Disturbance ecology

Disturbance is defined in the Oxford Dictionary as ‘the interruption of a settled and peaceful condition’.

Nature is everything but a ‘peaceful condition’. However, without disturbance it would be a settled one. In ecology, we use the term ‘climax vegetation’ for such a settled state: an ecosystem with climax vegetation has had enough time to develop to its final stage and find a dynamic equilibrium. Without disturbance, the system would stay the same (not exactly, as individuals will die and be replaced, but dynamically).

Disturbance however, from natural (e.g. grazing) or human origin, disrupts this equilibrium. It will  shuffle the ecosystem in such a way that the previous hierarchy disappears. It is the ecological version of a coup, a revolution. It creates instability, removes those in power and provides opportunities for a new order, all of a sudden.

This sudden reshuffling of the cards in the struggle for survival is exactly what makes disturbance one of the most interesting ecological processes to study. As modern humans amply showed they are the kings of major disturbing events, those unstable conditions are rapidly taking over the world.

In alpine and arctic tundra vegetation, large-scale antropogenic disturbances are relatively new. The slow-growing tundra plants are more than any other species ill-prepared to deal with its consequences (see also the results of this paper).

The local effects of disturbance on tundra are crystal-clear. There do however remain some important questions: how is the adjacent undisturbed tundra going to react on the disturbance? Can we expect a snowball effect, where the initial climax vegetation starts deteriorating and gets replaced by a new disturbance-adapted species set? Also: is there an alternative possible vegetation on the highest elevations, or is it tundra or nothing?

All of these effects will be driven by the changes in abiotic conditions caused by this disturbance. Removing the vegetation will change temperatures, alter the duration of the snow cover or change the soil conditions. Plants will have to cope with these changes, or perish…

And in this light, the smallest scale is surprisingly relevant. Within one disturbance event, biotic interactions (both negative through competition and positive through facilitation) change over a range of only tens of centimeters. Together with these biotic interactions, the microclimate changes impressively. Over an elevational gradient, these interactions will shift the best location for gap colonisation from the gap center to the gap edge. All on this theory can be found here.

    

On a slightly larger scale, disturbance will have a big impact on the distribution of species: it was already known that species had a climatic niche and that, for example invasive species would be limited by the cold-climate in the mountains. Yet, our paper in Ecography showed how they can use large-scale disturbances that cross a large climatic gradient (i. e. mountain roads) to change their niche. More surprisingly: even alpine species change their niches along these mountain roads, they see a downward shift in their distribution.

We also showed this positive effect of disturbance on non-native species experimentally: when we varied all possible drivers of plant invasion in the mountains along an elevation gradient, disturbance came out as the big number one: if we disturb nature, invaders quickly follow. At low elevations, however, the native vegetation might be more resistant against this invasion, as it can recover much faster than at higher elevations.