Inferring the absence of an incipient population during a rapid response for an invasive species

Abstract

Successful eradication of invasives is facilitated by early detection and prompt onset of control. However, realizing or verifying that a colonization has occurred is difficult for cryptic species especially at low population densities. Responding to the capture or unconfirmed sighting of a cryptic invasive species, and the associated effort to determine if it indicates an incipient (small, localized) population or merely a lone colonizer, is costly and cannot continue indefinitely. However, insufficient detection effort risks erroneously concluding the species is not present, allowing the population to increase in size and expand its range. Evidence for an incipient population requires detection of ≥1 individual; its absence, on the other hand, must be inferred probabilistically. We use an actual rapid response incident and species-specific detection estimates tied to a known density to calculate the amount of effort (with non-sequential detections) necessary to assert, with a pre-defined confidence, that invasive brown treesnakes are absent from the search area under a wide range of hypothetical population densities. We illustrate that the amount of effort necessary to declare that a species is absent is substantial and increases with decreased individual detection probability, decreased density, and increased level of desired confidence about its absence. Such survey investment would be justified where the cost savings due to early detection are large. Our Poisson-based model application will allow managers to make informed decisions about how long to continue detection efforts, should no additional detections occur, and suggests that effort to do so is significantly higher than previously thought. While our model application informs how long to search to infer absence of an incipient population of brown treesnakes, the approach is sufficiently general to apply to other invasive species if density-dependent detection estimates are known or reliable surrogate estimates are available.

Invasive brown treesnakes (Boiga irregularis) move short distances and have small activity areas in a high prey environment

“…Additionally, increased small mammal encounters observed immediately after the snake suppression augments evidence that ectothermic predators regulate prey populations 44 , 46 . Importantly, our findings suggest this invasive snake could exhibit markedly reduced movements in habitats outside of Guam where prey densities are much higher 30 and where there is a real threat of establishment 21 , 68 , 69 . This suggests that eradication of incipient populations may require a major labor investment that warrants continued development of detection tools for snakes at low densities and/or occupying high prey environments.…”

“…BTS invaded the island of Guam shortly after World War II 19 , 20 where it caused the local extinction of at least 15 vertebrate species 19 , 20 . Due to their high abundance and ability to stow away in shipping containers, there is a high risk that BTS could become established on nearby snake-free islands 21 . Currently, BTS are managed on Guam using snake traps, toxic bait, and visual surveys 22 .…”

Use of visual surveys and radiotelemetry reveals sources of detection bias for a cryptic snake at low densities

“…Using recent models developed to infer the amount of search effort required to detect BTS at varying densities, and assuming a hypothetical population of BTS at a density of 0.5 snakes/ha with a detection probability of 0.07, visual survey teams should have a 93.5% chance of detecting any snake after visually surveying 95.04 km of transects and a 99.6% chance after surveying 190.08 km of transects (Yackel Adams et al. ). Thus, our results are the first to confirm these modeling estimates and support the effectiveness of visual surveys to obtain information on population demographics, occupancy, rate of expansion, and general habitat use.…”

Detecting target species: with how many samples?

“…The approach followed, based on that of Green & Young [ 1 ], is applicable to any invasive species with seasonal variation in surveillance effectiveness. One more example: the authors of Yackel Adams et al [ 7 ] introduce a Poisson-based model application to report how long to look to infer the absence of an incipient population of brown tree snakes ( Boiga irregularis ), and claim that their approach applies to other invasive species.…”