Mycorrhizal types influence global island biogeography: invasion risks and effects of plant naturalizations

Background/Question/Methods

Although island plant biogeography has historically focused on abiotic drivers, plant colonization on islands could also be limited by biotic drivers, such as the absence of symbionts. Most plants form symbioses with mycorrhizal fungi, whose limited dispersal to islands could act as a colonization filter for plants. Many plant species are highly dependent on mycorrhizal fungi, however, these fungi disperse independently of their plant hosts and therefore the absence of mycorrhizal fungi can limit the colonization of mycorrhizal plant species. In addition, plant colonization may be differentially limited by mycorrhizal types, being more limited by mycorrhizal fungi with limited dispersal ability. Here, we test for differences in the strength of the mycorrhizal filter across plant species associating with the three major mycorrhizal types: arbuscular mycorrhizal (AM) fungi, the most common type of mycorrhizae, ectomycorrhizal and ericoid mycorrhizal (EEM) fungi, and orchid mycorrhizal (ORC) fungi. We tested for such differential island colonization across mycorrhizal types within angiosperm floras worldwide, first within native and then within invasion floras.

Results/Conclusions

We found evidence that arbuscular mycorrhizal (AM) plants experience a stronger mycorrhizal filter than other mycorrhizal or non-mycorrhizal plant species, with decreased proportions of native AM plant species on islands relative to mainlands. This effect increased with island isolation, particularly for non-endemic plant species. We further found evidence of humans overcoming the initial mycorrhizal filter, presumably due to co-introduction of plants with their symbionts. Naturalized floras showed higher proportions of AM plant species than native floras, a pattern that increased with increasing isolation and land-use intensity. This work provides evidence that mycorrhizal fungal symbionts shape plant colonization of islands, subsequent diversification, and invasion risks. Importantly, this work shows that biotic interactions between plants and soil mutualists determine global plant biogeography, shifting the longstanding abiotic centric perspective in island biogeography to include plant-associated mutualists.