Introduced species have a bad reputation. It has been believed 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 by Dov Sax of Brown University, thus asks how the benefits of introduced species might be better assessed, so that opinions can be more informed. Specifically, he identifies several aspects for that.

Initially, whether introduced species provide direct human advantage is taken into account. Dr. Sax and his colleagues ignored crops, since their benefits are obvious. But they included transplanted grass species that have gone wild，yet provide grazing(牧草)for domestic animals, and introduced forest trees that produce wood for construction.

Another factor is their possible benefit to the ecosystem into which the introduction has happened. Such introduction is sometimes made to reduce the risk of a localised species becoming extinct. Pyne’s ground plum(李子)，native to a handful of sites in the central basins of Tennessee but now transplanted to others, falls into this category.

The last value is experienced on an emotional rather than a practical level. Lots of people feel good about native wildlife, which is generally the main motive for its conservation. But that feel-good factor can extend to introduced species as well. Such value can cut both ways, however. For example, ring-necked parakeets, an Asian and African species, have been spreading through Britain for several decades. Some find them a colourful addition to the local wildlife, others a noisy competitor for native birds.

In light of their analysis, Dr. Sax and his team therefore suggest that researchers studying introduced species should in future create a clear distinction in their studies between changes that have happened and judgments about the value of those changes. In addition, when making those judgments, they should acknowledge all types of values, rather than focusing narrowly on one or two of them.

That done, many species will surely still be accused of possible damage. But others, badly thought of in the past, may not.

A recent study confirms that several turtle species(海龟种类) have either greatly dropped or disappeared from various sections of the Australian Murray River. “The problem is that the long life of turtles makes people unable to sense the seriousness of the situation,” says Ricky Spencer, a co-author of the study, which was published in February in Scientific Reports. “It’s human nature that only when something is gone do we start missing it.”

Spencer and his workmates recorded the population of three once common turtle species at 52 sites along the southern areas of the river. The researchers inferred the species’ population sizes from the number of individuals they caught in a given amount of time. They found the turtles have disappeared in places where they were previously rich, and most of the individuals they managed to catch elsewhere were large—and likely old—adults. Spencer and his workmates blame the losses on continuing nest predation(捕食) by foxes, accompanied by other problems, including a worse and worse environment and serious drought(干旱) in the 2000s.

“We have known about the sudden sharp drop of the population of the turtles for tens of years, and although media have covered a lot about the ‘trouble of our rivers’, nothing has been done,” says Rick Shine from Macquarie University in Sydney. “This paper is a wake-up call that unless we begin to do something about turtle protection, we may lose an attractive part of our native creatures.”

The turtles could recover quickly if action is taken to protect nests from foxes and bring back living areas, Spencer notes. But governments tend to respond only when losses reach crisis levels, and the Murray River species currently lack the government’s protection, he says. He and his workmates have a solution, however. “Our next step is to start designing community protection efforts for common turtle species,” he explains, “so people can actually do things without having to wait for the government’s support.”

Throughout history, many species of animals have been threatened with dying out. When Europeans first arrived in North America, more than 60 million buffalo (水牛) lived on the continent. Yet hunting the buffalo was so popular during the 19th century that by 1900 the animal’s population had fallen to about 400 before the government stepped in to protect the species. In some countries today, the elephant faces a similar challenge, as illegal hunters kill the animals for the ivory (象牙) in their tusks.

Yet not all animals with commercial value face this threat (威胁). The cow, for example, is a valuable source of food, but no one worries that the cow will soon die out. Why does the commercial value of ivory threaten the elephant, while the commercial value of beef protects the cow?

The reason is that elephants are a common resource, while cows are private goods which only belong to their owners. Elephants wander freely without any owners. The hunter has a strong motivation to kill as many elephants as he can find. Because the number of illegal hunters is big, each has only a little desire to protect the elephant population. By contrast, cattle live on farms that are privately owned. Each farmer makes great effort to keep the cattle population on his farm because he gets the benefit of these efforts.

Governments have tried to solve the elephant’s problem in two ways. Some countries, such as Kenya and Uganda, have made it illegal to kill elephants and sell their ivory. Yet these laws have been hard to put into effect, and elephant numbers have continued to reduce. By contrast, other countries, such as Malawi and Namibia, have made elephants private goods and allowed people to kill elephants, but only those who own these elephants.

With private ownership and the profit (利润) now on its side, the African elephant might someday be as safe from dying out as the cow. The ancient Greek philosopher Aristotle pointed out the problem with common resources: “What is common to many is taken least care of, for all men attach greater importance to what is their own than for what they have together with others.”