If you can't beat them, blend in? How do invasive plants and native plants coexist?

Produced by: Science Popularization China

Author: Le Haichuan, Xie Zongqiang (Institute of Botany, Chinese Academy of Sciences)

Producer: China Science Expo

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In "The Origin of Species", in addition to proposing the famous theory of evolution, Darwin also proposed the theory of naturalization of alien species, which has troubled later ecologists.

On the one hand, Darwin believed that alien species that are closely related to native species will adapt more easily to new environments because they tend to have similar ecological needs. This is just like us humans, if we go to a place with a climate and environment similar to our hometown, we will adapt more easily. This phenomenon is called the "pre-adaptation hypothesis."

On the other hand, Darwin also believed that alien species that are distantly related to native species are more likely to successfully invade and find their own "living space." If alien species and native species are "too close," they will face fierce competition, just like two brothers fighting over a toy. If alien species and native species are "unfamiliar," they can avoid this competition. This view is called the "Darwin naturalization hypothesis," and the two seemingly contradictory hypotheses he proposed constitute the "Darwin naturalization puzzle."

Some scientists have evidence to support the idea that "strangers are more likely to succeed." In places such as the Azores, South Africa, and the Colorado River in the United States, scientists have found that the more distant the relationship between alien species and native species, the easier it is for them to successfully invade.

Schematic diagram of the Darwinian naturalization puzzle: red represents the preadaptation hypothesis and blue represents the Darwinian naturalization hypothesis.

(Image source: Reference 2)

Another group of scientists has evidence to support the idea that "relatives are more likely to succeed." In studies in California, New Zealand, and the Mediterranean, they found that alien species that are closely related to native species are more likely to invade successfully.

As research continues to deepen, more and more evidence is found to support or oppose Darwin's two hypotheses, and the controversy over Darwin's naturalization mystery has intensified. In order to solve this problem, scientists have begun to explore various possible explanations, such as spatial scale, invasion stage, environmental gradient, and measurement methods of species differences, trying to unify the seemingly contradictory research results.

Small stage, big competition; big stage, big screening

On a small scale, competition between species is an important factor affecting the invasion of alien species. Species that are closely related often have similar ecological niches. Ecological niche is the general term for the environment in which a species lives and its living habits, including factors such as the species' lifestyle, resource utilization, and behavioral habits. These factors jointly determine the species' position in the ecosystem. Species with similar ecological niches will compete more fiercely. Darwin's naturalization hypothesis has been verified on a small scale.

However, when we expand our perspective from a single forest to an entire region or even the world, the role of environmental screening becomes more important. Species that are more adaptable to their native environment are more likely to successfully invade, which is consistent with the pre-adaptation hypothesis.

It is important to note that both at small and large scales, interactions between organisms affect the invasion of alien species. Therefore, the invasion of alien species is a complex process that is affected by multiple factors and cannot be simply attributed to a single mechanism.

Imitation is the way to survive, innovation is the magic weapon to win

Alien species invasion can go through stages such as introduction, colonization, naturalization and invasion, and the ecological processes that determine the success of the invasion may be different at each stage. Studies have shown that in the early stages of invasion, alien species tend to choose a living environment similar to that of native species in order to quickly integrate into the group, showing pre-adaptation characteristics; in the later stages of invasion, alien species gradually gain a foothold in the new environment and begin to look for new development opportunities. In order to avoid fierce competition with native species, they will gradually develop into different ecological niches.

The researchers found that in harsh environments, such as cold and drought, environmental filters dominate the coexistence of species and keep each other together for warmth, supporting the pre-adaptation hypothesis. In warm and humid conditions, abundant resources and a large number of species lead to fierce competition, which is conducive to the coexistence of distant species in different ecological niches, which is consistent with Darwin's naturalization hypothesis.

Pathways to reconciliation of Darwin's naturalization puzzle.

(Image source: Reference 3)

The Rashomon of Relationships and Traits

Both the preadaptation hypothesis and Darwin's naturalization hypothesis imply the assumption that closely related species share similar traits. However, divergence between closely related species and convergence between distantly related species make this assumption less reliable. In simple terms, scientists once assumed that relatives tend to have many similar habits.

