A Multi‐scale Test of the Forage Maturation Hypothesis in a Partially Migratory Ungulate Population

Abstract

Abstract. The forage maturation hypothesis (FMH) proposes that ungulate migration is driven by selection for high forage quality. Because quality declines with plant maturation, but intake declines at low biomass, ungulates are predicted to select for intermediate forage biomass to maximize energy intake by following phenological gradients during the growing season. We tested the FMH in the Canadian Rocky Mountains by comparing forage availability and selection by both migrant and nonmigratory resident elk (Cervus elaphus) during three growing seasons from [2002][2003][2004]. First, we confirmed that the expected trade-off between forage quality and quantity occurred across vegetation communities. Next, we modeled forage biomass and phenology during the growing season by combining ground and remote-sensing approaches. The growing season started 2.2 days earlier every 1 km east of the continental divide, was delayed by 50 days for every 1000-m increase in elevation, and occurred 8 days earlier on south aspects. Migrant and resident selection for forage biomass was then compared across three spatial scales (across the study area, within summer home ranges, and along movement paths) using VHF and GPS telemetry locations from 119 female elk. Migrant home ranges occurred closer to the continental divide in areas of higher topographical diversity, resulting in migrants consistently selecting for intermediate biomass at the two largest scales, but not at the finest scale along movement paths. In contrast, residents selected maximum forage biomass across all spatial scales. To evaluate the consequences of selection, we compared exposure at telemetry locations of migrant and resident elk to expected forage biomass and digestibility. The expected digestibility for migrant elk in summer was 6.5% higher than for residents, which was corroborated with higher fecal nitrogen levels for migrants. The observed differences in digestibility should increase migrant elk body mass, pregnancy rates, and adult and calf survival rates. Whether bottom-up effects of improved forage quality are realized will ultimately depend on trade-offs between forage and predation. Nevertheless, this study provides comprehensive evidence that montane ungulate migration leads to greater access to higher-quality forage relative to nonmigratory congeners, as predicted by the forage maturation hypothesis, resulting primarily from large-scale selection patterns.

Warming, plant phenology and the spatial dimension of trophic mismatch for large herbivores

“…In migratory herbivores, however, arrival at the breeding grounds is not the end of the story. Herbivores track phenological progression of forage plants by moving across the local landscape to maintain intake of newly emergent species, individual plants or even tissues within plants (Klein 1970(Klein , 1990Skogland 1989;Hebblewhite et al 2008). Such observations suggest that any spatial constriction of the plant growing season might constrain the ability of migratory herbivores to prolong intake of high-quality forage during peak resource demand.…”

Spatial and Temporal Changes in Vegetation Phenology at Middle and High Latitudes of the Northern Hemisphere over the Past Three Decades

“…Phenology has been extensively studied in different areas of research, such as climate change [7], biodiversity [8], wildlife ecology [9], snow dynamics [10], fires [11], and crops [12]. Land surface phenology (LSP) is an essential factor for measuring dynamic changes in vegetation landscapes.…”

Asynchronous vegetation phenology enhances winter body condition of a large mobile herbivore

“…For large-bodied herbivores, the phenology of vegetation is a critical determinant of diet quality (Van Soest 1982, Crawley 1983) that has been linked to diet choice, individual movement and performance (Hjeljord et al 1990, Albon andLangvatn 1992;Herfindal et al 2006;Hebblewhite et al 2008;Mysterud et al 2008;Hamel et al 2009;Martinez-Jauregui et al 2009;Bischof et al 2012;Nielsen et al 2012;Singh et al 2012;Giroux et al 2014, as well as population processes such as survival, reproduction and density-dependence Wang et al 2006;Pettorelli et al 2007;Wittemyer et al 2007. The ability of landscapes to support herbivores is ultimately limited by the total amount of aboveground net-primary production (ANPP) available for consumption (McNaughton et al 1989;Cebrian and Lartigue 2004).…”

“…Emerging evidence suggests that limits set by ANPP are modified by the spatial pattern and timing of plant growth. In particular, there is evidence that heterogeneity in plant communities expressed over space, particularly heterogeneity that induces variation in time by influencing plant phenology, offers fundamentally important nutritional benefits to foraging herbivores that enhance the performance of individuals and their populations (Pettorelli et al 2007;Hebblewhite et al 2008;Mysterud et al 2008, Searle et al 2010Giroux et al 2014, Hurley et al 2014, Iversen et al 2014. For instance, fine scale dynamics of vegetation green-up across landscapes may determine the length of time during spring when high quality forage is available for ungulates.…”