Mountain biking is nowadays one of the most important leisure activities in Switzerland. In urban areas, trails and paths in the forest are preferred. As a result of the increased usage pressure, illegal and informal trails are sometimes built off the official routes. This development can lead to soil compaction, soil erosion, damage to vegetation and wildlife. The research group Environmental Planning deals with the effects of unofficial paths and buildings on nature and the environment. Part of the unofficial trail network on urban areas was included in this small-scale project and briefly assessed with regard to its ecological impact.