Dartmouth professor presents research on invasive species over lunch

Rebecca Irwin, associate professor of biological sciences at Dartmouth College, spoke about the impacts invasive species have on native populations during a lecture yesterday for the  Environmental Studies Lunch & Learn Program. During the event, which was sponsored by both the Environmental Studies Program and Tufts Institute of the Environment, Irwin spoke about the interdisciplinary research on which she has collaborated. 

Irwin divided the lecture into three separate discussions focusing on the causes and then the consequences of invasive species, as well as the solutions for more sustainable management of the invasive plants. She started by underscoring the role humans have played in facilitating the spread of such species.

“Humans are really noteworthy in both altering the landscape and moving species around,” she said.

Invasive species have been a leading driver of environmental change and loss of biodiversity, particularly in the United States, Irwin added.

She explained that the research team looked to examine what factors affect the susceptibility of communities to invasion, noting that by correctly identifying contributing factors, they would be able to create a link to invasion.

In the past, researchers have looked closely at the human population size relative to invasive species, and they have identifying with birds, fish and plants, according to Irwin. The aim of their research was to demonstrate that there are important things that humans do to the landscape that may be linked to the rise of invasive species.

Irwin showed graphs indicating that as the world population increases, the rates of carbon dioxide in the atmosphere grows, and as state populations grow, the number of exotic plants per state grows. She said that this begs the question of what humans and population size are doing to the landscape to cause the change — a question that she sought to answer.

“When combined, ecological properties and economic activities predict the number of exotic plants in an ecosystem,” she hypothesized, noting that economic activity usually gives a good indication of general human activity.

Irwin explained that the development of a model required a mix of human, economic and ecological factors.

“There are certain aspects of economic activity that might be strongly linked to invasions,” she said, pointing in particular to real estate development and how it disturbs landscapes.

Overall, Irwin showed that her team’s research demonstrated a positive relationship between real estate gross state product (GSP) and the number of exotic plants.

She posited two models as explanations — a population model and an exotic-economic model — both of which used data concerning individual U.S. states’ year of entry into the union, the number of native plants, their latitude and their population size.

According to Irwin, the models revealed a positive relationship between the number of years a state has been a member of the union and the number of exotics, indicating that older states have more exotic plants. Additionally, the models  showed a negative relationship between number of native plants and exotic plants, indicating that biodiversity may provide some biotic resistance to the success of exotic species.

Irwin compared the models’ predictions to the number of exotic plants in a handful of countries with available data, including Australia, New Zealand and the United Kingdom, as well as across Canada’s provinces. She concluded that the model accurately predicted the number of exotics in 85 percent of cases.

Irwin next moved on to the consequences of invasive species, explaining that the team looked at how such species affect native communities and ecosystem services. In particular, her team examined how invasive species can affect pollination among native plants, focusing on the role played by pollinators, such as bumble bees, flies and hummingbirds.

Irwin’s research looked at the species Linaria vulgaris — a variety of flowering plant that is an invasive species in Colorado — which she described as a “fairly extreme invader.” In the region examined, native species were holding on, and although there was no significant decline in numbers, the region has experienced a decline in evenness among species, according to Irwin. She said the researchers then honed in on the effect of competition for pollination given a shared supply of pollinators.

“Not all flower visit interactions will respond in the same direction or with equal magnitude,” Irwin explained, noting that some native species rely more heavily on bumble bees — which was the common regional pollinator of Linaria — while other species depend more on other pollinators, such as hummingbirds, flies or other types of bees.

Irwin explained that Linaria produces many flowers, plenty of sugar and loads of protein with significantly more pollen grains, and bumble bees may thus be more attracted to these sites and more attracted to the invasive plant.

The researchers looked at two plots of flowers — one with Linaria present, and one with the Linaria blossoms clipped. They found that areas with Linaria had more insects coming in, whereas native plants with similar pollinators had more individual visitation in areas with no Linaria present.

To look beyond pollination to actual seed production, Irwin added that they found that seed production per plant with similar pollinators was higher in areas without the invasive plant, showing that the invasive species is in fact luring pollinators away.

However, for flowers like the Ipomopsis, which is pollinated more by hummingbirds, the invasive species actually had a positive effect, possibly because it drew away organisms that were robbing nectar but not pollinating.

“Not all plants are going to respond the same way to the same invader,” she concluded.

Finally, turning to solutions, Irwin said her team looked at how individuals can combine economics and ecology to best conduct invasive species management.

Researchers looked at invasive species and examined the treatment of their growth and the spread of their seeds across properties through both collective and independent management.

Irwin explained that the aim was to “solve for the optimal number of invaders removed and natives planted over time that generate the largest net benefits … for collective management and independent management.”

The bioeconomic model that the researchers developed showed that collective management was able to more rapidly remove invasive plants through “over-managing” early on.

“You see a higher rate in invasive plant removal initially, and they eradicate that invader earlier on in time compared to individuals managing independently,” Irwin said.

In order to influence individual behavior to promote the more effective style of collective management, she added that policymakers can either levy a penalty or levy a subsidy, but the ultimate goal is to optimize the management strategy based on net benefits.