10961 species of plants and animals are currently classified by the EU as alien species, a significant minority of which are dangerously invasive. Some of them were introduced deliberately, some escaped from farms or gardens and some hitched a ride in order to sneak in surreptitiously. Some of them are invisible to all but the most attentive observer, some can’t be ignored, like the swathes of Japanese knotweed tracking down the banks of European rivers and choking out the existing vegetation, some are attractive and colourful and some cute and well-loved like the American grey squirrels so familiar in English parks and gardens.
Without the usual constraints of their native ecosystems to keep them in check they have become established and spread out from their point of introduction, frequently out-competing the local species, sometimes to the point of threatening their extinction. And once established eradication is virtually impossible.
Venice, too, is invaded every two years by a tide of artists, curators, connoisseurs and culture vultures. Are they welcome? Are they invited? Do they make a useful contribution to the life of the city? Do they leave a lasting effect?