Living With Aliens
A pair of copulating bugs march across Scott Carroll's palm, the plump female dragging the smaller male behind her. The bodies of these soapberry bugs are a vivid scarlet, their wings a deep black. Carroll, director of the Institute for Contemporary Evolution and a researcher at the University of California, Davis, has spent decades studying the remarkable ways the bugs, which feed on soapberry seeds, adapt to exploit exotic plants on their home turf. He believes that native soapberry bugs can act as soldiers in the battle against invasive species.
Introduced species are all around us, and many of them cause no serious harm. But some plants and animals that people have carried to new regions spread explosively, overwhelming native competitors and often destroying long-established ecosystems. One notorious example is the Asian long-horned beetle, whose larvae feed on cambium, the growth layer inside tree trunks. Since its introduction to the United States in the 1990s, the beetle has caused major damage to hardwood trees.
Mindful of such disasters, many conservationists advocate removing all introduced species. But Carroll sees a critical lesson in the story of the soapberry bug. It's time, he says, to come to terms with the fact that in a world swamped with invasive species, many have become permanent residents that cannot be eradicated. Using adaptable native species to help control populations of invasives, he believes, may be more effective and sustainable than trying to eliminate them altogether. He calls this concept "conciliation biology -- the process of letting go constructively."
The beak of the soapberry bug is a long, needle-like probe, used to drill through the outer wall of soapberry fruits (balloon vine, lychee, and box elder are all members of the soapberry family) and into the seed inside. Carroll has compared bugs living on native plants in Australia and Florida with those he found colonizing invasives. In each case, native soapberry bugs living on introduced plants evolved radically different beak lengths. When the American balloon vine invaded Australia, for instance, the local bugs evolved longer beaks that allowed them to reach its seed, held in an inflated capsule. This striking bit of adaptation took less than 40 years. Carroll recommends deliberately breeding long-beaked soapberry bugs and releasing them into invaded Australian rainforests, where balloon vine is smothering native plants despite conventional efforts at control.
Soapberry bugs living on introduced plants also display dramatic changes in their rate of sexual development and in the size, timing, and number of eggs they lay. All these shifts help them adjust to the fruiting patterns of an exotic host. The balloon vine native to Florida fruits year-round, for example, but the invasive golden rain tree, enthusiastically adopted by local soapberry bugs, produces seed during a single season. Bugs living on golden rain trees now produce their young seasonally, to match the availability of seeds. These changes correlate with alterations in the bugs' genetic makeup and echo the differences found between subspecies. "The genetic recipe for building a bug adapted to the invasive host," explains Carroll, "has changed so much from the ancestral state that it's a whole new flavor."
In rangelands besieged by invasive plants in the western United States, researchers have found similar patterns among native grasses. Ann Hild, a shrub-land ecologist at the University of Wyoming, has shown that native alkali sacaton grasses that have survived for decades among invasive Russian knapweed have changed noticeably. Clumps of sacaton grass look smaller, but their root systems have grown larger and are more robust. There are also genetic differences between "experienced" grass populations that have coexisted with invaders and those that have not -- and those hardened by life with Russian knapweed cope better with another invader, Canada thistle. Meanwhile, ecolo-gists at the University of Nevada–Reno have tracked changes in native grasses that manage to coexist with invasive cheatgrass. Cheatgrass has triggered sweeping changes in the vegetation of the West, leading to the loss of soil nutrients and to more frequent wildfires. But in another case of fast evolution, native grasses have begun to green up earlier, allowing them to compete with cheatgrass for scarce water supplies.
Such findings raise the possibility that native grasses surviving alongside invasives might become an important source of seed for restoration projects. Harvesting seeds of experienced plants would mean laborious collection by hand, but it could eventually prove a valuable addition to restoration. Still, Hild says, the idea will likely take time to work out. "The heart of Carroll's argument will be hard for many to accept," she notes, "because it means we're no longer striving for a pristine community of native plants." That's the most practical approach in some cases, but preventing new invasions still seems most vital. Hild and her colleagues will work hard to battle new exotics, including yellow star thistle, which are now expanding their range toward Wyoming's open lands.
Yet even skeptics can appreciate Carroll's point that evolution continues all around us, not least in habitats undergoing invasion. In his lab, the entomologist keeps colonies of the soapberry bug populations he has studied over the years. Infant bugs -- tiny, wingless creatures of bright red -- scurry alongside adults with seed-piercing beaks. An Australian bug waves the wiry, newly redesigned tool that allows its brethren to devour many millions of seeds of the invasive balloon vine. Conciliation biology offers a valuable insight, Carroll says -- not that we must give up the fight against invasive species but that we can enlist strong allies in the wild.
