In the Galápagos, the swamp hibiscus has long been high on conservationists' hit lists. This pale, yellow-flowered bush, thought to have invaded the islands from elsewhere in the Pacific, has been rapidly overtaking native habitat in the Ecuadorian archipelago. Eradication and control plans were in the works.
No longer. A study of fossil pollen collected from Santa Cruz Island in the center of the Galápagos has revealed that the swamp hibiscus was growing in the islands thousands of years before humans arrived. It is a native plant making a comeback, not an invasive on the rampage, says study co-author Alan Tye, a conservation biologist at the South Pacific Regional Environmental Program, based in Samoa. "Now we think [the spread] is natural."
The Galápagos are home to 500 native flowering plants and ferns, and 95% of the islands' biological diversity is still intact. Yet extinction rates are among the highest in the world. Invasive plants that transform habitat are the most worrisome because they may disrupt conditions that are necessary for a native species' survival; trees that move into grassland, for example, can turn the area into a forest, blocking sun from the creatures that depend upon it. Likewise, patches of swamp hibiscus (Hibiscus diversifolius) seemed to be crowding out the island's ferns and sedges.
To determine how recently the swamp hibiscus and more than 50 other weedy plants arrived in the Galápagos, Jacqueline van Leeuwen of the University of Bern, Switzerland, and her colleagues dug into the fossil record. The group analyzed the pollen in sediment cores taken from four bogs in Santa Cruz's highlands. They also looked at seed fragments from one site.
Radiocarbon dating established the ages of the sediments, some of which were 8200 years old. The researchers identified 159 different plants. Six species thought to have arrived since Europeans first landed on the islands 500 years ago proved to be native, van Leeuwen and colleagues report tomorrow in Science. One was the swamp hibiscus; another is the buttercup, Ranunculus flagelliformis, which was not noticed until the 1970s, despite intensive botany surveys before that.
The findings "demonstrate that scientific opinion about species origins is often based on circumstantial evidence and can be wrong," says Chris Buddenhagen, coordinator of the Hawaii Invasive Species Council in Honolulu, another island plagued by invasive species.
Tye now thinks that swamp hibiscus was once more common but may have been mostly destroyed by fires during the middle of the 20th century. Now it is reestablishing itself with a vengeance. "Presumably, the other plants and animals were adapted to the hibiscus" and are ultimately not in any real danger of being pushed out by its reappearance, Tye adds.