Warming strengthens food web effects of predator phenotypic variation

Intraspecific variation modifies ecological processes and ecosystem
functioning. Still, we know relatively little of how the nature and
strength of ecosystem effects caused by intraspecific variation may
interact with climate change. We conducted a mesocosm experiment to test
if, and to what extent, ocean warming modifies the ecological impacts of
intraspecific variation in a predatory fish. The mesocosms consisted of a
simplified coastal food web with threespine stickleback (Gasterosteus
aculeatus) as the top predator, from a population where two stickleback
phenotypes with either complete or incomplete lateral armour plating
coexist and display differentiated predation behavior: The completely
plated phenotype often feeds more on invertebrate herbivores compared to
the incompletely plated phenotype. Presence of stickleback reduced biomass
of arthropod shredders (crustaceans, insect larvae). Warming (+4 °C)
strengthened this predation, releasing benthic primary producers (diatoms)
from top-down control, causing a trophic cascade. This trophic cascade was
attributed to one of the plate phenotypes: the completely plated
stickleback increased their predation on shredders under warming, while
the incompletely plated stickleback instead decreased their predation.
Diatom biomass responded accordingly: warming increased diatom biomass in
the presence of completely plated stickleback but not when incompletely
plated stickleback were present. Our results suggest that different plate
phenotypes of threespine stickleback differentially affect lower trophic
levels, and that warming may exacerbate these cascading effects. These
trait-dependent effects on trophic cascades highlight the consequences of
intraspecific variation on ecosystem functioning.