Abstract
Prey at risk of predation may experience stress and respond physiologically by altering their metabolic rates. Theory predicts that such physiological changes should alter prey nutrient demands from N-rich to C-rich macronutrients and shift the balance between maintenance and growth/reproduction. Theory further suggests that for ectotherms, temperature stands to exacerbate this stress. But, the behavior of many prey species facing perceived predation risk is the opposite of these predictions, consuming more N-rich resources. Here we revisit the original Threshold Elemental Ratio (TER) theoretical framework that inspired the idea of shifts in elemental (C:N) stoichiometry in response to chronic predation stress to reconcile the different prey responses. We examine the interactive effects of predation stress and temperature stress by exploring mathematically how the component physiological variables that determine TER vary individually with temperature. These functional relationships are then embedded into the equation for TER to predict how C and N intake should vary with and without predation stress across temperature gradients. This new theory reconciles the different prey responses and explains why and when species ought to consume more N vs more C when stressed by perceived predation, depending on the nature of their thermal performance with rising temperature. The theory also points to new ways to conduct experimental evaluations testing the temperature sensitivity of prey to predation stress.
Highlights
The classic concept of adaptive foraging-predation risk trade-off has served as a straightforward generalization to enhance understanding of how species interactions shape community structure (Werner and Peacor, 2003; Preisser et al, 2005; Peckarsky et al, 2008) and ecosystem processes (Schmitz et al, 2008; Schmitz, 2010; Trussell and Schmitz, 2012)
The mechanism determining this plasticity is encapsulated by the concept of the threshold elemental ratio (TER)—the dietary mixture where growth limitation switches from one element to another (Sterner, 1997; Frost et al, 2006; Cross et al, 2015)
It has been hypothesized that for ectotherms, increasing temperature and thereby increasing RC, should exacerbate the effects of predation stress (Schmitz, 2013). This may happen (Culler et al, 2014; Schmitz et al, 2016), but it is not universally true: the idea that temperature effects on organismal stoichiometry should conflate effects of stress from predation is based on the implicit assumption that all variables other than RC that determine the magnitude of GGEC and TERC:X remain constant in the face of perceived predation risk and changes in other environmental conditions
Summary
The classic concept of adaptive foraging-predation risk trade-off has served as a straightforward generalization to enhance understanding of how species interactions shape community structure (Werner and Peacor, 2003; Preisser et al, 2005; Peckarsky et al, 2008) and ecosystem processes (Schmitz et al, 2008; Schmitz, 2010; Trussell and Schmitz, 2012). Theory predicts that whenever mass-specific metabolic rate of prey exposed to chronic predation risk increases, prey should generally shift their nutrient intake and consume more carbohydrate than non-stressed prey (Hawlena and Schmitz, 2010a). This prediction derives from the idea that stressed animals should switch their life history strategies from one devoted to protein demanding growth or reproduction (production) to one of fulfilling energy demanding maintenance functions to cope with the added stress (Hawlena and Schmitz, 2010a). This can lead to new predictions of how changing environmental temperature and predation risk, both singly and in combination, influence organismal elemental stoichiometry
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