Alien plants and animals are not all bad.
Introduced species have a bad rap.
From American grey squirrels displacing European red ones, to Japanese knotweed displacing just about everything everywhere, their purported negative effects on nature are there for all to see.
But it is only the human eye which prefers the arboreal rodents in a particular place to be red rather than grey.
Ecologically, both occupy the same niche.
Nor might people fret about knotweed growing at other plants’ expense if it did not also undermine human constructions such as buildings and roads.
Until the middle of the 20th century, moving species around the world was reckoned a normal, often valuable, thing to do, while the consequences of their accidental movement were rarely considered.
It was not until the publication in 1958 of “The Ecology of Invasions by Animals and Plants”, by Charles Elton, one of the founders of scientific ecology, that values began to change.
Conservation is, as the name implies, a conservative business.
Those involved often value “nativism” - the idea that the species mix in a particular place should remain as unchanged as possible.
But this is just an opinion.
Other opinions are possible.
A study published recently in Trends in Ecology & Evolution by Dov Sax of Brown University, in Rhode Island, thus asks how the benefits of introduced species might be better assessed, so that opinions can be more informed.
Specifically, he and his compadres have recruited the vocabulary of moral philosophy.
They thus identify three sets of reasons why an introduced species might be valuable: instrumental, intrinsic and relational.
Instrumental values are easiest to grasp.
They pertain to things that provide direct human advantage.
Dr Sax and his colleagues ignored crops, since these are heavily managed by human beings and their benefits are obvious.
But they included transplanted grass species that have gone wild, yet provide grazing for stock animals, and introduced forest trees that yield timber for construction.
One little-regarded but important example of instrumental value is the transport of Old World earthworms to North America, parts of which were left worm-free after the last ice age.
The presence of these is reckoned to have increased agricultural productivity by as much as 25% in previously worm-deprived areas, though recent work suggests other invertebrates have suffered.
Honeybees, too, are an Old World species introduced into the New.
Their instrumental benefits in the form of honey and wax are obvious.
But they also pollinate flowering plants, including many crops.
Honeybees’ role as pollinators also makes them pertinent to the second category, intrinsic values.
These pertain to the ecosystem into which the introduction has happened.
Since honeybees pollinate wild plants as well as domesticated ones, they have a positive intrinsic effect on their adopted habitats.
Introduced species can bring other benefits.
Sometimes, in a manner reminiscent of the nursery rhyme in which an old lady swallows a fly, a spider to catch it, and a bird to catch the spider, an introduction may be made to undo a previous harm.
Several introductions of damaging insects have been attacked successfully by further introductions of critters that eat them - though this has not worked so well for voracious introduced molluscs called giant African land snails.