Succession is a natural process in which simple ecosystems are replaced with more complex ones because of changing circumstances often caused by those earlier ecosystems. But sometimes succession can also be reversed. Sometimes animals can halt succession by eating away certain plants forcing the ecosystem to stay at a certain stage or sometimes even making it to return to a earlier stage. Greylag Geese are one of those species. In our climate zone a shallow lake will be overgrown with water plant, later being replaced by reed and eventually being turned into a wet forest. Greylag Geese feed on reed, especially its roots, taking this away and keeping large parts of marshland open.
In this picture you see a Greylag Geese flying over such a reedland, probably on its way to feed some more.