UPDATE: Just to be clear, disturbance, and environmental variability more generally, can promote stable coexistence. They just can’t do so via the mechanisms that the Professor is trying to teach in this dialogue. I’ve been clear about this in previous posts, but it was suggested to me that anyone who only reads this post (and doesn’t read the comments) could get the wrong idea.
The scene: An undergraduate ecology lecture. The Professor has been teaching students about the effects of disturbance on competitive exclusion.
Professor: In summary of this section of the course, the great diversity of species to be found in a community is one of the puzzles of ecology. In an ideal world the most competitive species (the one that is most efficient at converting limited resources into descendants) would be expected to drive less competitive species to extinction. However, this argument rests on two assumptions that are not necessarily always valid.
The first assumption is that organisms are actually competing, which in turn implies that resources are limiting. But there are many situations where disturbance, such as predation, storms on a rocky shore, or frequent fire, may hold down the densities of populations, so that resources are not limiting and individuals do not compete for them.
The second assumption is that when competition is operating, one species will inevitably exclude the other. But in the real world, when no year is exactly like another, the process of competitive exclusion may never proceed to its monotonous end. Any force that continually changes direction at least delays, and may prevent, an equilibrium or a stable conclusion from being reached. Any force that simply interrupts the process of competitive exclusion may prevent extinction and enhance diversity.*
Clever student: [raises hand]
Professor: Yes, a question in the back.
Clever student: I’m confused about both of those assumptions. I just don’t understand how they prevent competitive exclusion.
Professor: Can you be more specific? What exactly don’t you understand?
Clever student: Well, with the first assumption, if a species is experiencing really high mortality rates from predation or fires or whatever, how come it doesn’t just go extinct?
Professor: Because if its density is low, then resource levels will be high, which allows the species to have a very high reproductive rate.
Clever student: So there’s really rapid mortality, but really rapid reproduction, and the two balance out?
Clever student: So then wouldn’t anything that reduces reproduction even a little, or increases mortality even a little, like just a little bit of competition, still lead to extinctions? I mean, in that simple resource competition model from that Tilman guy that you showed us to teach us about competitive exclusion, there were per-capita mortality rate parameters for each competitor. You can jack up those mortality rate parameters, and yeah, you reduce the equilibrium abundance of the dominant competitor, and you raise the equilibrium resource level, but you still get competitive exclusion. The species that can reduce the limiting resource to the lowest equilibrium level relative to its competitors still wins. It’s just that everybody’s R* value increases as mortality rates increase.
Professor: [pauses for thought] Hmm… I see what you mean. But remember, we’re envisioning a situation in which mortality is so high that resources aren’t limiting, so there’s no competition at all.
Clever student: You mean, species are there and they’re consuming resources, but their densities are so low that their consumption doesn’t reduce resource levels at all? How can that be? If they’re there, they have to be consuming some resources, right? And if they’re consuming some resources, then they’re surely at least slightly reducing resource levels, which means there’s at least some competition, right? Plus, isn’t it super-unlikely that mortality rates would be just high enough to reduce species to near-zero density, so that there’s no competition, but no higher? So that species can still persist rather than just being totally wiped out because they’re getting killed faster than they can possibly reproduce?
Professor: I think you’re over-thinking things. Rather than thinking about hypotheticals, think about real natural systems. Think of harsh environments like rocky shores and alpine meadows. There is in fact a lot of disturbance and mortality in those environments, which does reduce population densities, and species do coexist in those environments. So there you go.
Clever student: Yes, I know all that, but that doesn’t answer my question. I want to know why they coexist. I mean, how do we know they’re not just coexisting for some reason that doesn’t have anything to do with their densities being low? ‘Cause I read some microcosm papers where they manipulated mortality rates of competing species and found that you got just as much if not more competitive exclusion at high mortality, and if you didn’t it wasn’t just because competitor densities were low.
Professor [not sure how to answer so he bails out]: We’re running a bit short of time, and I’m not sure I’m familiar with the papers you refer to, although I don’t know that microcosm studies are relevant to what happens in nature. Why don’t you come to my office hours later and we’ll talk more about it? Now, you said you also had a question about the other assumption?
Clever student: Yes, I don’t understand the whole “interruption” thing. Like, if I give my buddy $2 today, and he gives me $1 tomorrow, then I give him $2 the next day, and we keep alternating like that, eventually I’ll run out of money and he’ll end up with all the money. Even though every other day my losses are interrupted by him giving me $1.
