By Whitney Heins
The hemlock woolly adelgid. Asian carp. Kudzu. What do these three things have in common?
They are dangerous killers transported from other parts of the world years ago that are now disrupting the ecological equilibrium in several areas of the United States.
These invasive species are wreaking economic, ecological, and environmental havoc because no natural predators, parasites, or diseases can keep their populations in check.
Dan Simberloff, distinguished professor and the Gore Hunger Professor of Environmental Studies in Ecology and Evolutionary Biology, has dedicated his life to studying the threat of invasive species, which continues to grow as the world’s borders erode.
“I’ve always been interested in how certain species fit together or don’t fit together in ecological communities,” explained Simberloff, who, as a boy growing up in rural Pennsylvania, collected and pinned insects, read insect books, and started his own ‘Society of Entomology’ at the age of four. “That is how I got interested in biological invasions, because you have these species being inserted into a previously existing ecological community, and either they survive or they don’t.”
Simberloff’s curiosity spurred a distinguished fifty-year career that earned him the highly coveted election to the National Academy of Sciences, the Eminent Ecologist Award in 2006, and the Ramon Margalef Prize in Ecology in 2012.
In 1971, he won the Mercer Award for his work with his Harvard University doctoral advisor, Edward O. Wilson, which tested the theory of island biogeography.
Simberloff’s research has informed policy, including President Bill Clinton’s executive order 13112 to create the National Invasive Species Council.
The 800-page Encyclopedia of Biological Invasions, co-edited with Marcel Rejmánek of the University of California, Berkeley, is a direct result of Simberloff’s investigations. He also has written a new book, Invasive Species: What Everyone Needs to Know, to help the public better understand the threat.
An Island Conundrum
Simberloff, who also directs UT’s Institute for Biological Invasions, has been around the world with shovels, nets, and traps studying fire ants, moths, mongooses, and weasels.
One of his recent stops was an island sitting in a large glacial lake in the Patagonian region of Argentina. What piqued Simberloff’s interest was its population of more than a hundred types of trees from every continent but Antarctica.
“Imagine clumps of ponderosa pines intermingled with fruit trees, Douglas firs, and maples,” Simberloff said. “Trees sandwiched inside a small park on a small island only twenty kilometers long and four kilometers wide.”
Known as Isla Victoria, it was the site of a government experiment initiated in the 1920s to find the fastest growing trees. The Argentines abandoned the project a decade later and let the greenery run amok.
Today, the island may look unharmed. Some might even call it beautiful. But beneath its beauty lurks the potential for ecological disaster.
“Many of these tree species, when introduced, lead to invasions in which they turn prairies into forests, cause conservation problems, and threaten the existence of native species,” Simberloff said.
In 2001, Simberloff and Martin Nuñez, an Argentinian graduate student and later post-doctoral researcher at UT, pulled on their hiking boots, grabbed some clippers, and headed out to census the implanted trees. They counted 1,643 trees, including sixty-two broad-leaf tree species–about a third were invasive–and seventy-three conifers, including sixty percent of the world’s most invasive conifer species.
While these species have had dire ecological consequences in the past, strangely this was not what Simberloff observed on Isla Victoria. “Even if they were famous invaders, they weren’t spreading,” Simberloff said. “This was puzzling.”
Instead, only two of the many known invasive conifers–Douglas firs and junipers–were thriving. So the real question became, why weren’t the supposedly invasive conifers more invasive?
This led to a series of projects that pointed to a surprising answer buried deep within the ground.
First, Simberloff and his team looked high, asking the most logical question–is the climate responsible for the lack of growth? They conducted a literature review to examine the climate the trees needed in order to live and determined this was not the problem.
Next, they looked lower. As it happens, two European deer species were introduced at the same time the plantation was established. The researchers built exclosures to study the impact of the deer’s presence on the trees. They also performed feeding experiments to observe what they were eating. Although the deer browsed, trampled, and killed some trees, the impact was not significant enough to explain the phenomenon Simberloff was witnessing.
If deer weren’t the answer, maybe another species was responsible. Simberloff and Nuñez wondered if rodents were eating the seeds. To test this hypothesis, they placed seeds in areas with and without rodents to find out which seeds the rodents preferred, and the distances they carried them. The team concluded rodents were probably impeding the invasion, but only minimally.
Finally, Simberloff and Nuñez looked below the ground at something unseen but vital to ecosystems–mycorrhizal fungi. These fungi develop a symbiotic relationship with the root systems of living plants. Networks of mycorrhizal filaments envelop the seedling’s root structure, supporting the plant’s own ability to extract water and nutrients from the soil.
“The associations between trees and fungi are often host-specific,” Simberloff explained. “Mycorrhizal species will frequently associate only with a certain type of conifer tree, so the impact of mycorrhizal fungi and their absence can be staggering. The trees might stop growing and start dying.”
A series of greenhouse and field experiments tested tree growth with and without the fungi. They found new trees planted in both environments thrived with fungi while the ones without fungi did not grow.
Simberloff and Nuñez concluded that the lack of vital fungi was being caused by yet another missing element: squirrels.
“In a normal environment, fungi would be spread by an animal that would dig and spread it, like squirrels,” Simberloff said. “But this area does not have a native animal to do that.”
In essence, the mystery of why the conifer trees have not spread throughout Isla Victoria was solved. But true to his nature, Simberloff is not done asking questions. His studies continue with a secondary investigation of the impact of a wild boar invasion on the island. The boar are eating the fungi, defecating, and thus possibly accelerating the conifer population after all.
As the global economy expands, more and more invasive species will undoubtedly be revealed as scientists unravel the complicated network of interactions that lead to invasion impacts. Simberloff’s work will continue to help counter these threats and restore nature’s delicate balance.