Intraspecific competition and spatial heterogeneity alter life history traits in an individual-based model of grasshoppers*1

Abstract

An individual-based model (IBM) was developed to examine the effects of intraspecific competition and spatial structuring of food on life history traits of grasshoppers inhabiting temperate-zone grasslands. Each individual carried real-valued genes which determined size of offspring and size at maturity, and which were passed on to its offspring. Size at maturity was a plastic trait, depending on an individual’s growth rate, whereas size of offspring was a fixed trait. Individuals with more successful combinations of traits produced more offspring that eventually came to dominate the population. Populations were food limited and intraspecific competition was either exploitative or size-based interference. Growth rates and fecundity depended on food quality, which declined within season as a function of day of year and the proportion consumed by grasshoppers. Three different spatial distributions of food quality were examined: uniform, random, and clumped. Optimal egg size was larger under interference competition and spatially clumped resources. Reaction norms of size at maturity were strongly affected by type of competition, and, by spatial distribution of resources within exploitative competition, but not under interference competition. The IBM shows promise as a means of analyzing life history evolution in grasshoppers in relation to processes that arise from localized interactions between individuals.