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 management. Our predation threshold metric provides an important management tool to help estimate target BTS suppression levels that can be used to determine when bird reintroduction campaigns might begin and serves as a model for other systems to match predator control with reintroduction efforts.

Keywords: Aplonis opaca; Boiga irregularis; Corvus kubaryi; Hypotaenidia owstoni; Island endemic avifauna; Mariana Islands; Pacific Islands conservation; Ptilinopus roseicapilla; Rhipidura rufifrons; Todiramphus cinnamominus; nonnative predators; reintroduction.

FIGURE 1

Life cycle diagrams showing two of several configurations we used to represent Guåhan avifauna. Juveniles in their first year go through five or six 2‐month time steps before adulthood. In life cycle (a), new adults are assigned a sex, and females are either pair bonded or not pair bonded. Only pair‐bonded females can progress through three stages of breeding. In life cycle (b), juveniles enter the subadult stage before being assigned a sex. Females in life cycle (b) pass through three breeding stages without regard to pairing. In life cycle (c), females are either breeding or nonbreeding. Here, only those females with nest success enter the breeding stage. Matrices at the right of each life cycle display the transition rules among breeding stages (Appendix S1). All columns sum to one.

FIGURE 2

Comparison of demographics and life history among Guåhan avifauna (No, Nosa‘; Ch, Chichirika; Si, Sihek; Så, Såli; to, Totot; Åg, Åga; Ko, Ko‘ko‘). Boxplots display the quantiles and range of output from 200 simulated populations. (a) Annual growth rate was calculated in four consecutive 5‐year intervals (e.g., 0–5, 6–10 years). (b) Annual fertility (number of young fledged per year), (c) net reproductive rate, (d) life expectancy (in years), (e) generation time (in years), (f) probability of survival to adulthood (black outline) shown together with probability of survival to first reproduction (color coded outline), and (g) age of first reproduction (in years).

FIGURE 3

Sensitivity of annual rate of stochastic population growth for each bird species to nightly chance of being encountered by a brown treesnake. Solid grayscale lines depict increasing probability of consumption according to inset graph in top right. Colored lines trace the expert‐defined mortality risk on encounter. Dashed colored lines delineate the range of encounter rates defined by the range of consumption rates that yield replacement population growth, and the arrow marks the best estimate. Results from simulated nest boxes are marked with a triangle and “NB.”

FIGURE 4

Translation of a nightly contact rate to the probability of a bird being encountered by brown treesnake at least once within either a two (black) or one (gray) month period. Dashed lines trace encounter rates between periods. In red is the contact rate measured by Yackel Adams et al. (2019) and Pollock et al. (2019). Species‐specific lines correspond to arrows in Figure 3 and trace maximum bimonthly encounter rates under which each bird could persist back to a nightly encounter rate.

FIGURE 5

Threshold (minimum) brown treesnake contact rate at which each bird species might persist. Diamonds indicate the results using expert estimates for mortality risk on encounter (McElderry et al., 2021), and confidence intervals represent the range of mortality bracketed in Figure 3. Dotted lines with open diamonds represent individual level encounters, and solid lines and diamonds represent family‐level encounters. Results from simulated nest boxes are marked with a triangle and “NB.”