A week or so ago, I wrote a blog post in praise of Hutchinson’s seminal 1961 paper “The Paradox of the Plankton,” while simultaneously but subtly singing praise of the “intermediate disturbance hypothesis.” As ecologist and blogger Jeremy Fox pointed out in the comments to that post, the idea of the “intermediate disturbance hypothesis” has been largely superseded by other mechanistic explanations. I’ll take a few moments to summarize Jeremy’s argument, provide a few comments of my own, and draw parallels to another longstanding tenant of ecological theory.
But first, I want to make it clear that I think Hutchinson’s paper is seminal not only for the idea it proposed (whether or not it has stood the test of time) but for presenting testable hypotheses and instigating the rise of modern ecology. Prior to the mid-20th century, ecology was largely the purview of the “natural philosopher” who was far more interested in observing and cataloging nature than in explaining it (obviously, there are notable exceptions to this statement, including Charles Darwin, Charles Elton, and others). It wasn’t until Joseph Connell started planting cages in the rocky intertidal in the late 1950s that the field migrated into the realm of the experimental ecologist, where it experienced rapid growth and attention, morphing into the multidisciplinary behemoth we know today. It is clear that Hutchinson’s original ideas were hugely influential on Connell and other ecologists working at the time, and for that he deserves some credit.
But as Jeremy points out in his post Zombie ideas in ecology, just because something is venerable doesn’t mean it’s correct, only that it’s harder to shake. Here are his arguments against the IDH, which invoke a lot of complicated math that I’m unable to convey, but hopefully I can get the gist of his ideas down:
- The idea: Intermediate disturbance reduces species’ densities and weakens the strength of competition, allowing coexistence. Too much disturbance leads to local extinction and too little disturbance allows competitively dominant species to take over. The rebuttal: Weaker interactions also leads to reduced threshold for competitive exclusion.
- The idea: Intermediate disturbance slows competitive exclusion, preventing an equilibrium state from being reached, and subsequently allowing all species to increase. The rebuttal: Disturbance prevents equilibrium, but also changes the long-term average mortality rate. This changes the difference in growth rates between competitively superior and inferior species, which may slow competitive exclusion but does not prevent it.
- The idea: The dominant competitor changes based on fluctuating environmental conditions, and thus over long time scales, no species ever remains dominant long enough to exclude others (Hutchinson’s hypothesis). The rebuttal: Environmental fluctuations change the relative fitness of species, but over long time periods, the species that is favored on average will come to dominate the assemblage. This point is a little more subtle and deserves closer attention. Fox argues that “a change in environmental conditions that creates an opportunity for one competitor necessarily creates the opposite of an opportunity for the previously favored competitor. [But] in order to persist in the long run, a species must be able to grow and compete sufficiently under all the conditions it will experience.” This is logical mathematically but ignores consideration of what relevant timescales might be for the system under investigation. Of course if the system is allowed to go on indefinitely, the best competitor on average will reign, but this approach ignores the fact the study systems are frequently of interest on much smaller timescales, where the IDH may be an appropriate explanation for the observed patterns (although here I am reticent to cherry pick examples that support this idea!). I also can’t help but feel timescales are necessarily limited because of other outside factors, such as recruitment (not to be confused with the colonization-competition trade-off, where disturbance clears patches that allow inferior competitors to recruit and reproduce before being excluded, and which Jeremy addresses in his recent TREE publication).
I actually agree with the majority of Jeremy’s points, and as an emerging ecologist, my job is to challenge the status quo (actually the job of all scientists!). It’s worth having a look at his post on the topic if you haven’t already, where he articulates his points far better than I, and reading his TREE publication. (Though I suppose I’m a little guilty of violating #7 on his list of “Rebuttals to my rebuttal” post.) His paper reminds me a lot of Don Strong’s 1992 paper criticizing trophic cascades, which generated a lot of debate on whether this too was a zombie idea in ecology (later meta-analyses showed, in fact, that trophic cascades were not “all wet” but this does not seem to be the case for IDH, see Mackey & Currie 2001). I’m curious to see what other young ecologists feel about this challenge to the IDH. Sound off in the comments.
Fox, J. 2012. The intermediate disturbance hypothesis should be abandoned. Trends in Ecology and Evolution 28(2): 86-92.
Mackey, R.L., and D.J. Currie. 2001. The diversity-disturbance relationship: is it generally strong and peaked? Ecology 82: 3479-3492.