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Gordon H. Rodda

Publications and source records attributed to Gordon H. Rodda.

At least 19 recordsLinked to original sources
USGS2026

Demographic responses to experimental manipulation of an enclosed population of invasive brown treesnakes (Boiga irregularis)

Understanding how invasive species populations respond to perturbations can give insight into effective management. However, opportunities to test theoretical population responses are rare, as experimental perturbations often run counter to management goals (e.g., suppression). On the island of Guam, a 5-ha outdoor enclosure (with no immigration or emigration) constructed in 2003 serves as a unique population-scale study area for invasive brown treesnakes ( Boiga irregularis ). Capture-mark-recapture studies (CMR) and several experiments have taken place since 2004. Here, we analyze a subset of the CMR data collected between 2008 and 2012, a time period encompassing two experimental perturbations: (1) supplemental feeding to determine if the population is food limited and (2) targeted removal of adult and large-subadult cohorts to quantify the timing and magnitude of the population's demographic response to a simulated control effort. We used the CMR data to fit a Pradel model in a Bayesian framework to compare the population growth rate, survival probability, and recruitment rate prior to, during, and post perturbations. We found that increasing food resources increased population growth and recruitment rates, but that survival remained relatively constant. The removal perturbation resulted in a decrease in population growth rate and survival immediately after the removal, but population growth rate and recruitment began increasing 1 year after the removal perturbation. Our results suggest that brown treesnakes in the enclosed population are prey limited, likely constraining population growth, and that the suppression of population growth after removal of adults and large sub-adults is short-lived (i.e., less than a year).

Guam↗
USGS2025

Growth rate variation in Brown Treesnakes (Boiga irregularis): An invasive species of conservation concern

Somatic growth rate is a fundamental trait that influences metabolism, lifespan and reproductive maturity and is critical for understanding population dynamics and informing management actions. Brown Treesnakes ( Boiga irregularis ) introduced to Guam are highly invasive and can reproduce year-round without discrete cohorts. We compared snake size trajectories described by the conventionally used von Bertalanffy growth function versus the Gompertz model. Using quantile regression with a regularized effect for individual snakes we modeled growth rates of 270 marked, wild snakes as a function of size. The Gompertz model explained more of the variation in growth and rendered more realistic predictions of asymptotic sizes than did the von Bertalanffy model. With the Gompertz model, growth rates were 1.05–1.16× faster in males than in females. Females reached asymptotic sizes at shorter snout-vent lengths than males. Growth rate was positively correlated with amount of precipitation, and modeling wet-dry seasonality on Guam as a sinusoidal function identified a growth peak in September—October. Effects of seasonality and precipitation, however, were minor compared to individual and sex related differences in size-adjusted growth rates. We estimated that the 50th (and 5th, 95th) growth-rate percentile males in our study population become sexually mature at an age of 33 (∞, 15) months, while females mature at 41 (∞, 18) months, where ∞ indicates that the slowest growing snakes never reach maturity. However, 50% of the snakes mature at a size below the median, and age at maturity may be as low as 10.4 (males) and 13.7 (females) months for average-sized hatchlings that grow fast. Our results have implications for the timing of management options for this species and our approach can be broadly applied to animals where repeated growth data are obtained and age is unknown.

Ecology and Evolution↗
USGS2025

Behavioral plasticity in detection height of an invasive, arboreal snake based on size, condition, and prey

Context Animals may adjust their behavior in predictable ways to balance tradeoffs between resource acquisition and survival or fecundity. Microhabitat selection based on individual traits or environmental conditions is one measure of risk–reward tradeoffs by individuals. Aims We used data from observational and manipulative studies to investigate whether an arboreal snake (brown treesnake, Boiga irregularis ) had context-dependent behavior based on the relationship between estimated prey availability, body condition, size, and detection height (microhabitat use) in two Mariana Islands. Methods We used observational data collected in four study sites and data from a manipulative study that we collected over a 5-year period. The observational data focused on four sites with different counts of three prey types, including lizards, birds, and small mammals. During the manipulative study we removed snakes, which resulted in increased prey counts over time. Using these two approaches, we tested whether prey counts predicted body condition and then evaluated how prey counts, snake size, and snake condition interactively predicted the detection height of captured individuals. Key results We found that body condition was greater at sites or in years with greater prey counts across both the observational and manipulative studies. We also found that snakes displayed differential microhabitat use based on both their condition and size. Larger snakes tended to be detected lower than smaller snakes, but only at sites or during years with few bird or small mammal counts. Snakes at sites with greater mammal and bird counts had a positive relationship between size and detection height. Snakes with greater condition scores tended to be detected higher irrespective of size, but this was also dependent on prey counts. At sites with low bird counts, snakes that were in better condition tended to be closer to the ground. Conclusions Brown treesnakes modified microhabitat use based on their condition, size, and the number or type of prey available. Our findings were consistent with a hypothesis that they optimized habitat use to secure food resources and maximize survival. Implications Context-dependent behavioral plasticity may be an important consideration for management of reptiles for population control or growth.

