Predator cannibalism can shift prey community composition toward dominance by small prey species

Cannibalism among predators is a key intraspecific interaction affecting
their density and foraging behaviour, eventually modifying the strength of
predation on heterospecific prey. Interestingly, previous studies showed
that cannibalism among predators can increase or reduce predation on
heterospecific prey; however we know less about the factors that lead to
these outcomes. Using a simple pond community consisting
of Hynobius retardatus salamander larvae and their
associated prey, I report empirical evidence that cannibalism among
predators can increase predation on large heterospecific prey but reduce
that on small heterospecific prey. In a field-enclosure experiment in
which I manipulated the occurrence of salamander cannibalism, I found that
salamander cannibalism increased predation on frog tadpoles but reduced
that on aquatic insects simultaneously. The contrasting effects are most
likely to be explained by prey body size. In the study system, frog
tadpoles were too large for non-cannibal salamanders to consume, while
aquatic insects were within the non-cannibals’ consumable prey size range.
However, when cannibalism occurred, a few individuals that succeeded in
cannibalising reached large enough size to consume frog tadpoles.
Consequently, although cannibalism among salamanders reduced their
density, salamander cannibalism increased predation on large prey frog
tadpoles. Meanwhile, salamander cannibalism reduced predation on small
prey aquatic insects probably because of a density reduction of
non-cannibals primarily consuming aquatic insects. Body size is often
correlated with various ecological traits, for instance, diet width,
consumption and excretion rates, and is thus considered as a good
indicator of species’ effects on ecosystem function. All this considered,
cannibalism among predators could eventually affect ecosystem function by
shifting the size composition of the prey community.