Local trees offer potent defense against invaders: study

A new study has found that healthy ecosystems provide a powerful defense against invasive species — and the added disruption they bring.

According to findings published Wednesday in Science, invasive tree species tend to enter landscapes from nearby human settlements, especially coastal and riverine ports. 

But if the ecosystems that surround them are full of life, invasive species struggle to find a foothold, the scientists found.

“We found that native biodiversity can limit the severity or intensity of non-native tree species invasions worldwide,” Camille Delavaux, lead author of the study, said in a statement. 

The team’s finding has a grim side: It means that distressed ecosystems provide opportunities for invasive species.

About 10 percent of introduced species worldwide become “invasive” — a catch-all term for species that escape containment and go feral in surrounding ecosystems.

Once there, they can wreak havoc — displacing local trees and the networks of species that live off them. 

In some cases, these new entrants can reshape the land around them. 

In Hawaii, for example, the ornamental albizia or mimosa — imported from Indonesia by 19th-century ranchers — creates a dense canopy that “shades out” native vegetation.

To make matters worse, the tree adds nitrogen to the soil — changing the chemistry of the generally nitrogen-poor soils of the islands so that it poisons native plants.

Then, in the newly-fertilized dead zones beneath the mimosa’s leaves, new invasives grow.

Or take fire-loving eucalyptus, another Pacific island import brought to dryland regions worldwide for timber, paper production, and erosion control.

In its native country, eucalyptus relies on fire to wipe out competitors, fertilize the soil with their ashes and even trigger the opening of their seeds, allowing seedlings to take advantage of the newly open forest.

Where introduced eucalyptus is controlled in strictly managed plantations, it’s not a risk. But when it escapes those plantations, it brings with it a new risk of fire: It was a prime cause of deadly blazes like the one that killed dozens in the Portuguese highlands in 2017. And it’s now ubiquitous across Southern California.

The scientists found different patterns of invasion depending on whether a region’s climate was harsh (either hot or cold) or temperate. 

In extreme climates, they found, the invasive trees that survive and spread are the ones that can imitate the characteristics of their local rivals — while the trees that successfully escape into more hospitable temperate ecosystems tend to be ones that can differ from local trees in key ways.

But in all cases, these trees can only spread where they can gain purchase in the ecosystems around them: They need what biologists call a “niche,” or a space in an ecosystem, that they can enter. 

If those niches are empty — or at least not fully locked down — near where a foreign tree species is introduced, they can be an open doorway into nearby ecosystems that potential invaders can use to enter and spread. 

The difference between uncontrollable invasive species and introduced species that stay in their plots, in other words, may have less to do with the trees themselves than with the ecosystems that surround them.

For land managers, that’s good news, Delavaux said: The steps taken to bolster biodiversity within ecosystems can also help keep invasives out. “The extent of invasion can be mitigated by promoting greater native tree diversity,” she said.