Note from Jeremy: This is a guest post from Douglas Sheil and David Burslem, authors of a recent letter to Trends in Ecology and Evolution (TREE) responding to my argument that the intermediate disturbance hypothesis (IDH) should be abandoned. They were kind enough to correspond with me before submitting their letter to TREE, leading to an amicable ongoing discussion which I’ve found valuable. I thought readers might find it valuable as well, so I invited them to write a guest post on Dynamic Ecology. I’m very pleased that they agreed, and I’m sure you will be too. They have a lot to say, touching on a range of larger issues that go well beyond the IDH. Thanks to both of them for taking the time to write this.
Just FYI: I’m probably not going to comment on this post, unless someone specifically asks me to do so (and maybe not even then). I’ve already said a lot about the IDH, so I’m just going to read and think about what Douglas, David, and the commenters have to say. If in future I have some things to say that haven’t already been said, I’ll write my own post.
Thanks to Jeremy for the invitation to provide more context for our views concerning the intermediate disturbance hypothesis (IDH). The letters at TREE appear to be open access (here and here) so we won’t repeat them (and Jeremy already made a summary) We’re keen to hear other views too.
Initially we didn’t see a need to respond to Jeremy’s article. We agreed with most of what he was saying, even if not the headline idea that the IDH theory should be “abandoned”. We reckoned that the abandonment proposal was just a punchy way to grab attention. Later, on reading Jeremy’s blog we realised he really intended us to discard the theory – and that those who continued to refer to it risked disapproval. So we decided we should respond.
We don’t dispute the common confusion of disturbance-diversity ideas. We had an extended commentary piece a decade earlier (also in TREE) that also attempted to highlight and address some muddles and contradictions in tropical forest studies. We have published various studies and commentaries on closely related topics that in our view would support a careful and well defined use of the IDH in the sense that Connell originally described it (see list at end of post).
We have always considered the IDH to be a theory about succession – the idea that it includes a broader set of non-successional ideas and mechanisms doesn’t match our experience. This may reflect our own biases: we’re both terrestrial plant community ecologists who have focused on tropical forests. But we suspect (from our biased sampling) that others who work on different systems might agree. When we (DB) teach the IDH to students we use Wayne Sousa’s work on rocky intertidal communities (see here and here; it’s based on elegant observational and experimental work).
Connell’s key example was a study of a forest in Uganda based on tree plots established in the 1930s and 1940s. One of us (DS) relocated and re-measured some of these plots in the 1990s and located and read a lot of the old literature about this site and study in the process. People had recognised since the 1930s that the forest was spreading into the surrounding woodlands and thus younger forest occurred on the edges and older forest in the interior. They realised that the tree community initially became richer and then more species poor as the succession progressed. They also realised that if the old-growth forest was disturbed other species would establish (they were keen to do this as the better timber trees were associated with early to mid-succession, the timbers of the late succession species though durable were dense and difficult to work). The ecological understanding of these patterns was illustrated and written up by Joe Eggeling, a forester and botanist who established a series of plots (nearly 2 ha each) to provide data. He published his study after the second-world war (1947), and Joe Connell used this to illustrate the IDH. Anybody interested in the history of IDH ideas may need to go back and look at those old Ugandan observations and records (they pre-date other claims we have seen, e.g. if you can find it see “Harris, C.M. (1934) Provisional Working Plan Report for the Bunyoro Forests, Uganda. Uganda Forest Department”). These forest managers were sharp-eyed researchers and their observations remain as valuable and as worthy of recognition as anyone else’s.
As Jeremy says, the IDH was presented as a verbal theory and that lacks the kind of precision that many might want now. But that is true of much older theory, and in any case, the most powerful part of Connell’s presentation was the illustration: the figure, with its hump-backed unimodal curve makes clear that the purpose was to link and synthesise a complex set of issues. Evaluating such a theory requires careful attention to the assumptions, predictions and methods.
A question we didn’t explore in our recent TREE comment, due to word limits and our own uncertainties, is when it is appropriate to “abandon” a theory and what that implies. It’s worth thinking about. All major ecological theories are misunderstood, misapplied and mis-explained from time to time. If the IDH were discarded on this basis, and we want a consistent approach, then presumably many other theories would have to go too (Darwin and Wallace’s theory of evolution comes to mind). And what do we refer to after that when we see a rise and fall in species richness over a successional series? What are we actually suggesting? Are theories with valid elements and applications ever “abandoned”? It seems simpler to highlight the problems, suggest fixes and refinements, and encourage precise approaches. Imprecise theories tend to fall out of use when challenged with empirical data, not because they are abandoned –but because they are refined and evolve, they don’t become extinct.
Alternatively, if the IDH, as its name implies, is genuinely a hypothesis and not a theory (we suspect it is both … but let’s leave that aside here), then it is subject to testing that would allow it to be falsified, or (more likely) the scope of its relevance to be sharpened based on the ecological contexts in which it is supported. By extension, if the IDH is genuinely a hypothesis then it cannot be abandoned – it is simply falsified sufficiently often that the weight of evidence against it becomes overwhelming. But this subject runs into deep semantic waters (what are falsification, rejection, revision etc. and should we necessarily expect consensus on when such choices are justified, and is such consensus even desirable?) – it’d be interesting to hear what others think on that. Do any philosophers of science read this blog?
The realisation that disturbance can lead to increased species richness is crucially important in a world where such species richness is often used to guide conservation choices. For example, believers in the IDH should not be surprised that plots in logged over forests are frequently richer in species than unlogged forests, and this perspective allows us to recognise that 1) this is a transient state and 2) the extra species are often (but not always) “weedy” species of lower conservation significance than the species they replace — see e.g. here. In different contexts, grazing is often used to maintain a high diversity of forbs in grassland communities that would otherwise become dominated by competitively superior grasses, and management is applied because the additional species are valued for conservation. These widespread practices acknowledge that disturbance is a natural property of plant communities and that local scale, short-term stability is neither desirable nor something that conservation managers should seek to impose. As an aside, the prevalence of practices that manipulate disturbance regimes (grazing, fire etc) to enhance the diversity of terrestrial plant communities provides anecdotal support for the idea that conservation managers think they achieve this impact (we recognise that understanding such effects requires much more than Connell’s IDH).
One issue of key importance is to identify the kinds of data that might represent a valid test of the IDH (or of its underlying assumptions and requirements). Spatial scale is fundamental because the IDH suggests that sampling should match the spatial scale of the disturbance regime. Successional change also has an implied temporal dimension, and appropriate scales may not be intuitive or easily studied. Pollen core records capturing community composition over long time sequences and containing evidence of disturbance (such as charcoal) may be a source of data for testing large-scale IDH predictions if data of appropriate species-level resolution and sufficient spatial replication can be found – are there any useful studies out there?
Anyway, that’s a few thoughts to get things going. You know what Jeremy thinks, and now what we think too, but what do you think? We look forward to hearing your thoughts,
-Douglas and David
Related papers from Douglas and David