Wildlife Research↗
USGS2021

Can we prove that an undetected species is absent? Evaluating whether brown treesnakes are established on the island of Saipan using surveillance and expert opinion

Detection of invasive species and decisions centered around early detection and rapid response (EDRR) are notorious challenges for decision makers. Detection probability is low for cryptic species, resources are limited, and ecological harm (especially for island ecosystems) can result from failure to remove invasive species due to inadequate or delayed surveillance efforts. Due to the proximity to the U.S. territory of Guam and inter-island traffic, the Commonwealth of the Northern Mariana Islands (CNMI) is at high risk of colonization by the invasive and cryptic brown treesnake ( Boiga irregularis ; BTS). There have been numerous reports of snakes and 7 confirmed specimens secured at ports of entry on the island of Saipan in the CNMI over the last four decades, raising the possibility that a population might be established. Establishment of BTS on Saipan is a major concern, as evidenced by the ecological and economic disruption that occurred on Guam. We evaluated the possibility of a small localized population on Saipan using evidence from surveillance efforts in 1999, 2007, 2009, 2016, and 2018, and from results of expert assessment of the credibility of non-confirmed reports of snakes for the period 1982–2013. For active surveillance efforts, we use a Poisson-based model to estimate the 95% probability of at least one snake being detected at a stated density given the level of sampling effort and detection probability. Based on this collective evidence we conclude there is a low probability that Saipan currently has an incipient population of BTS. However, with the continued presence of BTS on Guam, continuing commercial and military transportation in the region, and relief shipments responding to increased storm intensity, Saipan remains highly vulnerable to accidental introductions. Effective surveillance remains a crucial element for detection of any species, but this may be particularly true for a cryptic snake that is difficult to control once established.

Management of Biological Invasions↗
USGS2018

Habitat type and structure affect trap capture success of an invasive snake across variable densities

Detection represents an important limitation of accurately estimating population size, abundance, and habitat suitability for wildlife, which can be especially true for cryptic animals. Moreover, for reptiles, juveniles are often less likely to be detected than later life stages. In the case of invasive species, preventing false negatives early in the invasion process can be critical for improving outcomes of control measures. We evaluated habitat structure in relation to catch per unit effort (CPUE) and mean size of trapped invasive brown treesnakes ( Boiga irregularis ) on Guam. We used a 5‐ha enclosure containing a known, closed population of brown treesnakes to identify key habitat variables that related to CPUE and mean size of trapped snakes over six years. We then tested the relationship of those variables to CPUE and mean size of trapped snakes at three sites with suppressed snake populations as a proxy for low‐density populations anticipated to occur during early detection of invasive populations. We found that a coarse measure of habitat structure represented by three forest types correlated with trap detections, as well as finer measures of habitat structure, such as distance to nearest branch and the type of trap support structure used. On average, smaller snakes were captured in traps placed higher in the tree canopy. Some, but not all, habitat variables identified as predictive of CPUE and mean size within the enclosed population pre‐suppression were also predictive at the snake‐suppressed (low‐density proxy) sites. Habitat structure around the sampling unit (a trap) affected detection probability and the size of detected individuals independently of the demographic structure of the population. Measuring wildlife‐habitat relationships of invaders in their novel environments may be one method to improve early detection during invasive species management.

Ecosphere↗
USGS2017

Experimental landscape reduction of wild rodents increases movements in the invasive brown treesnake (Boiga irregularis)

