Feedbacks through time for Microstegium vimineum: invasion persistence or community integration?

Background/Question/Methods

A lack of coevolutionary history has been proposed as a mechanism for invasion of native communities. Advantages for the invader could be in the form of escape from pest and pathogens, novel weapons, or unique niche requirements. It is unknown whether these advantages would persist through time. The evolutionary advantage could be lost through time due to changes in the soil microbe community, such as an accumulation of pathogens. A loss of advantages could be understood through feedbacks between the invader and its local environment (does increasing abundance lead to greater or less per capita fitness). Positive feedbacks (positive density dependence) would lead to population growth while negative feedbacks (negative density dependence) would cause population decline. We explored this idea using Microstegium vimineum, a common invasive, annual grass. We established 12 sites throughout the eastern US that varied in age since invasion by M. vimineumand collected data pertaining to fitness; percent cover, individual biomass and mortality, and soil nutrients and community diversity. We hypothesized that negative density dependence would increase as invasion age increased and would be coupled with changes in soil microbe communities. This would support the idea of a loss of evolutionary advantages as time increases. 

Results/Conclusions

Comparing M. vimineum percent cover and percent mortality within plots at each site suggests changes in density dependence that are correlated with age of invasion. Across all sites, mortality generally showed negative density dependence (increasing with increasing cover of M. vimineum). However, this pattern varied with the invasion history at each site. In young populations, mortality was low at low density, increasing only at very high densities. In older populations, mortality was high even at low densities, and neared 100% at the highest densities.  Fungal community structure also showed a significant pattern with age. These preliminary results suggest that there could be a possible loss of evolutionary advantages as invasion age increases. This could be due to changes in the soil microbe community, possibly an accumulation of fungal pathogens. This study supports further investigation of soil microbe communities and other drivers of density dependence and how these could feed back to affect growth of M. vimineum populations through time.