Abstract
BackgroundTheory predicts that prey facing a combination of predators with different feeding modes have two options: to express a response against the feeding mode of the most dangerous predator, or to express an intermediate response. Intermediate phenotypes protect equally well against several feeding modes, rather than providing specific protection against a single predator. Anti-predator traits that protect against a common feeding mode displayed by all predators should be expressed regardless of predator combination, as there is no need for trade-offs.Principal FindingsWe studied phenotypic anti-predator responses of zebra mussels to predation threat from a handling-time-limited (crayfish) and a gape-size-limited (roach) predator. Both predators dislodge mussels from the substrate but diverge in their further feeding modes. Mussels increased expression of a non-specific defense trait (attachment strength) against all combinations of predators relative to a control. In response to roach alone, mussels showed a tendency to develop a weaker and more elongated shell. In response to crayfish, mussels developed a harder and rounder shell. When exposed to either a combination of predators or no predator, mussels developed an intermediate phenotype. Mussel growth rate was positively correlated with an elongated weaker shell and negatively correlated with a round strong shell, indicating a trade-off between anti-predator responses. Field observations of prey phenotypes revealed the presence of both anti-predator phenotypes and the trade-off with growth, but intra-specific population density and bottom substrate had a greater influence than predator density.ConclusionsOur results show that two different predators can exert both functionally equivalent and inverse selection pressures on a single prey. Our field study suggests that abiotic factors and prey population density should be considered when attempting to explain phenotypic diversity in the wild.
Highlights
Phenotypic plasticity is a common response of organisms to environmental variation and an important source of diversity within and across populations [1]
Our results show that two different predators can exert both functionally equivalent and inverse selection pressures on a single prey
Predation rarely results from a single predator but from a community including a multitude of predators [4,5,6], each with different feeding modes that can lead to opposing or functionally inverse selection pressures on prey phenotypes [5, 7]
Summary
Phenotypic plasticity is a common response of organisms to environmental variation and an important source of diversity within and across populations [1]. Predation rarely results from a single predator but from a community including a multitude of predators [4,5,6], each with different feeding modes that can lead to opposing or functionally inverse selection pressures on prey phenotypes [5, 7]. Principal Findings: We studied phenotypic anti-predator responses of zebra mussels to predation threat from a handling-time-limited (crayfish) and a gape-sizelimited (roach) predator. Both predators dislodge mussels from the substrate but diverge in their further feeding modes. Field observations of prey phenotypes revealed the presence of both anti-predator phenotypes and the trade-off with growth, but intra-specific population density and bottom substrate had a greater influence than predator density.
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