Potential drivers of mutual invasibility in Rumex congeners
Theory suggests that environmental heterogeneity should increase coexistence, by generating multiple niches within a community. However, experiments often find the opposite pattern, where heterogeneity decreases diversity. If heterogeneity is spatiotemporal, varying in space and time simultaneously, then heterogeneity can reduce coexistence, potentially explaining this apparent contradiction. Plant-soil feedbacks are by nature spatiotemporal, because they are influenced by the plant growing in that patch of soil, which influences the performance of the next plant to experience that patch. We used a mutual invasibility experiment with Rumex crispus and R. obtusifolius to ask how abiotic and biotic environmental heterogeneity created by plant-soil feedbacks may influence coexistence. The field experiment included a factorial combination of homogeneous and heterogeneous soils crossed with ambient and high turnover resident populations. We measured plant performance for resident and invader populations across these treatments. We also used DNA fragment analysis to quantify differences in soil microbial communities (bacterial, fungal) among soil patches in each treatment, and soil samples were analyzed for differences in soil chemistry. Finally, field measures of soil moisture in each patch were used to isolate effects of soil moisture on invasibility patterns.
Invaders of both species were able to establish under all four experimental conditions, and invader plant size was greatest under homogeneous soil and high turnover treatments for both invaders. Soil heterogeneity treatments influenced soil bacterial communities, and mixing soils from Rumex crispus and R. obtusifolius resulted in bacterial communities that were not intermediate between the two soil origins. Soil ammonium levels were also 1.5´ greater in homogeneous soils than in either heterogeneous soil patch type. Homogeneous and heterogeneous soils also differed in soil moisture holding capacity. Non-additive effects of soil mixing on plant performance could result from effects on the bacterial community, nitrogen availability, or soil moisture. Finally, we found that heterogeneity can reduce coexistence when it is generated by plant-soil feedbacks, potentially helping to explain a long-standing controversy in ecology.