Foiled foes could be one secret to the success of ever-growing armies of alien species that invade new lands, decimate the natives and take a $314 billion bite out of the world economy each year, two investigations released Wednesday reveal.
Escaping enemies that had held them in check on their home front has enabled many naturalized plants, microbes and animals -- virtually all of them transplanted by ships, planes and other modes of modern commerce and travel -- to rule the roost on their new turf at a great ecological and financial cost, researchers report in the Feb. 6 issue of the British journal Nature.
In side-by-side studies, scientists from Cornell University in Ithaca, N.Y., Princeton University in Princeton, N.J., and the University of California, Santa Barbara, reached similar conclusions on why the interlopers -- which can wreak mischief and misery -- often have the edge over the natives, and how to turn their advantage into their Achilles' heel.
An analysis of plant-health records on both sides of the Atlantic showed among 473 alien species that emigrated from Europe to the United States, those deemed "most successful" packed fewer plant diseases from their native land. The worrisome weeds -- a $50 billion a year headache to U.S. agriculture -- also were more immune to plant diseases prevalent in their new world, Charles Mitchell and Alison Power of Cornell discovered.
The findings parallel those obtained by the Princeton and Santa Barbara investigators, who studied an array of invasive animals, from mollusks to mammals.
"Introduced species may be such a problem because they leave their parasites and pathogens (disease-causing organisms) behind, just as we may have as we expanded our range across the globe," study co-author Kevin Lafferty, assistant adjunct professor of biology at UCSB and marine ecologist with the U.S. Geological Survey, told United Press International.
"Although this is bad news, it helps us to identify the Achilles' heel of introduced parasites, a weakness we may turn to our advantage if safe biological controls can be found," Lafferty added.
The results come at a time of increased efforts to control sprawling contingents of foreign organisms cutting a catastrophic course around the globe.
Scientists blame these encroachers for the extinction of dozens of local species, disruption of native ecosystems and dastardly effects on agriculture and human health.
"Invasive species are responsible for ... 42 percent of the endangered and threatened species in the U.S.," the second greatest threat to biodiversity, after habitat destruction, said David Pimentel, a Cornell ecologist not involved in either study and author of the book, "Biological Invasions: Economic and Environmental Costs of Alien Plant, Animal, and Microbe Species."
In the United States, Pimentel continued, "invasive species are causing more than $137 billion in damages each year, and in just seven nations a conservative estimate is $314 billion per year."
Non-natives comprise more than 70 percent of the weed species and 65 percent of the plant pathogen species that are the bane of farmers' existence, Pimentel, professor emeritus of ecology and evolutionary biology, told UPI.
"Invasive species are a major threat to public health worldwide, and I mention only a few species: malaria, flu, tuberculosis, AIDS, West Nile virus," he said.
The consequences can range from decreased food production and increased need for pesticides to multi-million-dollar-a-year economic losses for such remedial work as cleaning pipes clogged by zebra mussels around the Great Lakes area, Lafferty explained.
"Invasive species can be a real bother," said biologist Keith Clay of Indiana University in Bloomington, who analyzed the findings in an accompanying commentary. "Much effort is devoted to controlling them after they are established, but a better understanding of why species become invasive offers the possibility of taking pre-emptive measures."
The new findings should encourage strategists to look for weed-control agents in the invaders' native and adopted lands, the ecologists said. However, biological measures, which could threaten native species, are no panacea, cautioned Power, professor of ecology and evolutionary biology and dean of Cornell's graduate school.
She and her team studied records on viral and fungal infections of 473 European plant species, including leafy spurge, sulphur knapweed and Russian thistle, in their native and adopted habitats. They found those species that had moved abroad, on average, have 77 percent fewer diseases than do those that stayed home. In addition, the most parasite-infested invaders posed the least threat to agriculture, being less likely than their healthier brethren to turn into widespread, costly weeds.
"Thus, plant pathogens provide a free 'ecosystem service' of pest control for humans," Mitchell told UPI.
"This provides a ray of hope; if invasive species accumulate parasites in their naturalized range relatively rapidly, the problems they cause may be temporary," Clay said.
In the second survey, UCSB and Princeton investigators studied rates of infection by flatworms and roundworms in 26 animal species -- mollusks, crustaceans, fish, birds, mammals, amphibians and reptiles -- that had crossed over to the United States from their native Europe.
In general, the introduced populations had only half as many parasites as the natives, they found.
"On average, an animal has 16 parasites at home, but brings less than three of these to new areas that it invades," said first author Mark Torchin, assistant research biologist at UCSB. "In the new region, parasites are not well matched to novel hosts, and only about four parasites will successfully attack an invading species."
Parasites -- which can sicken, castrate, derange or even kill the hand that feeds them -- are so pervasive their lifestyle of mooching off their host is the most common on Earth, explained Armand Kuris, professor of biology at UCSB.
By leaving parasites behind, introduced species have a strong advantage over less fit native competitors encumbered by their unwanted lodgers, he noted.
The research reconciles two long-standing and much-debated theories that date back to British naturalist Charles Darwin in 1859, Power said.
"The enemy-release hypothesis argues that invaders' success results from reduced attacks by natural enemies from their native habitat, while the biotic-resistance hypothesis says invaders' impacts are limited by interactions with native species, including natural enemies, in their new habitat," she explained.
"Our study found that both factors -- enemy release and biotic resistance -- are important in determining whether an invading plant species thrives to become a noxious weed or struggles to survive."
Propelled by the globalization of commerce, alien life forms are swooping down on growing numbers of native populations, draining Earth of its local flavor and spreading a bland sameness in their wake, Mitchell said.
"Invasive species can drive out native species the same way that big corporations drive out local mom-and-pop businesses," he explained.
The consequences could be a dangerously homogenous Earth ruled by a handful of aggressive species, Clay foretold.
"In other words, if you are in India or Indiana, you will find the same rats, roaches and weeds (presuming they are tolerant of the climate)," he told UPI.
The problem is a man-driven one.
"Some species naturally migrate as part of their life cycle (e.g., birds in the winter) or to find a mate or a better food supply," Clay said. "But what we are seeing now is largely the result of humans accidentally or deliberately moving species around."
Perhaps the most famous example, he said, was the ill-fated intentional introduction of kudzu into the United States. The fast-growing vine that originated in Japan was planted in the 1930s and '40s to conserve soil, but now strangles more than 7 million acres of the deep South.
"Native species can either deal with and tolerate invasive species, or they may go extinct," Clay said. "Oftentimes, it seems that the only remaining populations of some species hang on only in areas where invasive species have not yet colonized."
An estimated 50,000 "exotics" are ensconced in the United States and more than 70,000 have invaded the United Kingdom, Australia, India, South Africa and Brazil, Pimentel said.
These include giant hogweed, a perennial herb in Britain that can severely irritate skin; root-rot fungus, a plant disease spread through Australia by feral pigs, which themselves are non-native down under; paddy brown spot, a plant disease that arrived in India on imported seeds and caused the Bengal famine of 1943 that killed 2 million people.
Next, the ecologists will study the role of herbivorous insects and soil-borne plant diseases in controlling invasive plant species.
