Fire life histories and the seeds of chaos

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Abstract
Changes in plant population size are usually attributed to exogenous factors such as drought or herbivore outbreaks. However, in theory, populations may oscillate from endogenous density-dependent causes alone. Under some conditions these oscillations are very irregular and are called chaos. There are very few examples of endogenous oscillations in plants and these are controversial. Large fluctuations in population size occur after fires in many species of Proteaceae in South African and Australian shrublands. These non-sprouting species form single-aged cohorts with recruitment dating from the last fire. Variation in cohort size has usually been attributed to exogenous factors, especially the length of the fire interval, fire season, and post-burn weather conditions. We studied the importance of endogenous density-dependence in generating population oscillations in these proteoid shrubs. Using a variety of approaches, we show that density-dependent reduction in fecundity may lead to endogenous oscillations in some species. We explore the “seeds of chaos” by defining attributes of species and populations most likely to generate endogenous oscillations. Finally, we note the implications of the phenomenon for research in, and management of, these systems.