Invasion success of three cool-season grasses in the northern prairie : a test of three hypotheses

Abstract

Empirical invasion ecology is laden with high context dependency. If general mechanisms underlying invasion success exist, they should be detectable in species that share biological and ecological characteristics. We carried out a growth experiment with Agropyron cristatum, Bromus inermis and Poa pratensis (subsp. angustifolia), to better understand the mechanisms underlying the invasion success of cool-season grasses in northern prairie grasslands of North America. By using a home–away approach, we tested whether 1) non-native plants have a higher performance than native plants, and whether invasiveness is 2) mediated by interactions with soils, such as a release from pathogens or enhanced mutualism, or 3) an adaptation to local soils. We compared seed size and weight of populations in Canada (non-native range) and Eurasia (native range) and carried out an experiment, in which seeds from the non-native and native ranges were planted into sterilized soil (control) and soil from a population in Canada or Eurasia, or local soils, respectively. We found inconsistent effects across species and response variables. Seed size and weight were not significantly different between native and non-native populations. The experiment showed a seed origin effect in A. cristatum (root and total biomass) and B. inermis (root biomass), with non-native populations outperforming native ones. Soil-mediated effects were supported in A. cristatum (root biomass) and local adaptation in B. inermis (root and total biomass). Germination across all species and biomass in P. pratensis did not respond to treatments. Despite the high similarity of our study group, our results indicate that invasiveness might be driven by idiosyncratic causes at the species level. Mechanisms not considered in our study, such as high propagule pressure and preadaptation could also potentially explain the invasion success across species.