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Fire ecology
Fire ecology

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1.1 The effects of fire on an ecosystem

Given the 2020 wildfires on Australia and the obvious devastation they have caused, the perception of wild fires is often that they are destructive disturbances. However, wildfires are an important part of healthy ecosystems and play a key role in shaping the composition, structure and function of many ecosystems on Earth.

Activity 1  The effects of fire on an ecosystem

Timing: Allow 30 minutes for this activity.

Watch Video 1, which is the excerpt ‘Fire as an agent of perturbation’ from the BBC programme The Living Planet, Northern Forests to give you an introduction to fires as an agent of ecosystem disturbance. As you watch the video, think in particular about the effects of fire on:

  • the dominant kind of vegetation
  • the overall biodiversity.

You should also consider what effects the frequency of fire might have on:

  • the structural complexity of the habitat
  • the resistance and resilience of the habitat to fire, i.e. its susceptibility and the time taken to recover from fire respectively.

You should refer to the following notes and questions as you watch the video. You may like to use the text box below to record your notes.

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Video 1  Fire as an agent of perturbation, with Sir David Attenborough. Fullscreen mode is recommended (last button on right of video control bar).
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Sir David Attenborough introduces the excerpt by showing that there are two dominant types of tree in the woodland.

What are the two dominant types of tree in the woodland, and how do they differ in terms of their fire resistance?

Answer

They are Pinus spp. (pines) and Quercus spp. (oaks). It is pointed out that the Quercus seedlings are destroyed by fire, while the Pinus seedlings are more resistant to fire because they have a protective thatch of pine needles around the growing tips.

Quercus spp. trees are stronger competitors than Pinus spp. trees for light and nutrients.

What would happen to the dominant tree type in the woodland if fire was suppressed over many decades?

Answer

The woodland would come to be dominated by Quercus spp. trees.

Video 1 also reveals that litter composed of resin-rich pine needles is more combustible than fallen Quercus spp. leaves.

Given this fact, what would you expect to happen to the likelihood of fire starting as the time since the last fire increases?

Answer

The probability of fire starting would fall as the proportion of Quercus spp. leaves in the litter layer increased.

Conversely, in a pine-dominated woodland, the litter layer is dominated by pine needles, and the more pines there are, the more needles there are, and so the more likely it is that fires will start.

Satellite imagery now makes it possible to view the vast areas of the Earth that have been, and are currently, subjected to fire:

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Video 2  The locations of actively burning fires around the world (Please note this video has no audio.)
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The map shows the locations of actively burning fires around the world on a monthly basis, based on observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]  on NASA’s Terra satellite. The colours are based on a count of the number (not size) of fires observed within a 1,000-square-kilometer area. White pixels show the high end of the count — as many as 30 fires in a 1,000-square-kilometer area per day. Orange pixels show as many as 10 fires, while red areas show as few as 1 fire per day.

Many of these fires have, in recent times, been catastrophic, destroying lives, properties and infrastructure. In 2016 and 2017, large areas of Canada, the western United States, southern Europe, Greenland, Indonesia, Chile, Australia and South Africa were affected by wildfires and this trend is set to continue with rising global temperatures, something that will be returned to later in the course.

Although in some cases, the economic and social effects of wildfires can be devastating, as you saw in the previous video, fire is also an important natural ecological phenomenon, regulating and influencing community composition and ecosystem function in many parts of the world. At present, the type of community (e.g. whether grassland or forest) and the relative abundance of species present on over half the land’s surface is believed to be determined largely by the interaction of fire and climate.