Droughts, downpours, pest outbreaks – disturbances like these are commonplace in all natural ecosystems. But how well can biocenoses recover subsequently? This was the subject of a study by Professor Helmut Hillebrand and Charlotte Kunze of the Institute for Chemistry and Biology of the Marine Environment (ICBM) and the Helmholtz Institute for Functional Marine Biodiversity (HIFMB) at the University of Oldenburg.
In a meta-analysis, the two environmental scientists evaluated 508 existing studies, primarily experiments in which natural or near-natural ecosystems were briefly brought out of equilibrium and subsequently observed. As the researchers report in the journal Ecology Letters in January 2020, the biocenoses performed their function in the ecosystem – for example as oxygen and biomass producers – almost as well at the end of the experiments as in the undisturbed controls. However, the species composition did not recover.
“While the number of species was generally almost as high at the end of the experiments as before the disturbance, they were not the same species,” is how Hillebrand summarises a central result of the analysis. “In other words, even if the original environmental conditions were present after a disturbance, the same species did not necessarily re-establish themselves.”
At the end of the experiments, the disturbed systems had on average 82 per cent of the original biomass. Certain indicators of biodiversity also returned to high levels. By contrast, the species composition was almost as far-fetched from equilibrium as it was directly after the disturbance. The researchers conclude from this that the composition of ecosystems recovers only slowly after disturbances such as storms, floods or pest outbreaks.
Human interventions such as ploughing, mining or fishing put ecosystems under additional pressure, just like climate change or large-scale eutrophication of waters. Environmental scientists have observed that the number and scale of disturbances is currently increasing, and entirely new kinds of disturbances are arising compared to the pre-industrial era. “As a result, it’s becoming increasingly important to understand what consequences disturbances have for a biocenosis,” Hillebrand continues.
In their analysis, Hillebrand and Kunze covered experiments on all types of ecosystems – marine, land and freshwater. The experiments took place worldwide, but were concentrated in moderate regions.
The two researchers have presented the most comprehensive overview of this issue to date. Their central result – that the functions of ecosystems recover faster than their species composition – is very important for the management of disturbed ecosystems, for example where a biocenosis has to be restored.
Helmut Hillebrand and Charlotte Kunze: “Meta-analysis on pulse disturbances reveals differences in functional and compositional recovery across ecosystems”, Ecology Letters (2020)