Linking Vital Rates of Landbirds on a Tropical Island to Rainfall and Vegetation Greenness

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.

Hot temperatures during the dry season reduce survival of a resident tropical bird

“…Vast differences in average annual rainfall, which exceeds 5000 mm in northeast Costa Rica compared with under 3000 mm in Ecuador, might explain the opposite effects of SOI on manakin survival if manakins in Ecuador are less adapted to cooler and wetter conditions compared with those in northeast Costa Rica which experience almost double the annual rainfall. Whatever the cause of the discrepancy, variation in responses to climate among and within species seen by comparing our results with those of previous studies [10][11][12][13] highlights the need to consider the local environment, such as forest type (open versus closed and dry versus wet) and elevation [55], and species traits, such as their diet and foraging behaviour [11], when predicting and interpreting effects of climate on the dynamics of tropical bird populations. The observed diversity of responses to climatic variation similarly highlights the need to study a broader range of species that represent the range of life histories and habitats in the tropics.…”

“…usgs.gov/data_access/daac2disk). Similar to Saracco et al [13], we removed cloud-contaminated pixels (pixel reliability code ¼ 3), extracted monthly EVI values for the six 1 km 2 pixels overlapping our study area using R [36] and package 'raster' [37], and then averaged values for the six pixels to acquire a monthly average EVI. We related monthly EVI to cumulative rainfall for the previous month using local polynomial regression implemented using the 'loess' function in R with a smoothing parameter (or span) equal to 0.9 (figure 2c).…”

“…To date, the few long-term, longitudinal studies aimed at elucidating the effects of climate on demography of tropical bird species have revealed widely varying population-level responses to environmental variation both within and among species depending on habitat and species traits, such as diet and geography [9][10][11][12][13][14]. For example, adult white-collared manakins Manacus candei in Costa Rica exhibited higher annual survival during cooler and wetter (positive) phases of the Southern Oscillation Index (SOI), but this effect was only apparent in young forests and not mature forest [10].…”

Yearling proportion correlates with habitat structure in a boreal forest landbird community

“…We examined relationships between vegetation greenness and: (a) in-situ habitat parameters, (b) % natural cover and (c) probability of capturing a landbird yearling at each station. The EVI is a composite metric which incorporates structural and seasonal components of habitat quality including primary productivity (leaf chlorophyl content), leaf area, canopy cover and vegetation complexity (Glenn et al, 2008), and which has previously been correlated with landbird occurrence and vital rates (Saracco et al, 2016). We extracted EVI cell values using the MODIS Subsets function of the MODIS Tools package 30 (Tuck et al, 2014) in the statistical software package R (R Core Team, 2015) at the 0.25 × 0.25 km scale for 81 cells surrounding each MAPS station, a grid extending 1.13 km in cardinal directions from station centers and a projected area (5.11 km 2 ) estimated as being sampled by a MAPS station (DeSante & Kaschube, 2009).…”

“…While trends in landbird abundance are useful for highlighting species of conservation concern, the assessment of vital rates may help identify causes of declining trends; for example, whether they are driven by factors on or away from breeding grounds (Newton, 2004;Albert et al, 2016). Data on vital rates collected at constant-effort landbird capture stations can also be used to predict population viability (Ryu et al, 2016) and can be modeled as functions of habitat variables to inform conservation-management strategies (DeSante, 1995;Saracco et al, 2016Saracco et al, , 2018, including reclamation and restoration programs (Foster et al, 2017). To date, modeled vital-rate terms have primarily included those of productivity, often indexed as the probability that a captured bird at a station had fledged that year and survivorship of breeding adults, as estimated from capture-mark-recapture models (Saracco, DeSante & Kaschube, 2008).…”