When does a species become invasive? A 1920s scientist showed us, partly by accident
In 2000, Forest and Kim Starr, biologists with the University of Hawaii, spotted a lone tree with large, umbrella-like leaves. It was bingabing, or Macaranga mappa.
If you’ve been to Hilo, you may know the plant; it lines roadsides and low elevation forests. But even after driving all the roads of Maui, the Starrs only found the lone plant upcountry. Why hadn’t it invaded the roadsides here?
Bingabing could spread, it was just early in the process; the Starrr’s were actively searching for invasive plants like bingabing that hadn’t yet spread. They eventually drove all the roads of Maui at 5-10 miles per hour, finding plants like bingabing. If you were to draw a graph representing the spread of an invasive plant or animal over time, you get an invasion curve.
University of Hawaii professor and researcher Curtis Daehler studies what factors contribute to a plant’s invasiveness. “There are thousands of introduced species present in Hawaii that aren’t invasive,” he says. “The goal of a plant is to thrive, so what keeps them from spreading? Certain species have a missing pollinator, or the plant is in the wrong location,” explains Daehler.
Some species haven’t yet “If there are only a few individuals, it will take time before the population starts to increase dramatically,” explains Daehler. This is known as the lag phase, it’s an important factor for eradication. But it’s also the most difficult time to find a pest because their numbers are so low. Having better information about how quickly a species might start to spread can help with prioritizing targets.
Estimates of lag time for introduced plants were once presumed to be very long, more than 100 years in some cases. These early estimates were based on indirect evidence and didn’t focus on plants from a tropical environment. To better understand lag times, Daehler says you’d need to intentionally plant a species and watch for its spread. It turns out the Lyon Arboretum in Manoa Valley on Oahu had done exactly that.
A hundred years ago, the slopes of Manoa Valley were bare. For decades, feral cattle had grazed the slopes, causing rain to run off the land rather than fill the aquifers. Alarmed, sugar plantation sought the help of plant pathologist, Harold Lyon. He advocated for protecting the remaining native forests, but also recognized the urgent need to restore vegetation. On a plot of land that eventually became the Lyon Arboretum, he planted hundreds of species from around the world looking for ones that spread swiftly to stabilize the soil and support aquifer recharge. His staff collected detailed information as the plants spread onto nearby land and up low-elevation slopes.
Today, the ability to spread quickly into new areas is a hallmark of an invasive species. Realizing the value of Lyon Arboretum data, Dr. Daehler examined the historic planting records and the documented spread to obtain lag times for 23 plant species known to be invasive in Hawaii. He found that lag times were much shorter than previously estimated. “As soon as they reach maturity, these trees start spreading.” He published his findings to help others address known invaders early.
One of the trees Lyon studied was bingabing. Foresters planted bingabing to support reforestation on Oahu; it is now widespread in low-elevation forests. On Hawaii Island, bingabing was seeded by aircraft following a fire in Hilo. It has continued to spread from that initial introduction and today, dense stands crowd the forests surrounding Hilo.
The history of bingabing on Maui appears to be limited to single introductions. Crews from the Maui Invasive Species Committee removed the tree on Olinda Road, and plants from two other sites as well as on Lanai. Bingabing is one of a dozen plant species eradicated from Maui it could spread.
Harold Lyon may have sought out plants that would spread but he was concerned about watershed health; had he known the outcome of some of his plantings, he likely would have chosen differently.
Today, we have a choice, and a wealth of information about plants’ invasiveness, thanks in part to Dr. Daehler’s work. He helped launch the Hawaii Pacific Weed Risk Assessment (HPWRA), which serves as a background check for plants by predicting the likelihood a species will become invasive in Hawaii.
Resource managers, horticulturalists, and farmers can use the HPWRA before choosing to purchase or plant a new species, helping to prevent the introduction of a new invader. The only thing better than eradicating an invasive species early is preventing its introduction in the first place. Learn more and take advantage of the HPWRA at www.plantpo no.org.
Lissa Strohecker is the public relations and education specialist for the Maui Invasive Species Committee. She holds a biological sciences degree from Montana State University. Kia‘i Moku, “Guarding the Island,” is prepared by the Maui Invasive Species Committee to provide information on protecting the island from invasive plants and animals that can threaten the island’s environment, economy and quality of life.