Some people think gray squirrels are a problem because they eat seed meant for birds, and in some cases eat the feeders, too. A recent study in Great Britain shows that it’s much worse than that.
In the study, two woodland plots were used, both attractive to nesting woodland birds. Squirrels as well as birds populated one plot. Squirrels had been removed from the other plot. (How did they do that?)
Woodland birds had greater breeding success in the plot without squirrels. Squirrels are known to eat bird eggs.
The study report said that deer are a problem as well. They eat vegetation that provides cover for birds that nest on or near the ground.
And then there are the non-native earthworms we have in Minnesota (not mentioned in the British report). Most of the earthworms you see here are introduced species. They create problems by eating the leaves that fall to the forest floor. With luck you can actually watch leaves being pulled underground by the worms. It’s sort of spooky, no cause in sight, the leaf quietly disappearing into a tiny hole.
The nutrients the leaves contain are important to the health of the forest. If you haven’t heard of this before, keep an eye on the ground when walking in woods. See if the forest floor is covered with duff or looks freshly raked. It should not look raked.