Allometric Regression of Snake Body Length from Head Image Measurements

As in many fields of wildlife research and management, camera devices and photogrammetry have become an integral part of the toolkit for exploring otherwise‐unseen aspects of the biology, behavior, and control of the invasive brown treesnake (Boiga irregularis) on Guam. Because brown treesnakes are cryptic and nocturnal, and nearly all aspects of their ecology are influenced by snake size, methods are needed to estimate snake size from images captured by infrared wildlife cameras. Unfortunately, it is difficult to capture images of an entire snake’s length at a controlled distance from a simple camera setup. Here, I describe the allometric relationships between brown treesnake body length and potential predictors: head measurements, sex, and body condition. Head length (HL) was the most important predictor of body length, alone accounting for 95.9% of the variation in brown treesnake snout‐vent length (SVL). We provide simple regression equations for predicting brown treesnake length from head measurements, an example of how to extract measurements from images, and a convenient lookup table for predicting SVL and 80% prediction intervals from HL alone. Coupled with a simple camera setup that controls subject distance and includes size standards in the image, we can estimate brown treesnake body size from images that include only the head when photographed from above. These methods have been developed to enable ongoing assessments of brown treesnake predation risk following landscape‐scale suppression efforts that could enable the reintroduction of extirpated native wildlife

Relative acceptance of brodifacoum pellets and soft bait sachets by Polynesian rats (Rattus exulans) on Wake Atoll

Removing invasive rats from island ecosystems using rodenticides has proven conservation benefits and is an important management tool for conserving and restoring island ecosystems. However, rodenticide-based eradications can fail if not all rats consume enough bait to result in lethal toxicosis. A recent post-operational review of a failed attempt to eradicate rats from Wake Atoll suggested that some individuals may not have ingested a lethal dose of rodenticide due to potential dietary and/or sensory preferences developed via regular access to anthropogenic food sources. These food sources may be higher in fats and oils, possessing different sensory properties (e.g., softer, chewier, etc.) than the harder pellet formulation of the rodenticide Brodifacoum 25W Conservation (B-25W) used in the eradication attempt. To test this theory, we captured rats from two areas on Wake Island where they may have regular access to human food sources, as well as an uninhabited part of island where rats presumably have less access to human-based food sources and therefore are less likely to be preconditioned for these food types. We subjected them to a head-to-head two-choice bait selection trial between a “soft” sachet formulation of a brodifacoum-based bait, FINAL Soft Bait with Lumitrack® (FINAL), versus the harder pellet formulation of B-25W. Regardless of which habitat rats were captured in, rats overwhelmingly preferred the pellet formulation. No rats in the head-to-head trail consumed any of the FINAL bait, and 100% of the rats that consumed B-25W died. Of the rats in a separate no-choice trail of just FINAL bait, 5 failed to eat any bait; of the rats that did consume some of the FINAL bait, 80% died. Our results demonstrate that Polynesian rats on Wake Atoll do not prefer this soft formulation of brodifacoum-based rodenticide bait. Our results suggest that baiting strategies in the inhabited regions of the atoll, for a proposed eradication attempt, should continue to focus on utilizing traditional pellet formulations. While these results are unequivocal in our test case, we suggest caution in making inference to other situations where dietary preferences of local rodent populations may differ, and local environmental conditions may make other baiting choices more appropriate and efficacious

Ontogenetic and ecological variation in invasion risk of Brown Treesnakes (\u3ci\u3eBoiga irregularis\u3c/i\u3e) on Guam

Size structure within populations of invasive species may have consequences for relative risk at all stages of the invasion process, with implications for management interventions such as interdiction, suppression, and eradication. To assess relative distributions of invasive Brown Treesnakes (Boiga irregularis) among demographic categories of management interest, we undertook the most comprehensive and controlled sampling in \u3e 25 years of research into this ecologically and economically destructive introduced predator. We collected a seasonally-balanced sample of 100 snakes from each of 18 sites, stratified by six habitat types, encompassing the species’ entire extralimital range. Samples indicated significant differences in distributions of female and male snakes among management classes (juvenile, transitional, and mature) by site and habitat. We found substantial heterogeneity in localized population characteristics over relatively small geographic distances, only modest influence of habitat type, higher prevalence of reproductively mature snakes in savanna and urban habitats, and an alarmingly high proportion of snakes that are too small to be effectively targeted by current rodent-baited control tools (mean = 38.2%, range = 19 to 72%). Failure to account for such variability in high risk demographic fractions may hinder successful interventions

