Linear and non-linear impacts of a non-native plant invasion on soil microbial community structure and function

Biological invasions can alter ecosystem functions such as litter decomposition and nutrient cycling, but little is known about how invader abundance influences the impact on the ecosystem. It is often assumed that impacts are proportional to invasion density, but this assumption has never been tested and has little justification. We tested the hypothesis that the microbial community structure and function of a mixed hardwood forest soil changed after invasion by Japanese barberry (Berberis thunbergii), an invasive shrub commonly found in eastern hardwood forests, and that changes were proportional to the density of invasion. We constructed microcosms with mixtures of native and invasive leaf litter, and measured microbial community structure (phospholipid fatty acids) and function (litter decomposition). Decomposition was linearly related to the degree of invasion (R2 = 0.945), but the ratio of bacteria to fungi exhibited a strongly non-linear, threshold response (R2 = 0.513). These results indicate that impacts of Japanese barberry invasion are not always proportional to invasion density. This finding has implications for the study of biological invasions as well as practical implications for the management of exotic invasive species.