Earthworm invasion reduces above-belowground biodiversity and ecosystem multifunctionality
This study demonstrates that earthworm invasion in North American forests reduces overall ecosystem multidiversity and multifunctionality through both direct and indirect pathways, highlighting the critical need for a whole-ecosystem perspective to fully understand the complex consequences of biological invasions.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer
In the quiet, dark world beneath the forest floor, a complex web of life operates largely out of sight. This subterranean realm is home to a staggering variety of organisms, from microscopic bacteria and fungi to tiny worms and larger soil animals, all working together to keep the ecosystem healthy. Scientists call the variety of these different life forms "biodiversity," and they measure how well the soil performs its many jobs—such as recycling nutrients, storing carbon, and supporting plant growth—as "multifunctionality." When this delicate balance is disrupted, the entire forest can suffer. For decades, researchers have watched how human activities like deforestation or pollution threaten these underground communities, but one major force has often been overlooked: the arrival of earthworms in places where they do not naturally belong. While earthworms are famous for helping farmers in their native ranges, in the forests of northern North America, they are invasive guests that arrived after the last ice age, fundamentally reshaping the soil from the ground up.
A team of researchers set out to understand exactly how these invasive earthworms change the forest floor, moving beyond simple observations to map the complex chain of cause and effect. They traveled to four forests in Canada and the United States, areas known for having clear boundaries where earthworms have recently invaded. In these locations, they established small plots of land, some heavily populated by earthworms and others with very few or none. In each plot, they did not just look for worms; they conducted a massive inventory of the entire underground world. They measured the soil's physical properties, such as how much water it held and how loose or compact it was. They counted the different types of microbes, plants, and animals living there, and they tested sixteen different functions the soil performed, from how fast it decomposed leaves to how well it held together against erosion. By gathering this vast amount of data, the scientists could build a detailed picture of how the presence of earthworms rippled through the entire system.
The researchers found that when earthworms invaded a forest plot, the total variety of life in the soil dropped significantly. This loss of biodiversity was not just a minor side effect; it was a central part of the problem. The earthworms acted as powerful engineers, burrowing through the soil and eating the layer of fallen leaves that usually covers the ground. This activity changed the environment, particularly by reducing the amount of water the soil could hold and altering how much light reached the forest floor. These changes in the physical environment, combined with the direct loss of species, led to a decline in the soil's ability to perform multiple functions at once. In short, the invaded forests became less capable of supporting the complex web of life and the essential services that keep the ecosystem running.
However, the story is more nuanced than a simple case of "worms bad, soil good." When the scientists looked closer at the specific groups of organisms, they discovered that the earthworms did not affect everyone equally. The animal life in the soil, such as tiny mites and larger insects, suffered the most, likely because the earthworms removed the leaf litter that these creatures relied on for food and shelter. Surprisingly, the variety of microbes did not show a clear decline in the same way, suggesting that the impact on the microscopic world was more complex or perhaps masked by the mixing of different soil layers. Furthermore, when the researchers looked at individual soil functions, they found that some were hit hard while others remained untouched. For instance, the ability of the soil to store carbon and nitrogen was negatively affected, and the growth of woody plants like tree seedlings was reduced. Yet, other functions, such as the growth of grasses and herbs, were not significantly changed.
A key discovery in this study was that the negative impact of earthworms on the forest's overall health could not be explained by looking at just one factor. The researchers used a sophisticated method to trace how the invasion spread its effects. They found that the earthworms hurt the forest in two main ways: by directly changing the soil's physical conditions and by reducing the total diversity of life. Interestingly, when they tried to explain the damage by looking at specific groups of animals or plants separately, the connection to the overall loss of function disappeared. This suggests that the true danger of the invasion lies in the combined loss of many different types of life working together, rather than the loss of any single group. The earthworms disrupt the entire system, creating a cascade of changes that weakens the forest's resilience.
The study concludes that while earthworms are vital for nutrient cycling in their native habitats, their introduction to northern North American forests acts as a major stressor that reduces both the variety of life and the health of the ecosystem. The invasion does not just swap one species for another; it restructures the environment in a way that makes it harder for the native community to thrive. By measuring the forest from the soil microbes up to the trees, the researchers showed that the loss of biodiversity and the degradation of soil function are deeply linked. This work provides a clearer, more complete understanding of how biological invasions can silently undermine the integrity of our forests, highlighting that protecting these ecosystems requires looking at the whole picture, not just the individual parts.
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