My collaborators and I just published “Population extinctions can increase metapopulation persistence“. New Scientist did a piece on it, which is the first time any media outlet other than my local newspaper has written up my work. I’m chuffed about this, because I think this is the coolest paper I’ve ever done by some distance.
Or, maybe it’s just a cute result–a fun curiosity. I could even imagine someone arguing that it’s oversold fluff. So why do I think it’s so cool? And what’s the difference between “cool” and “cute”?
Here’s the gist of the paper (the core of which I spoke about at ESA a couple of years ago). The paper’s about what we’re calling “the spatial hydra effect” on metapopulation persistence. In Greek mythology, the hydra is a multi-headed monster that grows two new heads every time one is cut off. It’s the ultimate example of the old saying that that which does not kill you makes you stronger. Peter Abrams coined the term “hydra effect” to refer to situations in which increasing a population’s mortality rate increases its abundance. Our paper is about a spatial hydra effect that arises for reasons unrelated to those Abrams considered. When you have a spatial hydra effect, population extinctions actually increase metapopulation persistence rather than reducing it. They do this because they’re desynchronizing. It’s well known that metapopulation persistence depends on sufficient asynchrony. If all populations crash at once, there’s no source remaining for recolonization (or for a rescue effect to prevent local extinctions). The spatial hydra effect arises because local extinctions (say, due to a severe local disturbance) can be very desynchronizing; they’re like especially severe perturbations to the local population’s abundance. A population that goes extinct and then later gets recolonized isn’t likely to be in sync with the surrounding populations, even if it was in sync with them pre-extinction. The resulting asynchrony ensures that extinct populations quickly get recolonized by emigrants from other currently-abundant populations. It’s an irony: on the one hand local extinctions create extinction risk for the entire metapopulation–but on the other hand they create the asynchrony that metapopulations need to persist.
Put another way, the optimal rate of local extinction for many metapopulations is an intermediate rate, not a low rate. If the local extinction rate is too high, populations go extinct faster than they can be recolonized and the metapopulation doesn’t last long. But if the local extinction rate is too low, the metapopulation doesn’t last long because all populations will sync up and then crash at once.
In our new paper, we show that the spatial hydra effect increases metapopulation persistence in three different stochastic metapopulation models (a single-species model, a host-parasitoid model, and a predator-prey model), and in experimental predator-prey metapopulations in laboratory microcosms. So it’s a generic effect that operates independent of many system-specific ecological details. And it’s not a subtle effect: raising the local population extinction rate can increase expected metapopulation persistence time by 50%, 100%, or even 1000% depending on the ecological details. The key precondition for the spatial hydra effect is local population cycles that can be spatially-synchronized (“phase locked”) by dispersal. Which, ok, isn’t the most common population dynamic in the world. But there are species we care about exhibit periodic outbreaks that can be synchronized by dispersal, including both species of conservation concern and terrible human diseases. See our paper for discussion of possible natural examples of the spatial hydra effect.
I think this result is extraordinarily cool, even profound.* It’s a great example of a “double-edged sword“, a situation in which a single cause (here, population extinctions) necessarily has two opposing effects (here, on metapopulation extinction risk), and interest centers on the relative magnitudes of those two effects. It’s also a great example of a result that’s counterintuitive until you think about it–and then it becomes intuitive. Results that replace our old intuitions about how the world works with better intuitions are some of the most important results, I think. I also think it’s cool because it’s a result that was kind of hiding in plain sight. We found the spatial hydra effect using bog standard, simple theoretical models that have been well-studied before. You might think there wouldn’t be anything left to learn about the behavior of simple metapopulation models–but you’d be wrong. I’m also proud of the way we demonstrated the result; I think we did a good job of combining theory and experiment, and of demonstrating the generality of the result. I also think we did a good job taking full advantage of the power of our model system; I doubt an experimental result this clean could’ve been obtained outside microcosms. And like any good result it raises interesting new questions. One of the biggest is whether the spatial hydra effect also works with endogenously-generated local extinctions, as opposed to extinctions due to exogenous disturbances. Think of local extinctions due to demographic stochasticity, or of local population cycles crashing to extinction of their own volition, as when a disease outbreak exhausts all the susceptible hosts in the local population and so burns itself out. If the spatial hydra effect works with endogenous extinctions too, then that broadens the range of natural systems in which we might expect a spatial hydra effect to operate. A spatial hydra effect that operates with endogenous extinctions would also just be the ultimate irony, because it would mean that the metapopulation itself generates the local extinctions that it needs in order to persist via asynchronous colonization-extinction dynamics.
But on the other hand, I wouldn’t blame you if you thought this was more “cute” than cool. If you don’t care about intuition and conceptual understanding, say because you only care about prediction, you won’t care that the spatial hydra effect sharpens our intuitions about metapopulation dynamics. If you think the only important task of ecology is to provide management advice for specific natural systems, then you’re going to think our new paper is as unimportant as any other bit of fundamental ecological research. If you’re the sort of person who thinks that a phenomenon isn’t interesting or important until it’s been shown to operate in nature, then you’re not going to be very impressed with our paper. If you’re the sort of person who thinks that a phenomenon isn’t interesting or important until it’s been shown to be common and strong in nature, you’re definitely not going to be very impressed with our paper! After all, most populations don’t cycle, metapopulations maintained by colonization-extinction dynamics don’t seem to be all that common, there are lots of things that affect metapopulation persistence besides the spatial hydra effect, and as far as I know there aren’t any natural examples of entire metapopulations that have gone extinct because the local populations all phase-locked and then crashed at once. If you’re the sort of person who thinks that multicausality is what makes ecology both challenging and fun (hi Brian!), you’re probably not going to be too excited about a highly-controlled experiment designed to isolate one determinant of metapopulation persistence among many. And if you’re the sort of person who thinks that describing and explaining large-scale patterns is what ecology is all about (hi Jim!), because the alternative is disappearing down a rabbit hole of reductionist research that festishizes internal validity, you’re not going to be very excited about a process-first (rather than pattern-first) research project based on laboratory microcosms.
I disagree with all of the views sketched (and hopefully not caricatured) in the previous paragraph. In particular, I disagree with the view that the only phenomena that are really worth caring about are those that are common and strong in nature.** But every view in the previous paragraph is defensible. Professional judgments about what research is most interesting or important or etc. are just that–judgments. They’re partially subjective, but also partially objective. They’re based partially on purely-personal preferences, but also partially on cogent reasons that other professionals can appreciate if not necessarily agree with. And we can’t do without such professional judgments, except at the cost of ecology ceasing to exist as a professional discipline.
So purely to indulge my own curiosity, here’s a little poll: how cool and interesting do you think our spatial hydra effect paper is on a scale of 1-10? Where 1 is “trivial, oversold rubbish”, 5 is a typical paper in a mid-level ecology journal, and 10 is “I would nominate this paper for the Mercer Award if you were still eligible, Jeremy.” As this post hopefully makes clear, you won’t hurt my feelings no matter how you vote, so be honest. 🙂
*Which is why we originally submitted to Nature, where the paper was reviewed, revised, and ultimately rejected. Which was disappointing, but fine. The ms was considered fully and fairly by the reviewers, which is all any author can ever ask.
**In part because the definitions of “common” and “strong” are unclear. For instance, bacteria, archaea, and viruses vastly outnumber macroscopic organisms. Does that mean that everybody except microbial ecologists studies organisms that are “rare” or “unusual” and therefore comparatively uninteresting or unimportant? As another example, human beings surely have stronger effects on the planet than any other species, by any plausible measure. Does that mean any ecologist who studies anything other than anthropogenic impacts is studying unimportant phenomena?