Wave-like Patterns of Plant Phenology Determine Ungulate Movement Tactics.

Animals exhibit a diversity of movement tactics [ 1 ]. Tracking resources that change across space and time is predicted to be a fundamental driver of animal movement [ 2 ]. For example, some migratory ungulates (i.e., hooved mammals) closely track the progression of highly nutritious plant green-up, a phenomenon called "green-wave surfing" [ 3–5 ]. Yet general principles describing how the dynamic nature of resources determine movement tactics are lacking [ 6 ]. We tested an emerging theory that predicts surfing and the existence of migratory behavior will be favored in environments where green-up is fleeting and moves sequentially across large landscapes (i.e., wave-like green-up) [ 7 ]. Landscapes exhibiting wave-like patterns of green-up facilitated surfing and explained the existence of migratory behavior across 61 populations of four ungulate species on two continents (n = 1,696 individuals). At the species level, foraging benefits were equivalent between tactics, suggesting that each movement tactic is fine-tuned to local patterns of plant phenology. For decades, ecologists have sought to understand how animals move to select habitat, commonly defining habitat as a set of static patches [ 8 , 9 ]. Our findings indicate that animal movement tactics emerge as a function of the flux of resources across space and time, underscoring the need to redefine habitat to include its dynamic attributes. As global habitats continue to be modified by anthropogenic disturbance and climate change [ 10 ], our synthesis provides a generalizable framework to understand how animal movement will be influenced by altered patterns of resource phenology. • Ungulates moved to track forage in landscapes with wave-like spring green-up • Patterns of green-up explained where migratory behavior occurred in many ecosystems • At the species level, migrants and residents received equivalent foraging benefits • Movement tactics represent behavioral adaptations to specific landscapes Using GPS-tracking from 61 populations of four ungulate species, Aikens et al. provide evidence that the dynamic nature of forage resources generates the diversity of movement tactics used by animals. Specifically, patterns of spring green-up shaped how closely animals tracked resources and where migration occurred across temperate ecosystems. [ABSTRACT FROM AUTHOR]