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

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 (U.S.A.). 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 highlight show 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.