An international research team led by the University of Massachusetts Amherst has conducted the first global assessment of the impacts to native species by invasive species. The experts wanted to measure the extent of the damage as invaders become more widespread and abundant.
“What surprised me most was the magnitude of some of these effects,” said study lead author and invasion ecologist Professor Bethany Bradley. “Invasive animal pests, like Emerald Ash Borer or lionfish, cause nearly a 50 percent decline in native populations. That’s the average case – on average, invasive pests will cut the populations of native species in half if we don’t prevent or control those invasions.”
According to Professor Bradley, as alien garlic mustard invades forest understory in New England, the number of native sugar maple seedlings declines. Across the region’s wetlands, invasive purple loosestrife is linked to declines in native red-winged blackbirds and song sparrows.
“Our principal finding is that if the invasive species is at a higher trophic level, that is, they are likely eating the native species, then just a few invasive individuals can cause a sharp decline in native populations,” said Professor Bradley. “Once those invaders reach higher abundance, the damage has long ago been done. This has big implications for management. Early detection becomes critical.”
The investigation was based on the analysis of 1,258 case studies from 201 published research papers. Professor Bradley said that one of the primary goals was to understand how impacts accumulate during an invasion in order to support evidence-based management.
“We know that invasive species have negative impacts, but past meta-analyses have only looked at studies where invaders were absent vs. present.”
Professor Bradley explained that for native species faced with an invader at the same trophic level, where the two are likely competing for resources, there is a “strong negative linear response,” but no sharp initial decline. “As you increase invasive species’ abundance, the native species decline in abundance and in community diversity. The management implications are that if you control the invasion at any point, it’s a win for ecosystems.”
On the other hand, when an invader enters at a lower trophic level than the native species, the results were not as clear.
“It could be creating new resources or it could compete against resources that the natives need. It’s not clear whether the effects will be positive or negative, unless you’re talking about terrestrial plants. Invasive plants are likely to have negative effects on native insects.”
Ultimately, the study authors concluded that “increasing the abundance of invasive alien species has pronounced negative impacts on native species populations and communities.” They said the fact that rapid declines in native populations have been found to occur at initial stages of the invasion process “highlights the increasing need for early detection and rapid response (EDRR) to new invasive alien species.”
“Avoiding the ecological impacts of invasive species will require a much stronger commitment to proactive policies designed to prevent novel introductions as well as increased management targeting the early stages of invasion.”
The study is published in the Proceedings of the National Academy of Sciences.
Image Credit: CJB Sorte/UC Irvine