Posted by: Jeremy Fox | April 16, 2012

Yes, IDH zombies are still worth worrying about (UPDATED)

Via Twitter, Nate Hough-Snee asks whether zombie ideas about the IDH are actually taught any more, and suggests that the idea that the IDH is still popular is itself a zombie idea. That’s a question I’ve asked myself about another zombie. But I was wrong. And in this case, well, sorry Nate, I sincerely wish you were right, but…

Searching Web of Science for “intermediate disturbance hypothesis” yields the following plot of the number of citations/year, which includes citations only to items indexed in Web of Science:

See any sign of declining popularity there? Neither do I.

And here’s a plot of the citations/year in the Web of Science database of two key IDH papers. Connell (1978) coined the term “intermediate disturbance hypothesis”, and Huston (1979) is the source of a key zombie idea about how disturbance affects coexistence. I left out Hutchinson (1961) because it’s often cited for reasons that have nothing to do with Hutchinson’s zombie idea about how disturbance can promote coexistence. Also plotted are data for several papers theoretically or empirically refuting zombie ideas about the IDH (Chesson and Huntly 1997, Pacala and Rees 1998, Mackey and Currie 2001, Roxburgh et al. 2004, Shea et al. 2004). Note the log scale on the y-axis.

See any sign that non-zombie ideas about disturbance are replacing zombie ideas? Or that people are losing interest in zombie ideas about how disturbance affects coexistence and so are no longer citing the papers that first developed those ideas? Neither do I.

As to whether zombie ideas about the IDH are still taught in undergraduate curricula, they aren’t any more at Calgary, but I doubt we’re typical. Certainly, zombie ideas about the IDH are still in our textbooks. The current (4th) edition of Begon et al.’s Ecology: From Individuals to Communities* continues to summarize an entire section (8.5) of the chapter on competition with statements like “Even when interspecific competition occurs it does not necessarily continue to completion”, and continues to devote an entire subsection (8.5.3) to a summary of Hutchinson’s zombie idea about how fluctuating environments promote coexistence. The 4th edition of Ricklefs and Miller’s Ecology includes in its online review material for the chapter on competition theory the claim that predators can promote coexistence by reducing competitor populations to a low level, so that “resource limitation is no longer a factor”, and repeats this claim in a review question for the chapter on competition in nature.

Believe me, I would love to discover that my crusade against these zombie ideas is unnecessary because everybody actually agrees with me already, and nobody teaches these zombies anymore. But all the evidence indicates that many, many ecologists still believe in these ideas, still base ongoing research on them, and still teach them. So yeah, the IDH zombies are still worth worrying about.

*UPDATE: In fairness, the 4th edition of Begon et al. actually does a better job on disturbance effects on coexistence than the second edition does. The introductory passages, while not zombie-free, are improved, and unless I missed it there’s no longer any discussion of Huston (1979).

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Responses

  1. Jeremy, thanks for clearing that up with a bit of data. I wonder if the sheer increase in the volume of papers over your timeframe of interest could be driving the increased citation of the IDH -and- Connell and Huston’s ’78 and ’79 contributions? Perhaps the number of IDH citations as a proportion of total ecology papers is a better metric? Maybe this number is bottoming out and will continue to decline? Also, a citation does not necessarily mean an endorsement – how many of those papers in your figures were actually arguing contrary to the IDH? Finally, does a citation (supporting or refuting) lead to embedding this zombie into standardized ecology curricula? I may have more questions than answers here, but that’s part of the fun, right?

    I’m not sure why I’ve thought of the idea that the IDH is popular in undergraduate education as a straw man (since straw men are boring though, a zombie!). I guess I’m just fortunate to have never had to teach, nor been taught the IDH as dogma. Viva zombie slaying!

    • Yes, the total volume of papers is increasing. I couldn’t be bothered to factor that in; you get the research you pay for on this blog. ;-) But I doubt it would reverse the conclusion.

      Note that increases in the total volume of papers published can’t explain why Connell 1978 and Huston 1979 are currently cited *much* more often (an order of magnitude more often, in the case of Connell 1978) than papers refuting their ideas.

      And sadly no, it’s not that lots and lots of people are citing Connell 1978 and Huston 1979 in a negative way these days. If they were, they’d also be citing the refutations, especially Chesson and Huntly 1997 and Mackey and Currie 2001.

      I doubt citation rates have much to do with what gets taught in undergrad ecology classes. That’s very much down to the preferences and interests of individual instructors, and to the preferences and interests of the small number of ecologists who choose to write textbooks. Many instructors have similar preferences, of course–but I don’t think those preferences have much to do with citation rates.

  2. Hi Jeremy, I came over your blog whilst trying to avoid revising for my second year undergraduate Ecology exam. As an undergraduate, I can safely say that the IDH is still on the curriculum here (Lancaster University) – several lecturers have taught us about it. I’m not completely sure what a zombie theory is (google came up with a lot of interesting but irrelevant stuff!) but I think I can guess… As for now I’ll have to do my best to remember the concept, but I might put in a sentence about the critisism of it (nice “evidence of extra reading” as they call it)!

    • Hi Hanna,

      A “zombie idea” is an idea that should be dead, but isn’t.

      Good luck with your revisions. Perhaps your essay will convince your instructors to read this blog–it sounds like they should! ;-)


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