Browsing by Author "Berkowitz, S. Paul"

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    Monitoring Hawaiian Biodiversity: Pilot Study to Assess Changes to forest birds and their habitat
    (2017-12-12) Gorresen, P. Marcos; Camp, Richard J.; Gaudioso-Levita, Jaqueline M.; Brinck, Kevin W.; Berkowitz, S. Paul; Jacobi, James D.
    Biological diversity, or biodiversity, is the variety and abundance of species in a defined area, and is one of the oldest and most basic descriptions of biological communities. Understanding how populations and communities are structured and change over space and time in response to internal and external forces is a management priority. Effective management practices and conservation strategies depend on our understanding of the relationship between changes in biodiversity and ecological drivers such as invasive species, land use and climate change. To demonstrate how changes in biodiversity may be monitored over a large (400 km2) tract of native forest habitat, we compared bird and plant community composition and structure in an upper montane region of Hawai‘i Island originally surveyed in 1977 as part of the Hawai‘i Forest Bird Survey (Scott et al. 1986) with a comprehensive sample of the same region in 2015. Our findings suggest that across a region spanning an elevation range of 600 to 2,000 m considerable changes occurred in the plant and bird communities between 1977 and 2015. Endemic and indigenous plants species richness (i.e., total number of species) decreased dramatically in the low and middle elevations below an invasive weed front, whereas naturalized plant species richness did not change between the two periods at any elevation. Endemic bird abundance decreased and two species were lost in the lower elevations (< 1,100 m) between 1977 and 2015, while naturalized bird abundance and the numbers of species increased in the same area. In addition to changes in community composition, the structure of the forest showed evidence of changes in dominant and sub-dominant tree canopy cover, shrub and herbaceous cover, dominant tree canopy height, and matted fern cover. Biodiversity monitoring helps to define specific conservation targets and to measure progress towards reaching those targets. It is difficult to ascribe causative factors to a change in biodiversity without directly manipulating the environment. Forest habitat in a variety of settings (i.e., islands and regions with differing land-use histories and elevation ranges), however, can provide opportunities to evaluate the influence of ecological drivers. Declines in native bird biodiversity in low-elevation areas may be attributed to invasive species as land use and climate conditions have remained relatively similar over the 40-year period. Thus, the shift from an endemic-naturalized co-dominated community in 1977 to one dominated by naturalized, alien birds in 2015, and reduction in native bird abundance over that period, may reflect increasing dominance by naturalized plants within this forested area. Inferences drawn from analyses of region-wide surveys, especially with replicate datasets, will facilitate the identification of broad-scale changes in biodiversity, and provide a needed current datum in Hawaiian plant and bird biodiversity monitoring.
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    Status of forest birds on Rota, Mariana Islands.
    (2016-01-25) Camp, Richard; Brinck, Kevin; Gorressen, P. Marcos; Amidon, Fred; Radley, Paul; Berkowitz, S. Paul; Banko, Paul
    The western Pacific island of Rota is the third largest human inhabited island in the Mariana archipelago, and is designated an Endemic Bird Area. Between 1982 and 2012, 12 point-transect distance sampling surveys were conducted to assess 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. Occupancy models of the 2012 survey revealed general patterns of habitat use and detectability among 11 species that could be reliably modeled. The endangered Mariana crow (Corvus kubaryi) was dispersed around the periphery of the island in steep forested habitats. In contrast, the endangered Rota white-eye (Zosterops rotensis) was restricted to the high-elevation mesa. Precision of detection probabilities and occupancy estimates and effects of habitat types, sampling conditions, and specific observers varied considerably among species, indicating that more narrowly defined classifications and additional observer training may improve the accuracy of predictive modeling. Population estimates of five out of ten native bird species, including collared kingfisher (Todiramphus chloris orii), Mariana crow, Mariana fruit-dove (Ptilinopus roseicapilla), Micronesian myzomela (Myzomela rubrata), and white-throated ground-dove (Gallicolumba xanthonura) declined over the 30-year time series. The crow declined sharply to fewer than 200 individuals (upper 95% confidence interval). Trends increased for Micronesian starling (Aplonis opaca), rufous fantail (Rhipidura rufifrons mariae), and white tern (Gygis alba). Rota white-eye numbers declined from 1982 to the late 1990s, but returned to 1980s levels by 2012. The trend for the yellow bittern (Ixobrychus sinensis) was inconclusive. The alien Eurasian tree sparrow (Passer montanus) apparently increased in number despite an unreliable trend assessment. Declines were noted in the other two alien birds, black drongo (Dicrurus macrocercus) and island collared-dove (Streptopelia bitorquata). Total bird densities on Rota were similar to those on Saipan and Tinian, which were lower than densities on Aguiguan. Overall, bird trends on Rota declined, whereas trends observed for the same period on Saipan and Tinian were mixed, and trends on Aguiguan were stable to increasing. We identified several sampling design and protocol procedures that may improve the precision of occupancy, status, and trend assessments. Continued monitoring and demographic sampling are needed to understand why most bird species on Rota are declining, to identify the causative agents, and to assess effectiveness of conservation actions for rare species, especially the Mariana crow.