The influence of winter severity, predation and senescence on moose habitat use

Abstract

Summary 1.Habitat use is widely known to be influenced by abiotic and biotic factors, such as climate, population density, foraging opportunity and predation risk. The influence of the life-history state of an individual organism on habitat use is less well understood, especially for terrestrial mammals.2. There is good reason to expect that life-history state would affect habitat use. For example, organisms exhibiting poor condition associated with senescence have an increased vulnerability to predation and that vulnerability is known to alter habitat use strategies. 3. We assessed the influence of life-history stage on habitat use for 732 moose (Alces alces) killed by wolves (Canis lupus) over a 50-year period in Isle Royale National Park, an island ecosystem in Lake Superior, USA. We developed regression models to assess how location of death was associated with a moose's life-history stage (prime-aged or senescent), presence or absence of senescent-associated pathology (osteoarthritis and jaw necrosis), and annual variation in winter severity, moose density and ratio of moose to wolves, which is an index of predation risk. 4. Compared to senescent moose, prime-aged moose tend to make greater use of habitat farther from the shoreline of Isle Royale. That result is ecologically relevant because shoreline habitat on Isle Royale tends to provide better foraging opportunities for moose but is also associated with increased predation risk. During severe winters prime-aged moose tend to make greater use of habitat that is closer to shore in relation to senescent-aged moose. Furthermore, moose of both age classes were more likely to die in riskier, shoreline habitat during years when predation risk was lower in the preceding year. 5. Our results highlight a complicated connection between life history, age-structured population dynamics and habitat-related behaviour. Our analysis also illustrates why intraspecific competition should not be the presumed mechanism underlying density-dependent habitat use, if predation risk is related to density, as it is expected to be in many systems.

Trophic Cascades in a Multicausal World: Isle Royale and Yellowstone

“…Moose density is typically three to six times greater in IRNP regions where short fir are common, compared with regions where fir is rare or absent (Montgomery et al 2014). Wolf density is also approximately two times greater in regions where moose are more abundant (Montgomery et al 2012(Montgomery et al , 2014. This tendency for consumers to aggregate in portions of a landscape where resources are most dense would seem to correspond most with a bottom-up process-the antithesis of a trophic cascade (but see Supplemental Sidebar 3).…”

“…An estimated slope that is significantly less than −1 could indicate undetected synergy between predation and winter severity or may result from population growth that is influenced by the reduced foraging efficiency of moose during years of high predation risk. The influence of risk-sensitive foraging is another plausible explanation, given that predation risk and winter severity impact moose habitat selection (Montgomery et al 2012). Finally, other instances in which cause-specific mortality is more than additive have been documented, for example, harvest mortality in NR elk during 1995-2004 ; see also Sandercock et al 2011).…”

Individual Variability

“…Importantly, individuals can use different habitats depending on their age or size (Englund and Krupa, 2000;Heithaus and Dill, 2002;Montgomery et al, 2013;Sweitzer and Berger, 1992), nutritional need (Barten et al, 2001), life history strategy (Daverat et al, 2006) or reproductive status (Berger, 1991), and such co-variation can underpin a certain amount of individual variability in predation risk. Bighorn ewes with offspring are, for example, less likely to utilize dangerous foraging areas than lone females (Berger, 1991), while predation threat by toadfish Opsanus tau on mud crabs Panopeus herbstii led smaller crabs to consistently use refuges more than larger ones (Toscano et al, 2014).…”

Habitat use and selection by male and female moose (Alces alces) in a boreal landscape.

“…Reducing exposure to mortality risk has obvious benefits to individual moose survival. Associated with predators, there also can be energetic costs such as reduced foraging efficiency caused by increased vigilance or movement, and by choosing safer habitats that may have less forage (Molvar and Bowyer 1994, White and Berger 2001, Montgomery et al 2013.…”