Alternative resources for apex predators limit their population growth and top-down control of their prey

Background/Question/Methods

Catastrophic collapses of apex predators have revealed how their top-down control of prey structures marine ecosystems through trophic cascades. After collapsing many of these predator populations fail to recover. For Atlantic cod the lack of recovery is often claimed to result from competition for zooplankton between prey fish species and juvenile stages of cod, which is increased due to the decreased predation pressure on the prey populations. This explanation is referred to as the cultivation/depensation theory.

The cultivation/depensation theory only considers a zooplanktivorous and piscivorous life-stage in cod, while it is known that alternative, benthic resources are an important component of its diet. After a phase of zooplanktivorous feeding, cod switches to benthic resources, and subsequently expands its foraging to include piscivory. Here, we explicitly model three ontogenetic foraging stages in cod: zooplanktivory, benthivory, and combined benthivory/piscivory. Using a physiologically structured population model, which accounts for size-dependent processes and interactions between cod and its prey species, we analyze the effect of an additional benthic resource for the occurrence of cultivation effects and on coexistence dynamics with prey (trophic cascades).

Results/Conclusions

Our analysis shows that additional resources for predators may profoundly influence predator-prey community structure: Firstly, including a benthic resource for cod results in regulation of its population through a growth bottleneck in the intermediate life-stage. Competition between zooplanktivorous cod and prey species did not change this outcome. Release from competition with prey for zooplankton can then never lead to increased predator growth capacity, which is the key prediction from the cultivation/depensation theory.

Secondly, top-down control of prey only occurs when the benthic resource represents a constant food source, independent from the feeding impact of the predator. When this feedback is accounted for, single cohort dynamics in cod prevents the build-up of high predator abundances and top-down control of prey. Addition of a benthic resource for cod can thus preclude a trophic cascade.

Thirdly, when piscivory is essential for maturation, cod has reduced potential to recover from low population densities, because at low densities and high benthic productivity growth through the intermediate life-stage is faster. As a consequence cod needs suitably sized prey in a period when these are not sufficiently available. This timing mismatch between prey and predators hinders cod re-establishment.