Geology topics

Research about Guam, Mariana Islands

Source-linked reports with geographic coverage including Guam, Mariana Islands.

3 recordsLinked to original sources

USGS2024

Evaluation of data collected by Guam Division of Aquatic and Wildlife Resources during population establishment and monitoring of ko'ko' (Hypotaenidia owstoni) on Rota, Commonwealth of the Northern Mariana Islands, and wildlife monitoring datasets on Cocos Island and Guam

Efforts to recover the critically endangered ko’ko’ (Guam rail, Hypotaenidia owstoni) through establishing an experimental population on the island of Rota in the Commonwealth of the Northern Mariana Islands have been ongoing for three decades. The U.S. Geological Survey collaborated with the Guam Division of Aquatic and Wildlife Resources and the Government of Guam to evaluate whether objectives for three projects can be met with current protocols. The aim of this report was to evaluate existing data provided on (1) ko’ko’ population monitoring on Rota; (2) ko’ko’ population establishment on Rota; plus (3) evaluation of three wildlife monitoring datasets for ko’ko’ on Cocos Island, endangered pulattat (Mariana common moorhen, Gallinula chloropus guami) on Guam, and introduced ungulate species on Guam. Data sources included playback call surveys, point count surveys, release events and studbook information, telemetry of radio-marked birds, as well as landcover classes, storm events, and Oceanic Niño Index information to relate environmental factors to ko’ko’ persistence. Major findings were that reaching objectives was constrained by limited data availability and quality. Suggestions for future study include developing detailed protocols for surveys and data collection, standardizing training procedures for observers, improving data organization and archiving, using methods like distance sampling that account for imperfect detection, and collecting additional data on nests and prey resources to understand drivers of ko’ko’ density and survival. While the current data provide a preliminary assessment, improved sampling designs and consistent protocols are needed to fully address objectives related to the recovery of the ko’ko’. The report provides a roadmap for enhancing data collection and analysis to support management decisions and reach conservation translocation goals about this endangered species and related projects.

Guam, Mariana Islands↗

USGS2022

Chew-cards can accurately index invasive rat densities in Mariana Island forests

Rats ( Rattus spp.) are likely established on 80–90% of the world’s islands and represent one of the most damaging and expensive biological invaders. Effective rat control tools exist but require accurate population density estimates or indices to inform treatment timing and effort and to assess treatment efficacy. Capture-mark-recapture data are frequently used to produce robust density estimates, but collecting these data can be expensive, time-consuming, and labor-intensive. We tested a potentially cheaper and easier alternative, chew-cards, as a count-based (quantitative) index of invasive rat densities in tropical forests in the Mariana Islands, an archipelago in the western North Pacific Ocean. We trialed chew-cards in nine forest grids on two Mariana Islands by comparing the proportion of cards chewed to capture-mark-recapture density estimates and manipulated rat densities to test whether the relationship was retained. Chew-card counts were positively correlated with rat capture-mark-recapture density estimates across a range of rat densities found in the region. Additionally, the correlation between the two sampling methods increased with the number of days chew-cards were deployed. Specifically, when chew-cards were deployed for five nights, a 10% increase in the proportion of cards chewed equated to an estimated increase in rat density of approximately 2.4 individuals per ha (R 2 = 0.74). Chew-cards can provide a valid index of rat densities in Mariana Island forests and are a cheaper alternative to capture-mark-recapture sampling when relative differences in density are of primary interest. New cost-effective monitoring tools can enhance our understanding and management of invaded islands while stretching limited resources further than some conventional approaches, thus improving invasive species management on islands.

NeoBiota↗

USGS2020

Isotopic and geochemical assessment of the sensitivity of groundwater resources of Guam, Mariana Islands, to intra- and inter-annual variations in hydroclimate

Assessing the sensitivity of groundwater systems to hydroclimate variability is critical to sustainable management of the water resources of Guam, US territory. We assess spatial and temporal variability of isotopic and geochemical compositions of vadose and phreatic groundwater sampled from cave drip sites and production wells, respectively, to better understand the vulnerability of the freshwater lens on Guam to variability in hydroclimate. We independently evaluate the existing conceptual model of the Northern Guam Lens Aquifer that is largely based on physical, as opposed to geochemical, observations. Sampling was conducted from 2008 to 2015, over which rainfall gradually increased. Major ion geochemistry and Sr isotope values of groundwater show varying influence from soil, limestone bedrock, and seawater. Geochemical modeling that can explain spatial variability in groundwater Na+ and Mg2+ concentrations and Sr/Ca and 87Sr/86 Sr values indicates that groundwater compositions are dominantly controlled by mixing of freshwater with seawater and water-rock interaction. Differences between amount-weighted annual average precipitation δ18 O values and groundwater δ18 O values indicate a recharge bias toward the wet season, consistent with other tropical carbonate island aquifer settings. Intra- and inter-annual variations in Na+ concentrations and δ18 O values in groundwater reflect sensitivity of recharge to seasonal variations in rainfall amount and changes in annual rainfall amounts. Our results indicate the influence of multiple modes of recharge on groundwater compositions and spatial variability in the sensitivity of groundwater to seawater mixing. This sensitivity of the freshwater lens points to the vulnerability of groundwater resources to changes in recharge associated with climate, land-use change, and increases in population.

Journal of Hydrology↗