Tools for increasing visual encounter probabilities for invasive species removal: a case study of brown treesnakes

Abstract

Early detection and rapid response (EDRR) are essential to identifying and decisively responding to the introduction or spread of an invasive species, thus avoiding population establishment and improving the probability of achieving eradication. However, detection can be challenging at the onset of a species invasion as low population densities can reduce the likelihood of detection and conceal the true extent of the situation until the species is well established. This is doubly challenging if the invading species displays cryptic behavior or is nocturnal, thus further limiting opportunities for its discovery. Survey methods that maximize a searcher’s ability to detect an incipient population are therefore critical for successful EDRR. Brown treesnakes (Boiga irregularis) on Guåhan are a classic cautionary example of the dangers of not detecting an invasion early on, and the risk of their introduction to other islands within the Marianas, Hawai’i and beyond remains. Nocturnal visual surveys are known to detect brown treesnakes of all sizes and are the primary detection tool used by the Brown Treesnake Rapid Response Team, but detection probability remains low in complex forest habitats. As such, we investigated the use of two potential enhancements to nocturnal visual surveys – a live mouse lure and spray scent attractant – that may create hotspots of increased detection probability during surveys. We found that, while brown treesnake detection probabilities were low for all surveys, visual surveys conducted on transects with live mouse lures resulted in detection probabilities that were 1.3 times higher than on transects without live mouse lures. Conversely, the spray scent attractant did not increase the probability of detecting brown treesnakes compared to transects without scent, and in fact had detection probabilities that were 0.66 times lower, though the reasons for this phenomenon are unclear. Unlike scent attractants, live mouse lures likely provide both visual and olfactory cues that attract brown treesnakes to transects and thus provide more opportunities to detect and capture them. These enhancements were trialed on Guåhan, where prey populations are depressed. It remains unclear whether live mouse lures will be as effective for EDRR applications in prey-rich settings.

Mtor not Mt: Temporal variation in detection probability in spatial capture-recapture and occupancy models

“…I re-analyzed the brown tree snake data set described in Amburgey et al (2021a) collected along 27 transects in a fenced area inside Andersen Air Force Base, Guam. Data are freely available from Amburgey et al (2021b).…”

“…Further, in occupancy, temporal changes in detectability can be used to study species phenology, e.g., when birds are detected by song, which is indicative of territorial behavior and mate attraction during the breeding season (Strebel et al, 2014; Furnas & McGrann, 2018). Temporal variation in detection can also inform the design of futures studies, by revealing the most suitable times or conditions (i.e., associated with high detection probability) for sampling (e.g., Chambert et al, 2012; Amburgey et al, 2021a). As a side note, this study only refers to variation in detection across a set of secondary sampling occasions all nested within the same primary sampling period (in the simulation and case studies, only a single primary period is considered).…”

Burmese pythons in Florida: A synthesis of biology, impacts, and management tools

“…Additionally, development of a facility in southern Florida for captive and small-scale manipulative trials could be useful to refine and optimize the application of control tools and explore techniques better equipped to handle low detection. Unlike the 5-ha enclosure built to enumerate a wild popula-tion of brown treesnakes and evaluate detection probability under various control approaches (e.g., Tyrrell et al 2009;Christy et al 2010;Amburgey et al 2021), a similar enclosure in southern Florida could not feasibly enclose an experimental population because pythons are much larger and more mobile than brown treesnakes (35-160 cm SVL, Tyrrell et al 2009;Christy et al 2010). Instead, a facility in southern Florida with large, enclosed areas (e.g., 75 m 2 ) could facilitate targeted experiments in a more natural environment, with a similar climate to the native range, for studies of behavior and physiology (e.g., attractants, see Trapping section).…”