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The number of simultaneously acting global change factors has a negative impact on the diversity of plant communities—regardless of the nature of the factors. This is one of the findings of a recent study by ecologists from the University of Konstanz.

Global warming, as a component of climate change, is probably one of the best-known risks to the ecological balance of ecosystems and global biodiversity. However, ecosystems worldwide are also exposed to many other human-induced global change factors (GCFs)—with the number and intensity of simultaneously acting factors increasing. Examples include phenomena such as light pollution, i.e. the brightening of the night sky caused by artificial light sources, or the accumulation of pesticides, such as fungicides, in the environment.

"We have a pretty clear picture of how some of these factors individually can affect parts of an ecosystem, such as a plant community. In fact, their individual impacts on a community can be quite different and even opposite," explains Benedikt Speißer, first author of the recent study and doctoral student in the laboratory of Mark van Kleunen in the Department of Biology at the University of Konstanz.

What happens when an ecosystem is exposed to several of these factors simultaneously has been less well studied, although this is likely the case in most natural ecosystems.

Experiments under controlled conditions

To address this lack of knowledge, the ecologists led by Mark van Kleunen investigated how simultaneously acting GCFs affect the composition and productivity of plant communities and what role the sheer number of factors plays in this.

For this purpose, they created small artificial ecological communities—generally known as mesocosms—consisting of nine different grasses and forbs native to Central Europe, where the selected species are widespread and often co-occur. Under controlled conditions, the researchers subsequently exposed these mesocosms to different numbers—0, 1, 2, 4, or 6—of GCFs for a duration of several weeks.

"For our experiments, we selected GCFs that do indeed often act simultaneously on an ecosystem, but differ greatly in their respective chemical and physical natures," explains van Kleunen. Besides the GCFs already mentioned—climate warming, light pollution and fungicide accumulation—microplastic pollution, eutrophication, which is the accumulation of nutrients in an ecosystem, and soil salinization were additional factors studied.

Quantity not quality

The researchers found that as the number of GCFs acting simultaneously increases, so does biomass production in the plant communities. "The more GCFs, the higher the probability to include a highly influential factor, such as eutrophication. In these cases, one could expect higher productivity due to the high availability of nutrients," Speißer explains. However, the researchers' analyses showed that interactions between other factors can also contribute to this effect.

With respect to plant community diversity, the researchers found that species diversity in the mesocosms decreased as the number of GCFs acting simultaneously on the community increased—regardless of the quality of the factors involved. What is more, when considered individually, none of the GCFs studied had a negative effect on diversity within the mesocosms.

"This suggests that new effects can arise when multiple GCFs act simultaneously. Effects that cannot be predicted based on the effects the individual factors have," van Kleunen says. "Considering that the number and intensity of simultaneously acting GCFs is very likely to increase in the future, it is important to better study such 'multifactorial processes' to avoid unpleasant surprises."

The work is published in the journal Nature Communications.