SUPER-INDIVIDUALS A SIMPLE SOLUTION FOR MODELING LARGE POPULATIONS ONAN INDIVIDUAL BASIS

Modelling populations on an individual-by-individual basis has proven to be a fruitful approach. Many complex patterns that are observed on the population level have been shown to arise from simple interactions between individuals. However, a major problem with these models is th at the typically large number of individuals needed requires impractic ally large computation times. The common solution, reduction of the nu mber of individuals in the model, can lead to loss of variation, irreg ular dynamics, and large sensitivity to the value of random generator seeds. As a solution to these problems, we propose to add an extra var iable feature to each model individual, namely the number of real indi viduals it actually represents. This approach allows zooming from a re al individual-by-individual model to a cohort representation or ultima tely an all-animals-are-equal view without changing the model formulat ion. Therefore, the super-individual concept offers easy possibilities to check whether the observed behaviour is an artifact of following a limited number of individuals or of lumping individuals, and also to verify whether individual variability is indeed an essential ingredien t for the observed behaviour. In addition the approach offers arbitrar ily large computational advantages. As an example the super-individual approach is applied to a generic model of the dynamics of a size-dist ributed consumer cohort as well as to an elaborate applied simulation model of the recruitment of striped bass.