Most earthworms feed on dead leaves. Some earthworm species live aboveground in the leaf litter layer, while others live belowground and dig tunnel systems. In doing so, these subterranean earthworms drag leaf litter from the surface into deeper soil layers. This improves aeration, water supply and nutrient availability in the soil, thus promoting plant growth. Earthworms also create new habitat for other soil animals and microorganisms by digging. They are thus often referred to as ecosystem engineers.
What is good here is a problem in the deciduous forests of North America. These forests have been free of earthworms since the last ice age and are not adapted to these animals. Earthworms introduced from Europe disturb the ecosystems in North America, reducing the diversity of plants and soil animals. Scientists at the iDiv research centre are investigating this phenomenon. The exhibits explain this in more detail.