Current land bird distribution and trends in population abundance between 1982 and 2012 on Rota, Mariana Islands

The western Pacific island of Rota is the fourth largest human-inhabited island in the Mariana archipelago and designated an Endemic Bird Area. Between 1982 and 2012, 12 point-transect distance-sampling surveys were conducted to assess bird population status. Surveys did not consistently sample the entire island; thus, we used a ratio estimator to estimate bird abundances in strata not sampled during every survey. Trends in population size were reliably estimated for 11 of 13 bird species, and 7 species declined over the 30-y time series, including the island collared-dove Streptopelia bitorquata, white-throated ground-dove Gallicolumba xanthonura, Mariana fruit-dove Ptilinopus roseicapilla, collared kingfisher Todiramphus chloris orii, Micronesian myzomela Myzomela rubratra, black drongo Dicrurus macrocercus, and Mariana crow Corvus kubaryi. The endangered Mariana crow (x̄  =  81 birds, 95% CI 30–202) declined sharply to fewer than 200 individuals in 2012, down from 1,491 birds in 1982 (95% CI  =  815–3,115). Trends increased for white tern Gygis alba, rufous fantail Rhipidura rufifrons mariae, and Micronesian starling Aplonis opaca. Numbers of the endangered Rota white-eye Zosterops rotensis declined from 1982 to the late 1990s but returned to 1980s levels by 2012, resulting in an overall stable trend. Trends for the yellow bittern Ixobrychus sinensis were inconclusive. Eurasian tree sparrow Passer montanus trends were not assessed; however, their numbers in 1982 and 2012 were similar. Occupancy models of the 2012 survey data revealed general patterns of land cover use and detectability among 12 species that could be reliably modeled. Occupancy was not assessed for the Eurasian tree sparrow because of insufficient detections. Based on the 2012 survey, bird distribution and abundance across Rota revealed three general patterns: 1) range restriction, including Mariana crow, Rota white-eye, and Eurasian tree sparrow; 2) widespread distribution, low abundance, including collared kingfisher, island collared-dove, white-throated ground-dove, Mariana fruit-dove, white tern, yellow bittern, black drongo, and Micronesian myzomela; and 3) widespread distribution, high abundance, including rufous fantail and Micronesian starling. The Mariana crow was dispersed around the periphery of the island in steep forested land-cover types. In contrast, the Rota white-eye was restricted to the high-elevation mesa. Only for the white-throated ground-dove was there a significant difference among cover types, with lower occupancy in open field than in forested areas. Vegetation was included in the best-fit occupancy models for yellow bittern, black drongo, Micronesian myzomela, and Micronesian starling, but vegetation type was not a significant variable nor included in the top models for the remaining five species: white tern, island collared-dove, Mariana fruit-dove, collared kingfisher, and rufous fantail. Given declining population trends, the Rota bird-monitoring program could benefit from establishing threshold and alert limits and identifying alternative research and management actions. Continued monitoring and demographic sampling, in conjunction with ecological studies, are needed to understand why most bird species on Rota are declining, identify the causative agents, and assess effectiveness of conservation actions, especially for the Mariana crow

Population trends of the forest bird community on the Pacific island of Rota, Mariana Islands

[EN] The Pacific island of Rota is part of the Mariana archipelago, and is located approximately 60 km north of the island of Guam. Two Rota endemics, the Mariana Crow (Corvus kubaryi) and the Rota Bridled White-eye (Zosterops rotensis), have declined dramatically in the last 20 years. We examined trends in abundance of eight terrestrial bird species (six native, two exotic) on Rota between 1982 and 2004, and found that seven of them declined significantly, with five species showing declines >50%. Only Micronesian Starlings (Aplonis opaca) increased in abundance. Declines occurred in species abundant in both forested and open habitats, suggesting that the declines were unlikely to be simply the result of deforestation. While the introduction of the brown tree snake (Boiga irregularis) on Guam caused the collapse of that island's avifauna, we do not believe that Rota's declines are due to the establishment of a snake population. Other, as yet unidentified, agents are likely to be responsible. We suggest that future research into the causative agent(s) of decline focus on the comparatively common declining species, rather than studying small populations of endangered species.[ES] La isla de Rota es parte del archipiélago de las Marianas en el Pacífico, y está situada aproximadamente a 60 km al norte de la isla de Guam. En Rota, dos especies endémicas de aves, Corvus kubaryi y Zosterops rotensis, han experimentado fuertes disminuciones poblacionales en los últimos 20 años. Examinamos las tendencias temporales de ocho especies de aves terrestres (seis nativas y dos exóticas) en Rota entre 1982 y 2004. La abundancia de siete de las ocho especies disminuyó significativamente durante el periodo de estudio; cinco de éstas tuvieron disminuciones de más del 50%. Sólo una especie, Aplonis opaca, aumentó su abundancia en el periodo estudiado. Entre las especies que disminuyeron había tanto especies forestales como especies típicas de hábitats abiertos, lo que sugiere que las disminuciones no son sólo el resultado de la deforestación. En Guam, la introducción de la serpiente Boiga irregularis causó el colapso de la avifauna insular. No obstante, no creemos que las disminuciones en Rota estén asociadas al establecimiento de una población de serpientes, sino a otros agentes, no identificados hasta la fecha. Sugerimos que los estudios para identificar los agentes causales de estas disminuciones se concentren en las especies en disminución más comunes, en vez de trabajar con poblaciones pequeñas de especies amenazadas.Peer reviewe

Potential priority areas for forest bird habitat restoration.

