A new study shows that invasive species can have a dramatic impact on native species — and that a strong proactive response can help mitigate those impacts.
Dr. Amanda Bates is the Canada Research Chair in Marine Physiological Ecology and an associate professor in the Department of Ocean Sciences, Faculty of Science.
She is part of an international research team that has conducted the first global meta-analysis of the characteristics and size of invasive species’ impacts on native species as invaders become more abundant.
The study, which saw team members analyze findings from 1,258 case studies from 201 research papers, was released in a paper in the online issue of Proceedings of the National Academy of Sciences, on April 29.
The researchers found that those impacts depend strongly on the invader’s position in the food chain, also known as tropic level. Invasive species at higher trophic levels have the greatest impact early in the invasion.
“What surprised me most was the magnitude of these effects,” said Bethany Bradley, an invasion ecologist at the University of Massachusetts Amherst. “Invasive animal pests, like the emerald ash borer or lionfish, will, on average, cut the populations of native species in half if we don’t prevent or control these invasions.”
Dr. Bates says when just a few invasive individuals higher in the food chain show up and begin eating native species, there’s an immediate sharp decline in native populations.
“Removing these species immediately will make a major difference, especially if they are top predators,” she said. “This has big implications for management. Early detection is critical but if you can control the invasion at any point, it’s a win for ecosystems.”
Competition for resources
For native species faced with an intruder at the same level of the food chain, competition for the same resources does not lead to a sharp initial decline. But, as the number of invaders increases, native species decline in abundance and community diversity.
“Invasives reduce other species around them because they’re feeding on them, or they’re competing for space and taking up resources,” said Dr. Bates.
“Here in this province, the green crab not only competes with lobster and has impacted the lobster fishery, but green crabs also bury themselves in the sediment and feed on the roots of seagrasses, which can decimate seagrass beds that are important habitat for juvenile fishes.”
Commitment and target early
The team hopes their findings will encourage governments around the world to make a stronger commitment to proactive policies designed to prevent the introduction of invasive species, as well as increased management targeting the early stages of invasion.
“The green crab . . . feed on the roots of seagrasses, which can decimate seagrass beds that are important habitat for juvenile fishes.”
Dr. Bates says the research supports the huge green crab mitigation efforts being undertaken as part of a five-year project funded by the federal government’s $71-million Coastal Restoration Fund.
The Marine Institute’s Centre for Fisheries Ecosystems Research is entering its third green crab removal season as part of a larger project to restore eelgrass habitats in historically rich areas.
The goal of that part of the project is to decrease the overall population of green crab below a threshold that will allow the ecosystem in areas of Placentia Bay to once again thrive as a nursery habitat for juvenile fish, stabilize sediment and water quality.
A short fall fishery in 2017, in the first year of the project, gave optimistic results, seeing a slight decrease in the size of green crab caught over the fishing period from September-December. The first full fishing season (July-December) was completed in 2018 and will be duplicated again in 2019. Results are currently being analyzed for the 2018 season.
“Our study suggests this is a fantastic strategy for managing the impacts of this crab in our waters,” said Dr. Bates.