When non-native plants invade an ecosystem, their interactions with new insects and microorganisms accelerate carbon cycling, according to a paper published in the journal Science.
Although the unwelcome guests were already known to destabilise carbon balance, the new study reveals the process is more complex than previously thought, says lead author Lauren Waller, from Lincoln University, New Zealand.
From the infamous prickly pear (Opuntia striota) that throttled Australia to the Japanese knotweed (Reynoutria japonica) that infiltrated English and US ecosystems, humans have paid dearly for our love of exotic flowers and shrubs.
As a warming climate extends their flowering seasons, the global threat of invasive species is growing as they fortify their stronghold over native plants and damage to biodiversity and ecosystems.
To unravel what causes their impact on carbon cycling, Waller and colleagues created a complex experiment with 160 mini-ecosystems in large outdoor pots, manipulating interactions between plants, herbivorous insects and soil microbiota.
Each community contained 20 combinations of eight different plant species that varied incrementally in their proportion of invasive plants, from zero to 100%, and native woody plants.
They were grown in their own or foreign soils with or without invertebrates such as leafhoppers, aphids, moths, beetles and slugs.
As exotic plants increased, so did soil respiration – but only when herbivores and microbes were present in the foreign soil, showing that the reshuffling of old and new friends is responsible.
“Previously, we assumed that invasive plants speed up carbon cycling because they are so abundant and they possess traits that increase microbial activity,” says Waller.
“Our results support that, but take it a step further, showing that it is the different interactions that these plants form with herbivorous insects and soil microorganisms that drive their impacts.”
The combined impact of invasive species and increased carbon cycling on global warming creates a double whammy.
“Climate change and invasion by exotic plants are two of the biggest environmental crises facing the globe,” Waller says, and appear to have strong feedback loops.
To quantify the latter, scientists estimate that kudzu (genus Pueraria) alone, one of the US’s most invasive species, causes 4.8 metric tonnes of carbon to be released per year, equivalent to that stored in nearly 4.8 hectares of forest or released by burning 2.3 billion kilograms of coal.
In addressing this, Waller’s study highlights the importance of understanding links between plant invasions and ecosystem processes, write Carlos Urcelay, from the National University of Cordoba, and Amy Austin, from the University of Buenos Aires, Argentina, in a related commentary.
“Researchers must delve further into this complexity to better predict how these links will respond to global changes.”
Natalie Parletta is a freelance science writer based in Adelaide and an adjunct senior research fellow with the University of South Australia.
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