Posted by: Jeremy Fox | November 3, 2011

Where are the wholehearted defenders of the IDH zombies?

One thing I’ve been struck by, in the responses to my repeated attacks on the zombie ideas that comprise the core of the intermediate disturbance hypothesis, is that nobody’s come forward with a full-on defense of those ideas–direct counterarguments to any of my arguments. I mean, I’ve come out and said that three hugely-famous, widely-cited ideas, which are in all the textbooks, are flat-out wrong, indeed so wrong that I’m perfectly comfortable not just arguing against them but making zombie jokes about them. And I’ve basically gotten no serious pushback! What disagreement I’ve gotten has mostly been either requests for clarification, or based on misunderstanding of my original posts or of the relevant theoretical literature (as evidenced by the fact that, when I clear things up, people stop disagreeing with me).

I’m happy to keep clarifying matters and addressing residual doubts, for folks who aren’t yet entirely convinced by my arguments (so keep those questions coming in the comments). But what would really be fun would be to hear from someone who just thinks I’m totally wrong, and is willing to explain why. Surely there’s somebody out there who’s willing to go to the mat for the IDH zombies?

If so, you’d better speak up. Because the available evidence says that I’m a pretty good zombie slayer. And I have no plans to stop anytime soon.

Or are the core ideas of the IDH not so much zombies, as naked emperors, whose nakedness is obvious to everyone once it’s pointed out?

p.s. Nobody’s come on to this blog with wholehearted (or even any) defenses of r-K selection or the unimodal diversity-productivity relationship either. Where are the zombies, and why aren’t they fighting back? I haven’t actually seen many zombie movies, but it wasn’t my impression that zombies avoid destruction by going into hiding when attacked, in the hopes that the attackers will just put away their shotguns and go home. ;-)

About these ads

Responses

  1. Speaking only for myself, I just don’t know that much about the topic, and therefore it’s not possible to mount any sort of objection. If I had the time and energy I would read the papers you’ve cited and get into it. But I can’t–I’m already over-extended. I’m in learning mode, and you do a real good job of bringing up and explaining these issues in an easily accessible way. I’m just glad there are folks out there who think in depth about a given topic and then also make the serious effort of explaining many of its ins and outs. Mostly I’m interested because, well, you’re interested, and well informed. If you weren’t, I probably wouldn’t be either. You’re doing a very good job here. If more people did this, we could all learn a lot quickly from each other, were we to pay attention.

    There are so many topics in ecology, so one has to pick one, or a few at most, and go with them. I started out with a strong interest in ecological genetics but somehow or other ended up as a generalist without a strong interest in any particular theoretical issue such as community assembly. I’m more interested in applied questions, such as how, and how much, are human beings impacting ecosystems (forests in my case), on multi-decadal to centennial time scales and large spatial scales. Therefore much of my energy is spent on non-theoretical topics and involve conceptually straight forward things such as just getting a better estimate of some quantity (e.g. historic population structure, or species composition, spatio-temporal changes therein, etc), or figuring out how to extract useful information from some particular data set that wasn’t originally collected for the purposes to which I want to apply it. Or at best, how to address some hypothesis about the amount, or the cause(s), of change in a given system, given some data set(s). To me, those are the questions of interest, and if a particular topic area of “traditional ecology” doesn’t bear directly on those questions, I’m not going to be spending much time on it. I would if I could, but I can’t, so I don’t.

    So it’s sort of like going to a seminar, or reading a paper in Nature, about plate tectonics or ocean acidification. Some of us are just going to sit there and absorb whatever we can. If we look dumber than stumps it’s at least partly because we are. I’ve been fortunate enough to expose myself to a number of different topics in ecology, genetics and earth sciences (now trying to wrap my head all around climate change–hooo boy, mistake!). I’m just trying to see how the myriad jigsaw puzzle pieces might orient themselves to each other, after I learn something about the pieces.

    Bottom line is I read your blog because you talk about method and approach to answering a given question, which is what it’s all about if you want to get anywhere, no matter what your topic area of interest.

    • Thanks for the kind words Jim. This post wasn’t really aimed at folks like you–I wouldn’t expect just anyone to have either the knowledge or the inclination to push back hard against my views on the IDH. I was just having a bit of fun, trying to goad someone like Michael Huston into commenting, in the unlikely event that he reads this blog.

      I hear from Peter Adler that there are some folks out there who really do believe that the unimodal diversity-productivity relationship is very common, or at least very important. So there are some wholehearted defenders of that zombie idea, they just haven’t shown up on this blog.

  2. Personally, I find your counter-arguments as arcane as the IDH literature or the educational psychology literature having been discussed.

    For example, you wrote: “Yes, disturbances reduce species’ densities and thereby weaken competition. But as Chesson and Huntly (1997) point out, they also reduce the strength of competition needed for exclusion!”

    Leave the reference away and it becomes: “Yes, disturbances reduce species’ densities and thereby weaken competition. But they also reduce the strength of competition needed for exclusion!”

