Lack of disturbance-dependence explains early-successional beachgrass invasion better than pathogen escape, associations with mutualists, or plant-soil feedbacks
Introduced species invade established communities via mechanisms such as outcompeting established species, pathogen escape, or facilitation by beneficial microbes. However, determining the most critical mechanisms underlying invasions remains a challenge. In early succession, many plant species are disturbance-dependent, and an invading species able to establish in undisturbed communities stands to gain a considerable competitive advantage. Moreover, low-nutrient soils in early succession suggest increased reliance on mutualists and/or susceptibility to pathogens, yet the effects of mutualists and pathogens depend on how rapidly they accumulate during succession. We investigated the roles of disturbance-dependence, pathogens, mutualists, and soil-feedbacks on the invasion of the beachgrass Ammophila breviligulata in primary successional dunes along the USA Pacific Northwest coast. A. breviligulata invasion in these dunes – long-dominated by its introduced congener A. arenaria – reduces native diversity and coastal flood protection. First, we conducted a field experiment that compared the effects of disturbance and competition with resident species on biomass of A. breviligulata, A. arenaria, and the native Elymus mollis. We then quantified associations with parasitic nematodes and arbuscular mycorrhizal and endophytic fungi. Second, we conducted a growth chamber study to investigate the effects of microbes isolated from A. arenaria soil on biomass of both Ammophila species.
In the field experiment, we found that A. breviligulata did not exhibit disturbance-dependence, while A. arenaria and E. mollis did. A lack of disturbance-dependence likely has allowed the A. breviligulata to more readily establish in A. arenaria dominated dunes and ultimately displace A. arenaria. Furthermore, the native E. mollis harbored greater abundances of parasitic nematodes and arbuscular mycorrhizal fungi than either Ammophila species, but A. arenaria and A. breviligulata showed no differences. A. breviligulata demonstrated more frequent associations with dark septate endophytes than either of the other plant species. In the growth chamber experiment, the presence of microbes generally led to an increase in aboveground biomass and slight decrease in belowground biomass for A. arenaria, and an increase in belowground biomass for A. breviligulata. However, total biomass was not significantly affected by the microbe treatment. Overall, we conclude that A. breviligulata likely invades early successional dunes because, unlike its established competitors, its performance is not disturbance-dependent. In early succession when microbe and pathogen abundances may still be relatively low, a species’ ability to colonize independent of disturbance may be a critical mechanism by which it invades.