Gordon H. Rodda

The impact of the introduction of the colubrid snake, Boiga irregularis, on Guam's lizards

The extirpation of Guams forest avifauna has been attributed to the accidental introduction and subsequent irruption of the brown tree snake, Boiga irregularis. However, recent dietary studies of this nocturnal arboreal snake indicate that it now preys primarily on lizards, not birds. We evaluated the effect the snake has had on Guams lizards by contrasting lizard communities on Guam with those on adjacent snake-free islands and by comparing the extant lizard communities on Guam with those that were present before the snake arrived. Both comparisons revealed radical reductions in abundance of Guams native nocturnal lizards and the extirpation of several species. The effect of the snake on diurnal lizards (skinks) is more equivocal. Skinks are still common on Guam, but several species no longer exist on the island. Identification of causes of these extirpations is complicated by the snakes elimination of an important avian skink predator, the concurrent irruption of a shrew, and the effects of predation and competition between the native skinks and an introduced skink. In the last five years a large number of papers have addressed the ecology of biological inva-

What parts of the US mainland are climatically suitable for invasive alien pythons spreading from Everglades National Park

The Burmese Python (Python molurus bivittatus) is now well established in southern Florida and spreading northward. The factors likely to limit this spread are unknown, but presumably include climate or are correlated with climate. We compiled monthly rainfall and temperature statistics from 149 stations located near the edge of the python’s native range in Asia (Pakistan east to China and south to Indonesia). The southern and eastern native range limits extend to saltwater, leaving unresolved the species’ climatic tolerances in those areas. The northern and western limits are associated with cold and aridity respectively. We plotted mean monthly rainfall against mean monthly temperature for the 149 native range weather stations to identify the climate conditions inhabited by pythons in their native range, and mapped areas of the coterminous United States with the same climate today and projected for the year 2100. We accounted for both dry-season aestivation and winter hibernation (under two scenarios of hibernation duration). The potential distribution was relatively insensitive to choice of scenario for hibernation duration. US areas climatically matched at present ranged up the coasts and across the south from Delaware to Oregon, and included most of California, Texas, Oklahoma, Arkansas, Louisiana, Mississippi, Alabama, Florida, Georgia, and South and North Carolina. By the year 2100, projected areas of potential suitable climate extend northward beyond the current limit to include parts of the states of Washington, Colorado, Illinois, Indiana, Ohio, West Virginia, Pennsylvania, New Jersey, and New York. Thus a substantial portion of the mainland US is potentially vulnerable to this ostensibly tropical invader.

Biology and impacts of Pacific island invasive species. 2. Boiga irregularis , the Brown Tree Snake (Reptilia: Colubridae)

ABSTRACT The Brown Tree Snake, Boiga irregularis (Merrem, 1802), was accidentally transported to the island of Guam shortly after World War II. Over the following two decades it spread throughout the island with little public or professional recognition of its extent or impacts. This secretive nocturnal arboreal snake occurs in all habitats on Guam, from grasslands to forests. Under the right conditions, it is capable of high rates of reproduction and population growth. The Brown Tree Snake caused the extirpation of 13 of Guams 22 native breeding birds and contributed to the extirpation of several species of native bats and lizards. Guams 12 forest birds were especially impacted, with 10 species eliminated and the other two severely reduced. In addition, the snake continues to substantially impact domestic poultry, pets, the islands electrical power infrastructure, and human health. To protect other vulnerable Pacific islands, the U.S. government annually spends several million dollars inspecting cargo outbound from Guam to exclude Brown Tree Snakes. Cargo destinations most at risk are in Micronesia, especially the Northern Mariana Islands, but Guam also has direct air transportation links to Hawai‘i that will soon be supplemented with direct ship traffic. Ultimately, all Pacific islands are at risk but especially those obtaining cargo through Guam.

