Presenter: Anne Willem Omta (MIT)

Description:
There exists considerable uncertainty about the most appropriate functional form to describe mortality at the highest trophic level in marine ecosystem models. Although linear and quadratic formulations predict strongly different dynamics, it is unclear which of these formulations is more realistic. We introduce an implicit predator population feeding on zooplankton that we parameterize using commonly observed predator-prey scaling relations. Thus, we arrive at a hyperbolic zooplankton mortality formulation that is a hybrid between the linear and quadratic forms. This hybrid closure can be applied to ecosystem models with any number of explicitly represented trophic levels. To investigate the stability properties, we have performed a bifurcation analysis of a Nutrient-Phytoplankton-Zooplankton (NPZ) model with the hyperbolic formulation for zooplankton mortality. A salient result is that limit cycles are primarily associated with a top-heavy ecosystem structure (high zooplankton, low phytoplankton), rather than with nutrient enrichment. We discuss the mechanism behind the relationship between top-heaviness and instability from both a mathematical and an ecological perspective.