Posted by: Jeremy Fox | January 24, 2012

Zombie ideas in ecology: “neutral” = “dispersal limitation”

Following on from the previous post, another way in which community ecologists often misinterpret neutral models is by mixing up neutrality with dispersal limitation. This leads to mistakes like testing for neutrality by testing for a community ecology equivalent of “isolation by distance”, where more widely-separated communities are more different in species composition, independent of any environmental differences.

I honestly have no clue how this zombie idea could’ve gotten started. Sewell Wright’s classic paper defining isolation by distance talks at length about how migration rate among subpopulations affects their isolation, implying that isolation by distance is not a signature of neutrality. Even in a neutral world, different subpopulations can be as different or similar as you want, depending on the migration rate. Neutral models can perfectly well be panmictic. Wright’s paper also has a whole section on how selection affects isolation by distance, showing that you can still have isolation by distance even in a non-neutral world. Much the same is true in the neutral model with which community ecologists are most familiar, Hubbell’s neutral model. That model remains neutral even if the migration rate is set so high that every newborn individual in the local community is an immigrant from the “metacommunity”, so that there’s no “dispersal limitation” at all. More broadly, ecological theorists have for decades considered all sorts of non-neutral models with all sorts of dispersal rates, from completely closed systems (=zero dispersal) to highly open ones. And that’s before we even start talking about things like sophisticated habitat selection behavior on the part of migrants, which can generate isolation by distance in species composition even in non-neutral, drift-free systems. So how did anyone ever get the idea that low migration rates or isolation by distance are synonymous with, or even tend to be associated with, neutrality?

Don’t get me wrong, dispersal limitation is an interesting and important phenomenon. But it has nothing to do with neutrality vs. non-neutrality.

HT to Brian McGill for beating me to the punch on this in a comment on the previous post.

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Responses

  1. This doesn’t seem quite as much of a zombie as the last one to me, while at the same time I wholeheartedly agree with Brian M that “there can be highly non-neutral dispersal limitation”. Just that finding evidence for neutrality is a different beast from defining what processes neutral models include.

    I’m not sure how else to define neutral ecology other than dispersal limitation+speciation+demographic stochasticity, with no species differences or stabilizing mechanisms. It is fair to emphasize the latter two features, but without the processes of dispersal limitation or speciation there wouldn’t be much left. (You can always remove the impact of dispersal by taking a certain limit, but this seems like a special case to me.)

    • Fair enough James, although as the post indicates I don’t entirely agree. In my view, neutrality = no selection (this includes no frequency dependent selection, which is the evolutionary equivalent of what ecologists call stabilizing mechanisms). Defining neutrality any other way just confounds selection (or more precisely, the lack of it) with other quite distinct processes. Yes, it’s true that, as both you and Jim point out, a neutral world without stochastic drift, dispersal limitation, or speciation doesn’t have very interesting dynamics. But I guess I don’t see why the uninteresting behavior of one particular limiting case should dictate how we define our concepts. Why should “neutrality” be defined as “no selection, plus whatever other processes are needed to produce interesting dynamics when there’s no selection, even if those processes also operate when there is selection”?

      Just to be clear, I don’t think it would be interesting to test a model that just included neutrality, but didn’t include stochastic drift, dispersal limitation, or speciation. But so what? Most every model that’s of any interest to ecologists includes more than one distinct process. I’m perfectly happy for people to test models that lack selection but include other processes. I’m even happy for people to call such models “neutral models”. After all, we have to call these models *something*, and “neutral” is probably the best name, since what’s distinctive about these models is precisely that they lack selection and hence are neutral. But what’s not ok is to just lump together all the distinct processes in such models and *define* the whole amalgamation as “neutral”, as if it were *necessarily* the case that all those distinct processes went hand in hand.

      Don’t get me wrong, I’m pleased that you, like Brian and I, recognize that dispersal limitation does not equal neutrality. But given that you recognize that, I just don’t quite follow why you’re still comfortable defining neutrality as “no selection + dispersal limitation + speciation + demographic stochasticity”.

      • I would also define neutrality as the absence of selection, roughly speaking, which maybe means we completely agree. I’ve just written an encyclopedia definition to this effect, so really I have to agree 😉

        On the other hand, I sometimes use neutrality as a shorthand for neutral ecology, which I interpret as the application of this principle to an ecological system. That still leaves you with some important ecological processes remaining, including dispersal and speciation.

        We also agree that looking for evidence for neutrality alone is futile, as I think you are (almost) always going to find species differences and stabilizing mechanisms if you look hard enough. I think what is more interesting is the hypothesis that the remaining processes of dispersal and/or speciation are `more’ important than selection in shaping empirical patterns, and this is at least what I mean when speaking of dispersal limitation and neutrality.

      • Thanks for your further comments, it sounds like we are very much on the same page.

      • Yes, I think so. It’s really just that I don’t see neutrality vs non-neutrality alone as a useful question.

      • Oh, I agree with you there. But if you *do* see it as a useful question (and many folks do), it is incumbent on you to ask it properly. If you think you’re asking about neutrality vs. non-neutrality when really what you’re asking about is dispersal limitation vs. non-dispersal limitation, or stochasticity vs. non-stochasticity, there’s going to be tears before bedtime. 😉

  2. Dear Jeremy,
    Do you have a reference fot this: “And that’s before we even start talking about things like sophisticated habitat selection behavior on the part of migrants, which can generate isolation by distance in species composition even in non-neutral, drift-free systems.” ?

    Thank you

    • No, but I don’t need one (though I’m sure they exist). It’s not hard to come up with scenarios where this would happen. For instance, if dispersal is sufficiently costly (for instance, because it entails substantial mortality risk), adaptive dispersers will settle near their natal site, thereby creating isolation by distance. That’s true whether or not there’s spatial variation in post-settlement relative fitness (i.e. a non-neutral world with spatial variation in selection).

  3. […] Further, even very experienced people do sometimes make serious mistakes (like believing in zombie ideas!), so maybe my annoyance here is based on the false premise that there are some mistakes that […]


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