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.

Biology and Impacts of Pacific Island Invasive Species. 11.Rattus rattus,the Black Rat (Rodentia: Muridae)

“…Several species of civets ( Viverridae), which are nocturnal mammals, are predators of R. rattus in the Philippines (Rickart et al 1993). Monitor lizards (Varanus indicus) and the brown tree snake (Boiga irregularis) are predators of R. rattus in the Mariana Islands ( Wiewel et al 2009) …”

“…Phylogenetic restructuring of the "R. rattus complex" (Oceanic and Asian groups) continues to progress, and based on molecular evidence there are almost certainly multiple species within what has historically been identified as Rattus rattus (e.g., five to seven species in the R. rattus complex described by Robins et al [2007] and Pagès et al [2010], respectively). In the Mariana Islands, Wiewel et al (2009) reported that all species in their sampling that had been previously believed to be R. rattus or R. tanezumi were most closely related to the R. diardii group described by Robins et al (2007). Without molecular analysis of individuals within the R. rattus complex, it is very difficult to separate the species, and R. rattus and R. tanezumi are almost impossible to distinguish morphologically .…”

Excluding Nontarget Species from Brown Tree Snake, Boiga irregularis (Reptilia: Colubridae), Bait Stations: Experimental Tests of Station Design and Placement

“…Thus, of the three rat species we studied, the roof rat is probably the most arboreal, which is in line with our findings. Unfortunately, recent surveys raise the possibility that the roof rat (or the closely related R. diardii [ Wiewel et al 2009]) may be by far the most common of the three rat species on Guam (Wiewel et al 2009). The Polynesian rat is probably more arboreal than the Norway rat, but this was not evident in our study.…”

“…The necessity of obtaining confident estimates of numbers of snakes removed creates concern when other nonnative invasives, particularly Rattus spp., may be removing baits. At least three rat species have been documented on Guam: the roof rat, Rattus rattus (L.); Norway rat, Rattus norvegicus (Berkenhout); and Polynesian rat, Rattus exulans (Peale) (Baker 1946, Crabb and Emik 1946, Barbehenn 1974, Savidge 1987, although good estimates of current density are available only for R. rattus (Rattus diardii [ Wiewel et al 2009]; taxonomy sensu Robins et al [2007]). No information is available on propensities of these species to take DNM baits.…”

Burmese Pythons

“…The consequent loss of avian ecosystem function has resulted in cascading ecological consequences, including disturbance of plant reproduction (Mortensen et al 2008;Rogers 2011), forest regeneration (Perry and Morton 1999), and arthropod release (Rogers et al 2012). Predation by brown tree snakes has also negatively impacted nearly all native vertebrate populations on Guam as well as nonnative and domestic animals (Fritts and McCoid 1991;Wiewel et al 2009). …”