Invading a habitat is easier when you have a friend to help. De La Cruz and colleagues have investigated an intriguing mechanism of plant interaction among exotic species in central-northern Chile. Their research focuses on two exotic halophyte species, Mesembryanthemum crystallinum and Mesembryanthemum nodiflorum, investigating how the former may indirectly support the establishment of the latter by increasing soil salinity to suppress native competitors. Their work, published in Biological Invasions, has implications for understanding how these exotic species may unintentionally promote each other’s proliferation, leading to biodiversity loss and the displacement of native species.
De La Cruz’s team found that Mesembryanthemum crystallinum increases soil salinity, suppressing the growth of salt-susceptible native competitors, such as Helenium urmenetae. This increased salinity did not hinder but instead seemingly favoured Mesembryanthemum nodiflorum, allowing it to flourish alongside Mesembryanthemum crystallinum. The researchers also observed that a salt-tolerant native plant, Amblyopappus pusillus, could endure this change in soil conditions. These findings open up a new understanding of how certain exotic species could indirectly facilitate the establishment of other exotic species. This process could lead to significant biodiversity loss if unchecked.
The concept of indirect facilitation among exotic species is not new. It’s a phenomenon that has been observed in different forms, such as increased light availability, increased canopy shade cover, or modification of soil microorganisms. What’s new about De La Cruz’s research is the experimental evidence on how increased soil salinity can promote this kind of interaction among halophytic, or salt-tolerant, exotic species.
To arrive at these findings, the researchers carried out field co-occurrence surveys at Quebrada El Romeral coupled with greenhouse germination and competition experiments. They found all four of their study plants in the Chilean desert, but not happily side-by-side. They found that both Mesembryanthemum crystallinum and Mesembryanthemum nodiflorum tend to co-occur, suggesting a form of mutual facilitation. One native plant, the salt-tolerant Amblyopappus pusillus, could grow next to Mesembryanthemum patches. However, another native plant, Helenium urmenetae, could not. A study of the soil in the Mesembryanthemum patches found it was more saline under Mesembryanthemum crystallinum than elsewhere.
Germination experiments confirmed that the Mesembryanthemum plants had a strong advantage in saline soils. This demonstrated that if Mesembryanthemum crystallinum can increase the salinity of soils, then other halophytes, salt-tolerant plants, can invade and establish, say De La Cruz and colleagues in their paper.
The present study, set in the framework of the invasional meltdown (Simberloff and Von Holle 1999; Simberloff 2006), supports the notion that the global, progressive accumulation of exotic plants cannot be explained solely by the ecological responses of single exotic species to environmental factors (MacDougall and Turkington 2005). Exotic species may facilitate the establishment of other exotic species, promoting their high co-occurrence, and likely resulting in a greater impact on native species diversity (Braga et al. 2018; Stotz et al. 2020).De La Criuz et al. 2023.
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De La Cruz, H.J., Salgado-Luarte, C., Stotz, G.C. and Gianoli, E. (2023) “An exotic plant species indirectly facilitates a secondary exotic plant through increased soil salinity,” Biological invasions. Available at: https://doi.org/10.1007/s10530-023-03061-z.