Dynamic selection for forage quality and quantity in response to phenology and insects in an Arctic ungulate

Abstract

Spatiotemporal variation in forage is a primary driver of ungulate behavior, yet little is known about the nutritional components they select, and how selection varies across the growing season with changes in forage quality and quantity. We addressed these uncertainties in barren‐ground caribou (Rangifer tarandus), which experience their most important foraging opportunities during the short Arctic summer. Recent declines in Arctic caribou populations have raised concerns about the influence of climate change on summer foraging opportunities, given shifting vegetation conditions and insect harassment, and their potential effects on caribou body condition and demography. We examined Arctic caribou selection of summer forage by pairing locations from females in the Central Arctic Herd of Alaska with spatiotemporal predictions of biomass, digestible nitrogen (DN), and digestible energy (DE). We then assessed selection for these nutritional components across the growing season at landscape and patch scales, and determined whether foraging opportunities were constrained by insect harassment. During early summer, at the landscape scale, caribou selected for intermediate biomass and high DN and DE, following expectations of the forage maturation hypothesis. At the patch scale, however, caribou selected for high values of all forage components, particularly DN, suggesting that protein may be limiting. During late summer, after DN declined below the threshold for protein gain, caribou exhibited a switch at both spatial scales, selecting for higher biomass, likely enabling mass and fat deposition. Mosquito activity strongly altered caribou selection of forage and increased their movement rates, while oestrid fly activity had little influence. Our results demonstrate that early and late summer periods afford Arctic caribou distinct foraging opportunities, as they prioritize quality earlier in the summer and quantity later. Climate change may further constrain caribou access to DN as earlier, warmer Arctic summers may be associated with reduced DN and increased mosquito harassment.

Adverse effects of Diptera flies on northern ungulates: Rangifer, Alces, and Bison

“…Rangifer tarandus in poor condition are less likely to survive and reproduce; recruitment to the population therefore diminishes with loss of body mass and body stores (Colman et al 2003, Kutz et al 2012, Witter et al 2012a, Raponi et al 2018). Harassment by flies affects foraging of female Rangifer tarandus and thus the annual productivity of herds in Alaska (Johnson et al 2018, 2021). The timing is particularly detrimental, as the end of fly season, and the resulting time of reduced body condition for Rangifer tarandus , is close to the start of winter (Mallory & Boyce 2018).…”

Critical summer foraging tradeoffs in a subarctic ungulate

“…Past studies in the arctic have shown mosquitoes ( Culicidae ) alter forage selection and induce behavioral responses by caribou (e.g, grouping and movement; Johnson et al, 2021 ; Joly et al, 2020 ; Witter, Johnson, Croft, Gunn, & Gillingham, 2012 ; Witter, Johnson, Croft, Gunn, & Poirier, 2012 ). The avoidance behaviors we frequently observed (e.g., muzzle to the ground, head shaking, stomping and scratching), however, suggest harassment by oestrids ( Oestridae ) and tabanids ( Tabanidae ).…”

“…, our work shows that interior caribou face similar costs of insect harassment as coastal populations.Past studies in the arctic have shown mosquitoes (Culicidae) alter forage selection and induce behavioral responses by caribou (e.g, grouping and movement;Johnson et al, 2021;Joly et al, 2020;Witter, Johnson, Croft, Gunn, & Gillingham, 2012;F I G U R E 7The mean proportions of six forage functional types (lichen, shrub, graminoid, forb, Equisetum spp. and moss) estimated in the summer diets of caribou of the Fortymile Caribou Herd Alaska, USA and Yukon, Canada, 2011-2019.…”

Survival and reproduction in Arctic caribou are associated with summer forage and insect harassment

“…Arctic caribou exhibit the longest terrestrial migrations on earth to reach their summer ranges (Fancy et al, 1989;Joly et al, 2020b) which provide access to important forage resources (White et al, 1975;Griffith et al, 2002;Johnson et al, 2021). As capital breeders, caribou must maximize their foraging opportunities during the short Arctic summer to regain body stores lost during the previous winter and amass stores for the upcoming winter and reproductive cycle (Barboza and Parker, 2008;Parker et al, 2009;Taillon et al, 2013).…”

“…Arctic caribou subsist on lichen and senesced vascular vegetation most of the year (Russell et al, 1993;Joly et al, 2015), a diet that is generally high in digestible energy (DE) but very low in digestible protein [indexed by digestible nitrogen (DN)]. During the short Arctic summer growing season, recent studies have found that DE appears to be sufficient in caribou forage, while DN is only abundant in newly emergent vegetation, declining below maintenance levels after mid-summer (Barboza et al, 2018;Johnson et al, 2021). Given this constraint, Barboza et al (2018) hypothesized that caribou may be limited by their ability to store adequate protein when it is available, which could have effects on their subsequent reproductive success.…”