Wikipedia defines bioturbation as the reworking of soils and sediments by animals or plants. Here is a video of a system with and without soil fauna such as earthworms, mites and isopods over a 15 week period: this is what is happening to the fallen leaves all over the reserve.
Southwick Country Park Nature Reserve 
Fascinating video – thanks for sharing!
Our autumnal gardens should be seething with life; so many so-called good gardening practices are, in fact, destructive of our invertebrate wildlife.
The Park’s soil should be like the right hand video. I wonder what the soil in the upper part of Simpson’s Field is like with all the cars that have parked on it.
Good question, David.