The densest terrestrial vertebrate

An understanding of the abundance of organisms is central to understanding ecology, but many population density estimates are unrepresentative because they were obtained from study areas chosen for the high abundance of the target species. For example, from a pool of 1072 lizard density estimates that we compiled from the literature, we sampled 303 estimates and scored each for its assessment of the degree to which the study site was representative. Less than half (45%) indicated that the study area was chosen to be representative of the population or habitat. An additional 15% reported that individual plots or transects were chosen randomly, but this often indicated only that the sample points were located randomly within a study area chosen for its high abundance of the target species. The remainder of the studies either gave no information or specified that the study area was chosen because the focal species was locally abundant. In many environments, lizards form important elements of the food web. In many cases, they have been used as model organisms with which to examine

THERMAL AND ENERGETIC CONSTRAINTS ON ECTOTHERM ABUNDANCE: A GLOBAL TEST USING LIZARDS

Population densities of birds and mammals have been shown to decrease with body mass at approximately the same rate as metabolic rates increase, indicating that energetic needs constrain endotherm population densities. In ectotherms, the exponential increase of metabolic rate with body temperature suggests that environmental temperature may additionally constrain population densities. Here we test simple bioenergetic models for an ecologically important group of ectothermic vertebrates by examining 483 lizard populations. We find that lizard population densities decrease as a power law of body mass with a slope approximately inverse to the slope of the relationship between metabolic rates and body mass. Energy availability should limit population densities. As predicted, environmental productivity has a positive effect on lizard density, strengthening the relationship between lizard density and body mass. In contrast, the effect of environmental temperature is at most weak due to behavioral thermoregulation, thermal evolution, or the temperature dependence of ectotherm performance. Our results provide initial insights into how energy needs and availability differentially constrain ectotherm and endotherm density across broad spatial scales.

Size bias in visual searches and trapping of brown treesnakes on guam

Abstract The accidental introduction of the brown treesnake (BTS; Boiga irregularis) to the island of Guam after World War II set off a chain of bird, bat, and lizard extirpations. Fortunately, many of the eliminated species have the potential to be restored following population reduction or eradication of the snake. The primary operational tool for population reduction is an effective snake trap, but areas subjected to long-term trapping continue to support BTS, suggesting that some adult snakes are refractory to trapping. We closed a 5-ha area to BTS emigration and immigration and surveyed the population using trapping and visual surveys to determine whether a refractory stratum of adult snakes existed and if trapping was effective for snakes of all sizes. Our surveys included 101 trapping occasions and 109 visual surveys over 309 days, resulting in 2,522 detections of 122 individuals. We detected 44 of 45 supplemented snakes by this intensive sampling effort, which also revealed that trapping was fully effective for snakes >900 mm in snout–vent length (SVL), partially effective for snakes 700–900 mm SVL, and totally ineffective for smaller juveniles (350–700 mm SVL). Visual searching was effective for snakes of all sizes. As BTS mature at approximately 950–1,050 mm SVL, continuous trapping should suffice to eliminate recruitment in the absence of immigration. Immigration or inadequate effort is most likely responsible for the persistence of BTS in areas subject to long-term trapping. Thus, current efforts to capture trap-refractory adult snakes with alternate control tools are less likely to be successful than immigration barriers alone or in combination with elevated capture effort.

Distribution, density, and biomass of introduced small mammals in the Southern Mariana Islands.

Abstract: Although it is generally accepted that introduced small mammals have detrimental effects on island ecology, our understanding of these effects is frequently limited by incomplete knowledge of small mammal distribution, density, and biomass. Such information is especially critical in the Mariana Islands, where small mammal density is inversely related to effectiveness of Brown Tree Snake (Boiga irregularis) control tools, such as mouse-attractant traps. We used mark-recapture sampling to determine introduced small mammal distribution, density, and biomass in the major habitats of Guam, Rota, Saipan, and Tinian, including grassland, Leucaena forest, and native limestone forest. Of the five species captured, Rattus diardii (sensu Robins et al. 2007) was most common across habitats and islands. In contrast, Mus musculus was rarely captured at forested sites, Suncus murinus was not captured on Rota, and R. exulans and R. norvegicus captures were uncommon. Modeling indicated that neophobia, island, sex, reproductive status, and rain amount influenced R. diardii capture probability, whereas time, island, and capture heterogeneity influenced S. murinus and M. musculus capture probability. Density and biomass were much greater on Rota, Saipan, and Tinian than on Guam, most likely a result of Brown Tree Snake predation pressure on the latter island. Rattus diardii and M. musculus density and biomass were greatest in grassland, whereas S. murinus density and biomass were greatest in Leucaena forest. The high densities documented during this research suggest that introduced small mammals (especially R. diardii) are impacting abundance and diversity of the native fauna and flora of the Mariana Islands. Further, Brown Tree Snake control and management tools that rely on mouse attractants will be less effective on Rota, Saipan, and Tinian than on Guam. If the Brown Tree Snake becomes established on these islands, high-density introduced small mammal populations will likely facilitate and support a high-density Brown Tree Snake population, even as native species are reduced or extirpated.

