Intermediate Disturbance Hypothesis at Small Scale

Back in the summer of 2000 I had a great field course as part of my undergrad degree. A few of us formed an informal research collective dubbed ‘The Young Lions of Science’. We took as our motto, “rar”.

Our first and perhaps best work was this study of trail-side diversity gradients. We tested Connell’s classic intermediate disturbance hypothesis, which posits that levels of species diversity will be highest in areas of medium disturbance, because you get a mix of both generalist invaders and specialist local experts. I like the study because a) it was dead simple and b) to my very limited knowledge, IDH hadn’t been demonstrated at scales anywhere close to that small and c) it worked, at least statistically. Of course, we didn’t come close to demonstrating that the relationship we observed (highest diversity in the areas that got walked on a medium amount) was either mechanistically really the IDH at work, or that it isn’t a trivially obvious application if it is, but I still have an affection for this particular bit of research.

The Paper

Testing the intermediate disturbance hypothesis at a small scale: trailside vegetation diversity. Hugh Stimson, Melissa Cameron, Tim Irvin, 2000.

“….This study could be completed by observing patterns of trail use, to determine if disturbance really does decrease steadily away from the centre of the trail. For now, we will accept that this is true, based on our personal experience and informal observations: an individual walking a trail is most likely to stick to the centre, but may on occasion have reason to stray off the edges – to observe something, to pass an oncoming trail user, or while walking two or three abreast. The further from the centre, the less often a trail user will have strayed there. Given a broad definition of disturbance – any event which kills or damages the resident biota – we make the further assumption that the differences in species richness are a consequence of disturbance levels. This seemed plausible, as there was only one observable biotic or abiotic factor, other than disturbance, which varied with distance from the trail centre. This possible confounding variable is sunlight, but the impact of this should have been minimized by our explicitly chosing trails with little tall vegetation on either side.

We therefore feel confident that high species diversity is being maintained in the middle distances as a result of disturbance, in the field area. Whether this is a result of the ecological processes suggested by the intermediate disturbance hypothesis (simultaneous presence of both colonizer and competitive species) cannot be directly inferred from this result.”