But the reality is that some relatives are very different from each other, just like a pair of twins, one likes sports and the other likes reading, with completely different interests and hobbies. Some species that are not closely related are very similar, such as dolphins and fish. Although one is a mammal and the other is a fish, their appearance and living habits are very similar. They both have streamlined bodies and live in water. Therefore, comprehensively considering the relationship and trait differences between alien species and native species can help us better understand the mystery of Darwin's naturalization.

The evolution of traits in a species can show either conservative (left) or convergent (right) patterns.

(Image source: Reference 4)

Survival of the Fittest

However, compared with natural environments, human disturbances usually have a wider range and greater intensity, suggesting that the trajectory of species composition may be different from that under natural conditions. The Three Gorges Dam has changed the pattern of water level changes in the Yangtze River. Unlike the previous naturally formed water level changes, the water level in the dam now changes in the opposite direction throughout the year, with low water levels in summer and high water levels in winter. The dam also causes the Yangtze River riparian zone to be flooded for longer and deeper each year, causing the riparian zone soil to experience frequent flooding and exposure, making it very vulnerable to invasion by alien plants.

Our team is dedicated to studying the impact of large-scale water conservancy projects on ecosystems and ecological restoration. We conduct plant community surveys in the Three Gorges Reservoir area to explore the coexistence relationship between alien plants and native plants.

Aerial photo of the Three Gorges Reservoir water level drop

(Photo source: Xinhua News Agency)

We found that, on the one hand, frequent water level fluctuations in the drawdown zone (the transition zone between terrestrial and aquatic ecosystems) led to a decrease in native plant diversity, and most native species were concentrated in a few families with strong representation, such as Gramineae and Salicaceae. This aggregation would create niche vacancies in the community, allowing distantly related alien species to take advantage, which supports Darwin's naturalization hypothesis from the perspective of kinship.

On the other hand, the functions of alien species in the drawdown zone are similar to those of native species. They have adapted to waterlogging stress. Annual herbaceous plants adapt to this environment by producing a large number of seeds, waiting for the water level to drop so that they can germinate and grow quickly, completing their life cycle in the short dry season.

In contrast, perennial plants integrate resources through clonal growth and improve environmental tolerance, which supports the pre-adaptation hypothesis from the perspective of functional traits. At the same time, the shape similarity of species in the reservoir riparian zone increased over time, indicating that the continuous water level fluctuations of the Three Gorges Reservoir may have weakened the competition among species in the reservoir riparian zone.

Schematic diagram of the Darwin naturalization puzzle in the reservoir riparian zone

(Image source: Reference 6)

Conclusion

Darwin's naturalization puzzle tells us that biological invasion is a complex process affected by many factors. The success of alien species' invasion depends not only on their relationship with native species, but also on their ecological adaptability, environmental conditions, and human activities.

Our research results provide a new explanation for the classic Darwinian naturalization puzzle, emphasizing that periodic water level fluctuations play an important role in driving the colonization of invasive species and the adaptation of native species. This will help to better predict which alien species are more likely to invade reservoir riparian zones and which communities composed of native species are more likely to be invaded, thereby protecting riparian biodiversity and reducing the threats posed by invasive alien species.

References

[1] Li Shaopeng, Fan Shuya, Meng Yani, et al. The mystery of Darwinian naturalization in invasive alien species. Science China: Life Sciences, 2024, 54: 723–738

[2] Darwin, C. (1859). On the Origin of Species. London: John Murray.

[3] Cadotte, MW, Campbell, SE, Li, SP, Sodhi, DS, & Mandrak, NE (2018). Preadaptation and Naturalization of Nonnative Species: Darwin's Two Fundamental Insights into Species Invasion. Annual Review of Plant Biology, 69, 661-684.

[4] Cavender-Bares, J., Ackerly, DD, Baum, DA, & Bazzaz, FA (2004). Phylogenetic overdispersion in Floridian oak communities. The American Naturalist, 163(6), 823-843.

[5] New, T. & Xie, Z. (2008). Impacts of large dams on riparian vegetation: applying global experience to the case of China's Three Gorges Dam. Biodiversity and Conservation, 17, 3149-3163.

[6] Le, H., Mao, J., Cavender-Bares, J., Pinto-Ledezma, JN, Deng, Y., Zhao, C., Xiong, G., Xu, W. & Xie, Z. (2024). Non-native plants tend to be phylogenetically distant but functionally similar to native plants under intense disturbance at the Three Gorges Reservoir Area. New Phytologist.