In the mountains we usually study, plant invasion is often only in its earliest phase, with no more than a few individuals established at high elevations. In these circumstances, the measurable impacts of plant invasion are currently virtually zero. To see the impressive impacts plant invasions can have on an ecosystem, we will have to turn to other study systems for an example.
A very clear example of these impacts is given by the pine encroachment in the otherwise treeless steppe communities of Chilean Patagonia. Pine plantations (in this case Pinus contorta) in this area date back to the second half of the previous century. As the pines appeared to like the climate in the region and the open space in the native steppe, these trees tended to escape easily and happily from the boundaries of their plantations to invade the openness around them.
Needless to say that this pine invasion has a dramatic effect on the vegetation. Where an open, low-statured vegetation had been for ages, now darkness increased. For the typical steppe species, this shady canopy is not a welcome change, as they were adapted to the sunny conditions they used to live in.
The negative effects become obvious really fast: 50% reduction in species diversity and important shifts in the species community, with the few species that do like the shade having important advantages.
With the invasion of these pines, we thus get a virtually complete shift in the species community, as a whole new system is formed. And that, my friends, is a serious impact for just one introduced species, with possibly severe implications for the conservation of biodiversity and ecosystem functioning wherever they invade.
Bravo-Monasterio P, Pauchard A, Fajardo A (2016). Pinus contorta invasion into treeless steppe reduces species richness and alters species traits of the local community. Biological invasions.