Enhanced recruitment of larger predators in the presence of large prey

Most carnivores undergo diet shift from smaller to larger prey items
during ontogeny. The trophic relationship between a growing carnivore and
larger prey is representative of a size-structured predator-prey
interaction. The strength of this interaction is, in part, determined by
the recruitment of individuals from smaller predatory size classes into
larger predatory size classes. Therefore, it is interesting to investigate
how larger prey alter the recruitment of smaller predator size classes
into larger predator size classes, since this can affect their own future
predation risk.  Past empirical studies have exclusively
documented that large prey reduce predator recruitment by decreasing
growth and/or survival of the smaller predators. In this study, we provide
empirical evidence of the contrasting pattern: large prey enhance the
recruitment of smaller predators into larger predators even though they
increase cannibalism mortality of the smaller predators. We have done this
here by studying the trophic interaction between predatory salamander
larvae (Hynobius retardatus) and the frog tadpoles (Rana pirica) that
represent their large prey.  In a field experiment in which
salamander hatchlings were exposed to the presence or absence of large
frog tadpoles, we found that more giant salamanders emerged in the
presence of frog tadpoles than in their absence. Re-assignment of frog
tadpoles (to both treatments) in the subsequent experimental period showed
that the enhanced emergence of giant salamanders in the presence of frog
tadpoles leads to the intensification of salamander predation on the frog
tadpoles.  In an additional laboratory experiment, to better
understand the underlying mechanisms, we manipulated both the presence of
frog tadpoles and the occurrence of cannibalism between salamander
hatchlings. This experiment revealed that frog tadpoles intensify the
cannibalism of salamander larvae during their hatchling stage, thus
allowing more salamander larvae to become large-sized predators. Our
results suggest that frog tadpoles can inadvertently intensify their own
future predation risk by intensifying cannibalistic interactions among
predatory salamander hatchlings, thereby enhancing the degree of predator
recruitment to a larger size class. Hence, large prey can enhance the
recruitment of individuals from small predator size classes into larger
predator size classes.