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Some disturbance can be beneficial, but too much causes ecosystem unsustainability.

Disturbance

An avalanche of mud crashes down a hillside, already dotted with the scars of previous slips. Rocks and muddy debris sluice through the village, pushing aside coconut palms and other plants, before plunging into the ocean. How could a disturbance like this be beneficial?

Disturbance upsets the normal structure and function of an ecosystem. This happens on both the large and small scale. Examples of natural disturbance events include:

  • Storms
  • Earthquakes
  • Fire
  • Impacts of herbivores or predators

Surprisingly, some disturbance can be beneficial, but too much erodes an ecosystem's ability to maintain itself sustainably. Ecosystem sustainability depends on a balance between too much and not enough disturbance.

 Disturbance is good?

The right amount of disturbance can help maintain ecosystem diversity by creating areas where organisms that might otherwise be overgrown can live.

Some organisms prefer to grow in disturbed areas, and are first to colonize after a disturbance. This is the first step in repairing ecosystem damage.

These colonizing organisms may be overgrown by other organisms. This next group may also be overgrown and so on. Recovery from disturbance can be thought of as a succession of organisms changing the disturbed area until it resembles undisturbed habitat. However, subsequent disturbance can restart the process before it completes.

Diversity in an ecosystem increases if a series of disturbance events create a patchwork of areas at different stages of recovery.

Not enough disturbance lowers diversity because organisms that like disturbed areas are usually overgrown by others under settled conditions. If there are no disturbed areas for these organisms they will be overgrown and lost from the ecosystem.

While disturbance can increase diversity, too much lowers it.

 Disturbance is bad?

Disturbance events can kill and/or displace organisms, and disrupt the ecosystem. Some species cannot tolerate much disturbance and go extinct if an ecosystem is disturbed too much. This lowers the diversity of the ecosystem.

Diversity loss can feed back on itself because the ability of an ecosystem to resist or recover from disturbance (its resilience) depends on diversity. If disturbance lowers ecosystem resilience then the chance of losing more diversity increases.

Disturbance can also lower productivity, because a diverse ecosystem is normally more productive than one that has lost diversity.

 Disturbance and people

Many of our actions add disturbance to ecosystems. Examples include:

  • global warming
  • over-fishing
  • pollution
  • coastal development
  • ocean acidification
  • invasive species

These activities can add to the level of disturbance in the ecosystem, and too much can tip the balance from ecosystem sustainability to unsustainability.

 Conclusion

Diversity is maximized if a balance between too much and too little disturbance is maintained. Some disturbance is beneficial, but too much lowers diversity, productivity and resilience. Our actions can add disturbance to an ecosystem and tip from sustainable into unsustainable.

 References

Cardinale, B. J. et al. (2005). Diversity-productivity relationships in streams vary as a function of the natural disturbance regime. Ecology 86(3): 716-726.

Connell, J. H. 1978. Diversity in tropical rain forests and coral reefs. Science 199:1302-1310.

Horn, H. S. (1976). Succession. In Theoretical Ecology: Principles and applications (May, R. M. ed.). Saunders: Philadelphia.

Moral, R. & Walker, L. R. (2007). Environmental disasters, Natural recovery and human responses. Cambridge University Press: New York.

Potter, C., Tan, P., Klooster, S. & Kumar, V. (2006, March 21). Background on Disturbances. NASA-CASA Project. Retrieved 30 April 2008 from http://geo.arc.nasa.gov/sge/casa/background.html

Wikipedia. (2003, September 20). Ecological succession. Wikipedia. Retrieved 30 April 2008 from http://en.wikipedia.org/wiki/Ecological_succession

Wikipedia. (2005, June 6) Intermediate disturbance hypothesis. Wikipedia. Retrieved 30 April 2008 from http://en.wikipedia.org/wiki/Intermediate_Disturbance_Hypothesis


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