Functional responses and ecosystem dynamics: how clearance rates explain the influence of satiation, food-limitation and acclimation

Modellers have long been aware that the mathematical form of zooplankton mortality, or closure, significantly affects the dynamics of planktonic ecosystem models. Another important formulation is the functional response, i.e. how ingestion rates change with prey density. Here we explain why different grazing responses can have profoundly differing influences on modelled dynamics, and how common practices may limit models due to misguided characterization of feeding behaviours. Use of different ingestion functions in a Nutrient-Phytoplankton-Zooplankton (NPZ) model results in oscillating versus constant densities. Contrary to the conclusions of previous studies, it is shown that these results are not due to zooplankton satiation versus non-satiation. Analysis of a predator-prey model is used to derive the necessary condition for ecological stability, which is related to food-limited clearance rates. Sensitivity studies demonstrate that zooplankton clearance rates have a strong influence on the dynamics of more complex models. Moreover, it is shown that acclimation time lags can dramatically alter results from those where zooplankton instantly adapt to changing prey densities due to the corollary effect on clearance rates. These results are discussed in terms of practical advice to modellers who face uncertainty in choosing expressions for the functional response.