Predation risk and resource abundance mediate foraging behaviour and intraspecific resource partitioning among consumers in dominance hierarchies

25 January 2019

Naman, Sean; Ueda, Rui; Sato, Takuya

Dominance hierarchies and the resulting unequal resource partitioning among individuals are key mechanisms of population regulation. The strength of dominance hierarchies can be influenced by size-dependent trade-offs between foraging and predator avoidance whereby competitively inferior subdominants can access a larger proportion of limiting resources by accepting higher predation risk. Foraging-predation risk trade-offs also depend on resource abundance. Yet, few studies have manipulated predation risk and resource abundance simultaneously; consequently, their joint effect on resource partitioning within dominance hierarchies are not well understood. We addressed this gap by measuring behavioural responses of masu salmon (Oncorhynchus masou ishikawae) to experimental manipulations of predation risk and resource abundance in a natural temperate forest stream. Responses to predation risk depended on body size and social status such that larger fish (often social dominants) exhibited more risk-averse behaviour (e.g., lower foraging and appearance rates) than smaller subdominants after exposure to a simulated predator. The magnitude of this effect was lower when resources were elevated, indicating that dominant fish accepted a higher predation risk to forage on abundant resources. However, the influence of resource abundance did not extend to the population level, where predation risk altered the distribution of foraging attempts (a proxy for energy intake) from being skewed towards large individuals to being skewed towards small individuals after predator exposure. Our results imply that size-dependent foraging-predation risk trade-offs can weaken the strength of dominance hierarchies by allowing competitively inferior subdominants to access resources that would otherwise be monopolized.