People often think first of climate change with regard to global environmental changes, i.e., global warming caused by humans, which is increasingly leading to extreme weather conditions. Next they think of environmental pollution, over-fertilization, mass application of pesticides, soil sealing, light pollution, or microplastics. Yet these are just some of the many anthropogenic influences that affect the planet’s biosphere and permanently change it.

“For some time now, I’d been thinking about setting up a classification system because nobody knows exactly how many factors there are and what happens when they all occur together,” says Matthias Rillig, a biology professor at Freie Universität Berlin. His preliminary research showed the need for such a classification system. An analysis of more than 4000 scientific publications dealing with global change showed that in 98 percent of the work, the researchers only examined one or two factors.

Global Change Biology Lacks Overview

“Over the years, the field of global change biology has split up into so many subdisciplines that there is no overview,” explains Rillig, and continues, “For example, people who do research on invasive plants rarely speak to those who work on the effects of pesticides, increased carbon dioxide concentrations in the atmosphere, or heavy metal contamination.”

As a soil ecologist, Rillig initially concentrated on his own field. He began by considering an imaginary plot of soil with any plant on it. It could be a daisy in a park, a corn plant in a field, a gentian off an alpine hiking trail, or a barren shrub in a desert.

List of Factors to Create a Matrix

Rillig compiled a list of influencing factors that soon consisted of 30 items ranging from biocides to drought, warming, particles, heavy metals, over-fertilization, flooding, and salination. As different as these factors are, it was a bold idea to put them all into one matrix. To this end, Rillig drew up a list of questions, which also happened to come to 30, that could be asked about each factor. The questions can be clearly answered with yes or no and describe the characteristics of the factor. Together, they define the nature of the influencing factor, its effect, and its mechanism.

Rillig explains the system using fire as an example: Does the factor act simultaneously on all living beings in the system? Yes, it acts on plants as on soil organisms. Is it limited in time? Yes. Is it osmotic? No. Toxic? No, because the ecosystem regenerates after the forest fire is over. Is it a biological factor? No. A chemical one? No. A physical one? Yes. All 30 questions are to be answered following this example.

30 Factors Multiplied by 30 Questions

30 factors times 30 questions: A complex pattern of 900 boxes was created on paper. Even a layperson can see at first glance that some factors have very similar characteristics although they are fundamentally different. The effects of heavy metals, for example, are similar to land use changes, floods, or fire. Then, assuming that all factors were equal, the team ran algorithms over it.

A tree emerged from the box pattern, a kind of family tree of human-made environmental changes. Similar to the evolutionary family tree of plants, it illustrates which influencing factors are more closely “related” to one another, i.e., which are similar in terms of their characteristics.

Does Noise Affect the Soil?

Rillig makes no claim that his list of factors is complete. So one or another factor could still be added to the list. For example, what about noise pollution? “So far, noise has not been considered important for soil. We published the only work on the subject of noise, and it is being ignored by all of our colleagues,” he says with a laugh.

There is, however, experimental evidence that noise pollution is a problem in aboveground ecosystems and in the oceans. For example, ship engines and sonar devices influence the communication between dolphins and whales. Rillig immediately goes through his catalog of questions: Noise would be a mechanical factor, a physical one, and, yes, a form of energy.

Classification Leads to New Research Questions

“We want to use the classification system to illustrate how many different ways we are damaging our planet as well as how long the different factors have been playing a role,” says soil ecologist Matthias Rillig. “A lot more is involved than just climate change. Of course, global warming is an important aspect. But there are many other things, such as artificial light at night, the expansion of urban spaces, chemical pollution, microplastics, and even the coronavirus. After all, the coronavirus is also an invasive species.”

His classification of anthropogenic influences has led Rillig to consider completely new research questions. He says, “Suppose I let four factors act concurrently on the soil–plant system: Does it make a difference whether their traits are very similar, i.e., close together in the ‘tree’ or whether they are completely different? And what is less bad in the end? We simply do not yet know.” He points out that concurrent anthropogenic influences on ecosystems are not an exception, but rather the rule.

To test his ideas, Rillig’s team carried out extensive experiments in the laboratory. In one experiment, they allowed ten of the 30 global change factors to act in various combinations on soil samples without plants. The findings were published in Science in 2019. Interestingly, in his concept Rillig had been thinking of an abstract soil–plant system, but his assumptions were reflected surprisingly well in the experiments. What is more, his assumptions made it possible to predict the outcome of the experiments.

In the test series, the researchers investigated the effects of drought stress, the weed killer glyphosate, antifungal agents, microplastic fibers, table salt, and the heavy metal copper. There would have been 1024 possible combinations with ten factors alone. A small lab does not have the means to carry out so many experiments.

Therefore, Rillig randomly let one, two, five, eight, and finally all ten factors “patter” onto sample containers, each with 30 grams of soil. The result was sobering. “We saw a progressive decline in biodiversity and ecosystem functions, such as the degradation of matter and the formation of new soil structures. The more factors that were at work, the worse it got, regardless of which ones. The water-repellent properties of the soil increased dramatically once five factors were included. Rillig says he expects there will be many ecological surprises like this, but research does not currently focus on these multiple effects at all.

Rillig is now in the process of honing his classification system for anthropogenic environmental changes to consider different biological levels such as the individual, community, and ecosystems. He says that more knowledge about the extent of the anthropogenic effects is crucial to avoid being taken by surprise in the future.

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This text originally appeared in German on April 24, 2021, in the Tagesspiegel newspaper supplement published by Freie Universität Berlin.