Eliminating goats on the islands that once inspired Charles Darwin have helped boost native species.
Giant tortoises and albatrosses on Galapagos have made a comeback.
Meanwhile, many native plants on the islands are threatened by invasive, non-native plants.
The eradication of non-native goats have helped native animals thrive.
The islands that inspired Charles Darwin to formulate his famous theory of evolution have seen in an impressive comeback among its giant tortoise and albatross populations, while many of its unique plant species remain threatened by non-native varieties.
In inhabited areas of the Galapagos, non-native plant species now outnumber native plant species by a factor of almost 1.6 to one, according to a survey published in the journal PLoS ONE.
Meanwhile, giant tortoises, albatross and native cactus are doing well on the uninhabited island of Española, according to a new assessment of their numbers, following the eradication of goats introduced to the island centuries ago, and a successful tortoise breeding program.
"We're winning on the uninhabited islands and we're losing on the inhabited islands," said Alan Tye of the Secretariat of the Pacific Regional Environment Program in Samoa, who studied invasive species on the Galapagos for more than a decade.
"The problem is that the inhabited islands are four of only seven islands that have humid highlands, and they are the largest. If things carry on as they are, we're going to see quite a lot of extinctions," he said.
The new plant survey took a detailed look at the inhabited islands, visiting nearly every property to catalog the species found there. The team catalogued 754 different alien plants, Of these, 257 were not previously known to be in Galapagos. Some of the worst invaders include guava, the quinine tree and a type of blackberry.
The humid highlands, the islands' agricultural areas, host the greatest abundance of invasive species, said study authors Anne Guézou and Mandy Trueman of the Charles Darwin Foundation in Galapagos.
In the towns, non-native species are mainly ornamental plants found in people's yards, but some of these could become invasive if they escape and start reproducing on their own, they said.
"Plants are a lot more insidious than animals," said Mark Gardener, Director of Terrestrial Sciences at the Charles Darwin Foundation, but not a part of the study. "Plants aren't like goats, unfortunately. Plants have seeds and you can't find every seed."
While plants may not capture the public's heart like giant tortoises, they provide essential habitat for tortoises and other species. Darwin's famous finches rely on trees that have suffered from clearing of land and from invasive blackberries, Trueman said.
The good news is that on the uninhabited island of Española, several native species are thriving after the elimination of feral goats on the island.
"I think there are something like double the albatrosses there were before, which is great news," Gardener said.
"There are very few invaders on the uninhabited islands," Tye said. "Goats are gone and we're starting to work on the rats and we're seeing fabulous regeneration. Once you get rid of those animals the vegetation comes back pretty resilient."
Giant tortoises on the island represent a huge success, bouncing back from only 15 individuals to more than 2000 with the help of captive breeding. The survey showed that the tortoises released on the island are now breeding on their own.
Surveyers also found that a cactus native to Española, devastated by goats, was coming back now that goats are gone.
Conquering the problems on inhabited islands won't be easy, Tye said. "There are fundamental problems in Galapagos with land use planning an management of what people bring into the islands and how they manage domestic plants and animals. It's fundamentally a social issue."
Bringing non-native plants to Galapagos has been banned since 1999. "People are bringing less," Guézou said, "but it is still happening. The quarantine system is not 100 percent perfect."