「经济学人」Long-standing mystery

Non-native species are not always harmful

THE MALAGASY baobab tree, whose thick trunks and tiny branches dot Madagascar’s landscape, should not, by rights, have survived to the present day. Scientists believe that its large seeds were once dispersed by the giant tortoises and gorilla-size giant lemurs that roamed the island. When these species went extinct over one thousand years ago owing to human activity, the baobab tree should have vanished too. It did not. Seheno Andriantsaralaza at the University of Antananarivo in Madagascar and Onja Razafindratsima at the University of California, Berkeley, now think they may know the reason why.

Together with their colleagues, the scientists monitored 15 tree canopies in a western region of Madagascar, to identify any animals that might have claimed the mantle of baobab-seed disperser. The researchers also set up camera traps around seed-containing fruits lying on the ground, and searched any faeces that they encountered along the way for the presence of seeds.

They report in the journal Biotropica that a native rodent known as the western tuft-tailed rat was caught on camera handling whole fruits on four occasions. Although there was no footage of the rat breaking the fruits open, the team did stumble upon 13 fruits that had been chewed into and had their seeds removed. Though the bite marks were not clear enough to identify a culprit, this was clear evidence that a seed-distributing animal was out there. They then found the ecological equivalent of a smoking gun: baobab seeds in seven different piles of bush-pig dung.

The discovery is not without irony. Scientists believe bush pigs were brought to Madagascar by humans sometime between 1,000 and 5,000 years ago, just as the giant lemurs and tortoises were going extinct. Their arrival was a lucky break for the baobabs—especially as scientists have determined that a seed’s journey through the porcine digestive system does not hamper its ability to germinate.

While the finding is important in its own right, it also provides valuable evidence that introduced species may not be entirely harmful. Madagascar’s pigs, for example, though not native, have made themselves essential to the survival of truly native species. Similar relationships are suspected to hold in South America between rabbits native to Europe and plants with no seed distributors. For Dr Andriantsaralaza, that suggests the full ecological role of introduced species should be considered before talk of extermination begins.(Sep 18th 2024)

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