- 2018年01月04日14:23 来源：互联网
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Invasive species——Thorny questions
The ecological effects of buckthorn in North America
CONSERVATIONISTS, being by definition conservative, usually view the introduction of new species into an environment with horror.
If such a species is successful, it is described as invasive—a rather pejorative word.
But how much change such species actually cause is often moot.
So two recent studies of European buckthorn in North America, which attempt to find this out, are a welcome shaft of light on the debate.
Buckthorn was imported from Europe to America in the 19th century, to make garden hedges.
Since then it has spread through the country's woodlands, which many naturalists think has been bad for local wildlife.
The new studies suggest they are right—if you are a frog or a small bird.
If you are a coyote or a raccoon, though, buckthorn is a good thing.
Seth Magle, of Lincoln Park Zoo, in Chicago, suspected these predators are commoner where buckthorn grows.
He therefore set up motion-activated cameras in 35 woods near the city.
Some sites had a lot of buckthorn. Some did not.
He did indeed find more coyotes and raccoons where buckthorn is common—particularly in spring, when birds are nesting.
Buckthorn is notorious for overshadowing other plants and thus inhibiting their growth.
That thins the forest understorey and increases visibility which, Dr Magle reckons, helps predators spot nests that denser vegetation would hide.
So the predators come looking.
Besides overshadowing its neighbours, buckthorn also engages in chemical warfare.
Its leaves are full of a toxin called emodin that discourages browsers.
Allison Sacerdote-Velat, one of Dr Magle's colleagues at Lincoln Park Zoo, and Richard King of Northern Illinois University, wondered what effect this chemical has on the world when the leaves fall off.
Emodin, they found, when they tested it on frogspawn, kills embryonic frogs. And when they analysed some local ponds, they discovered the chemical in them. Amphibian numbers have been falling in many places, including America, and many causes have been suggested. Emodin is surely not the only one. But it probably does not help.