"The fact that the goats and hogs destroyed all the young trees as they sprang up, and that in the course of time the old ones, where were safe from their attacks, perished from age, seems clearly made out. Goats were introduced in the year 1502; eighty-six years afterwards, in the time of Cavendish, it is known that they were exceedingly numerous. More that a century afterwards, in 1731, when the evil was complete and irretrievable, an order was issued that all stray animals should be destroyed... There can be little doubt that this great change in the vegetation affected not only the land-shells, causing eight species to become extinct, but likewise a multitude of insects." Darwin, Voyage of the Beagle
Darwin has left England, sailed around South America, up to Galapagos, out to Pacific Islands, down to Australia, over to Mauritius, around Cape Horn and is now back in the Atlantic. St. Helena is a small island between Africa and South America.
We often think of the problem of invasive species is one of modern times. Indeed it is. But invasive species have been a problem as long as Europeans have been sailing around the globe - since the 16th century.