Ecology Explained: The Intermediate Disturbance Hypothesis

The Intermediate Disturbance Hypothesis is one of the staples of ecology, especially marine ecology. The Intermediate Disturbance Hypothesis was first proposed by Connell, a well-known intertidal and general ecologist, in 1978 (See his article “Diversity in Tropical Rain Forests and Coral Reefs”). But what is it exactly? Let me explain!

Let’s start with this handy diagram from Wikipedia.

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As you can see, we have Level of Disturbance on the x-axis. That simply describes the level of disturbance present in the system. It could be any sort of disturbance…fires, hurricanes, waves, human or animal trampling, wind, sun, and so on. The level of disturbance increases from left to right. So, the area marked with I has less disturbance than area II.

Species Diversity on the y-axis is more of a general term. Sometimes this is “species richness,” which is a pure count of species present in the ecosystem. Sometimes this is actually referring to “species diversity,” which takes the number of species from species richness and combines it with how the species are distributed in the system. But that’s a subject for another blog post! All you need to know now is that species diversity in the system increases from the bottom to the top.

Now let’s look at those areas marked by Roman numerals.

Area I is first up. In that section, we have a low amount of disturbance, which results in an okay amount of diversity. Why is that?

Ecosystems typically have a successional pathway–a pattern across time of species that are present. Think about a forest directly after a forest fire. Early-regrowth forests are going to look totally different from established forests! Those established forests will most likely have one or more competitive dominants–the species that compete the best. That’s great for the competitive dominants because they do well in those established systems, but it’s not so good for diversity. The competitive dominants compete so well that there isn’t much room for other species.

(Sometimes you will see similar graphs that are a simple bell curve with Area I showing the same low, low diversity that Area III exhibits. This can also happen–if there is no disturbance in the system at all, the species diversity is going to be extremely low. But, from my perception, it’s probably not going to get as bad as Area III. It’s all up to interpretation.)

Area II looks great! There is an intermediate amount of disturbance and the maximum amount of diversity. In Area II, there is enough disturbance in the system to stop the competitive dominants from over-dominating. Organisms earlier in the successional pathway that are poorer competitors (but still play an important role) are able to survive. This results in the maximum amount of species diversity! Hence the name “Intermediate Disturbance Hypothesis!” This area is experiencing an intermediate amount of disturbance, but not enough to push it to Area III.

Area III is not a good place to be. Area III exhibits a very high amount of disturbance and a low amount of diversity. It’s pretty easy to understand why! Think of a coral reef that’s constantly being battered by huge hurricanes or a forest that has repeated, huge fires. The succession pathway barely even gets a chance to begin before another huge disturbance sweeps through. This will result in very low diversity.

So, the Intermediate Disturbance Hypothesis shows us that with an intermediate level of disturbance we can expect a high amount of diversity. With low and high levels of disturbance, not so much!

Do you have a lingering question? Ask it in the comments section and I will be happy to help as best I can!

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