Fire as a global ‘herbivore’: the ecology and evolution of flammable ecosystems

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It is difficult to find references to fire in general textbooks on ecology, conservation biology or biogeography, in spite of the fact that large parts of the world burn on a regular basis, and that there is a considerable literature on the ecology of fire and its use for managing ecosystems. Fire has been burning ecosystems for hundreds of millions of years, helping to shape global biome distribution and to maintain the structure and function of fire-prone communities. Fire is also a significant evolutionary force, and is one of the first tools that humans used to re-shape their world. Here, we review the recent literature, drawing parallels between fire and herbivores as alternative consumers of vegetation. We point to the common questions, and some surprisingly different answers, that emerge from viewing fire as a globally significant consumer that is analogous to herbivory.

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Parallels between fire and herbivory

Ecologists and biogeographers generally assume that plant distribution, abundance and, therefore, community composition, structure and biomass, are determined largely by climate and soils. This is implicit in current attempts to model species range shifts in response to climate change [1]. However, nearly 50 years ago, Hairston et al. [2] suggested that the properties of ecosystems are instead determined by the regulation of herbivores by predators. In the absence of predators, herbivore

Fire and consumer control of ecosystems

Polis [3], in a review of the ‘green world’ hypothesis, argued that terrestrial vegetation is determined largely by climate, locally modified by low-nutrient soils, with consumer control by herbivores sometimes occurring but being localized in space and time. How can the global importance of consumers (herbivores and fires) versus resources (climate and soils) in shaping vegetation be evaluated? A useful alternative to meta-analyses of experimental studies (often limited in space, time,

Fire and consumer control of species composition

Trophic cascades, measured as large changes in species composition, are an expected consequence of predator removal in ecosystems where consumers have the potential to proliferate in their absence. Although evidence for trophic cascades in terrestrial ecosystems is disputed [7], cascading changes in species composition are commonplace where fire is the consumer. For example, in tropical forests, a single fire can reduce woody plant richness by a third to two-thirds depending on fire severity

The variable nature of fire as a consumer control

Flammable ecosystems include boreal forests, eucalypt woodlands, shrublands, grasslands and savannas. Why, if fire is such an influential consumer, is there such a diversity of growth form mixtures in flammable ecosystems? Fire ecologists have looked first to the diversity of fire regimes for answers. A fire regime includes the patterns of frequency, season, type, severity and extent of fires in a landscape (Box 1). Vegetation consumed and patterns of fire spread vary across landscapes, and

Fire and community assembly

Hairston et al. [2] predicted relatively little competition between plants where herbivores proliferate in the absence of predators, because plant growth would be limited more by consumption than by resources. Instead, community assemblages would comprise species that are best able to persist and thrive in the face of repeated defoliation. One of the striking features of the fire ecology literature is that there are many studies on life-history traits that enable the persistence of species in a

Fire as an evolutionary agent

There are few studies of the evolution of fire-adaptive traits, and many plant traits have been uncritically labeled as ‘fire adaptations’ without any rigorous analysis either as to the functional importance of the trait, or its phylogenetic origin. For example, post-burn sprouting is often seen as a ‘fire adaptation’, but sprouting per se is a widespread trait in angiosperms. Evolutionary interpretations of the loss or gain of sprouting in different fire regimes make no sense without

Fire and the origin of biomes

The large area occupied by flammable biomes, especially in the tropics and sub-tropics, has often been attributed to anthropogenic burning. Although anthropogenic fires have extended areas of flammable vegetation 48, 49, there is now evidence that natural fires occurred long before humans and that flammable ecosystems pre-date anthropogenic burning by millions of years 4, 50. Stable isotope evidence shows that C4 grassy ecosystems, the most extensive flammable formation worldwide, first

Conclusions

We have shown major similarities between fire and herbivory and argued for a more-inclusive view of top-down, or, in this instance, consumer control of biomes. We believe that the global extent of fire as a consumer, its many parallels with herbivory, its role in selecting for particular plant traits and in the evolution of biomes, is worthy of much wider attention from ecologists.

There is an added incentive for greater understanding of fire as a globally important consumer. Climate change,

Acknowledgements

We thank Jeremy Midgley, C.J. Fotheringham, Ian Woodward, Guy Midgley, Ross Bradstock, David Keith, Dave Bowman, Malcolm Gill, Alan Andersen and Herve Fritz for useful discussions. Jeremy Midgley and anonymous reviewers provided useful comments on the article.

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