Ontogenetic niche shifts and flexible behaviour in size-structured populations

Flexible behavior has been shown to have substantial effects on population dynamics in unstructured models. We investigate the influence of flexible behavior on the dynamics of a size-structured population using a physiologically structured modeling approach. Individuals of the size-structured population have a choice between living in a risky but profitable habitat and living in a safer but less profitable habitat. Each of the two habitats houses its own resource population on which the individuals feed. Two types of flexible behavior are considered: discrete habitat shifts, in which individuals instantaneously and nonreversibly shift from living in the safe habitat to the more risky/profitable habitat, and continuous habitat choice, in which individuals can continuously adapt their habitat choice to current resource/mortality conditions. We study the dynamics of the model as a function of the mortality risk in the risky/profitable habitat. The model formulation and parameterization are derived using data on Eurasian perch (Perca fluviatilis) and describe reproduction as a yearly event at the beginning of summer, while all other processes are continuous in time. The presence of two habitats per se, with unique resources that are shared among all consumers, does not change model dynamics, when compared to the one-resource situation. Flexible behavior increases the range of mortality levels for which the population can persist, because it allows individuals to hide from high mortality in the risky habitat. In contrast, flexible behavior does not significantly change the dynamics for mortality risks, where the consumer population also persists without it. Discrete habitat shifts result in model dynamics that are largely similar to the dynamics observed with continuous habitat choice, as long as individuals strongly respond to small differences in habitat profitability. In these cases, consumers spend an increasing part of their first year of life in the safe habitat, when mortality risks in the risky habitat increase. Ultimately, consumers are driven out into the risky habitat by intercohort competition from their successive year class. Therefore, major mortality and rapid growth occur among 1-yr-old individuals. Younger individuals exhibit retarded growth due to intracohort competition in the safe habitat, which may also induce large-amplitude fluctuations when the mortality risk is high in the risky habitat. With continuous habitat choice and a low responsiveness to habitat profitability, consumer persistence is increased as well, but large-amplitude fluctuations are absent. In this case, consumers always spend a significant part of their first year of life in the risky habitat, even at high mortality risks. Major mortality and rapid growth occur among individuals younger than 1 yr, while the shift to the risky habitat is mainly induced by intracohort competition for resources. The high mortality and rapid growth at younger ages lead to an increase in maximum size and fecundity of surviving individuals, as well as to larger total population biomasses. We argue that the pattern of individual habitat use is mainly determined by population feedback on resource levels.