Sometimes, a wingless insect that’s only slightly larger than the period at the end of this sentence is all it takes to throw an ecosystem out of whack.
Take the hemlock woolly adelgid. The insect, which is similar to an aphid, is ravaging Eastern hemlock forests by sucking nutrients from trees and simultaneously injecting them with a toxin against which they have no natural defense.
James D. Lewis, Ph.D., associate professor of biology, said this activity has the potential to disrupt a lot more than just the life of one type of plant.
“If we lose the hemlock, we’ll see a potentially dramatic shift in the structure of the forest. We’re going to lose a habitat for animals; we’re going to lose the coolness that’s associated with hemlocks in the summertime and the warmth that’s associated with hemlock in the wintertime,” he said.
Thanks to the research that Lewis and his colleagues are conducting at the Louis Calder Center Biological Field Station in Armonk, N.Y., people have a better understanding of threats to biodiversity such as the woolly adelgid. When a hemlock dies, for instance, it sets off a domino effect, much of which is hidden from the naked eye.
On the trees’ roots are mycorrhizal fungi, which help them absorb minerals in exchange for carbohydrates. If the trees die, the fungi starve, and with fewer fungi in the system, trees that are not hemlocks have an increasingly difficult time growing.
Then there’s the nitrogen released by the dead tree. It can be a valuable nutrient for other plants, but too much can pollute water systems. Excess nitrogen, Lewis said, is one of the biggest threats to the reservoirs that supply New York City.
“It’s part of the reason they’ve discussed building a $10-20 billion filtering plant,” he said.
If hemlocks do not survive adelgid infestations, they’ll join elms and chestnuts on a list of species that were once plentiful in the Northeast but now are rare. When it comes to invasive species, there is no shortage of subjects to study, as even maple trees, the source of fall foliage and syrup, are being attacked by Asian long-horned beetles.
In addition to invasive species, Lewis focuses on man-made threats to biodiversity, from the mundane (people trampling plants) to the massive (global warming). All pose threats that are poorly understood but potentially have grave consequences.
“Typically, communities that have more biodiversity are more stable,” Lewis said. “They persist for longer and they’re more capable of withstanding disturbances like a hurricane or a drought. Communities that have very little biodiversity are much more prone to going extinct if disturbed by climatic events, fires or humans.”
Along with nitrogen, phosphorus is the other key plant nutrient that Lewis has studied. A paper he co-wrote, “Phosphorus Supply Drives Nonlinear Responses of Cottonwood (Populus deltoides) to Increases in CO2 Concentration from Glacial to Future Concentrations” (New Phytologist, 2010), has received a great deal of attention in Australia and Africa, where phosphorus, in contrast to North America and Europe, is more commonly limiting than nitrogen.
He wanted to know how cottonwood—a tree widely used for paper and pulp—reacts when levels of carbon dioxide (CO2) rise but supplies of phosphorus dip. Experts predict that at the current rate it is being mined, phosphorus supplies will be exhausted in 20 years, so Lewis said it’s important to know if the levels of CO2 in the atmosphere—which have been rising—might help offset this loss.
“Cottonwood exists from the East Coast all the way to the West Coast, so we thought to look at that as a potential canary in the coal mine,” he said. “If we start seeing phosphorus limitation become an issue, what might happen, and how might the forest respond to it?”
The study, which took 20 years to complete, revealed that increased CO2 does not make a very large difference in dogwoods with poor levels of phosphorus, a finding that should serve as a wake-up call, he said.
Increased urbanization is another challenge to biodiversity, and for this research, Lewis said New York City is a great laboratory. From the heat island effects of concentrated asphalt, steel and glass to increases in nitrogen from industry and CO2 from vehicles, the influences that cities have on plants is voluminous.
“You have all these indirect effects from people, so teasing those things apart can be really challenging. But that’s what makes it exciting, because then you have all these things you can look at. It’s not just that we’re going to look at how water or nitrogen affects plants. We have all these things we can play with to determine how plants are being affected by humans.”
Lewis said working with local land managers also gives them a chance to make a real difference in the quest to preserve biodiversity.
“They’ll say, ‘We have a preserve that’s 50 acres and surrounded by subdivisions. What can we do to protect the species that are there? What do we do to manage the plants so we can have birds and butterflies and bees and the things that people really enjoy seeing?’” he said.
“We can go in and start to tease apart the threads, and start to see what the key things are they have to focus on.”
As for future research, Lewis is interested in the effects of droughts on local ecosystems. Although droughts are mostly a West Coast phenomenon in the United States, they do happen periodically in the East.
“If we have a drought, and then we layer on top of that the heat island effect, and we layer on top of that a temperature change associated with climate change, we’re potentially pushing these plants beyond what they can handle,” he said.
“We’ve started looking to see to what extent is drought killing the trees around here, and to what extent does drought make trees more susceptible to pests like the emerald ash bore and the hemlock woolly adelgid?”
“We deal with a lot of really morbid issues in the lab,” he joked. “We should call it the plant morbidity lab.”