Fear, snails, and funny-shaped barnacles: the complex workings of predation and competition in the rocky intertidal zone

Catherine M. Matassa, Patrick J. Ewanchuk, and Geoffrey C. Trussell

All animals have to eat in order to survive, but they must also avoid being eaten.  This poses a problem because the greatest quantity and quality of food is often in the riskiest habitat.  Animals must therefore choose carefully when and where to eat by assessing their risk of predation, but also considering their need to obtain energy for survival, growth, and reproduction. An animal that perceives a risk of predation may choose lower quality food in a safe habitat over higher quality food in a risky one.

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On rocky shores, crowded, fast-growing barnacles (Semibalanus balanoides) form hummocks, clusters of trumpet-shaped individuals that are highly susceptible to dislodgement by breaking waves. Carnivorous snails (Nucella lapillus) consume barnacles and reduce the likelihood of hummock development. However, by consuming snails or causing them to change their feeding behavior, predatory green crabs (Carcinus maenas) suppress the effects of snails on barnacles and allow for greater rates of hummock development. Photo credit: Catherine Matassa

On rocky intertidal shores, carnivorous snails must avoid their predators (crabs) while searching for and consuming barnacles.  When snails detect the presence of green crabs, they spend less time foraging on barnacles and more time hiding in cracks and crevices of their rocky habitat.  The result: snails avoid predation, and so do the barnacles!

However, predatory crabs are not the snails’ only problem.  Competitors rapidly deplete safe habitats of food, forcing snails to take greater risks in order to obtain the energy needed to survive.  Barnacles have the same problem: when crowded, barnacles run out of space.  Unable to continue growing in their characteristic “volcano” shape, crowded barnacles begin growing tall instead of wide, forming regularly-spaced mounds of trumpet-shaped barnacles called “hummocks” (see photograph).  But competition isn’t all bad.  There are also benefits to being in a crowded group. For example, a snail in a crowded group may “feel safer” and experience less stress, while barnacles can ‘lean’ on one another and invest less energy in building their shells.

In our paper, we test whether snails are ruled more by fear or competition and how these effects trickle down the food chain to impact barnacles.  We found that fear of crabs had a stronger impact on snails than competition, but both competition and fear were important for barnacles.  The fear-induced changes in snail feeding behavior caused barnacles to grow into hummocks more frequently.  Our study indicates that competition and predation can interact in complex ways across multiple levels in the food chain.

Read the paper here.

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