by Hari Sridhar
In a paper in The Annual Review of Ecology, Evolution and Systematics in 2000, Peter Chesson attempted to “tame” the wide array of models and ideas about species diversity maintenance, especially in the context of species coexistence in local communities. Chesson’s paper went on to become a cornerstone of modern coexistence theory. Sixteen years after the paper was published, I spoke to Peter Chesson about his motivation to write this paper, and how research on species coexistence has progressed since then.
Citation: Chesson, P. (2000). Mechanisms of maintenance of species diversity. Annual review of Ecology and Systematics: 343-366.
Date of interview: Questions emailed on 3rd September 2016; responses received on 18th September 2016
Hari Sridhar: Could you share with us your motivation for writing this paper? Was it written following an invitation from the journal?
Peter Chesson: I wanted to write this paper for several reasons. The first was that there was a general lack of appreciation of several important ideas in competition theory with regard to what it means for species to have similar niches. There seemed to be little appreciation that average adaptedness to the environment, what I called in this paper “average fitness,” needed to be considered very differently from the details of how a species uses the environment. It always seemed obvious to me, but it took me years to realize that it was not obvious to everyone. In addition, I felt there was not enough general appreciation of the theoretical advances that had been made to incorporate the role of environmental fluctuations in coexistence theory. Finally, I felt that although there are lots of common themes in coexistence theory, many people seemed to think of each hypothesis rather differently, when many of them could be united. These were my motivations. I was not invited by the journal to write the paper. I submitted a proposal three times in successive years before it was accepted.
HS: Stepping back a little, could you tell us how you got interested in the topic of species coexistence? Which came first: an interest in ecology or an interest in mathematical modelling?
PC: I have been fascinated by ecology since I was a small child. I developed an interest in mathematics as a teenager, and ultimately decided to do a mathematics degree with the idea of applying it in ecology. I got interested in species coexistence after seeing Bob May‘s treatment of it in his book, “Stability and Complexity in Model Ecosystems“. My imagination was captured by Robert MacArthur‘s utilization function idea, but also, I thought there could be a better development of the role of environmental fluctuations than appears in May’s book.
HS: How long did the writing of this paper take? When and where did you do most of the writing?
PC: It took me about two weeks of concentrated effort to write the paper as I rushed to meet the deadline for submission. Naturally, I had been thinking about the issues for a long time. Most of the writing was done in my office at UC Davis between teaching terms when I could focus on it.
HS: You acknowledge P. Abrams for his help on this paper. Can you tell us a little more about how he helped?
PC: After the paper was submitted, I sent it out to more than a dozen colleagues. The majority of people who responded were enthusiastic, but had no concrete suggestions. Peter Abrams gave a series of critical comments that helped me bring out more clearly some of the points that I was making. I was glad to get these comments from him because of his often critical appraisals of the field as a whole.
HS: At the time when you were writing this paper, did you anticipate at all that it would have such a big impact on the field?
PC: I certainly hoped it would have an impact, but I did not expect that the stabilizing-equalizing dichotomy that is highlighted in the abstract would have such a life of its own.
HS: Would you know what this paper mostly gets cited for? Would you say that most of the citations are appropriate, i.e. that people understand the theory correctly?
PC: The paper gets cited in two ways. Most commonly it is “coexistence theory” as a general reference or for a specific hypothesis that authors presume I have discussed in the paper, regardless of whether I actually have. Naturally, that leads to a large number of mis-citations. Some citations are for the exact opposite of what it does say. These are primarily for cases where it takes on the conventional wisdom with the regard to environmental fluctuations or natural enemies, and the citing authors have either not understood what I have said or have not bothered to read the paper. Other citations are for the stabilizing-equalizing dichotomy, which by the way does not refer to strict alternatives. A coexistence mechanism can be both stabilizing and equalizing, and this fact is clearly demonstrated with examples in the paper, but this subtlety is often lost. I think many people get the basic ideas in the paper, but we have a serious problem in ecology that education about theory and the use of models is not widespread or in depth. Thus, for most people, a deep understanding of the ideas in the paper is not possible. However, the paper itself seems to have inspired some labs and some intrepid graduate students to dig more deeply into theory, and has led to many applications of at least some of the ideas.
HS: Did this paper have any kind of a direct impact on your career? How did it influence the future course of your research?
PC: It has not changed any major plans in my research, but it does seem to have drawn more attention to my work.
HS: Could you reflect on the kind of impact this paper has had on empirical research?
PC: The most obvious impact I can see is various efforts to investigate the stabilizing-equalizing distinction. There have also been numerous effects to apply that idea in related areas of ecology and evolutionary biology. The major impacts may be more subtle in providing the understanding required for better interpretation of empirical studies.
HS: If you were to rewrite this paper today, would you update the theory in anyway?
PC: There is much to update really. The most obvious update would be the much more comprehensive understanding that we have today on the role of predation in species coexistence. The paper had just a small section on this. In effect, it has been updated though in my essay on “Species Competition and Predation” in Robert Meyers’Encyclopedia of Sustainability Science and Technology. Another big update would be more on the effects of spatial and temporal scale, especially spatial scale. Early in the paper, I make the statement, “Many models of species coexistence are thought of as models of coexistence in some defined local area. However, to make any sense, the area addressed must be large enough that population dynamics within the area are not too greatly affected by migration across its boundary (103). At some spatial scale, this condition will be achieved, but it may be much larger than is considered in most models and field studies.” This is actually a warning against the focus on the “local community” in empirical studies. At the time, I did not have much to say about how to get around that problem. But that has changed with the further development of Scale transition theory, which provides an adequate framework now for how to deal with multiple spatial and temporal scales including non-stationarity of the environment in space and time, which we need for addressing long-term climate change. Finally, an update would provide a better guide on how to use the various concepts empirically.
HS: In the last sentence of your paper you say “Allee effects in sparse (low density) populations and stochastic extinction in small populations both potentially limit how similar the niches of coexisting species can be when similar niches mean sparser or smaller populations. These possibilities deserve further study as they have the unique property that they would still work when species are equal in average fitness”. Subsequent to this paper, have these aspects been researched further?
PC: In fact, I am not aware of serious research by theoretical ecologists following up the point on Allee effects. However, the mathematical literature on dynamical systems in ecology has been investigating Allee effects mostly for the interesting nonlinear outcomes in dynamical systems that include them. None of this, however, is motivated by, or seriously addresses, niche relationships between coexisting species. The effects of stochastic local extinction on niche similarity of coexisting species have in effect been investigated by Dave Tilman and Schwilk and Ackerly, but I do not think they were influenced by the last sentence of my paper.
HS: Have you ever read this paper after it was published? If yes, in what context?
PC: I am constantly going back to it see if I actually spat out clearly some point that I know I had in mind at the time, so I can refer to it. I sometimes consult the paper when I see a strange citation of it, to see how the citing author could possibly have got that idea. I do not recall having read the whole thing through though, after I returned the proofs.
HS: Would you count this paper as one of your favourites, among all the papers you have published?
PC: Yes, I would count it as a favorite. I like it because it “says it like it is” without worrying about what reviewers might think. It seems to have communicated effectively, and I am proud of it.
HS: What would you say to a student who is about to read this paper today? What should he or she take away from this paper written 16 years ago? Would you add any caveats?
PC: The main thing I would say to a student today is, don’t just read it, make sure you understand why it comes to those conclusions. I would then refer them to updates. But really, the paper is aging well.