Food web framework for size-structured populations

Abstract

We synthesise traditional unstructured food webs, allometric body size scaling, trait-based modelling, and physiologically structured modelling to provide a novel and ecologically relevant tool for size-structured food webs. The framework allows food web models to include ontogenetic growth and life-history omnivory at the individual level by resolving the population structure of each species as a size-spectrum. Each species is characterised by the trait 'size at maturation', and all model parameters are made species independent through scaling with individual body size and size at maturation. Parameter values are determined from cross-species analysis of fish communities as life-history omnivory is widespread in aquatic systems, but may be reparameterised for other systems. An ensemble of food webs is generated and the resulting communities are analysed at four levels of organisation: community level, species level, trait level, and individual level. The model may be solved analytically by assuming that the community spectrum follows a power law. The analytical solution provides a baseline expectation of the results of complex food web simulations, and agrees well with the predictions of the full model on biomass distribution as a function of individual size, biomass distribution as a function of size at maturation, and relation between predator-prey mass ratio of preferred and eaten food. The full model additionally predicts the diversity distribution as a function of size at maturation.

Size-selective fishing drives species composition in the Celtic Sea

“…After each set of invasions, species that go extinct are removed. This method of gradually building up a foodweb resolves the marine foodweb modelling problem first described by Andersen and Ursin (1977): without sufficiently strong non-trophic intraspecific competition, empirically parametrized foodweb models with many species tend to be dynamically unstable (Andersen and Ursin, 1977;Loeuille and Loreau, 2005;Andersen and Pedersen, 2010;Hartvig et al, 2011). The PDMM assembly algorithm produces stable communities with inter-and intraspecific competition between consumers resulting only from trophic mechanisms that are well understood, namely resource-and consumer-mediated competition.…”

“…The first, the population-dynamical matching model (PDMM) of Rossberg et al (2008), was used to determine whether the observed relations between the LFI and the LSI relied on ecological mechanisms specific to the Celtic Sea fish community or on more general mechanisms that could be shared with other fish communities. The second, the fish community size-resolved model (FCSRM) of Hartvig et al (2011), was used to understand the observed intraspecific size structures and their responses to size-selective fishing, using generic model assumptions.…”

“…The FCSRM (Hartvig et al, 2011) extends the size-spectrum theory of Andersen and Beyer (2006) and represents a general temperate marine community of fish. Life-history parameters and trophic interactions between individuals are determined by individual body size and motivated by the observed strong sizedependence of life-history traits (Peters, 1983) and trophic level (Jennings et al, 2001(Jennings et al, , 2007 on body size.…”

“…A coupled system of these equations accounts for the dynamic balance between growth and mortality throughout the community, which in turn determines community biomass dynamics. Equations governing relationships between feeding rate and growth rate, and reproductive output and mortality rate, follow Hartvig et al (2011). Through these equations, the values of all rates emerge as properties of the organisms in their community context.…”

“…The smallest individuals feed on plankton, represented by a planktonic size spectrum, and larger individuals feed on smaller fish. The model is parametrized following Hartvig et al (2011), using empirical values for temperate marine ecosystems. As the model is used here to develop generic expectations for intraspecific size structure in fished communities, it has not been validated specifically for the Celtic Sea.…”