Like all obligately ectomycorrhizal plants, pines require ectomycorrhizal fungal symbionts to complete their life cycle. Pines introduced into regions far from their native range are typically incompatible with local ectomycorrhizal fungi, and, when they invade, coinvade with fungi from their native range. While the identities and distributions of coinvasive fungal symbionts of pine invasions are poorly known, communities that have been studied are notably depauperate. However, it is not yet clear whether any number of fungal coinvaders is able to support a Pinaceae invasion, or whether very depauperate communities are unable to invade. Here, we ask whether there is evidence for a minimum species richness of fungal symbionts necessary to support a pine/ectomycorrhizal fungus coinvasion. We sampled a Pinus contorta invasion front near Coyhaique, Chile, using molecular barcoding to identify ectomycorrhizal fungi. We report that the site has a total richness of four species, and that many invasive trees appear to be supported by only a single ectomycorrhizal fungus, Suillus luteus. We conclude that a single ectomycorrhizal (ECM) fungus can suffice to enable a pine invasion.

Biological invasions are often complex phenomena because many factors influence their outcome. One key aspect is how non-natives interact with the local biota. Interaction with local species may be especially important for exotic species that require an obligatory mutualist, such as Pinaceae species that need ectomycorrhizal (EM) fungi. EM fungi and seeds of Pinaceae disperse independently, so they may use different vectors. We studied the role of exotic mammals as dispersal agents of EM fungi on Isla Victoria, Argentina, where many Pinaceae species have been introduced. Only a few of these tree species have become invasive, and they are found in high densities only near plantations, partly because these Pinaceae trees lack proper EM fungi when their seeds land far from plantations. Native mammals (a dwarf deer and rodents) are rare around plantations and do not appear to play a role in these invasions. With greenhouse experiments using animal feces as inoculum, plus observational and molecular studies, we found that wild boar and deer, both non-native, are dispersing EM fungi. Approximately 30% of the Pinaceae seedlings growing with feces of wild boar and 15% of the seedlings growing with deer feces were colonized by non-native EM fungi. Seedlings growing in control pots were not colonized by EM fungi. We found a low diversity of fungi colonizing the seedlings, with the hypogeous Rhizopogon as the most abundant genus. Wild boar, a recent introduction to the island, appear to be the main animal dispersing the fungi and may be playing a key role in facilitating the invasion of pine trees and even triggering their spread. These results show that interactions among non-natives help explain pine invasions in our study area.