Experimental studies evaluating the effects of food availability on the movement of free-ranging animals generally involve food supplementation rather than suppression. Both approaches can yield similar insights, but we were interested in the potential for using food suppression for the management and control of invasive predators, in particular, the brown treesnake ( Boiga irregularis ) on Guam. However, understanding a species’ response to food resources is critical before employing such a strategy. We studied the movements of 24 radio-tagged B. irregularis initially caught within four 4-ha unfenced plots in rodent-abundant (control) and rodent-suppressed (treatment) grassland habitats over a 40-day period. Because monitoring duration differed among snakes, we also analyzed short-term (16-day) activity areas. Over the 16 days, snakes associated with rodent-suppressed plots had 86% larger activity areas (ha), 94% greater dispersal distances (m), and 43% greater movement rates (m/h) than snakes associated with control plots. Boiga irregularis moved extensively, but these movements were not always reflected in the size of the snake’s total activity area. Movement rates did not differ between sexes, but snakes in above-average body condition moved greater distances per hour than those in below-average condition irrespective of treatment. Our study indicates that a relatively small prey suppression effort can cause almost immediate and significant changes in B. irregularis movement. On Guam, prey suppression might enhance control efforts by either increasing trap capture success or discouraging snakes from entering areas of conservation or management concern. However, the outcome of using prey suppression as a control tool in areas threatened with the accidental introduction of the brown treesnake is more difficult to predict and might have negative consequences such as elevated dispersal rates.

Management of Biological Invasions↗
USGS2016

Detection rates of geckos in visual surveys: Turning confounding variables into useful knowledge

Transect surveys without some means of estimating detection probabilities generate population size indices prone to bias because survey conditions differ in time and space. Knowing what causes such bias can help guide the collection of relevant survey covariates, correct the survey data, anticipate situations where bias might be unacceptably large, and elucidate the ecology of target species. We used negative binomial regression to evaluate confounding variables for gecko (primarily Hemidactylus frenatus and Lepidodactylus lugubris) counts on 220-m-long transects surveyed at night, primarily for snakes, on 9,475 occasions. Searchers differed in gecko detection rates by up to a factor of six. The worst and best headlamps differed by a factor of at least two. Strong winds had a negative effect potentially as large as those of searchers or headlamps. More geckos were seen during wet weather conditions, but the effect size was small. Compared with a detection nadir during waxing gibbous (nearly full) moons above the horizon, we saw 28% more geckos during waning crescent moons below the horizon. A sine function suggested that we saw 24% more geckos at the end of the wet season than at the end of the dry season. Fluctuations on a longer timescale also were verified. Disturbingly, corrected data exhibited strong short-term fluctuations that covariates apparently failed to capture. Although some biases can be addressed with measured covariates, others will be difficult to eliminate as a significant source of error in longterm monitoring programs.

Andersen Air Force Base↗
USGS2015

Stability of detectability over 17 years at a single site and other lizard detection comparisons from Guam

To obtain quantitative information about population dynamics from counts of animals, the per capita detectabilities of each species must remain constant over the course of monitoring. We characterized lizard detection constancy for four species over 17 yr from a single site in northern Guam, a relatively benign situation because detection was relatively easy and we were able to hold constant the site, habitat type, species, season, and sampling method. We monitored two species of diurnal terrestrial skinks ( Carlia ailanpalai [Curious Skink], Emoia caeruleocauda [Pacific Bluetailed Skink]) using glueboards placed on the ground in the shade for 3 h on rainless mornings, yielding 10,286 skink captures. We additionally monitored two species of nocturnal arboreal geckos ( Hemidactylus frenatus [Common House Gecko]; Lepidodactylus lugubris [Mourning Gecko]) on the basis of 15,212 sightings. We compared these count samples to a series of complete censuses we conducted from four or more total removal plots (everything removed to mineral soil) totaling 400 m 2 (about 1% of study site) in each of the years 1995, 1999, and 2012, providing time-stamped quantification of detectability for each species. Unfortunately, the actual population trajectories taken by the four species were masked by unexplained variation in detectability. This observation of debilitating latent variability in lizard detectability under nearly ideal conditions undercuts our trust in population estimation techniques that fail to quantify venue-specific detectability, rely on pooled detection probability estimates, or assume that modulation in predefined environmental covariates suffices for estimating detectability.

Journal of Herpetology↗
USGS2015

General herpetological collecting is size-biased for five Pacific lizards

Accurate estimation of a species' size distribution is a key component of characterizing its ecology, evolution, physiology, and demography. We compared the body size distributions of five Pacific lizards ( Carlia ailanpalai, Emoia caeruleocauda, Gehyra mutilata, Hemidactylus frenatus , and Lepidodactylus lugubris ) from general herpetological collecting (including visual surveys and glue boards) with those from complete censuses obtained by total removal. All species exhibited the same pattern: general herpetological collecting undersampled juveniles and oversampled mid-sized adults. The bias was greatest for the smallest juveniles and was not statistically evident for newly maturing and very large adults. All of the true size distributions of these continuously breeding species were skewed heavily toward juveniles, more so than the detections obtained from general collecting. A strongly skewed size distribution is not well characterized by the mean or maximum, though those are the statistics routinely reported for species' sizes. We found body mass to be distributed more symmetrically than was snout–vent length, providing an additional rationale for collecting and reporting that size measure.

