That was a crude sign, regardless. The sign said "Warning! Damage caused by beavers".
This is the end of the article:
Beavers were once almost hunted to extinction in Poland. By 1928, it was estimated that only 235 remained in the country. However, thanks to postwar efforts to reintroduce the animals into the Polish wilderness, the beaver population has grown and is currently estimated at around 150,000.
Ecologists note that beavers play an important role in tackling drought by retaining water through the dams they build. Those dams also create “oases of biodiversity”, notes the state water agency PGW WP.
However, the rodents can cause damage to roads, houses and farmland, which has led some to call for culls to keep beaver numbers down.
From this description, it seems like beavers were building there first, and their constructions were more important to the ecology of the land, so isn't it more correct to say that it's the thing that humans did afterwards that is the damage, and that the beavers' constructions are the ones that are supposed to be there?
Also, even if you think that the beavers are causing damage, it's sort of rude to say that on the sign. If somebody called Daniel Tilles was to drive his car badly and cause damage to a guard rail along a road, are we going to put up a sign saying, "Warning! Damaged caused by Daniel Tilles." I think that would just be rude.