It's (ecologically) disturbing!

Lightning is one cause of ecological disturbances. Photo credit: Axel Rouvin via wikimedia commons

Lightning is one cause of ecological disturbances. Photo credit: Axel Rouvin via wikimedia commons

Disturbance comes up often in my work, which in the ecological sense refers to an event that changes an ecosystem in some way. Some examples are a hurricane, a pest outbreak, a fire, a tree falling down, and a timber harvest.

This comes up a lot because my research project involves New England cottontail habitat, which is young, brushy vegetation, which in many areas wouldn’t exist without disturbances. 

We don’t like change, and we don’t like to see things die, so naturally it is hard to reconcile the idea that fires, devastating hurricanes, and other events are required for some organisms to survive. 

The Northeast United States before humans is often imagined as a pristine old-growth forest, but this cannot be entirely true. There are many plants and animals that are native to the area that only grow in places that have recently been disturbed. For example, some trees require a lot of light, so they can’t establish themselves on the forest floor that is shaded by the adult trees. They must find their way to areas where the trees have fallen over, been removed, or burned.

New England cottontails require shrubs and young trees, which cannot grow in the shade of a mature forest. They also evolved here, so their preferred habitat must have been here for a long time.

 

A (grossly oversimplified) illustration of the effect of a disturbance, in this case a fire, on a forest. The number of species that occupy the site over time is higher because of the changes the site goes through. Image credit: Alena Warren

A (grossly oversimplified) illustration of the effect of a disturbance, in this case a fire, on a forest. The number of species that occupy the site over time is higher because of the changes the site goes through. Image credit: Alena Warren

Here’s another good reason that I got comfortable with disturbance:

The Intermediate Disturbance Hypothesis curve. Very high and very low disturbance frequency/intensity is associated with low species diversity. A moderate disturbance frequency/intensity is associated with the highest amount of species diversity. Image credit: Alena Warren

The Intermediate Disturbance Hypothesis curve. Very high and very low disturbance frequency/intensity is associated with low species diversity. A moderate disturbance frequency/intensity is associated with the highest amount of species diversity. Image credit: Alena Warren

There is a theory called the  Intermediate Disturbance Hypothesis  that suggests that a certain amount of disturbance maximizes species diversity. Without enough disturbance, a few species can out-compete the others because they are most adapted to the conditions- which never change. With too much disturbance, the reproductive success of all species is compromised.  The amount of disturbance that is just right provides opportunities and niches for many species.

 

The way we manage the land can sometimes interfere with natural disturbances, fire suppression being the obvious example. We also mimic disturbances to bring about certain plant communities that we might want, for wildlife habitat or for other reasons. Additionally, we inadvertently cause disturbances as a result of economic activities like timber harvesting, agriculture, etc.

Different disturbances bring about different changes. For example:

A hurricane blows over many large trees but usually not all of them, and it leaves the dead trees on the ground. This provides debris for new plants to grow in as well as animals and fungi. Smaller trees that weren’t destroyed by the hurricane have a chance to replace the big trees that were damaged or blown over.  Northern hardwood forests can deal with hurricanes. 

Human activities, like timber removal operations, are different. Unlike a hurricane, the wood is removed. There have been several studies looking at the ways organisms react differently to natural vs. anthropogenic disturbances. Hocking et al found that patch cuts and clearcuts are associated with lower densities of salamanders than undisturbed forest and forests damaged by ice storms [1]. Another study determined that some insect communities respond differently to fires and clearcuts, and some fauna that occupy forests post-fire are not found in forests post-clearcut [2]. 

A clearcut. Photo credit: TJ Watt via wikimedia commons

A clearcut. Photo credit: TJ Watt via wikimedia commons

So disturbances can be:

  • Natural or anthropogenic
  • An important vector for change, that sustains biodiversity
  • A tool that we use to create certain communities of organisms (e.g. create wildlife habitat)
  • A detriment to ecosystems if they are too severe or frequent 

References

1. Hocking, D. J, Babbitt, K., & Yamasaki, M. (2013). Comparison of silvicultural and natural disturbance effects on terrestrial salamanders in northern hardwood forests. Biological Conservation, 167.  

2. Buddle, C., Langor, D., Pohl, G.,  Spence, J. (2006) Arthropod responses to harvesting and wildfire: Implications for emulation of natural disturbance in forest management. Biological Conservation 128:3.