Mariana Islands↗
USGS2014

Movements and activity of juvenile Brown Treesnakes ( Boiga irregularis )

Understanding the spatial ecology and foraging strategy of invasive animals is essential for success in control or eradication. We studied movements and activity in juvenile Brown Treesnakes on Guam, as this population segment has proven particularly difficult to control. Distance between daytime refugia (from telemetry of 18 juveniles, 423-800 mm snout-vent length) ranged from 0-118 m ( n = 86), with a grand mean of 43 m. There were tendencies for shorter snake movements on nights directly following a full moon and on dry nights, but variation among snakes was of a larger magnitude and would greatly reduce chances to detect moon or rain effects unless corrected for. Snake activity was estimated from audio recordings of signals from “tipping” radio transmitters, analyzed for pulse period and amplitude. Activity was highest in the hours immediately after sunset, and gradually declined throughout the night before dropping abruptly in conjunction with sunrise. Snake activity was higher on rainy nights, and tended to be highest during waning moons and when the moon was below the horizon. We conclude that small Brown Treesnakes forage actively and appear to move far enough to regularly encounter the traps and bait used on Guam for control purposes, suggesting that alternative explanations are required for their low capture rates with these control tools.

Copeia↗
USGS2012

Do predators control prey species abundance? An experimental test with brown treesnakes on Guam

The effect of predators on the abundance of prey species is a topic of ongoing debate in ecology; the effect of snake predators on their prey has been less debated, as there exists a general consensus that snakes do not negatively influence the abundance of their prey. However, this viewpoint has not been adequately tested. We quantified the effect of brown treesnake ( Boiga irregularis ) predation on the abundance and size of lizards on Guam by contrasting lizards in two 1-ha treatment plots of secondary forest from which snakes had been removed and excluded vs. two 1-ha control plots in which snakes were monitored but not removed or excluded. We removed resident snakes from the treatment plots with snake traps and hand capture, and snake immigration into these plots was precluded by electrified snake barriers. Lizards were sampled in all plots quarterly for a year following snake elimination in the treatment plots. Following the completion of this experiment, we used total removal sampling to census lizards on a 100-m 2 subsample of each plot. Results of systematic lizard population monitoring before and after snake removal suggest that the abundance of the skink, Carlia ailanpalai, increased substantially and the abundance of two species of gekkonids, Lepidodactylus lugubris and Hemidactylus frenatus , also increased on snake-free plots. No treatment effect was observed for the skink Emoia caeruleocauda . Mean snout–vent length of all lizard species only increased following snake removal in the treatment plots. The general increase in prey density and mean size was unexpected in light of the literature consensus that snakes do not control the abundance of their prey species. Our findings show that, at least where alternate predators are lacking, snakes may indeed affect prey populations.

Ecology↗
USGS2011

Diet and conservation implications of an invasive chameleon, Chamaeleo jacksonii (Squamata: Chamaeleonidae) in Hawaii

We summarize information on current distribution of the invasive lizard Chamaeleo jacksonii and predict its potential distribution in the Hawaiian Islands. Potential distribution maps are based on climate models developed from known localities in its native range and its Hawaiian range. We also present results of analysis of stomach contents of a sample of 34 chameleons collected from native, predominantly dryland, forest on Maui. These data are the first summarizing prey range of this non-native species in an invaded native-forest setting. Potential distribution models predict that the species can occur throughout most of Hawaii from sea level to >2,100 m elevation. Important features of this data set are that approximately one-third of the diet of these lizards is native insects, and the lizards are consuming large numbers of arthropods each day. Prey sizes span virtually the entire gamut of native Hawaiian arthropod diversity, thereby placing a large number of native species at risk of predation. Our dietary results contrast with expectations for most iguanian lizards and support suggestions that chameleons comprise a third distinct foraging-mode category among saurians. The combination of expanding distribution, large potential range size, broad diet, high predation rates, and high densities of these chameleons imply that they may well become a serious threat to some of the Hawaiian fauna.

