreline. We could also express toad density as animals per km² of island, in which case we calculate an average density of individuals of 56/km² on Indian Island and 2899/km² on Horan Island.

Benefit of quarantine on Prioritized Australian Islands

Using our estimates of eradication costs per-kilometer of freshwater shoreline, we examine the economic benefit of cane toad quarantine on all toad-free islands (by jurisdiction), as well as the cost to restore all toad-inhabited islands to a toad-free state (Figure 2). The current economic benefit of quarantine on all prioritized toad-free islands is estimated to be between $43.4 [28.4–66.6] million (based on Horan Island) and $76.7 [42.6–123.6] million (Indian Island). We estimate it would cost, on average, between $6.0 [3.9–9.2] million (Horan Island) and $10.6 [5.9–17.0] million (Indian Island) to remove toads from all prioritized islands currently occupied by toads. Finally, we estimate the economic benefit of the ‘waterless barrier’ protecting the Pilbara to be between $80.5 [52.6–123.4] million (Horan Island) and $142.1 [79.0–229.0] million (Indian Island).

Figure 2.

Distribution of the benefit of cane toad quarantine across different jurisdictions within Australia. Toad present distributions denote areas where toads are known to occur and represent the cost to remove toads. No islands in either New South Wales, Western Australia or the Pilbara Bioregion have confirmed toad presence.

As the number of alien invasive species requiring management increases, practitioners must identify efficient strategies for allocating resources to various management activities. Although conventional wisdom places emphasis on prevention measures, the practice of valuing such actions in the face of non-economic costs can be challenging. Placing monetary value on a conservation benefit will most often require some value judgement as to the monetary worth of biodiversity. Using estimates of a species’ detectability, population density, and subsequent eradication costs, we aim to sidestep such value judgement when investigating the benefit of quarantine measures in combatting the impact of the invasive cane toad across Australia’s prioritized offshore islands.

Despite substantial community and research effort into cane toad removal via trapping and hand capture, there are only a handful of published detection estimates for the species (Griffiths and McKay 2007). Our detection estimate is, of course, specific to the details of our survey. Nonetheless, it is surprisingly low for our large-shoreline site (Horan Island). Here, the length of shoreline meant we only passed each location once per night, and individual toads in this closed system had, on average, a 0.10 [0.07–0.13] probability of being seen on any given night. This contrasts with our small-shoreline site (Indian Island), where we were able to make multiple passes of the same point each night. Here, individual toads had a 0.27 [0.22–0.33] probability of being detected on a given survey night. Whilst individual toads are relatively easy to see when they are active, our results suggest that this might give a misleading impression of their detectability, especially if the size of area surveilled prevents more than a single pass during each survey. Additionally, physiological correlates are likely to affect individual detection probability, with both sex and body condition linked to activity levels (and hence detectability) of adult cane toads (Yeager et al. 2014). Further work is required to examine how both physiological and environmental correlates influence cane toad detectability as they invade into, and interact with novel environments in Australia.

We compared two density metrics: linear density (per km) and areal density (per km²). Our areal density estimate for Horan Island (2 893 individuals/km²) is similar to estimates derived from previous studies of invasive cane toads in the Solomon Islands archipelago (1 035/km²; Pikacha et al. 2015), the islands of Papua New Guinea (3 000/km²; Zugg et al. 1975; Freeland et al. 1986), and density estimates of an analogous invasive toad on Madagascar (3 240/km²; Reardon et al. 2018). A single study conducted on the Australian mainland reported densities as high as 256 300 individuals per km² (Cohen and Alford 1993), but this estimate was predominantly of the metamorph life stage, which occurs at very high densities prior to dispersal. Metamorphs are strongly constrained to the edges of water bodies (Child et al. 2008), and typically suffer high mortality from predation and desiccation before reaching maturity (Ward-Fear et al. 2010). While an areal density would make sense in a habitat where animals are constrained by some factor that scales with area (e.g., primary productivity), it is clear that toads in northern Australia are often constrained by access to water in the dry season, and thus length of shoreline is more appropriate. Length of shoreline not only defines access to water, but also the density of infectious parasites (such as Rhabdias pseudosphaerocephala) that use moist conditions and high toad densities along shorelines as opportunities for transmission (Kelehear et al. 2011, 2013). It is also likely that the survival rate of emergent metamorphs is dependent on length of shoreline, because this will set the density of conspecifics and so moderate the rate at which these conspecifics cannibalize each other (Pizzatto and Shine 2008). In comparing the areal and linear densities between our sites, we find a large difference between sites in the areal metric, but a strikingly similar density value across sites in the linear metric. Our results suggest that across these two different systems, adult toads achieve a density of around ~350 adults per kilometer of shoreline.

Because toads in dry conditions require regular re-hydration (Seebacher and Alford 2002; Tingley and Shine 2011) it is a logical step to conduct removal efforts when toads are restricted to a subset of semi-permanent hydration points during drier sections of the year (Letnic et al. 2015). Given the ecological reasons discussed above, and the fact that the linear density metric is so concordant across sites, we suggest that the linear metric should be used to calculate eradication costs. Certainly, if we use the areal metric, we find a wide gulf in the possible eradication values relative to our shoreline metric (Suppl. material 3: Figure S2). Encouragingly, our cost estimates using the shoreline metric are similar to estimates derived from a successful eradication program associated with removing the American bullfrog from two locations in Canada ($8 200–$23 000 CAN per kilometer of freshwater shoreline).

