She the stranger, the foreigner, of alien blood and mind, did not share his power or his conscience or his knowledge or his exile. She shared nothing at all with him, but had met him and joined with him wholly and immediately across the gulf of their great difference: as if it were that difference, the alienness between them, that let them meet, and that in joining them together, freed them.
—Ursula Le Guin. Planet of Exile.
New species of plants and fungi can alter the interactions of native species. Here, the native hyperparasite of ergots, Fusarium heterosporum, has formed a new relationship with the ergot Claviceps spartinae and its grass host Sporobolus (Spartina) anglicus, both of which have been introduced to Ireland in the last 120 years.
This is not the norm. When new species are introduced to an area, they leave their old symbionts behind, and most of the species in the new range cannot interact with them. They have not coëvolved with the local community, so they are generally less connected to other species in the ecological network of interactions between species. This is one of the reasons why invasive species arise: in their introduced range, they are freed from the pressure of their native predators and parasites. This is the Enemy Release Hypothesis: release from their natural enemies gives non-native species an advantage that native species, which have coëvolved with local enemies, lack. Although some pathogens in the new range infect the invader, this is not enough to compensate for the lack of enemies from its native range1.
Himalayan Balsam is a notoriously invasive plant. Since the early 20th century, it has become extremely abundant in damp woodlands and riversides across Britain and Ireland2. Outside of its native range, it is free from its natural enemies, and so it easily outcompetes native plants in tall herb communities typical of damp ground. With explosive seedpods, it spreads its seeds far and wide. As an annual, it rapidly outgrows other species, shading them out, before dying back in the winter to leave bare soil that is easily eroded by rain. It has more than earned its reputation, leading to the erosion of riverbanks and a drop in the biodiversity of the communities it smothers.
All is not lost in the fight against Himalayan Balsam, though. If humans can introduce it to somewhere new, they can bring its natural enemies, too. In England, an ongoing trial involves introducing the rust fungus Puccinia komarovii var. glanduliferae in order to reduce the competitive dominance of Himalayan Balsam. Although anecdotal, early results have shown a remarkable reduction in the cover of Balsam at one trial site in the Tweed Catchment3.
to be continued...