<p>Map is based on the identification of currently converted forest bird habitat within locations remaining climatically suitable for high model reliability species between now and end of century. The pink overlay shows the spatial configuration of the main Hawaiian Islands. A similar figure including all extant species is included in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140389#pone.0140389.s007" target="_blank">S7 File</a>.</p

Large-Scale Range Collapse of Hawaiian Forest Birds under Climate Change and the Need 21^st Century Conservation Options

<div><p>Hawaiian forest birds serve as an ideal group to explore the extent of climate change impacts on at-risk species. Avian malaria constrains many remaining Hawaiian forest bird species to high elevations where temperatures are too cool for malaria’s life cycle and its principal mosquito vector. The impact of climate change on Hawaiian forest birds has been a recent focus of Hawaiian conservation biology, and has centered on the links between climate and avian malaria. To elucidate the differential impacts of projected climate shifts on species with known varying niches, disease resistance and tolerance, we use a comprehensive database of species sightings, regional climate projections and ensemble distribution models to project distribution shifts for all Hawaiian forest bird species. We illustrate that, under a likely scenario of continued disease-driven distribution limitation, all 10 species with highly reliable models (mostly narrow-ranged, single-island endemics) are expected to lose >50% of their range by 2100. Of those, three are expected to lose all range and three others are expected to lose >90% of their range. Projected range loss was smaller for several of the more widespread species; however improved data and models are necessary to refine future projections. Like other at-risk species, Hawaiian forest birds have specific habitat requirements that limit the possibility of range expansion for most species, as projected expansion is frequently in areas where forest habitat is presently not available (such as recent lava flows). Given the large projected range losses for all species, protecting high elevation forest alone is not an adequate long-term strategy for many species under climate change. We describe the types of additional conservation actions practitioners will likely need to consider, while providing results to help with such considerations.</p></div

Potential priority areas for forest bird habitat conservation.

<p>Map is based on the number of high model reliability species projected to maintain their range between now and end of century. Current protected areas are delineated in green (National parks, State parks, Natural area reserves, wildlife refuges, sea bird sanctuaries, Nature Conservancy lands and other major private conservation areas). The pink overlay shows the spatial configuration of the main Hawaiian Islands. A similar figure including all extant species is included in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140389#pone.0140389.s007" target="_blank">S7 File</a>.</p

Projected climate-based range change between 1990–2010 and 2080–2100 for Hawai`i `Ākepa (left) and Maui Parrotbill (right).

<p>The gridded overlay represents the distribution of primary vegetation types associated with the species (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140389#pone.0140389.s006" target="_blank">S6 File</a>). The pink overlay shows the spatial configuration of the main Hawaiian Islands.</p

Current (left) and future (right) forest bird number of species based on modeled range and available primary habitat of high model reliability species.

<p>The pink overlay shows the spatial configuration of the main Hawaiian Islands.</p

Projected changes between 1990–2010 and 2080–2100 in ranges of high model reliability species, limited to current available primary habitat.

<p>All range estimates are in km². Similar estimates for reduced model reliability species are included in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0140389#pone.0140389.s008" target="_blank">S8 File</a>.</p

Conceptual timeline for implementing novel forest bird conservation options.

<p>Diagram shows the latest possible start (dashed red line) of long-term novel options that decouple climate shifts from species decline. Missing this threshold implies the need of buying time options necessary for species persistence.</p

Modeling Hawaiian Ecosystem Degradation due to Invasive Plants under Current and Future Climates

<div><p>Occupation of native ecosystems by invasive plant species alters their structure and/or function. In Hawaii, a subset of introduced plants is regarded as extremely harmful due to competitive ability, ecosystem modification, and biogeochemical habitat degradation. By controlling this subset of highly invasive ecosystem modifiers, conservation managers could significantly reduce native ecosystem degradation. To assess the invasibility of vulnerable native ecosystems, we selected a proxy subset of these invasive plants and developed robust ensemble species distribution models to define their respective potential distributions. The combinations of all species models using both binary and continuous habitat suitability projections resulted in estimates of species richness and diversity that were subsequently used to define an invasibility metric. The invasibility metric was defined from species distribution models with <0.7 niche overlap (Warrens <i>I</i>) and relatively discriminative distributions (Area Under the Curve >0.8; True Skill Statistic >0.75) as evaluated per species. Invasibility was further projected onto a 2100 Hawaii regional climate change scenario to assess the change in potential habitat degradation. The distribution defined by the invasibility metric delineates areas of known and potential invasibility under current climate conditions and, when projected into the future, estimates potential reductions in native ecosystem extent due to climate-driven invasive incursion. We have provided the code used to develop these metrics to facilitate their wider use (Code S1). This work will help determine the vulnerability of native-dominated ecosystems to the combined threats of climate change and invasive species, and thus help prioritize ecosystem and species management actions.</p></div