TEXT B What makes for a successful invasion Often, the answer is to have better weapons than the enemy. And, as it is with people, so it is with plants—at least, that is the conclusion of a paper published in Biology Letters by Naomi Cappuccino, of Carleton University, and Thor Arnason, of the University of Ottawa, both in Canada. The phenomenon of alien species popping up unexpected parts of the world has grown over the past few decades as people and goods become more mobile and plant seeds and animal larvae have hitched along for the ride. Most such aliens blend into the ecosystem in which they arrive without too much fuss. (Indeed, many probably fail to establish themselves at all—but those failures, of course, are never noticed.) Occasionally, though, something goes bananas and starts trying to take the place over, and an invasive species is born. Dr. Cappuccino and Dr. Arnason asked themselves why. One hypothesis is that aliens leave their predators behind. Since the predators in their new homelands are not adapted to exploit them, they are able to reproduce unchecked. That is a nice idea, but it does not explain why only certain aliens become invasive. Dr. Cappuccino and Dr. Arnason suspected this might be because native predators are sometimes "pre-adapted" to the aliens’ defences, but in other cases they are not. To test this, they had first to establish a reliable list of invaders. That is not as easy as it sounds. As they observe, "although there are many lists of invasive species published by governmental agencies, inclusion of a given species in the lists may not be entirely free of political motivation". Instead, they polled established researchers in the field of alien species, asking each to list ten invasive species and, for comparison, ten aliens that just rubbed along quietly with their neighbours. The result was a list of 21 species widely agreed to be invasive and, for comparison, 18 non-invasive aliens. Having established these lists, they went to the library to find out what was known about the plants’ chemistry. Their aim was to find the most prominent chemical weapon in each plant, whether that weapon was directed against insects that might want to eat the plant, bacteria and fungi that might want to infect it, or other plants that might compete for space, water, nutrients and light. Botanists know a lot about which sorts of compounds have what roles, so classifying constituent chemicals in this way was not too hard. The researchers then compared the chemical arsenals of their aliens with those of native North American plants, to see if superior (or, at least, unusual) weaponry was the explanation for the invaders’ success. Their hypothesis was that highly invasive species would have chemical weapons not found in native plants, and which pests, parasites and other plants would therefore not have evolved any resistance to. The more benign aliens, by contrast, were predicted to have arsenals also found in at least some native species. And so it proved. More than 40% of the invasive species had a chemical unknown to native plants, just over 10% of the non-invasive aliens had such a chemical. Moreover, when they looked at past studies on alien plants that had examined how much such plants suffer from the depredations of herbivorous insects, they found that the extent of the damage reported was significantly correlated with the number of native species with which that alien shared its principal chemical weapon. For alien plants, then, the real secret of success—also as in human warfare—is surprise. It is not that the chemicals concerned are more toxic in any general sense (indeed, successful invaders are often rare in their own native habitats). Rather, it is that the locals just don’t see them coming.
A．It is difficult to find the chemical weapons of invasive species.B．The most prominent chemical weapons of invaders explain their success.C．The less chemical weapons invaders have, the more benign they become.D．Most native plants don’t have the same chemical weapons of aliens.
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