When loosestrife covers a wetland with purple flowers, it flaunts the invasive power of alien plants. While scientists have barely begun to understand the full ecological impact of alien plant invasions, the global mixing of plants and animals now going on could be a major channel for unplanned environmental change.
The ecological alterations that an aggressive alien may bring vary. It can co-opt soil microbes to enhance its growth. It can deny native animals their natural food and shelter. It can create a new wildfire hazard as alien grasses are doing in some North American deserts. New research shows it can even help a new insect species to emerge.
Research on one familiar alien, the Japanese honeysuckle, reported last week in Nature magazine, illustrates this point. People brought the bushy honeysuckle to North America more than 250 years ago. The US Department of Agriculture began promoting it as a garden and wildlife plant in 1880. The plant, which produces a berry-size fruit, is now part of the wild flora of the Northeastern United States.
The tephritid fruit fly loves this kind of plant. In fact, there's a fly species specifically adapted to exploit each species of berry-producing plants. For instance, the blueberry fly goes through its entire life cycle on blueberry bushes. It can't live on any other type of plant. That's how the fly-plant relationship has evolved naturally.
The Japanese honeysuckle also has its fly. But that fly didn't originate with the plant. A Pennsylvania State University research team traced its ancestry to a hybrid produced by flies that live on blueberry and snowberry plants, respectively. Normally, such a hybrid fly strain would die out. It can't compete with either of its parent species on their host plants. Honeysuckle offered a niche with no such competition where the hybrid became a new species. The family outcast found an empty house on the block and moved in.
The research team calls this development "a novel example of how invasive weeds can influence the evolution of native fauna."
Research team leader Dietmar Schwarz and his colleagues say their findings also illustrate how man-made changes can "offer new opportunities for hybridization" as previously isolated organisms come into contact with each other.
The ecological changes brought about by the Japanese honeysuckle also give a new perspective on animal evolution. The rise of species through hybridization is common in plants. But zoologists have considered it rare in animals. "There might be a lot more animal hybrid species than previously thought if researchers take a closer look," Dr. Schwarz says.
The subtle changes that invasive species bring locally may combine globally in ways that profoundly affect Earth's web of organic life.
That's why ecologists, including Peter Moore at King's College in London, have called for vigorous international research to find out what's going on.
Dr. Moore has noted, for example, that increasing concentration of carbon dioxide in the atmosphere might favor growth of an invasive weed over native plants. He warned last year in an essay in Nature that the influence wrought by invasive species could fuel an "explosion" of ecological change "to levels beyond all expectation."