Is summer food intake a limiting factor for boreal browsers? Diet, temperature, and reproduction as drivers of consumption in female moose

Abstract

Food intake may limit the ability of browsing mammals to gain body mass during the growing season when the leaves and stems of woody plants are most abundant. Moose are highly productive browsers with high demands for energy and nutrients, particularly during lactation. Using an indigestible marker, we estimated dry matter intake of free ranging adult female moose with and without calves over three growing seasons. During the same period, we analyzed forage quality. Intakes were highest in late spring (280 ± 19 g·kg-0.75·d-1) when forage quality peaked; however, intakes declined by 39% throughout the summer as temperatures increased and as acid detergent fiber content of browse increased. Digestibility of dry matter declined over summer from 71% to 57% among browse. Intakes were similar for moose with and without calves. Heat loads may impair the ability of moose to consume sufficient energy and nutrients. Warming and habitat change can adversely affect browser populations when poor forage qualities and low dry matter intakes combine to suppress digestible intakes of energy and nutrients.

Environment and physiology shape Arctic ungulate population dynamics

“…Different static winter climate metrics have been used, for instance, spring snow cover (% area coverage) was correlated with muskox calf recruitment (Schmidt et al, 2015), mid‐winter temperature correlated with fecundity rates in red deer ( Cervus elaphus ), Soay sheep ( Ovis aries ), and saiga antelope ( Saiga tatarica ; Coulson et al, 2000), cumulative daily winter precipitation was associated with elk ( Cervus elaphus ) calf recruitment (Garrott et al, 2003), and cumulative number of winter rain‐on‐snow events was associated with reduced survival in youngest age classes and overall population size and growth rates in Arctic reindeer (Hansen, Gamelon, et al, 2019; Hansen, Pedersen, et al, 2019). In non‐Arctic ungulates, summer forage has been identified as an important determinant of population fitness (Albon et al, 2017; Shively et al, 2019). Climate change is predicted to alter temperature and precipitation patterns in winter and summer variably across the Northern Hemisphere (Krasting et al, 2013).…”

The nutritional condition of moose co‐varies with climate, but not with density, predation risk or diet composition

“…heart and respiration rates) and behavior (e.g. habitat selection and forage intake rates (van Beest and Milner 2013, Shively et al 2019, Thompson et al 2020)). We hypothesized that moose would be more nutritionally stressed and invest more in detoxification following hot and dry summers and following springs with fewer growing degree days.…”

“…Our assessment of spatio–temporal variation in these two nutritional indicators was partly guided by the preceding background and a set of five specific hypotheses.1) Nutritional restriction and investment in detoxification would be greater following warmer summers and during winters with deeper snow. Those weather conditions are expected to increase metabolic and energetic costs, cause moose to reduce food intake rates, and seek shelter in habitats with dense canopy cover where the abundance and quality of forage is lower (Moen 1976, Parker et al 1984, van Beest and Milner 2013, Shively et al 2019, Thompson et al 2020). Moreover, warmer temperatures have been shown to influence the PSM profiles of at least some plant species that moose forage on in this region (Berini et al 2018).…”

Critical summer foraging tradeoffs in a subarctic ungulate

“…Previous studies have demonstrated the key role of summer nutrition, especially for arctic ungulates who experience short growing seasons (Barboza et al, 2009;Cook et al, 2004;Shively et al, 2019).…”

“…Previous studies have demonstrated the key role of summer nutrition, especially for arctic ungulates who experience short growing seasons (Barboza et al, 2009 ; Cook et al, 2004 ; Shively et al, 2019 ). Following the forage maturation hypothesis for large herbivores (Fryxell, 1991 ; Hebblewhite et al, 2008 ), caribou transition from a diet dominated by low‐quality lichen (winter) to a diet dominated by higher‐quality green vegetation (i.e., graminoids and shrubs) to meet the digestible energy and protein requirements for fetal growth (spring) and lactation (summer; Barboza et al, 2018 ; Crête & Huot, 1993 ; Denryter et al, 2020 ).…”