    This seemingly cuircular argument confused me a lot.

    I guess (I’m not sure) you were trying to point out that, if disturbance has the same average effect on every species then nothing will change in their interactions, on average. So no species that was previously dominant or rare will thereby become less dominant or rare.

    If that understanding is right (I don’t know), the alternative would be a hypothesis that disturbances that hit the dominant species the hardest should increase diversity. Let’s call it the cataclysmic disturbance hypothesis.

    Was Tchenobyl such a disturbance? After all, diversity could be higher there now, within the evacuated area, because most humans abandoned it. Fukuchima would be a chance to see whether it is true that diversity increases when humans (the dominant species) leave an area. Of course, one would want examples from systems where humans are not the dominant species.

    P.S.: I’m a layman on this, have no idea whether any of it is already discussed in the literature.

    • Hi Joe,

      The original post wasn’t really aimed at “laymen”, although I did try to summarize the zombie ideas, and the counterarguments, as clearly and simply as I could. So I’m not surprised that a layman would find the post somewhat arcane.

      Having said that, I didn’t make any circular arguments and I’m not clear why you think I did. A circular argument has the form “If A, then B” and “If B, then A”. Let me try to clarify. The first zombie idea, about disturbances reducing species’ densities and thereby weakening competition, is a zombie because it exclusively focuses on one effect of disturbance when in fact disturbance also necessarily has a second effect (reducing what your growth rate would be in the absence of competition, thereby reducing the amount of competition needed to push your growth rate into negative territory) that needs to be accounted for. If someone says to you “I’m moving from Bangladesh to Tokyo to take up a new job. It pays more than my old job, so now I’ll be rich!”, it is not circular to reply “But Tokyo is much more expensive than Bangladesh–you’ll have to spend much more money on living expenses, so you won’t necessarily be any richer than you were in Bangladesh.”

      Not sure why you think leaving out the citations changes the sense of this passage. I included citations because my counterarguments against the zombies are not original to me. I’m merely stating them (I hope) more clearly and forcefully than they’ve been stated in the primary literature. I needed to include the citations so that readers who were not fully convinced by my arguments could easily go to the primary literature and verify my claims for themselves. Nor are the zombie ideas themselves original to me, and I needed to include citations so that readers could verify for themselves, if they chose, that I’m not attacking straw men.

      Yes, disturbances that differentially affect the competitively dominant species absolutely are a good, non-zombie way in which disturbance can generate competitive coexistence that would not otherwise occur. This idea is much discussed in the literature. But the evacuation of Chernobyl isn’t a good example. Humans don’t just, or even mainly, compete with other species for resources like food and space. Humans are “dominant” in a much broader and more profound sense. So yes, I would expect that the absence of humans from Chernobyl has had many effects on the ecology of the area–but I doubt that it makes much sense to think about those effects simply in terms of the removal of a competitively dominant species.

      I recommend an undergraduate ecology textbook like Begon, Harper, & Townsend if you want to start digging into these ideas further. I’m always happy to address whatever questions anyone poses to me, but it’s not really the best or most systematic way to fill in the background on these issues for someone who’s never been taught this stuff.

  3. I have not said that you made a circular argument, but that a “seemingly circular argument confused me a lot” (emphasis on seemingly and me).

    Did neither mean that leaving out the citations changes the sense of the sentence, but it was easier (for me) to grasp the sense of the sentence that way and that’s why I’ve written it down that way.

    Sorry, if it came over otherwise. Difficult medium for communication this blogging, plus a flu interferes with me booting my brain today.

    My hunch is (emphasis on my and hunch) that the feeling of circularity (emphasis on feeling) often occurs among laypeople, when experts juggle with many technical terms at the same time. Just think of how often fitness has been mistaken to involve a circularity.

    Concerning the previous discussion in another thread of how to best teach this difficult subject: If I was one of your densest students, but you started out with the differential disturbance hypothesis, you’d have won me over immediately. Convincing me thereafter that the IDH is false would have been a matter of course, because that’s were I’d have been headed left on my own means.

  4. I think that despite the wide audience you reach, I imagine there’s a strong demographic skew towards younger people. Its something I learned about in text books 10 years ago as an undergraduate but never really came back to. Much of what I learned about as an ecology graduate student has come from Nick’s books (both of his primers) which don’t focus at all on the IDH and so I never received any serious treatment of it as a graduate student. I can’t think of any peers who have hung their hats on the IDH, and those folks who did make their careers on the hypothesis don’t read your blog I’m guessing. I think you might see more pushback if you were to take your ideas here and publish them. Why not a TREE paper called “Ecological zombies want your brains, and but here’s how you can fight back” or something like that. After all isn’t that an ideal way to use blogs? Push your ideas out here, get some feedback and then take them to more formal arenas. Or what about an Oikos special edition with people on both sides of the idea? Another facet of the demographic gap might also be how seriously people take blogs. While I might read something on a blog by you and think: “That Jeremy Fox is a sharp guy, and I’ve seen a picture of him with an axe so I’ll take him seriously”, I’m taking you seriously because I don’t have a bias about blogs as a serious form of scientific communication. At the same time take a full professor in my faculty and they might say: A. “What’s a blog?”, or B. “Blog? Isn’t that what fat nerds who live with their mom’s use to get mad at latest actor playing captain of the Enterprise?” (foot note: there’s really only, its Patrick Stewart, Picard was the best captain ever, and I don’t live with my mom I live with my wife, who’s nothing like my mom.) I’m not going so far as to say the “medium is the message”, but the medium will bias your audience and your audiences opinion of the message.

