Population dynamics and mortality tradeoffs of bold and shy foraging: A field study with sunfish

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Melissa Kjelvik
Gary Mittelbach
Laurel Lindemann


Recently, ecologists have found that a tradeoff between increased foraging activity and predation risk may lead to behavioral variation within a population, with individuals showing consistent, repeatable responses to foraging tradeoffs. Much of the current work on this subject has been conducted in laboratory experiments. It is important for tradeoffs studied in the laboratory to be tested in the field to quantify the effects on population dynamics and ecological interactions. To apply the concepts of this foraging activity-risk tradeoff in the field, we first assayed young-of-year bluegill (Lepomis machrochirus) in a laboratory setting and differentially marked individuals classified as bold or shy foragers. These fish (200 bold, 200 shy) were then stocked into a pond at the Kellogg Biological Station’s Experimental Pond Facility with a predator (largemouth bass, Micropterus salmoides). After two months, we drained the pond to collect all remaining individuals. Survivorship and growth were measured to determine whether the foraging activity-risk tradeoff was present in bluegill. We hypothesized that “bold” foragers would have a higher growth rate but lower survival than “shy” foragers in the presence of the predator. Physiological variables were also quantified to examine how foraging behavior may affect somatic growth efficiency and injury repair.
After draining the ponds we found that more shy fish survived predation pressure than bold fish (66 bold, 74 shy) and that bold fish were larger than shy fish, on average (bold mean length= 68.6±0.08, shy mean length=65.6±0.08). Shy fish also were able to repair injuries much more effectively than bold fish. Our data supports our predictions, suggesting the foraging activity-risk tradeoff is present in our study population. Differences in growth and survival show that population dynamics are affected by intraspecific variation in individual foraging behavior. Our results show that tradeoffs may be an important mechanism for maintaining biodiversity within a species. There are fitness costs and benefits for both bold and shy individuals, potentially equalizing fitness and allowing both behavioral types to be successful under predation pressure.

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