Intraspecific diversity in Atlantic cod drives divergent trophic cascades across a coastal food web

Intraspecific diversity can profoundly shape ecological interactions, yet
its role within complex food webs remains challenging to quantify. We
explored how eco-evolutionary divergence within Atlantic cod (Gadus
morhua), specifically between the resident Fjord and migratory North Sea
ecotypes, influences food web structure and function in a coastal marine
ecosystem. Using key trait differences, including growth rate, age, and
size at maturation, reproductive investment, and ontogenetic dietary
shifts, we parameterized an allometric trophic network (ATN) model with
empirical data from the Norwegian Skagerrak. The high-resolution network
comprised 148 trophic guilds and 2054 feeding links, permitting resolution
of ecotype-specific interactions. We found pronounced intraspecific
divergence: North Sea cod reached higher trophic position earlier,
exerting greater interspecific influence, whereas Fjord cod retained
higher early-stage biomass, intensifying intraspecific competition between
ecotypes. Together, these complementary roles indicate that variation
within a single species can both dampen and amplify food web interactions
across life stages, enhancing ecosystem connectivity, and triggering
cascading ecological effects. To sustain this natural buffer, conservation
and management must move beyond species-level metrics and actively protect
intraspecific diversity.