Highly invasive tree species are more dependent on mutualisms

Abstract

Citation: Moyano, J., M. A. Rodriguez-Cabal, and M. A. Nuñez. 2020. Highly invasive tree species are more dependent on mutualisms. Ecology 101(5):Abstract. Why some species become invasive while others do not remains an elusive question. It has been proposed that invasive species should depend less on mutualisms, because their spread would then be less constrained by the availability of mutualistic partners. We tested this idea with the genus Pinus, whose degree of invasiveness is known at the species level (being highly and negatively correlated with seed size), and which forms obligate mutualistic associations with ectomycorrhizal fungi (EMF). Mycorrhizal dependence is defined as the degree to which a plant needs the mycorrhizal fungi to show the maximum growth. In this regard, we use plant growth response to mycorrhizal fungi as a proxy for mycorrhizal dependence. We assessed the responsiveness of Pinus species to EMF using 1,206 contrasts published on 34 species, and matched these data with data on Pinus species invasiveness. Surprisingly, we found that species that are more invasive depend more on mutualisms (EMF). Seedling growth of species with smaller seeds benefited more from mutualisms, indicating a higher dependence. A higher reliance on EMF could be part of a strategy in which small-seeded species produce more seeds that can disperse further, and these species are likely to establish only if facilitated by mycorrhizal fungi. On the contrary, big-seeded species showed a lower dependence on EMF, which may be explained by their tolerance to stressful conditions during establishment. However, the limited dispersal of larger seeds may limit the spread of these species. We present strong evidence against a venerable belief in ecology that species that rely more on mutualisms are less prone to invade, and suggest that in certain circumstances greater reliance on mutualists can increase spread capacity.

Invasive trees rely more on mycorrhizas, countering the ideal‐weed hypothesis

“…Despite empirical support for the ideal weed hypothesis, there is evidence that suggests some invasive plants may be more reliant on mutualisms than expected. In a recent meta-analysis based on data of Pinus seedling growth with and without mycorrhizal fungi, we found that species that are more invasive were more dependent on this mutualism (Moyano et al 2020). According to this study, Pinus species with higher invasiveness and lower seed mass had a higher growth response to ectomycorrhizal fungi (EMF) inoculation.…”

“…Following the results of Moyano et al (2020), we predicted that species with higher invasiveness would show higher responsiveness to EMF inoculation.…”

“…In this regard, our recent meta-analysis proposed an interesting hypothesis (i.e. invasive plants depend more on mutualisms) (Moyano et al 2020) that now needs to be tested using controlled experimental approaches to account for biases and to validate previous findings.…”

Alien ectomycorrhizal plants differ in their ability to interact with co-introduced and native ectomycorrhizal fungi in novel sites

“…EcM plant species form a disproportionately high share of alien trees globally, and many of them are regarded as the most severe invaders [ 31 – 34 ]. Recently, Moyano et al [ 63 ] provided evidence that alien EcM plants that rely more on EcMF are more prone to become invasive in introduced regions. Therefore, our study, which provides the most comprehensive overview of strategies that alien EcM plants adopted to establish a mycorrhizal partnership in regions to which they were introduced, represents a substantial step forward in understanding the mechanisms of EcM plant naturalization.…”

Using ectomycorrhizae to improve the restoration of Neotropical coastal zones

“…Introduced native seedlings (via restoration) may not succeed if the native microbial communities are not introduced concurrently, especially those associated with initial colonization. Restoration efforts often involve controlling invasive species (Weidlich et al 2020), which may rely on mutualists (Vogelsang & Bever 2009; Moyano et al 2020), but little is known about the consequences of having exotic soil microbiota in areas where exotic plants have been removed. For ECMs, interactions between native plants and exotic fungi do not necessarily cease after the removal of exotic plant species (Lofgren et al 2018).…”