The Intermediate Disturbance Hypothesis Is Contested: What Actually Keeps a Habitat Diverse

The intermediate disturbance hypothesis (IDH) starts from a simple observation: a coral reef that never feels a storm does not stay a postcard. Left perfectly undisturbed, it slides toward a few winning species that crowd everyone else out. The same goes for a patch of old forest: without the occasional fallen tree opening a gap, a handful of dominant trees take the light and the roster of species shrinks. Joseph Connell put this together in 1978, looking at the two most diverse habitats on Earth, tropical rain forests and coral reefs, and drew a conclusion that still unsettles the textbook picture. Their high diversity is not a settled balance the system relaxes into. It is held up by repeated knocks. Stop the knocks and diversity drains away (Connell, 1978).

Why does disturbance increase diversity?

Why would damage help? Because competition has a winner, and the winner takes time to win. In a calm habitat the best competitor for the limiting resource (light on the reef, space on the rock, canopy in the forest) slowly excludes the rest. Diversity is highest while that race is still being run, before anyone has won.

A disturbance resets the race. A storm, a treefall, a grazer, a fire clears patches, knocks back the leaders, and reopens room for species that would otherwise be squeezed out. So the diversity a habitat can hold depends on how the timing of disturbance compares with the timing of competitive exclusion.

• Disturbances too rare or too mild: the best competitor has time to win, and the habitat settles to a few dominants.
• Disturbances too frequent or too severe: only the species that tolerate constant damage hang on, and everything else is wiped before it can establish.
• Disturbances in between: the race is perpetually interrupted and never finished, so many species coexist that could not coexist at equilibrium.

This is the intermediate disturbance hypothesis, and the word "intermediate" is precise. It is intermediate relative to a clock: the interval between disturbances set against the time a dominant needs to take over. Michael Huston framed it as a dynamic equilibrium between how fast disturbance arrives and how fast competitive displacement runs.

Does disturbance always produce a humped curve? Mostly no

Here the careful version parts ways with the poster version. The poster shows a clean hump, diversity rising to a peak at intermediate disturbance and falling off on either side. Real data are messier. When Mackey and Currie (2001) gathered the published diversity-disturbance studies and asked how often that hump actually appears, the peaked shape was a minority of cases. Plenty of systems show diversity simply rising with disturbance, or simply falling, or dipping in the middle. The hump is one outcome among several.

The mechanism took fire too. Jeremy Fox (2013), under the deliberately blunt title "the intermediate disturbance hypothesis should be abandoned," argued that the standard stories told to produce the hump do not, on inspection, predict a hump at all. Others pushed back and defended a broader reading of Connell's idea (Sheil and Burslem replied in the same journal). The argument is open, and that is the accurate state of the field: the disturbance-diversity link is real and important, the specific humped form is system-dependent, and the textbook mechanism is under live dispute.

What survives the critique of the intermediate disturbance hypothesis?

Strip the claim to what the data support on both sides and something solid is left, narrower than the poster. A habitat held in perfect stasis tends to lose diversity to competitive monopoly. A habitat hit too hard and too often keeps only the tolerant few. Sustained high diversity lives in a regime between those two failures. Where exactly the best point sits, and what shape the curve takes getting there, depends on the system. The disturbance regime is a condition for diversity. It is not a single law that fixes the amount.

How can you test the intermediate disturbance hypothesis?

Here is the version that can be checked rather than admired. There is really one master curve, a hump, and each habitat only ever shows you the slice of it that its own conditions allow. The slice you see depends on where a habitat's typical disturbance interval sits next to the time a dominant competitor needs to exclude its rivals. Where those two roughly match, the peak falls inside the window and you see the hump. Where disturbances come far slower than exclusion, the habitat is already monopolized and you are looking at the rising side of the hump, so adding disturbance RAISES diversity. Where they come far faster, you see only the over-disturbed floor, a flat held by tolerant species alone. If you measure that ratio across systems and the slices do not line up by it, the dynamic-equilibrium reading of the curve is wrong and should be dropped. If they do line up, the messy field of conflicting curves resolves into one underlying clock.

A failed sort would sink the clock-shape story, not the narrower point that some disturbance is needed at all. That one you test a different way: actually stop the storms and watch whether diversity collapses to a monopoly, or over-disturb and watch whether only the tolerant few hang on. The shape story is settled by the pattern across many systems; the bare condition is settled inside a single system by switching disturbance off.