Linking Dynamic Habitat Selection with Wading Bird Foraging Distributions across Resource Gradients: TFC Model

The temporal foraging conditions (TFC) models used daily mean hydrological characteristics calculated throughout the Everglades. Three explanatory variables representing hydrological conditions across a gradient of temporal scales were used as proxies for prey dynamics: days since drydown (DSD) was used as an indicator for long-term prey production (Trexler 2010), recession rate was used as an indicator for prey concentration dynamics (Russell et al. 2002, Beerens et al. 2011), and daily water depth was used as an indicator of short-term prey availability (Gawlik 2002, Beerens et al. 2011). These temporally-specific “resources” were considered available only when cell depths were in the foraging depth range of each species (Beerens et al, in press). The mean levels and heterogeneity (SD) of each resource were used to predict resource selection, which subsequently was used to predict the abundance of flocks and individuals across the landscape. Flock presence was defined as one or more birds of the target species (e.g., egrets, ibises, and storks) detected in a cell, whereas individual abundance counted the total number of birds present. Both individual and flock responses were modeled because wading birds are highly social and select foraging habitat based in part on the presence of conspecifics, a process that may increase or decrease individual fitness (Campomizzi et al. 2008). For the TFC, daily output summed over each region (Water Conservation Area (WCA) 1, WCA-2, WCA-3N, WCA-3S, Big Cypress National Park (BCNP), and Everglades National Park (ENP); Fig. 1) represents the mean patch quality of the landscape, within the suitable water depths of each species.

本研究所用的时间尺度觅食条件（Temporal Foraging Conditions, TFC）模型，以佛罗里达大沼泽地（Everglades）全域测算得到的日平均水文特征为基础。研究选取三类覆盖不同时间尺度梯度的水文条件解释变量，作为猎物动态的替代指标：自干涸起天数（Days Since Drydown, DSD）用以表征长期猎物生产力（Trexler, 2010），退水速率用以表征猎物浓度动态（Russell等, 2002；Beerens等, 2011），而日水深则用以表征短期猎物可获得性（Gawlik, 2002；Beerens等, 2011）。上述具有时间特异性的「资源」仅当栅格水深处于对应物种的觅食水深区间时，方可被判定为可利用资源（Beerens等，即刊）。以各类资源的平均水平及其异质性（标准差SD）为预测变量，构建物种资源选择偏好模型，进而推演研究区全域内鸟群与个体的丰度。鸟群存在性被定义为：单个栅格内检测到1只及以上目标物种个体（如白鹭、朱鹭、鹳类）；而个体丰度则统计栅格内鸟类的总数量。同时对个体与鸟群响应进行建模，原因在于涉禽具有高度的社会性，其觅食生境选择会部分受同种个体存在的影响，该过程可提升或降低个体适合度（Campomizzi等, 2008）。针对TFC模型，将各区域（1号水资源保护区（Water Conservation Area, WCA-1）、WCA-2、WCA-3N、WCA-3S、大柏树国家公园（Big Cypress National Park, BCNP）以及大沼泽地国家公园（Everglades National Park, ENP）；图1）的逐日输出结果求和，即可得到各物种适宜水深范围内的景观平均斑块质量。