Predation thresholds for reintroduction of native avifauna following suppression of invasive Brown Treesnakes on Guam

Abstract

The brown treesnake (BTS) (Boiga irregularis) invasion on Guåhan (in English, Guam) led to the extirpation of nearly all native forest birds. In recent years, methods have been developed to reduce BTS abundance on a landscape scale. To help assess the prospects for the successful reintroduction of native birds to Guåhan following BTS suppression, we modeled bird population persistence based on their life history characteristics and relative sensitivity to BTS predation. We constructed individual-based models and simulated BTS predation in hypothetical founding populations for each of seven candidate bird species. We represented BTS predation risk in two steps: risk of being encountered and risk of mortality if encountered. We link encounter risk from the bird's perspective to snake contact rates at camera traps with live animal lures, the most direct practical means of estimating BTS predation risk. Our simulations support the well-documented fact that Guåhan's birds cannot persist with an uncontrolled population of BTS but do indicate that bird persistence in Guåhan's forests is possible with suppression short of total eradication. We estimate threshold BTS contact rates would need to be below 0.0002-0.0006 snake contacts per bird per night for these birds to persist on the landscape, which translates to an annual encounter probability of 0.07-0.20. We simulated the effects of snake-proof nest boxes for Sihek (Todiramphus cinnamominus) and Såli (Aplonis opaca), but the benefits were small relative to the overall variation in contact rate thresholds among species. This variation among focal bird species in sustainable predation levels can be used to prioritize species for reintroduction in a BTS-suppressed landscape, but variation among these species is narrow relative to the required reduction from current BTS levels, which may be four orders of magnitude higher (>0.18). Our modeling indicates that the required predation thresholds may need to be lower than have yet been demonstrated with current BTS

Adaptive resource management: Achieving functional eradication of invasive snakes to benefit avian conservation

“…Applying such a strategy for the conservation of endangered species would, however, likely require a better understanding of the direct numeric relationship between the density of the invader and the functional response of the target species, which can be defined as the density‐impact relationship (Green & Grosholz, 2021). The density of snakes required to observe a response in birds was extremely low (2 snakes ≥900 mm SVL/ha), and comports with prior estimates based on vital rates and expert opinion for avian species on Guam (McElderry et al., 2022). We also did not achieve target densities or removals until we deployed live avian lures in our control programme.…”

“…The density of snakes required to observe a response in birds was extremely low (2 snakes ≥900 mm SVL/ha), and comports with prior estimates based on vital rates and expert opinion for avian species on Guam (McElderry et al, 2022). We also did not achieve target densities or removals until we deployed live avian lures in our control programme.…”

Gape‐limited invasive predator frequently kills avian prey that are too large to swallow

“…This is particularly important in light of the results of McElderry et al. ( 2022 ), which indicate that reintroduced native bird populations on Guam will have extremely low predation thresholds to allow population establishment.…”

“…that snake management for bird restoration would need to account for fatal predation attempts by snakes too small to successfully ingest their prey. This is particularly important in light of the results ofMcElderry et al (2022), which indicate that reintroduced native bird populations on Guam will have extremely low predation thresholds to allow population establishment.…”