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

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College of Forest Resources and Environmental Science


A fundamental question about the ecology of herbivore populations pertains to the relative influence of biotic and abiotic processes on nutritional condition. Nutritional condition is influenced in important, yet poorly understood, ways by plant secondary metabolites (PSMs) which can adversely affect a herbivore's physiology and energetics. Here we assess the relative influence of various abiotic (weather) and biotic (intraspecific competition, predation risk and diet composition) factors on indicators of nutritional condition and the energetic costs of detoxifying PSMs for the moose population in Isle Royale National Park (USA). Specifically, we observed interannual variation in the ratio of urea nitrogen to creatinine (UN:C), an indicator of nutritional restriction, over 29 years and the ratio of glucuronic acid to creatinine (GA:C), an indicator of energetic investment in detoxifying PSMs, over 19-years. Both UN:C and GA:C were measured in samples of urine-soaked snow. Most importantly, climatic factors explained 66% of the interannual variation in UN:C, with moose being more nutritionally stressed during winters with deep snow and during winters that followed warm summers. None of the biotic factors (density, predation, diet composition) were useful predictors of UN:C or GA:C. The absence of a relationship between diet composition and either UN:C or GA:C suggests that the nutritional ecology of wild herbivores is probably complicated by fine-scale variation in protein content and concentrations of PSMs amongst plants of the same species. UN:C increased with GA:C at both the individual and population-level. That result is consistent with detoxification being energetically costly, such that it impairs nutritional condition and also highlights how spatio-temporal variation in the intake and detoxification of PSMs may influence population dynamics. Lastly, because we observed interannual variation in nutritional condition over three decades and detoxification over two decades these findings are relevant to concerns about how herbivore populations respond to climate change.

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