After all the discussion on this blog surrounding Jon’s thoughtful post on Paradox of Phytoplankton and the Intermediate Disturbance Hypothesis (IDH), I was both excited and a little intimidated when I came across a review of evidence related to IDH as well as the Mass Ratio Hypothesis (MRH) I wondered, should I add more fuel to the fire, or is it best to let this article slip by? Though not my field of study, disturbance ecology and its applications to land management are interesting and as the authors point out, increasingly relevant as the frequency and intensity of disturbance are likely on the rise. Ideally, land managers could focus their disturbance management and restoration efforts in response to biodiversity and function targets that follow a predictable pattern.
My hope was that this new review would provide a cut and dry answer to the IDH debate (zombie idea?!). But as I’m learning more and more, the application of ecological hypotheses can be messy.
Here’s the problem: Even though previous reviews have shown scant evidence for ecosystems behaving consistently with the IDH (for example, see Mackey & Currie 2001), it is constantly cited and used for land management decisions. Kershaw and Mallik proposed that lack of support for IDH may be due to confounding factors such as ecosystem and landscape type and sought to test that hypothesis. They were also interested in the applicability of a lesser-cited theory, (and new to me) MRH, which suggests that diversity and productivity peak near high frequency/intensity/immediately following disturbance and then diversity drops and productivity levels-off as a result of a few, dominant species (see figure below).
Modified from Kershal and Mallik (2013), theoretical relationships between biomass, species diversity and disturbance according to the Intermediate Disturbance and Mass Ratio Hypotheses.
The authors found 60 papers that looked at terrestrial vegetative communities and evaluated their results for compliance with IDH and MRH, then looked at whether compliance correlated with ecosystem or landscape type (ie grassland vs forest, upland vs lowland, productive vs less productive) and though their results weren’t as enlightening as I’d hoped, they did differ from previous reviews and offered some credence to their suggestion that the applicability of IDH and MRH are dependent on other factors—in other words, it’s more complicated. Interestingly, they found that about half of the papers that looked at either IDH or MRH found evidence consistent with the theory in question, this contrasts with the mere 16%-21% rate of previous reviews.
Kershaw and Mallik conclude that IDH and MRH still have usefulness but in different contexts: MRH shows more applicability in low productivity systems with poor drainage and high organic content whereas IDH is more consistent with productive upland habitats. But why? Their restriction to terrestrial systems overall suggests that both of these theories hold greater applicability outside of the aquatic realm. But again, why? The article falls short at suggesting testable explanations for these results.
Ultimately, an understanding of the underlying unique mechanisms for responses to disturbance, rather than depending on one or more simplified theories that don’t yet have predictive power, may be more useful to land managers. Of course, a predictable relationship between ecosystem functions (eg productivity), biodiversity, and disturbance/stability would be neat and tidy, but the world is rarely neat and tidy.