Data from: Prey responses to fine-scale variation in predation risk from combined predators

While it is well documented that organisms can express phenotypic
plasticity in response to single gradients of environmental variation, our
understanding of how organisms integrate information along multiple
environmental gradients is limited in many systems. Using the freshwater
snail Helisoma trivolvis and two common predators (water bugs Belostoma
flumineum and crayfish Orconectes rusticus), we explored how prey
integrate information along multiple predation risk gradients (i.e. caged
predators fed increasing amounts of prey biomass) that induce opposing
phenotypes. When exposed to single predators fed increasing amounts of
prey biomass, we detected threshold responses; intermediate amounts of
consumed biomass induced phenotypic responses, but higher amounts induced
little additional induction. This suggests that additional increases in
predator-induced traits with greater predator risk offer minimal increases
in fitness or that a limit in the response magnitude was reached.
Additionally, the response thresholds were contingent on the predator and
focal trait. For shell width, responses were generally detected at a lower
amount of consumed biomass by water bugs compared to crayfish. Within the
crayfish treatments, we found that the shell thickness response threshold
was lower than the shell width response threshold. When we combined
gradients of consumed biomass from both predators, we found that the
magnitude of response to one predator was often reduced when the other
predator was present. Interestingly, these effects were often detected at
consumed biomass levels that were lower than the threshold concentration
necessary to elicit a response in the single-predator treatments.
Moreover, our combined predator treatments revealed that snails shifted
from discrete responses to more continuous (i.e. graded) responses.
Together, our results reveal that organisms experiencing multiple
environmental gradients can integrate this information to make phenotypic
decisions and demonstrate the novel result that an exposure to multiple
species of predators can lower the response threshold of prey.