Niche Difference Prevents Competitive Exclusion between the Invasive Submerged Macrophyte Elodea densa (Planch.) Casp. and Native Hydrilla verticillata (L.f.) Royle in a Large Plateau Lake

1. Biological invasions in freshwater ecosystems are increasingly severe, posing significant threats to ecosystem health and economic development. Hydrilla verticillata (L.f.) Royle and Elodea densa (Planch.) Casp. are two of the most aggressive invasive submerged macrophytes worldwide, and often regarded as similar species due to their growth forms and habitat requirements, although there are few field coexistence records. Hydrilla verticillata is a native species in the large plateau Lake Erhai, where the non-native E. densa has been documented since 2017. This study aims at exploring the colonisation process of E. densa as well as its niche overlap and interspecific interactions with native H. verticillata in Lake Erhai.
2. A continuous seven-year field investigation was conducted in Lake Erhai. Four indicators were used to assess the population distribution of H. verticillata and E. densa, including occurrence frequency, biomass, relative abundance and relative niche breadth. The logistic growth model was applied to analyse population dynamics. The Gaussian model was used to characterise their distribution with water depths. A stability index was employed to evaluate variations in measured indices across different water depths. Indicators of niche overlap and interspecific association were used to describe the coexistence and interactions between the two species.
3. Elodea densa established several stable populations in Lake Erhai after years of colonisation and naturalisation, primarily colonising deeper areas of the lake than H. verticillata. Elodea densa tended to thrive at depths of around 4.0 m, whereas H. verticillata typically grew at depths from 2.0 to 3.0 m. Hydrilla verticillata demonstrated greater stability across varying water depths than E. densa. Niche overlap between the two species was minimal.
4. Our findings indicated that in this large plateau lake, native H. verticillata exhibited higher competitiveness than alien E. densa. There was a distinct niche difference in water depths between H. verticillata and E. densa, which prevented competitive exclusion. The two species achieved stable coexistence at a lake-wide scale.
5. This study provided the first field evidence for coexistence between two globally recognized invasive species, showing that in habitats with sufficient environmental gradients and filtering pressures, submerged macrophytes similar in taxonomy and appearance could avoid competitive exclusion by occupying different ecological niches, leading to stable coexistence.