Is coexistence between invasive Microstegium vimineum and native plants dependent on invasion history of the soil?
Coexistence of species with large differences in average fitness is a result of stabilizing niche differences. Stabilizing niche differences are characterized by negative frequency dependence, decreasing per capita growth rate with increasing frequency of conspecifics, which can buffer species from extinction and prevent competitive exclusion of heterospecifics. Invasive species often competitively exclude native species in local areas, a result of higher average fitness and/or possibly weak negative frequency dependence. Invasive plants may have these traits due to their lack of coevolution with species in the invaded range; for instance, they may lack co-evolved specialist enemies in their new range. However, enemies may accumulate through time, which could reduce the average fitness differences between the invader and native species and increase the invader’s negative frequency dependence. If this occurs then competitive outcomes between invasive and native species may shift from dominance by the invader to coexistence between the species. We have performed an observational study, field experiment, and currently, a greenhouse experiment to investigate whether stabilizing niche differences are increasing and fitness is decreasing for invasive Microstegium vimineum across invasion time and whether this can be related to changes in the soil microbial community, specifically an accumulation of soil pathogens.
Studying the invasive grass, Microstegium vimineum, through the field experiment, we found evidence that suggests an increase in stabilizing niche differences and a decrease in average fitness differences between native species, Pilea pumila, and itself through invasion time. A soil manipulation in the field experiment showed these same patterns in both invaded and uninvaded soil, suggesting that direct conditioning of the soil by M. vimineum is not needed for these patterns across invasion time. Although, in the observational study we found changes in the general fungal community on M. vimineum roots and soil across invasion time and with survivorship of M. vimineum individuals, suggesting that soil microbes could be driving these changes through time. The final greenhouse experiment will allow us to directly estimate the two components of coexistence, stabilizing niche differences and relative fitness differences, for M. vimineum and native P. pumila and how they change based on the invasion history of the soil microbial community. We expect that stabilizing niche differences will increase and relative fitness differences will decrease with increasing invasion age of the soil community, increasing the potential for coexistence through invasion time.