Hawai'i↗
USGS2011

A field test of attractant traps for invasive Burmese pythons (Python molurus bivittatus) in southern Florida

Context: Invasive Burmese pythons (Python molurus bivittatus) are established over thousands of square kilometres of southern Florida, USA, and consume a wide range of native vertebrates. Few tools are available to control the python population, and none of the available tools have been validated in the field to assess capture success as a proportion of pythons available to be captured. Aims: Our primary aim was to conduct a trap trial for capturing invasive pythons in an area east of Everglades National Park, where many pythons had been captured in previous years, to assess the efficacy of traps for population control. We also aimed to compare results of visual surveys with trap capture rates, to determine capture rates of non-target species, and to assess capture rates as a proportion of resident pythons in the study area. Methods: We conducted a medium-scale (6053 trap nights) experiment using two types of attractant traps baited with live rats in the Frog Pond area east of Everglades National Park. We also conducted standardised and opportunistic visual surveys in the trapping area. Following the trap trial, the area was disc harrowed to expose pythons and allow calculation of an index of the number of resident pythons. Key results: We captured three pythons and 69 individuals of various rodent, amphibian, and reptile species in traps. Eleven pythons were discovered during disc harrowing operations, as were large numbers of rodents. Conclusions: The trap trial captured a relatively small proportion of the pythons that appeared to be present in the study area, although previous research suggests that trap capture rates improve with additional testing of alternative trap designs. Potential negative impacts to non-target species were minimal. Low python capture rates may have been associated with extremely high local prey abundances during the trap experiment. Implications: Results of this trial illustrate many of the challenges in implementing and interpreting results from tests of control tools for large cryptic predators such as Burmese pythons.

Florida↗
USGS2009

Giant Constrictors: Biological and Management Profiles and an Establishment Risk Assessment for Nine Large Species of Pythons, Anacondas, and the Boa Constrictor

Giant Constrictors: Biological and Management Profiles and an Establishment Risk Assessment for Nine Large Species of Pythons, Anacondas, and the Boa Constrictor, estimates the ecological risks associated with colonization of the United States by nine large constrictors. The nine include the world's four largest snake species (Green Anaconda, Eunectes murinus; Indian or Burmese Python, Python molurus; Northern African Python, Python sebae; and Reticulated Python, Broghammerus reticulatus), the Boa Constrictor (Boa constrictor), and four species that are ecologically or visually similar to one of the above (Southern African Python, Python natalensis; Yellow Anaconda, Eunectes notaeus; DeSchauensee's Anaconda, Eunectes deschauenseei; and Beni Anaconda, Eunectes beniensis). At present, the only probable pathway by which these species would become established in the United States is the pet trade. Although importation for the pet trade involves some risk that these animals could become established as exotic or invasive species, it does not guarantee such establishment. Federal regulators have the task of appraising the importation risks and balancing those risks against economic, social, and ecological benefits associated with the importation. The risk assessment quantifies only the ecological risks, recognizing that ecosystem processes are complex and only poorly understood. The risk assessment enumerates the types of economic impacts that may be experienced, but leaves quantification of economic costs to subsequent studies. Primary factors considered in judging the risk of establishment were: (1) history of establishment in other countries, (2) number of each species in commerce, (3) suitability of U.S. climates for each species, and (4) natural history traits, such as reproductive rate and dispersal ability, that influence the probability of establishment, spread, and impact. In addition, the risk assessment reviews all management tools for control of invasive giant constrictor populations. There is great uncertainty about many aspects of the risk assessment; the level of uncertainty is estimated separately for each risk component. Overall risk was judged to be high for five of the giant constrictors studied, and medium for the other four species. Because all nine species shared a large number of natural history traits that promote invasiveness or impede population control, none of the species was judged to be of low risk.

Open-File Report↗
USGS2009

Prey preferences and prey acceptance in juvenile Brown Treesnakes (Boiga irregularis)

On the Pacific island of Guam, control of the invasive Brown Treesnake (Boiga irregularis) relies largely on methods that use mice as bait. Juvenile B. irregularis feed primarily on lizards and their eggs, but little is known about their prey preference. We conducted an experiment to investigate preferences for, and acceptance of, dead geckos, skinks, and neonatal mice, in juvenile B. irregularis ranging from 290 mm to ca. 700 mm snout-vent length (SVL). Snakes of all sizes showed a preference for geckos over skinks and neonatal mice. Geckos were the first prey chosen in 87% of 224 initial trials (56 snakes subjected to four trials each; 33% would be expected from a random choice). The smallest snakes had the most pronounced preference. Although many of the snakes accepted neonatal mice and/or skinks, some snakes of all sizes were reluctant to feed on anything but geckos, especially when well fed. We also addressed the hypothesis that repeated encounters with a particular prey type increase a snake's preference for that prey. Our study does not support this hypothesis. Our results suggest that control methods relying solely on rodent bait may be inefficient for targeting snakes < 700 mm SVL and that individual heterogeneity in prey preference may cause a significant part of this juvenile cohort to be completely refractory to capture with rodent bait, even if the bait is dead and small enough to be readily swallowed.

Herpetological Conservation and Biology↗