Variation in response by grasses to soil communities varies with introduced status: A mycorrhizal perspective

Background/Question/Methods

A large body of theoretical and experimental work has explored the communities and environments in which invasive species are found.  Instead of focusing on characteristics of newly invaded environments, Darwin’s Naturalization Hypothesis proposes that genera previously unrepresented in a habitat will be best able to invade that habitat, and recently this has been demonstrated for California grasses (Strauss et al. 2006).  What mechanism could produce this result?  In any new environment, perhaps the first and strongest selection pressure a non-native species encounters is a new soil environment.  Among plant species, both native and introduced, there is wide variation in response to soil communities. Given demonstrated variation in plant response to soil, we might expect three different evolutionary responses to introduction:  selection for genotypes that associate with a wide variety of soil organisms allowing new arrivals to colonize soil containing any community, selection for plant genotypes with a reduced response to soil communities allowing them to invade any soil regardless of the presence of a community, or selection may not influence plant growth in response to novel soil communities.

Here we test for mechanisms producing Darwin’s Naturalization Hypothesis, and directly examine whether response to soil communities differs between invasive pest grass species, invasive non-pest grass species, and native grass species in two soils from Yolo County, California.  We grew 18 grass species (6 introduced-pest grass species, 6 introduced-non-pest grass species, and 5 native grass species (all congeners of the introduced non-pest species)) in a common background soil inoculated with sterile or live soil from two locations in Yolo County where most of the grass species have been at one time identified.   

Results/Conclusions

We find that overall invasive species achieve a greater biomass than native species.  In addition, native species have a greater response to soil communities than invasive grasses, whether categorized as pest or non-pest species, but this variation in response to soil communities varies with the soil community tested.  We explore potential mechanisms for this pattern, including variation in root architecture and arbuscular mycorrhizal fungal colonization, and discuss implications for plant invasive potential and native biodiversity patterns across landscapes.