Physiologically structured models - from versatile technique to ecologicaltheory

A ubiquitous feature of natural communities is the variation in size that c an be observed between organisms, a variation that to a substantial degree is intraspecific. Size variation within species by necessity implies that e cological interactions vary both in intensity and type over the life cycle of an individual. Physiologically structured population models (PSPMs) cons titute a modelling approach especially designed to analyse these size-depen dent interactions as they explicitly link individual level processes such a s consumption and growth to population dynamics. We discuss two cases where PSPMs have been used to analyse the dynamics of size-structured population s. In the first case, a model of a size-structured consumer population feed ing on a non-structured prey was successful in predicting both qualitative (mechanisms) and quantitative (individual growth. survival, cycle amplitude ) aspects of the population dynamics of a planktivorous fish population. We conclude that single generation cycles as a result of intercohort competit ion is a general outcome of size-structured consumer-resource interactions. In the second case, involving both cohort competition and cannibalism, we show that PSPMs may predict double asymptotic growth trajectories with indi viduals ending up as giants. These growth trajectories. which have also bee n observed in field data, could not be predicted from individual level info rmation, but are emergent properties of the population feedback on individu al processes. In contrast to the size-structured consumer-resource model, t he dynamics in this case cannot be reduced to simpler lumped stage-based mo dels, but can only be analysed within the domain of PSPMs. Parameter values used in PSPMs adhere to the individual level and are derived independently from the system at focus, whereas model predictions involve both populatio n level processes and individual level processes under conditions of popula tion feedback. This leads to an increased ability to test model predictions but also to a larger set of variables that is predicted at both the indivi dual and population level. The results turn out to be relatively robust to specific model assumptions and thus render a higher degree of generality th an purely individual-based models. At the same time. PSPMs offer a much hig her degree of realism, precision and testing ability than lumped stage-base d or non-structured models. The results of our analyses so far suggest that also in more complex species configurations only a limited set of mechanis ms determines the dynamics of PSPMs. We therefore conclude that there is a high potential for developing an individual-based, size-dependent community theory using PSPMs.