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The Yangtze River is one of the longest rivers in the world, flowing nearly 4,000 miles from the western interior of China to the coast in Shanghai, and the surrounding valleys are home to thousands of species of woody and flowering plants, many of them rare. But throughout the river valley, temperatures are warming too fast for many native plant species to migrate or adapt in place, and hardier, non-native varieties are taking their place.
As climate change transforms habitats worldwide, scientists are debating how large of a role non-native species invasions play in the extinction of native ones. Now, a new study published in Nature Plants suggests that in some cases, non-native plants might not be driving as much extinction of local flora as previously assumed.
Warming temperatures accounted for more than half of the decline in native species.
Researchers from the Wuhan Botanical Garden at the Chinese Academy of Sciences and Rice University analyzed field surveys tracking 142 different species over 1,000 miles along the Yangtze River in China, including 98 native and 44 “exotic” species. (The research team used the term “exotic” to describe any species that had not been historically found in the region, and “invasive” to describe species that have become abundant enough to directly impact native species’ survival.)
In this case study, they found that warming temperatures accounted for more than half of the decline in native species, while direct competition from exotic species accounted for only 15 percent of native species decline. They also found that “moderate” invasion increased communities’ overall species diversity. But they predicted that invasions will be more intense under future climatic conditions, particularly in the coolest areas along the river.
“These new conditions, independent of the exotic species, are just not very good for the native species there,” says Evan Siemann, a professor at Rice University in Texas and a co-author of the study.
While many studies focus on the changes to individual native or exotic species, the Yangtze River study researchers attempted to predict which species will thrive in the future by figuring out which factors most affect their survival on a community level. If native species are impacted most heavily by rising temperatures, interventions targeting invasive species alone are unlikely to protect them from decline.
“We often make the assumption that native species are directly and immediately threatened by non-native species, but we rarely actually know how much that’s truly the case,” says Jeff Dukes, a senior staff scientist at the Carnegie Institution for Science who was not involved in the study. “This paper is a good reminder that we need to be thinking more comprehensively, and that we actually have many simultaneous threats to our native systems.” 
Lead image: wxj651208 / Shutterstock
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