Reproductive Biology of the Brown Tree Snake, Boiga irregularis (Reptilia: Colubridae), during Colonization of Guam and Comparison with That in Their Native Range

ABSTRACT Since their introduction to Guam shortly after World War II, brown tree snakes, Boiga irregularis (Merrem), have seriously impacted the biota and human population of the island. Understanding the biology of this exotic species will likely be important to the success of control programs. We compared the reproductive biology of 782 B. irregularis caught on Guam during the 1980s with results from published studies of native-range populations. Average and maximum sizes of mature snakes on Guam were larger than those from Australian populations. The majority of female brown tree snakes matured at snout-vent lengths (SVLs) of 910–1,025 mm, and most males matured at SVLs of 940–1,030 mm on Guam. Based on growth rates from the early 1990s on Guam, sexual maturity is estimated to occur during a snakes third or fourth year. Only one female (0.3%) in our data set had oviductal eggs. Clutch size was estimated at 4.3 (SD = 2.2), based on large vitellogenic ovarian follicle (≥30 mm in length) and oviductal egg counts. Unlike their Australian counterparts, the Guam population reproduced year-round. Our data offer insights into the likely reproductive patterns of brown tree snakes should they infest other islands in the Pacific region.

Recent Records of Alien Anurans on the Pacific Island of Guam

ABSTRACT Eight anuran species were recorded for the first time in Guam in the period May 2003–December 2005, all apparently the result of arrivals to the island since 2000. Three of the eight species (Rana guentheri, Polypedates megacephalus, and Eleutherodactylus planirostris) had well-established breeding populations by 2005. A further three (Fejervarya cf. limnocharis, Fejervarya cancrivora, and Microhyla pulchra) were recorded from a number of individuals, but it is not known whether these species have established breeding populations. Two species (Kaloula pulchra and Eleutherodactylus coqui) appear to be incidental transportations to the island that have not established. Before 2003, five anuran species, all introductions, had been recorded from Guam. Three of these, Polypedates leucomystax, Pseudacris regilla, and Kaloula picta, were detected on Guam in incoming cargo but destroyed. Two species established: Bufo marinus was deliberately introduced and the Australian hylid Litoria fallax was probably an accidental introduction. Successful establishment of anurans on Guam has increased the risk of frog introductions to nearby islands. By providing additional food sources for the brown tree snake (Boiga irregularis), anuran introductions have increased the chance that B. irregularis might substantially increase in numbers and in turn increase the risk of the snake being accidentally transported to other islands.

Response of Brown Treesnakes to Reduction of Their Rodent Prey

Abstract Trapping brown treesnakes (Boiga irregularis; BTS) with live-mouse (Mus domesticus) lures is the principal control technique for this invasive species on Guam. Lure-based trapping is also used on other islands as a precaution against undetected arrivals and in response to verified BTS sightings. However, the effectiveness of lure-based trapping on other islands is questionable, as it has yielded no BTS despite other evidence of their presence. Some evidence suggests that high rodent numbers may interfere with BTS control. To test the relationship between rodent abundance and snake trappability, we conducted a controlled, replicated field experiment incorporating a rodenticide treatment during a BTS mark–recapture study. Using open population modeling in Program MARK, we estimated BTS apparent survival and recapture probabilities. Rodent reduction increased BTS recapture probabilities by 52–65% in 2002 and 22–36% in 2003, and it decreased apparent survival by <1% both years. This appears to be the first published instance of manipulating wild prey to influence snake behavior. Rodent reduction may enhance detection and control of BTS with traps on Guam and other islands. It may also amplify the effectiveness of oral toxicants against BTS.