Professor: Ok, I see where you’re confused. The point of the second assumption is that the interruptions slow down the rate of exclusion, here the rate at which you go broke, compared to what would happen if there were no interruptions. If you gave your friend $2 every day, with no interruptions, you’d go broke a lot faster.
Clever student: Yes, that’s true, but why is that the right comparison? I mean, I’d also go broke slower if I only gave my friend $0.50 every day, with no interruptions. I’m losing the same amount of money every day, I’m just losing less than if I give my friend $2 every day. That’s why I go broke slower when I give him $2 every other day, and he gives me $1 every other day–on average I’m only losing $0.50 per day in that case. So I go broke at the same rate, whether I give him $0.50 every day, or we alternate between me giving him $2 and him giving me $1. It seems like how much money I’m losing on average is all that matters. The interruptions are just noise, aren’t they? They don’t actually have any effect on anything.
Professor: Well, remember that we learned that the frequency of environmental change matters. Hutchinson said so. The environment has to switch from favoring one competitor to favoring another with intermediate frequency, on the same timescale as competitive exclusion. In your example, you and your friend are favored on alternate days, which is a really high switching frequency, not an intermediate frequency. In that kind of case, Hutchinson says that species just average across the fluctuations.
Clever student: But what if I give my friend $2 every day for, like, a week, and then he gives me $1 every day for a week, and so on? Or a month, whatever. I’m still losing $0.50 per day on average, so I’m still going broke at the same rate.
Professor: Hmm, yes, I see what you mean. But at least when you and your friend are favored for longer periods of time, those periods of time are economically relevant. I mean, at the end of a week when you were favored, you’ll have saved up enough money to buy a beer. [laughs] As ecologists, it’s often difficult to study what happens in the long term, so we just focus on shorter, ecologically-relevant timescales.
Clever student: [does not laugh] But I thought we were trying to explain coexistence. Like, real coexistence, not just temporary blips that sort of look like coexistence. I mean yeah, sure, the long run is hard to study–but I’d still have the same question even if the long run were really easy to study. So I’m even more confused now. Are you saying that species will go extinct no matter what the frequency of disturbance, but it’s ok, because along the way they’ll sometimes increase in density? Isn’t that totally changing the question?
Professor: Ok, you’re right, average conditions do matter. So consider a case in which you give your friend $2, and the next day he gives you $2, and so on. In that case, neither of you loses or wins any money on average.
Clever student: But that’s just the same as if we each give the other no money. Or we each give each other the same amount of money on the same day rather than alternating days. Or there’s a continuous steady flow of money from his bank account to mine, and an equal continuous steady flow of money from my account back to his. The day-to-day fluctuations in who gives money to who don’t matter at all, all that matters is the fact that we’re each breaking even on average.
Professor: You seem very focused on the average conditions and the long-term outcome, to the exclusion of the fluctuating dynamics that disturbance generates. Those fluctuations are a very interesting part of ecology, you can’t just ignore them. You can’t just ignore dynamics.
Clever student: [getting frustrated] I’m not ignoring dynamics–competitive exclusion is a kind of population dynamic. The abundance keeps going down until it hits zero. And I’d be happy to pay attention to fluctuating dynamics if you gave me a reason to. It’s you who said that “Any force that simply interrupts the process of competitive exclusion may prevent extinction and enhance diversity”. You didn’t say “Any force that simply interrupts the process of competitive exclusion creates interesting fluctuations in species abundances on the way to exclusion, which is something we can’t ignore even though it has no effect on diversity.”
Professor: Ok, I see your point on dynamics, you certainly do have a way with words. I think where you’re confused is that you’re trying to separate two effects of disturbance that just can’t be separated. Adding disturbance to a disturbance-free system both changes the average conditions, and interrupts approach to equilibrium. Those two things always go hand in hand, and you don’t really need to worry about separating their effects. It’s all just effects of disturbance.
Clever student: But they don’t always go hand in hand–there are ecological systems with similar average conditions and different amounts of variability around the average. And there are ecological systems with different average conditions but similar amounts of variability. Plus, we can do experiments to manipulate average conditions and variability independently of one another, can’t we? Don’t scientists do that all the time–do experiments to tease apart effects that usually co-occur?
