Fire Response Strategies Open Coexistence Opportunities in Fire-prone Heathlands

Fire is known to be an important factor in many ecological systems [Whelan 1995]. In plant communities, the combined effects of opening up space, burning biomass, and releasing nutrients to the soil offer a variety changing environmental conditions. When burns are relatively frequent, and a stable burning regime has persisted on evolutionary time scales, many species develop specific responses to fires [Wisheu et al. 2000]. Previous theory has shown that, when species respond differently to environmental fluctuations, coexistence can be promoted by certain types of interactions between environmental and competitive factors [Chesson 2000a]. In this paper, we demonstrate how response strategies to fire can form an axis of differentiation, allowing species to stably coexist. Essentially, the presence a frequent fire burning regime can generally provide a stabilizing (sensu [Chesson 2000b]) effect for the coexistence of species. Shrub communities in mediterranean heathlands, such as the Fynbos of South Africa, and the Kwongan of western Australia, display high levels of plant diversity, as well as at least two distinct fire response strategies [Wisheu et al. 2000]. In this instance, we refer to a species’ reaction to the set of conditions brought about by burning as a fire response strategy. In nature, we find many subtle fire response strategies [Wisheu et al. 2000], but we consider here only two, which are picked to represent opposing ends of the observed spectrum. Some species, known as ‘sprouters’, are not killed even in severe burns, and are able to re-sprout from surviving below ground structures, such as lignotubers, or epicormic buds. Other species, known as ‘seeders’ will die in a fire, but trigger the release of a seed bank stored in the canopy, often in the form of serotinous cones. We develop a mathematical model of the system which demonstrates how the action of frequent burning can allow a stable coexistence of two species which employ these opposing strategies.