Article: Durigan, G. et al. (2020) No net loss of species diversity after prescribed fires in the Brazilian savanna. Frontiers in Forests and Global Change, 3: 1-15. https://doi.org/10.3389/ffgc.2020.00013. Cover photo credit: Flickr – Fernando Tatagiba).
When thinking about fire, what is the first word that comes to mind? I can imagine that fire might be associated to words such as “camp fire,” “s’mores,” and “warmth.” If I add the environment to this scenario (Figure 1), would that increase your negative feeling towards fire, due to devastating wildfires in the past decade (see these previous Envirobites articles for more information: here and here)? However, you might be surprised to hear, or possibly marginally heard, that fire has been an instrumental ally (naturally or through humans) in maintaining a healthy habitat within certain vegetation types. Savannas (i.e., grassy plains with few trees) and grasslands (i.e., areas where vegetation is dominated by grasses) are highly dependent on seasonal fire to help them 1) propagate (e.g., fire can break the dormancy of seeds from various species), and 2) make space for grass, shrubs and small trees that might be out-competed by larger, invasive trees. This process is called “woody encroachment” and has been considered one of the biggest enemies to natural grasslands and prairies around the world. Not only are plants affected by this process, but various animals are also threatened by this vegetation shift. Some examples are grazing animals that feed on grasses and shrubs, and are not tall enough to eat leaves from trees (see this previous Envirobites post); or rodents will have less food sources and locations to hide if grasses are substituted by trees.
Fire: enemy or foe?
Even though the benefits of seasonal, controlled fires in these ecosystems are well known, there have been many resistances in some countries (especially in the tropics) that deem prescribed fires or fire containment (if it occurs naturally) as detrimental to natural systems. Indeed, fires that reach very dense forests can lead to catastrophes. That is why practices such as clearing forest floors of debris and thinning, or removing some trees, are both widely adopted to keep fires from spreading. However, completely suppressing fire from grassland and savannas can create a large imbalance that allows future fires to be hotter and spread more widely than they might otherwise. Hence, policies that inhibit fire suppression might lead to large biodiversity losses and vegetation shifts. To answer if fire indeed causes more damage than benefits, Dr. Giselda Durigan and collaborators quantified the effects of pre- and post-fire in plant and animal species occurrence in a Brazilian savanna and grassland (called collectively as “Cerrado” in Portuguese – Figure 2).
The authors monitored the number of plants (grasses, forbs, shrubs, sub-shrubs, and trees) and animals (ants, frogs, lizards, birds, and small mammals) pre- and post-prescribed fire. Small areas of forest, called experimental plots, were setup in savanna and grassland type of vegetation to see if the effects of fire was more significant in one than the other. The plots were not burned for at least 4 years before the experiment started. They were then burned once every year for three years. Post-fire plant and animal counts were done 6 months to one year after the last prescribed fire.
Surprisingly, burning did not significantly reduce species richness (i.e., number of species) of any of the groups analyzed, but had a slightly more positive effect on richness of grass species in grassland. Similarly, animal and plant abundance (i.e., number of individuals) within each species also did not show significant effects from the fire, with only number of frogs significantly reducing in grasslands. Furthermore, the overall differences between grasslands and savannas’ performance after burning showed that the number of forbs, grasses and subshrub plants were higher in grasslands than in savannas, which saw a decline. In summary, grassland species benefitted more from the fire than savanna species, although the latter was very little negatively affected. Authors’ attribute rapid re-propagation of savanna and grassland plant species, and need for feeding and strategies to hide from fire of certain animals (e.g., burrowing in the ground) as possible reasons for the abundance and richness not significantly varying after burning. Hence, confirming that even after such a stressful and destructive phenomenon that these species are highly resilient and can recover completely after a couple of years.
Better policies for healthier savannas
Even though this study was conducted in Brazil, these findings can be easily extrapolated to other savannas and grasslands in the world as they are under very similar climatic regimes and stressors. Hence, even though fire can be destructive if not handled appropriately, when it is applied and controlled in a responsible manner it can be beneficial in allowing a biome to maintain healthy and numerous populations of various plant and animal species. Therefore, better policies should be implemented so that 1) seasonal fire prescription is allowed, and 2) encourage and train for better practices to contain wildfires (not suppression – Figure 3) so that the biodiversity of these biomes can be maintained as land cover and climate change increase in the upcoming decades.