Smokey the Bear and Theoretical Discussion of Fires in the Prairie Ecosystem

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Smokey the Bear taught us that fires are bad. Period. The longest running public service advertising campaign, Smokey posters have been calling for the prevention of wildfires since 1944. Fires, though, have been a natural part of all ecosystems for uncountable years; human regulation of fire has often removed this important contributor to maintaining ecosystem health.

In tall grass prairies, fire contribues to biodiversity. By disturbing established community dynamics and productivity of individual species, fires modify the distribution of biomass and energy in ecosystems. In other words, fires change the local conditions of growth, allowing new species to recolonize the disturbed, burned regions and modifying species composition. Natural fires would occur at different frequencies and intensities and follow Connell's intermediate disturbance hypothesis demonstrated below:

Figure from Connell's "Diversity in Tropical Rain Forests and Coral Reefs" (1978)

His idea is that intermediate disturbance allows a mixture of colonizing species (the species that appear after a disturbance) and the best competitors (those that dominate communities long after disturbances), which promotes a high level of species diversity (represented as B in the diagram). Frequent disturbances only allow colonizers to flourish (represented as A) while infrequent disturbance allow the best competitors to dominate (represented as C).

Much research has been focused on understanding how fire acts as a disturbance. How does fire modify an ecosystem to promote biodiversity?

Hulbert (1969) demonstrated that the most important effect of fire is the removal of litter. Removing the layer of dead plant material has many implications. First, it allows a larger amount of sunlight to reach the ground, which should warm and dry the soil. This should promote the growth of species that are better suited to dry climates. By removing the litter, new shoots should have less competition and more sunlight. Therefore, burn plots should also have a higher amount of biomass, but this biomass should mostly consist of species with high growth rates (Connell's colonizers). Therefore, burning should allow grasses to infiltrate areas once dominated by forbs (better competitors). This prediction were confirmed by research by Abrams et al. (1986).

Therefore, fire promotes the growth of grasses while decreasing form dominance. How does this promote biodiversity? Following Connell's intermediate disturbance hypothesis, intermediate levels of fire will allow for a mix of colonizers and competitors (grasses and forbs). This leads us to an interesting question: What types of fires qualify as intermediate disturbances? Yearly? Bi-yearly? Understanding the mechanisms by which fire acts as an intermediate disturbance is important for conservation and reconstruction of tall grass prairies.

Sources used:

1. Abrams, Marc D., Knapp, Alan K., Hulbert, Lloyld, C. 1986. A Ten-Year Record of Aboveground Biomass in a Kansas Tallgrass Prairie: Effects of Fire and Topographic Positions. American Journal of Botany 73: 1509-1515.

2. Campaign History. Smokey Bear. http://www.smokeybear.com/vault/history_main.asp. 8 May 2014.

3. Connell, Joseph H. 1978. Diversity in Tropical Rain Forests and Coral Reefs. Science 199: 1302-1310.

4. Hulbert, Lloyd C. 1969. Fire and Litter Effects in Undisturbed Bluestem Prairie in Kansas. Ecology 50: 874-877.

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