I think modern coexistence theory, as developed by Peter Chesson and his co-authors, is one of the most important bodies of ideas in community ecology. And I’m far from the only one who thinks so–interest in modern coexistence theory is taking off. Which is great–the best sorts of bandwagons are those centered on really good ideas! But it’s also a bit scary, since modern coexistence theory is one of the most widely-misunderstood ideas in ecology, even among community ecologists who study species coexistence. Currently, people developing and applying modern coexistence theory mostly understand it really well. But if interest in modern coexistence theory takes off as I think and hope it will, that could change. Best case scenario: people who start working on it mostly get the hang of it. That’s more or less what I think happened with, say, resource competition theory in the wake of Dave Tilman’s early work. Worst case scenario: people who start working on it misunderstand it, and the whole research program goes off the rails, possibly even generating zombie ideas. That’s more or less what I think happened with limiting similarity after Robert MacArthur first proposed it.
I’d like to do what little I can to maximize the odds of the best case scenario. I’m not an expert on modern coexistence theory. But I do get the gist of it, and some though not all of the technical details.* I get it well enough to have taught bits of it to undergrads, and to have written a series of posts explaining those bits. I’m sometimes asked to review papers on modern coexistence theory, so other people besides me think I get this stuff.** And several of my best friends get it, which totally counts for something, right?***
So, got questions about modern coexistence theory? Ask away! I’ll answer them in the comments, and I’ll turn at least some of the questions and answers into future posts.
To get you thinking a bit–since the worst sorts of misunderstandings are those you don’t even know you have–below are some misconceptions about modern coexistence theory.**** I’m not going to explain why they’re misconceptions right now, but they are. So if something you think is true is on the list below, well, that should be all the incentive you need to ask me a question!🙂
But before I get to that list, here are some other, more comprehensive resources to help you learn modern coexistence theory. Start with these, not my past or forthcoming posts. My posts are not comprehensive and so aren’t a good entry point into the topic.*****
- Chesson 2000. Peter Chesson’s most accessible summary of modern coexistence theory
- Adler et al. 2007. An even more accessible summary of modern coexistence theory (UPDATE: link fixed)
- Chesson 1994. Chesson’s most formal, general, and least accessible derivation of modern coexistence theory. If you have mathematical chops and want to dig into the technical details of modern coexistence theory, this is the paper to read.
Misconceptions about modern coexistence theory
- It only applies to the specific models that Chesson and others often use as illustrative examples (e.g., the Lotka-Volterra competition model; the R* model of resource competition; a simple model of competing annual plants with a seed bank). This and the next six misconceptions all arise because most explanations of modern coexistence theory rely on the same few go-to examples. Preventing people from mistaking your specific examples for the broader concepts they illustrate is a difficult pedagogical problem.
- It only applies to models in which there are “competition coefficient” parameters. As opposed to models in which species compete indirectly, via shared resources, shared predators, competition for space, etc.
- It only applies to species that interact via competition for shared resources, as opposed to via interference competition, apparent competition, competition for space, or other direct and indirect mechanisms of interaction.
- It only applies to communities with a single trophic level.
- It only applies to species with structured life histories that include a long-lived stage such as a seed bank. A variant of this misconception is that the temporal storage effect (a particular class of coexistence mechanism identified by modern coexistence theory) only works with such species.
- It only applies to well-mixed or spatially-homogeneous systems.
- It only applies in fluctuating or disturbed environments. A misconception that may be unintentionally reinforced by leading community ecology textbooks.
- It doesn’t apply to species that experience intraspecific density dependence.
- It predicts that more species rich communities are those in which species coexist more strongly. I heard this one in a talk at the last ESA meeting. I worry that this sort of misconception could become very widespread very fast. Modern coexistence theory isn’t currently a theory of species richness, but a theory of species richness is what lots of ecologists want. There’s a high potential for errors to creep in if people who don’t fully understand modern coexistence theory start making intuitive-seeming leaps of logic to try to turn it into a theory of species richness.
- It’s the alternative hypothesis to neutral theory. In fact, a neutrally stable system is one particular special (limiting) case of the much larger range of cases covered by modern coexistence theory.
- It’s of purely theoretical interest because you can’t measure any of its terms in nature.
A closing bleg
*The fact that I get some of the technical details is why I also get the gist. To really “get the gist” of any idea–or to be able to tell if there’s even a “gist” to get–there’s often no substitute for getting the technical details. In particular, when an idea is expressed mathematically, you usually cannot get “the gist” by just reading the words and skipping over the math. The words are there to aid your understanding of the math, not as a substitute for the math. Reading the words while skipping the math just gives you what Noah Smith calls “the warm glow of understandiness”. The other reason I get the gist of modern coexistence theory is that I get the Price equation. Like modern coexistence theory, the Price equation is a partition (not in the combinatorics sense).
**Man, I sure hope those other people are right.
***If you eat dinner with Robin Snyder enough times you’re bound to get smarter. Sadly, this is not a scalable way to make every ecologist in the world smarter.
****All of the items on this list are misconceptions that I’ve encountered in real life. The length of the list illustrates just how difficult modern coexistence theory is. So although I’m pretty sure I get it–sure enough to write these posts!–there is definitely some non-zero chance that I have failed to understand some crucial details and will write something that will make Peter Chesson and Steve Ellner do this.
*****I thought about writing a comprehensive series of posts, but it’d be too much work. Sorry. If it’s going to happen, it’s only going to happen if I gradually accumulate lots of posts on this topic, which I later organize and edit into something comprehensive.