To our knowledge, there is only one instance in which the cost to eradicate cane toads from an island has been documented (Wingate 2011). Carried out on Nonsuch Island in Bermuda, this removal occurred over six years and included countless volunteer hours, hand collection and fencing methods, and an investment of $10 000 USD (~$14 330 AUD) to remove toads from an area of 0.6 km². In addition, two successful eradications from extralimital mainland sites have been documented, occurring beyond the southern border of the cane toads’ current range in Australia (White 2010; Greenlees et al. 2018). The low incidence of successful removals of the invasive cane toad mirrors a broad trend in the eradication of invasive amphibian populations globally (Adams and Pearl 2007; Kraus 2009; Beachy et al. 2011; Orchard 2011). As such, there is scant information available to guide policy makers and management agencies when evaluating the feasibility of implementing amphibian quarantine and eradication measures.

Hand removal of individuals is required if eradication is to be successful. In landscapes where hydration points are localized or scarce, the use of fencing to exclude individuals from waterbodies can be a cost-effective solution (e.g., Wingate 2011). In these cases, the effectiveness of fencing relies predominantly on the proportion of the population excluded outside the fence (those not excluded still need to be removed by hand), as well as the cost of materials and the person hours associated with installing and maintaining the fence (see Brooke et al. 2004 for a full costing). For small waterbodies where fencing is feasible, the cost will be directly reduced by the proportion of the population retained outside the fence. Our goal was to provide a general cost metric comparable across prioritized islands and jurisdictions, and to place a lower bound on the value of cane toad quarantine more generally. As such, we refrain from exploring a multi-method approach, although acknowledge this may reduce the overall cost of an eradication program in some instances.

If we are to shift away from tactical, post-invasion approaches, to a preventative strategic approach, management practitioners require an estimate of the economic value that quarantine holds. Our analysis of the feasibility and benefit of cane toad quarantine is timely, given renewed emphasis on Australia’s offshore islands as safe havens to buffer biodiversity against cane toad impacts. Sixty-two Australian offshore islands designated as ‘high conservation status’ fall within the cane toad’s predicted distribution; more than a third of these (21) have already been colonized by toads. Given our criteria (see Methods), we estimate the remaining value of toad quarantine across toad-free islands in northern Australia to be up to $77 [43–124] million. This value is conservative for a number of reasons. It is a reasonable expectation that as islands become home to increasing numbers of insurance populations or endangered species, the benefit of maintaining those islands as pest-free (measured as the cost of restoration) will increase. In addition, as toads establish themselves in an increasing number of these islands, those remaining toad-free will, by their scarcity alone, attain a greater environmental value.

At the same time, our estimate of the remaining value of toad quarantine across toad-free islands may overestimate the total quarantine benefit because it is unlikely that all islands without quarantine will be invaded. For example, islands that only contain hydration opportunities in the form of cultivated lawns or watering gardens (e.g., Darnly Island, Table 2) may be suitable for toads to invade, but reproduction and long-term persistence are unlikely. The benefit of quarantine within our dataset is held primarily by a few large islands (e.g., Melville Island, Table 2). These larger islands often have human settlements, competing management objectives (e.g., economic growth activities, multi-species quarantine), or more convoluted invasion pathways associated with anthropogenic activity. For those that contain large human settlements, the use of organized community groups to conduct local removals or population suppression may reduce costs, although eradication is unlikely without a defined management goal and coordinated effort. In short, quarantine needs to be prioritized and carefully managed on these large islands.

Eradication efforts for taxa other than toads have been successful on large islands, such as a goat eradication program on Santiago Island (5 465 km² at a cost of $7.08 million) (Cruz et al. 2009) or rat eradication carried out on Macquarie island (128 km² at a cost of $21.25 million) (Raymond et al. 2011). These efforts on larger islands require careful planning, intersectional management, and investment in post-eradication surveillance and monitoring (Moore et al. 2010; Rout et al. 2011; Carwardine et al. 2012) and the monetary cost associated with a successful eradication will vary depending on the biology of the target species in question.

The vanguard of the cane toad invasion is currently sweeping across Western Australia at ~50 km per annum, but recent research suggests that a waterless barrier between the Kimberley and the Pilbara could halt the toad invasion (Florance et al. 2011; Tingley et al. 2013; Southwell et al. 2017; Gregg et al. 2019). This barrier represents the only option remaining to exclude cane toads from realizing their entire potential distribution across the Australian mainland. Applying our results to this management strategy revealed that the benefit of quarantine over such an area ($80–142 M) is roughly double the value of quarantine across all offshore islands combined ($49–77 M). The cost of quarantine in this case has been rigorously estimated at around $5 million dollars over 50 years (Southwell et al. 2017), only a fraction of what we estimate it would cost to eradicate toads from this area.

Here we demonstrate the immense benefit of toad quarantine across northern Australia. We avoid value judgement and simply calculate the cost of eradication in the case of quarantine failure. Our valuation is certainly a lower bound on the true benefit, but valuing preventative management is important and will become more so as conservation actions increasingly rely on offshore islands and fenced areas as cost-effective avenues to protect biodiversity from the impacts of alien invasive species. Quarantine measures often protect against multiple potential invaders but our results suggest that even when considering a single species, the monetary value of quarantine can be substantial. Prevention, it seems, is worth more than we might naively guess, even with aphorisms to remind us.

We recognise and thank the Kenbi Traditional Custodians (Raylene and Zoe Singh) for land access permission. We thank Chris Jolly, John Moreen and the Kenbi Ranger Group for their aid in the field, and for logistical support. Corrin Everitt, John Llewelyn, Ruchira Somaweera, and Greg Clarke provided constructive comments and advice. We also thank Greg Smith from Lake Argyle Cruises for his input and local knowledge, and Jane Austen for the opening line. All procedures were approved by the University of Melbourne Animal Ethics Committee (1714277.1). This research was supported by an Australian Research Council Future Fellowship to BP (FT160100198) and an Australian Research Council DECRA to RT (DE170100601). Land access was granted via the Northern Land Council (permit 82368).