Does environmental variation structured by plant-soil feedbacks interact with species life history to influence invasibility?

Background/Question/Methods

The effects of environmental heterogeneity on plant populations are important for understanding the mechanisms governing invasibility and thus coexistence. In theory, environmental heterogeneity should interact with species’ life history to influence invasibility. Spatial heterogeneity in the soil microbiota, especially as a result of plant-soil feedbacks (PSF), could have important consequences for plant populations. PSF occur when a plant affects the properties of the soil environment and thus subsequent plant performance in that soil. However the consequences of this heterogeneity for plant populations are typically unknown.

To determine whether soil heterogeneity created by PSF can interact with species’ life history to influence plant population establishment and invasibility, we conducted a field experiment at Case Western Reserve University Farm with six perennial plant species in three congeneric pairs. We used experimental manipulations of resident population turnover rate, or the rate at which individuals die and are replaced in the population, as a proxy for life history. We manipulated soil heterogeneity experimentally with soils from different zones of root influences in the field (conspecific, congener), established resident populations of plants, manipulated the turnover rate of the resident population (ambient, high), and then experimentally invaded all plots with the resident’s congener.

Results/Conclusions

The effects of soil treatment depended on species, where the probability of an invader establishing at a given site within a plot was higher in heterogeneous soil environments than in homogeneous treatments for Rumex obtusifolius and Solanum dulcamara. In addition, the final size of invader populations was a function of invader species identity interacting with soil heterogeneity and the turnover rate of the resident population. For example, Rumex crispus had higher invader population sizes in high turnover treatments than in low turnover treatments, but only when in homogeneous soils. However, Solanum dulcamara had higher invader population sizes in the ambient turnover treatment than in the high turnover treatment in homogeneous soils. This is consistent with theoretical predictions that the speed of population turnover for resident populations is an important life history trait that can interact with environmental heterogeneity to influence patterns of invasibility.