And if you don’t separate effects of changes in the average from effects of changes in the variability around the average, how do you know that the variability is what matters? Because that’s how you explained it–you talked about variability, you talked about “interrupting” the approach to equilibrium. I mean, that’s why we call them “disturbances”, right? They disturb what we think of as the “normal” course of events. But based on what you’ve told us, disturbances don’t actually matter as disturbances at all, since what they do isn’t really to disturb the normal course of events, it’s to change what constitutes the normal course of events. That is, change the average conditions. Which of course you could also change without having any disturbances. So I don’t see what’s so special and unique about disturbances as opposed to just any old change in average environmental conditions.
So I guess that’s really my big question: what can introducing disturbances do that can’t be done by just making the equivalent change in average environmental conditions. Why does variability per se matter?
This post is directed at the many readers who are still on the fence about whether the intermediate disturbance hypothesis (IDH) is a zombie idea. My goal with this post is to try to force those fence-sitters to come down on the correct side, by reminding those readers that this isn’t a purely intellectual debate among academics. This is about what we teach to our students. And as teachers, we need to be able to answer our students’ questions. I don’t think the questions I’ve put in Clever Student’s mouth are at all unreasonable. Indeed, and in all honesty, they’re exactly the sort of questions that I’d expect my University of Calgary undergraduates to ask. They’re certainly the sorts of questions any undergraduate who reads this blog would ask (and there are many undergrads who do read this blog, at universities around the world). Maybe most students wouldn’t ask these questions in quite so articulate or pointed a manner, but they would ask them. They’re perfectly natural questions, that arise from the standard way in which the IDH is explained in textbooks.
So, for those of you who are still on the fence about whether the standard, textbook explanations of the IDH are zombie ideas: How would you answer Clever Student’s questions?
Note that you can’t answer Clever Student’s questions by claiming that the IDH actually has to do with competition-colonization trade-offs, or trade-offs between disturbance tolerance and competitive ability, or successional niches, or the storage effect, or other factors not mentioned by the Professor in his opening lines. Yes, those other factors are relevant to thinking about the effects of disturbance on coexistence. They’re also irrelevant here, because the way in which the Professor explains the IDH is the standard explanation that actually appears in numerous textbooks, and in the Introduction sections of many, many papers. If you think the way to answer Clever Student’s questions is to redefine the IDH by dropping both of the Professor’s assumptions and explaining the effects of disturbance completely differently, then you’re admitting that the standard, textbook understanding of the IDH is 100% wrong. Which I suggest ought to bother you as much as it bothers me. Yes, textbook explanations have to simplify and gloss over technical details–but surely not the point of inviting the sorts of questions Clever Student asks!
Note as well that Clever Student is very alert to attempts to change the question, which the Professor tries and fails to do several times. Note as well that all those attempts to change the question were actually tried by commenters on my original zombie ideas post, or folks who’ve corresponded with me privately. The Professor in this post is just trying to answer Clever Student’s questions the way commenters and correspondents have tried to respond to my original post. The Professor here is no straw man.
If you’re tempted to respond by arguing that Clever Student’s questions are somehow ambiguous or otherwise flawed, please be aware that Clever Student’s questions can be put much more rigorously and precisely. In particular, I would discourage you from trying to argue that “students exchanging money is nothing like species competing for resources”, unless you’re prepared to explain why the analogy is a bad one. Because in every relevant respect, the analogy is perfectly consistent with the Professor’s second assumption. So if you think the monetary exchange analogy is a bad analogy to the IDH, then what you actually think is that the standard, textbook explanation of the IDH is a bad explanation.
The Professor here is not an unreasonable or ignorant person. He’s smart, and he’s doing his best to answer Clever Student’s questions. But those answers just don’t cut it. Hence my curiosity whether any readers can come up with better answers. Our students–real students, not Clever Student–deserve no less.
Of course, I think Clever Student’s questions don’t have good answers. I think the only legitimate response to those questions is to stop teaching the Professor’s zombie ideas in the first place.**
*These lines of the Professor’s dialogue are an abridged quote from p. 740 of the second edition of Begon, Harper, and Townsend’s textbook Ecology: Individuals, Populations, and Communities. My abridgements are minor and do not alter the meaning of the passage.
**I wonder if anyone will try to argue for “teaching the controversy”–teaching both standard ideas about the IDH, and counterarguments. Personally, I think that’s about as good an idea as “teaching the controversy” between evolution and creationism. Remember, this isn’t a controversy between alternative logically-valid claims, which simply make different assumptions about how the world works, and which we can decide between by conducting an appropriate experiment. It’s a controversy about the logical validity of one set of claims. There are scientific controversies which can be usefully taught in science classes. But this isn’t that kind of controversy.