Invasive Coqui Frogs Are Associated With Differences in Mongoose and Rat Abundances and Diets in Hawaii

With the increasing rate of species being introduced to areas outside of their native ranges, non-natives are likely to interact in ways that influence each other’s populations. The high densities of invasive coqui frogs (Eleutherodactylus coqui) in Hawaii have been hypothesized to increase non-native mongoose (Herpestes auropunctatus) and rat (Rattus spp.) abundances, and in turn increase bird nest depredation rates. We compared the relative abundances of rats and mongooses and artificial bird nest predation rates at 12 sites that had plots with similar habitat invaded and not invaded by coqui frogs across the island of Hawaii. We interpret our results considering mongoose and rat stomach analyses and camera trap data collected to monitor coqui scavengers. We found that coqui presence was associated with 30% greater mongoose abundance and 17% lower Pacific rat (R. exulans) abundance. Based on our diet analyses and scavenging data, both mongooses and rats consume coquis, but mongooses were the most important consumers of coquis, which may have contributed to their increase in coqui plots. We speculate that coquis are competing with rats for invertebrate prey due to reduced Pacific rat abundance and greater amounts of fruit in rat stomachs collected in coqui-invaded compared to uninvaded plots. We did not observe any difference in bird nest predation rates in coqui-invaded and uninvaded plots. Our results suggest that the coqui invasion may increase or decrease non-native mammal populations, and non-native amphibians may serve as both novel prey and competitors to non-native mammals

An Introduction to the Special Issue: Island Invaders

This is the introductory letter from the associate editors of the special issue on Island Invaders

A Review of Rat Lungworm Infection and Recent Data on Its Definitive Hosts in Hawaii

Rat lungworm (Angiostrongylus cantonensis) is a zoonotic nematode that causes rat lungworm disease (angiostrongyliasis), a potentially debilitating form of meningitis, in humans worldwide. The definitive hosts for rat lungworm are primarily members of the genus Rattus, with gastropods as intermediate hosts. This parasite has emerged as an important public health concern in the United States, especially in Hawaii, where the number of human cases has increased in the last decade. Here we discuss the current knowledge of the rat lungworm, including information on the life cycle and host species, as well as updates on known infection levels. Three species of rats have been unintentionally introduced and become established in Hawaii (Rattus exulans, R. norvegicus, and R. rattus), all of which have been documented as definitive hosts of rat lungworm. Our recent findings indicate that infection levels in rats can vary by species and age. Based on these findings, we also suggest the possibility that R. rattus populations in Hawaii are capable of developing some form of acquired immunity to infection over time, which could have important management implications related to control operations. Information on rat lungworm infection levels and distribution in Hawaii is lacking, especially in rat definitive hosts, and the U.S. Department of Agriculture (USDA) National Wildlife Research Center and the University of Hawaii at Hilo are continuing efforts to help fill these gaps in knowledge

Quantile regression of microgeographic variation in population characteristics of an invasive vertebrate predator

