Large herbivores surf waves of green-up during spring

Abstract

The green wave hypothesis (GWH) states that migrating animals should track or 'surf' high-quality forage at the leading edge of spring green-up. To index such high-quality forage, recent work proposed the instantaneous rate of green-up (IRG), i.e. rate of change in the normalized difference vegetation index over time. Despite this important advancement, no study has tested the assumption that herbivores select habitat patches at peak IRG. We evaluated this assumption using step selection functions parametrized with movement data during the green-up period from two populations each of bighorn sheep, mule deer, elk, moose and bison, totalling 463 individuals monitored 1-3 years from 2004 to 2014. Accounting for variables that typically influence habitat selection for each species, we found seven of 10 populations selected patches exhibiting high IRG-supporting the GWH. Nonetheless, large herbivores selected for the leading edge, trailing edge and crest of the IRG wave, indicating that other mechanisms (e.g. ruminant physiology) or measurement error inherent with satellite data affect selection for IRG. Our evaluation indicates that IRG is a useful tool for linking herbivore movement with plant phenology, paving the way for significant advancements in understanding how animals track resource quality that varies both spatially and temporally.

Keywords: forage maturation hypothesis; green wave hypothesis; habitat selection; large herbivores; migration; normalized difference vegetation index.

Figure 1.

Relationship between odds of selection (with 95% CI in red, reported as odds ratio or exponent of the beta coefficient for IRG]) of a habitat patch and its IRG value for two populations each of elk, mule deer (MD), bighorn sheep (BHS), moose and bison in western Wyoming and eastern Utah between 2004 and 2014. Probability of selection based on predicted values of a Step Selection Function parametrized with GPS collar data in each population. (Online version in colour.)

Figure 2.

Selection for the leading edge, trailing edge or crest of the IRG wave by mule deer, bighorn sheep and moose in western Wyoming, and bison in eastern Utah between 2004 and 2014. The location on the wave was determined by calculating the local maximum of the quadratic relationship between relative probability of selection and NDVI. Probability of selection was based on a step selection function parametrized with GPS collar data in each population. (Online version in colour.)