We've read and heard a lot about invasive species. They are infamous for taking over where native species once were common. It's understood that the "invaders" outcompete the "natives" for a variety of reasons. And the general consensus is that these non-native or "exotic" species are somehow undesirable.
One thing that most of us don't think about is how long humans have been introducing "non-natives" to the landscape. It's arguable that as long as our species has existed we have participated in the movement and transplantation of exotic species from one part of the globe to the other. Certainly when transcontinental travel picked up in the 15th and 16th centuries new the rate of introductions increased. By the 20th century there were few if any places of human habitation without introduced species.
Another thing we don't think about is that most of the species we transport are small. Invisible invaders have changed the landscape more than larger organisms. I was thinking about this last week when I was up in Maine at Victor's place.
The prettiest little chestnut trees were lit up by the morning light in the understory of the forest, their dry brown leaves rustling in the gentle wind. I remembered that these small insignificant and rare trees were once the dominant species in the eastern woodlands of North America. What happened?
In the early 20th century some decorative trees from east Asia were brought to grace the grounds of the New York Botanical Gardens. The trees carried with them a fungus that was inconsequential to its natural host, but which soon spread to the nearby chestnut forests.
The microscopic fungus invaded the vascular tissue if the chestnuts, literally choking the trees with a proliferation of fungal growth. No chestnut was immune from the fungus which spread to the entire eastern chestnut forest. Within a decade. small and large trees alike were killed and the landscape was permanently changed as the chestnuts died out and other trees took their place in the first canopy.
Chestnuts resprout from underground tissue. The small ones we see today are the resprouted clones of older trees. They may survive for 10 or 20 years above ground, gathering energy and thriving but never making it to maturity. Ultimately microscopic fungal spores find their way to the young trees and cause their premature death.
It's kind if a sad story but one that tells us about how species interactions can work to alter the landscape. It also drives home the fact that the natural history of introduced species has been going on for longer than we might expect, a history with unexpected microscopic players like the fungus that destroyed the chestnut forest.