    • Way ahead of you, Ted. I turned the original zombie ideas post into a Forum article for Oikos. Should be getting a decision on the ms any day now!

  5. I’ll pass on the IDH argument, but Hutchinson’s solution to the plankton paradox ain’t a zombie, it’s mathematically sound. Why does everyone think otherwise?

    Two species compete for one resource, where the identity of the superior competitor alternates periodically. For small periods, your verbal argument holds: whichever species is favored more outcompete the other. For larger periods, there is a sizable coexistence region, determined by mutual invasibility. For even larger periods, one of the species’ densities falls below a minimum threshold and is driven extinct.

    Exactly what Hutchinson said in 1961. “Zombie” exonerated?

    • Finally! I knew someone would step up to the plate! ;-)

      Do you have a citation? Because the math with which I’m familiar (Wright 1948, and especially Chesson and Huntly 1997, equations 1-8 and discussion on pp. 531-533) backs me up. I was not making a verbal argument in the post; I was verbally summarizing the math of Wright 1948 and Chesson and Huntly 1997. Sorry if that wasn’t clear from the post.

      I would guess, although of course I can’t know for sure without knowing what model you’re thinking of, that whatever model you’re thinking of includes some sort of nonlinearity or nonadditivity sensu Chesson and Huntly 1997.

      I look forward to seeing the citation of the model you’re thinking of. I suspect that identifying the source of the contrast between whatever model you’re thinking of, and the models considered by Wright 1948 and Chesson and Huntly 1997, will highlight the importance of mathematical models as a tool forcing ecologists to make their arguments precise. Personally, I find the models of Wright 1948 and Chesson and Huntly 1997 to be quite plausible mathematical versions of Hutchinson’s argument. If there are other models, with different behavior, that could also be considered as plausible mathematical versions of Hutchinson’s argument, then that wouldn’t fully “exonerate” Hutchinson, but would merely show that Hutchinson’s verbal argument was too imprecise to be truly useful (except perhaps as inspiration for precise mathematical modeling).

      • Here are details. Let’s see if I can attach a figure this time…

        The resource competition model for two species (n1 and n2) and one resource (R):

        dn1/dt = (r1*R-m)*n1
        dn2/dt = (r2*R-m)*n2
        R = Rtot-n1-n2

        The forcing:

        r1 > r2 for iT < t < (i+phi)T
        r1 < r2 for (i+phi)T < t < (i+1)T

        where i is the (integer) period number, T is the period, and 0<phi<1 is the proportion of the period where species 1 is the superior competitor.

        Verbally, this is competition for one limiting resource, where the identity of the superior competitor periodically alternates between species 1 (phi proportion of the period) and species 2 (1-phi proportion). I’d say this is the simplest model that captures what Hutchinson was talking about. It’s even a Lotka-Volterra model, which Chesson & Huntley specifically say don’t allow fluctuation-mediated coexistence.

        Outcome as a function of phi and T here. The coexistence region was determined numerically through invasion criteria and the minimum population extinction border by simulating the two species coexistence. r1={2,1}, r2={1,2}, m=0.1, Rtot=1. If the coexistence region isn’t wide enough for you, just increase the variation in the r’s. Mathematica notebook available on request.

        This is something I worked out just now, but see Klausmeier 2010 section 4.3 for an analytical treatment in the limit of large T (ignoring minimum population size).

        I haven’t read Wright 1948, but I know that Chesson & Huntley 1997 make a similar argument to yours. That paper is justifiably widely read and widely cited, but I don’t think it’s as widely understood. At least not by me!

        Peter and I sat down at a meeting once and tried to get a similar resource competition model in his framework. We got it to work for fast environmental fluctuations but didn’t manage to do it for slower ones. As you say, it must involve some nonlinearity or nonadditivity, but where is it and how is it calculated? So if Peter or someone who uses his methods is reading, feel free to chime in!

        Personally, I still think Hutchinson nailed it.

  6. [...] the comments on a recent post, ace theoretician Chris Klausmeier (commenting as “lowendtheory”) argues that one of my three [...]


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

Follow

Get every new post delivered to your Inbox.

Join 4,091 other followers

%d bloggers like this: