Non-native mammals disperse non-native fungi that promote invasion of non-native trees

Background/Question/Methods

Biological invasions are complex phenomena since many factors determine their outcome. One key aspect is the interaction of the non-natives with the local biota. Interaction with local species may be especially important for exotic species that require an obligatory mutualism, such as the case of members of the Pinaceae family that need ectomycorrhizal (EM) fungi. EM fungi and seed of Pinaceae disperse independently, so different vectors can be involved in their dispersal. Mammals such as deer, wild boar, and rodents can disperse EM fungi. We studied the role of mammals as dispersal agents of EM fungi on Isla Victoria, Argentina.  On this island ca. 80 years ago 135 species of exotic trees, many of them Pinaceae, were introduced. Only a few of these species have become invasive, and they are found in high densities only near plantations, partly because Pinaceae trees cannot find EM fungi when their seeds land far from plantations. With greenhouse experiments using animal feces as inoculum, plus observational and molecular studies, we examined how local mammals disperse EM fungi. This project aimed to determine which animals can act as vectors of the mycorrhizal fungi. 

Results/Conclusions

We found that wild boar and deer, both non-native, are dispersing EM fungi. Approximately 30% of the seedlings growing with feces of wild boar and 15% of the seedling growing with deer feces were colonized by EM fungi. Seedlings growing in control pots were not colonized by EM fungi. We attempted to collect feces of rodents, but observational evidence confirms that rodents are very rare near and in the plantations of non-native trees, so their ecological role seems to be negligible in the zones studied. We found low diversity of fungi colonizing the seedlings, possibly owing to animal preferences and/or the differential ability EM species to germinate after being ingested. The hypogeous genus Rhizopogon was the most abundant genus found. Wild boar, a recent addition to the local fauna, seem to be the main agent dispersing the fungi, so they may be playing a key role in facilitating the invasion of pine trees and may contribute to triggering their spread. These results show that interactions among non-natives, in this case from three different kingdoms, seem to explain pine invasions at our study area.