Abstract
Remote tropical oceanic islands are of high conservation priority, and they are exemplified by range-restricted species with small global populations. Spatial and temporal patterns in rainfall and plant productivity may be important in driving dynamics of these species. Yet, little is known about environmental influences on population dynamics for most islands and species. Here we leveraged avian capture-recapture, rainfall, and remote-sensed habitat data (enhanced vegetation index [EVI]) to assess relationships between rainfall, vegetation greenness, and demographic rates (productivity, adult apparent survival) of three native bird species on Saipan, Northern Mariana Islands: rufous fantail (Rhipidura rufifrons), bridled white-eye (Zosterops conspicillatus), and golden white-eye (Cleptornis marchei). Rainfall was positively related to vegetation greenness at all but the highest rainfall levels. Temporal variation in greenness affected the productivity of each bird species in unique ways. Predicted productivity of rufous fantail was highest when dry and wet season greenness values were high relative to site-specific 5-year seasonal mean values (i.e., relative greenness); while the white-eye species had highest predicted productivity when relative greenness contrasted between wet and dry seasons. Survival of rufous fantail and bridled white eye was positively related to relative dry-season greenness and negatively related to relative wet-season greenness. Bridled white-eye survival also showed evidence of a positive response to overall greenness. Our results highlight the potentially important role of rainfall regimes in affecting population dynamics of species on oceanic tropical islands. Understanding linkages between rainfall, vegetation, and animal population dynamics will be critical for developing effective conservation strategies in this and other regions where the seasonal timing, extent, and variability of rainfall is expected to change in the coming decades.
Highlights
Remote oceanic islands are widely recognized as important reservoirs of regional and global biodiversity, and they are typified by endemic and range-restricted species with small globalPLOS ONE | DOI:10.1371/journal.pone.0148570 February 10, 2016Rainfall, Vegetation, and Avian Vital Rates
Our results indicated strong links between rainfall, vegetation greenness, and the demographic rates of three endemic island landbirds
Rainfall was positively associated with vegetation greenness in both dry and wet seasons, greenness was lower than expected at the highest rainfall levels recorded
Summary
Remote oceanic islands are widely recognized as important reservoirs of regional and global biodiversity, and they are typified by endemic and range-restricted species with small global. Climatic variability may be important in affecting the population dynamics of island species. Climate change impacts on populations may range from direct effects, such as sea-level rise or creation of climatic conditions beyond physiological tolerance limits [11], to indirect effects on the spatial and temporal availability of resources and related intra- and inter-specific interactions [12]. We expected that seasonal and annual variation in rainfall would result in concomitant changes in habitat phenology and landbird demography. A recent study of the Rota white-eye (Zosterops rotensis) on a nearby island suggested a positive relationship between bird density and leaf density [21] Such positive relationships would suggest that vegetation density and productivity affects resource availability for birds and could be directly related to demographic parameters. Positive effects of an unusually wet and green dry season on demographic rates (when resources might be most limiting) might reduce the magnitude of any wet-season greenness effects on demographic rates (i.e., a negative seasonal interaction)
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