Localized ecological conditions have the potential to induce variation in population characteristics such as size distributions and body conditions. The ability to generalize the influence of ecological characteristics on such population traits may be particularly meaningful when those traits influence prospects for successful management interventions. To characterize variability in invasive Brown Treesnake population attributes within and among habitat types, we conducted systematic and seasonally-balanced surveys, collecting 100 snakes from each of 18 sites: three replicates within each of six major habitat types comprising 95% of Guam\u27s geographic expanse. Our study constitutes one of the most comprehensive and controlled samplings of any published snake study. Quantile regression on snake size and body condition indicated significant ecological heterogeneity, with a general trend of relative consistency of size classes and body conditions within and among scrub and Leucaena forest habitat types and more heterogeneity among ravine forest, savanna, and urban residential sites. Larger and more robust snakes were found within some savanna and urban habitat replicates, likely due to relative availability of larger prey. Compared to more homogeneous samples in the wet season, variability in size distributions and body conditions was greater during the dry season. Although there is evidence of habitat influencing Brown Treesnake populations at localized scales (e.g., the higher prevalence of larger snakes- particularly males-in savanna and urban sites), the level of variability among sites within habitat types indicates little ability to make meaningful predictions about these traits at unsampled locations. Seasonal variability within sites and habitats indicates that localized population characterization should include sampling in both wet and dry seasons. Extreme values at single replicates occasionally influenced overall habitat patterns, while pooling replicates masked variability among sites. A full understanding of population characteristics should include an assessment of variability both at the site and habitat level

To cross or not to cross: modeling wildlife road crossings as a binary response variable with contextual predictors

Roads are significant barriers to landscape-scale movements of individuals or populations of many wildlife taxa. The decision by an animal near a road to either cross or not cross may be influenced by characteristics of the road, environmental conditions, traits of the individual animal, and other aspects of the context within which the decision is made. We considered such factors in a mixed-effects logistic regression model describing the nightly road crossing probabilities of invasive nocturnal Brown Treesnakes (Boiga irregularis) through short-term radiotracking of 691 snakes within close proximity to 50 road segments across the island of Guam. All measures of road magnitude (traffic volume, gap width, surface type, etc.) were significantly negatively correlated with crossing probabilities. Snake body size was the only intrinsic factor associated with crossing rates, with larger snakes crossing roads more frequently. Humidity was the only environmental variable affecting crossing rate. The distance of the snake from the road at the start of nightly movement trials was the most significant predictor of crossings. The presence of snake traps with live mouse lures during a portion of the trials indicated that localized prey cues reduced the probability of a snake crossing the road away from the traps, suggesting that a snake’s decision to cross roads is influenced by local foraging opportunities. Per capita road crossing rates of Brown Treesnakes were very low, and comparisons to historical records suggest that crossing rates have declined in the 60+ yr since introduction to Guam. We report a simplified model that will allow managers to predict road crossing rates based on snake, road, and contextual characteristics. Road crossing simulations based on actual snake size distributions demonstrate that populations with size distributions skewed toward larger snakes will result in a higher number of road crossings. Our method of modeling per capita road crossing probabilities as a binary response variable, influenced by contextual factors, may be useful for describing or predicting road crossings by individuals of other taxa provided that appropriate spatial and temporal resolution can be achieved and that potentially influential covariate data can be obtained

Evaluating lethal toxicant doses for the largest individuals of an invasive vertebrate predator with indeterminate growth

The brown treesnake (Boiga irregularis) was accidentally introduced to Guam and caused severe ecological and economic damages. Acetaminophen is an effective, low-risk oral toxicant for invasive brown treesnakes, and an automated aerial delivery system (ADS) has been developed for landscape-scale toxic bait distribution. A fixed dose of 80 mg of acetaminophen within a tablet inserted into a dead neonatal mouse (DNM) was lethal for all brown treesnakes in previous trials; however, these trials did not include very large individuals which are difficult to acquire for testing. Because most reptiles continue to grow throughout their lifespan, a small number reach much greater than average body sizes. Here, we tested effectiveness of 80 mg acetaminophen DNM baits for unusually large brown treesnakes as they became available. Our results confirmed that an 80 mg dose is lethal for the vast majority of snakes on Guam, but efficacy starts to diminish around 200 g of body mass. We also tested an alternative mouse bait configuration with 160 mg of acetaminophen that could be incorporated into the ADS to improve control of unusually large snakes. The 160 mg dose is expected to be effective for nearly all female snakes; males grow much larger and additional methods will be needed for extraordinarily large individuals. We describe a full dose-response curve for brown treesnakes to acetaminophen tablets and estimate the LD90 at 299 mg/kg and the LD99 at 578 mg/kg. To our knowledge, this is the first published dose-response curve for an invasive vertebrate with indeterminate growth

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

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 (\u3e0.18). 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