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

Background:

The stability of a fish population is influenced by the relationship between social dominance hierarchies and the partitioning of available resources among individuals. Understanding how behavioural mechanisms are affected by this relationship can improve our understanding of the drivers governing population structure. Red-spotted masu salmon (Oncorhynchus masou ishikawae) exhibit strong dominance hierarchies where larger dominant individuals exclude smaller subdominants from the most profitable foraging territories, and the species frequently experience significant predation risk from terrestrial predators. Naman et al. study how the behavioural trade-off between foraging success and predation risk is influenced by dominant-subdominant interactions and resource abundance.

 

Methods:

• The study was conducted in a stream of the Arida River in Kyoto University’s Wakayama Forest Research Station in Japan.
• The study stream was separated into six experimental reaches (n = 10 fish per reaches), consisting of three replicates of two resource treatments (control vs. elevated).
• Elevated resources were simulated by adding live mealworms to the stream.
• Predator simulations consisted of a crow decoy on a string, rigged to fly (swing) over the stream making brief contact with the water.
• Underwater videography was used to determine the proportion of time a fish was visible in the pool relative to the total footage recorded (Appearance rate), and the number of foraging attempts per minute that a given fish was visible (Frequency of foraging attempts).
• Foraging rate was quantified as the product of Appearance rate and Frequency of foraging attempts.
• Results were analysed for the effects of body size and resource availability on foraging rates before and after predator exposure.

 

Predictions (arrows indicate the direction of predicted shift in foraging rate):

• On an individual level (i.e. without dominant-subdominant interactions), larger individuals are more risk-averse compared to smaller individuals, because the energetic return for a given foraging intake decreases with increasing body size. Consequently, resource allocation should shift towards smaller individuals in the presence of a predator (Large → Small) (1).
• With dominant-subdominant interactions, larger dominants exclude smaller subdominants from the most profitable foraging territories in the absence of a predator (Large ← Small) (2).
• In the presence of a predator, the risk-averse behaviour of larger individuals (hypothesis 1) shifts resource allocation from dominants towards subdominants (Large → Small) (3).
• Foraging activity decreases with an increasing abundance of resources because of the lower marginal benefit of food intake. Consequently, elevated resources should exacerbate the risk-averse behaviour of larger individuals in the presence of a predator (hypothesis 1) and further shift resource allocation from dominants towards subdominants (Large →→ Small) (4).

 

Results (arrows indicate the direction of observed shift in foraging rate):

• On an individual level, foraging rates in the presence of a simulated predator declined with increasing body size, both with normal (control) and elevated resources (Large → Small) (a).
• With dominant-subdominant interactions, foraging rates in pools with normal resources increased with body size in the absence of a predator (Large ← Small) (b) and decreased with body size in the presence of a simulated predator (Large → Small) (c).
• In pools with elevated resources, foraging rates of smaller subdominants were equal to larger dominants, both in the absence and presence of a simulated predator (Large = Small) (d).

 

	Individual level	Dominant-subdominant interactions
Foraging rate shift	Predator present	Predator absent	Predator present
	Predictions
Normal resources	Large → Small (1)	Large ← Small (2)	Large → Small (3)
Elevated resources	Large → Small (1)	Large ← Small (2)	Large →→ Small (4)
	Results
Normal resources	Large → Small (a)	Large ← Small (b)	Large → Small (c)
Elevated resources	Large → Small (a)	Large = Small (d)	Large = Small (d)

 

Conclusion:

The results indicate that size-dependent trade-offs between foraging success and predation risk can mediate the strength of dominance hierarchies by allowing competitively inferior subdominants to access resources that would otherwise be monopolized. The results also indicate that individuals exposed to an abundance of resources may exhibit a ‘feast or famine’ behavioural response; accepting higher predation risk in order to meet their energetic demands. This response may override the effect of a predator on the risk-averse behaviour of larger individuals and reduce the foraging opportunities for subdominants.

 

Interest:

The study by Naman et al. is among the first to manipulated predation risk and resource abundance simultaneously and assess their joint effect on resource partitioning within dominance hierarchies. I am particularly interested in this study because it assesses the relationships between multiple environmental biotic factors (i.e., resource availability, dominant-subdominant interactions, and predation risk), and how they affect foraging behaviour in fishes.

 

Future directions:

Future studies could add additional dimensions to the experimental design by including abiotic factors such as elevated temperatures, salinity fluctuations, or aquatic hypoxia, and assess the extent to which these environmental stressors enhance or override the behavioural responses presented in this study. Naman et al. assessed the responses of fish in reaches containing 10 fish. It would be interesting is the behavioural responses are influenced by the number of fish in the experimental reaches or the ration of dominant/subdominant individuals.

 

doi: https://doi.org/10.1242/prelights.5273

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