26Dominance hierarchies and unequal resource partitioning among individuals are key mechanisms 27 of population regulation. The strength of dominance hierarchies can be influenced by size 28 dependent trade-offs between foraging and predator avoidance whereby competitively inferior 29 subdominants can access a larger proportion of limiting resources by accepting higher predation 30 risk. Foraging-predation risk trade-offs also depend on resource abundance. Yet, few studies 31 have manipulated predation risk and resource abundance simultaneously; consequently, their 32 joint effect on resource partitioning within dominance hierarchies are not well understood. We 33 addressed this gap by measuring behavioural responses of masu salmon to experimental 34 manipulations of predation risk and resource abundance in a natural temperate forest stream. 35 Responses to predation risk depended on body size such that larger dominants exhibited more 36 risk-averse behaviour (e.g., lower foraging and appearance rates) relative to smaller 37 subdominants after exposure to a simulated predator. The magnitude of this effect was lower 38 when resources were elevated, indicating that dominant fish accepted a higher predation risk to 39 forage on abundant resources. However, the influence of resource abundance did not extend to 40 the population level, where predation risk altered the distribution of foraging attempts (a proxy 41 for energy intake) from being skewed towards large individuals to being skewed towards small 42 individuals after predator exposure. Our results imply that size dependent foraging-predation risk 43 trade-offs can mediate the strength of dominance hierarchies by allowing competitively inferior 44 subdominants to access resources that would otherwise be monopolized. 45 46 47 48 49 Social dominance hierarchies and the maintenance of unequal resource partitioning among 50 individuals are key mechanisms of population regulation and stability (Hassell 1978, Lomnicki 51 1988). The strength and stability of dominance hierarchies depends on the behavioural 52 mechanisms mediating intraspecific competition (Weir and Grant 2004). Predators and resource 53 abundance appear to have particularly important roles in this context given that many animals 54 face a trade-off between maximizing resource intake while minimizing mortality risk (Werner 55 and Gilliam 1984), and that the optimum of this trade-off can vary among individuals as a 56 function of body size and social status (Lima and Dill 1990).
57Size or status dependent foraging-predation risk trade-offs are often attributed to the asset 58 protection principal (Clark 1994), which posits that larger individuals should be more risk-averse 59 than their smaller conspecifics due to their higher accumulated fitness 'assets' and the 60 diminishing energetic return for a given foraging intake with increasing body size. For 61 consumers in dominance hierarchies, this implies that reduced foraging rates by larger dominants 62 in the presence of predators could allow smalle...