Plant invasion success varies with latitude

The team examined how the invasive plant Alternanthera philoxeroides competed with the native plant A. sessilis in China. But rather than just pick a few spots, they examined a range of latitudes and environments, to see if latitude made a difference. For Professor Lu the study is part of a long programme to fight invasive species. He said, “I started to work on this species about 12 years ago when I was a PhD student. At the beginning, I just knew it as a common weed in China. As my study went on, I gradually realized that this species is one of most serious invasive plants in China. It has invaded a large area, ranging from 18 to 36.8 north degrees latitude, and its range is still expanding partly due to ongoing climate warming. In this area, it invades a diverse of habitats, aquatic and terrestrial habitats, in both agricultural and natural ecosystems. Consequently, it causes many losses in agriculture and imposes great threats to natural ecosystem functions and biodiversity. Also, it is hard to manage this species. This is especially true in terrestrial habitats, where it is highly tolerant to pesticides and herbivory, partially due to developed roots. Now, as a researcher, I think I need to do something to combat this weed. But, as the first step, I need to explore the reason underlying its success.”

The team expected the native plant to perform better at higher latitudes, as herbivores would be less common. With few herbivores, A. sessilis would be at less of a disadvantage. This was the case, but there was a surprise at the other end of the range, closest to the equator. Here, the ratio of A. philoxeroides to A. sessilis cover was lower than the middle of the range. This meant the ratio of A. philoxeroides to A. sessilis cover was hump-shaped with latitude. Professor Lu said, “The hump shaped distribution for the ratio of invasive to native plant cover is really interesting and unexpected. At the beginning, we predicted that the invasive species should be a “winner” when competing with the native congener across its invaded ranges. It is well known that a competitive advantage relative to co-occurring native species is an important reason underlying plant invasions. Now, I have set a common garden experiment at five latitudes in China to explore the reasons for this pattern. Personally, I think this pattern reflects the combined impacts of several factors, such as plant functional traits (e.g., species sensitive to environment changes), changes in soil and foliar biotic communities (e.g., microbials and insects), climates and their interactions across latitudes.”

While the results here apply to A. philoxeroides, Professor Lu says that the results have implications for other plant invasions. “I think our finding, latitudinal variations in invasiveness of the invasive plant and biotic interaction it involved, could be common for invasive plants. Similar results have been reported for the invasive genotype of Phragmites australis in North America and Spartina alterniflora in China. However, the underlying reasons have largely been unexplored. This year, I will examine this with the common garden experiment with eight invasive species at five latitudes in China, combined with field survey. We want to know whether their invasiveness changes with latitude and why.”

These results will be important in years to come. Professor Lu said, “This paper is relevant to people working on the mechanism of plant invasions and biogeographic variations in biotic interactions. Our results show that biotic interactions, plant phenology and overwintering ability change with latitude, and as a result the invasiveness and the key driving factor for non-native plant invasions may change with latitude.”