Invasiveness of Ammophila arenaria: Release from soil-borne pathogens?

The Natural Enemies Hypothesis (i.e., introduced species experience release from their natural enemies) is a common explanation for why invasive species are so successful. We tested this hypothesis for Ammophila arenaria (Poaceae: European beachgrass), an aggressive plant invading the coastal dunes of California, USA, by comparing the demographic effects of belowground pathogens on A. arenaria in its introduced range to those reported in its native range. European research on A. arenaria in its native range has established that soil-borne pathogens, primarily nematodes and fungi, reduce A. arenaria's growth. In a greenhouse experiment designed to parallel European studies, seeds and 2-wk-old seedlings were planted in sterilized and nonsterilized soil collected from the A. arenaria root zone in its introduced range of California. We assessed the effects of pathogens via soil sterilization on three early performance traits: seed germination, seedling survival, and plant growth. We found that seed germination was reduced by 12-16% in nonsterilized soil compared with sterilized soil; similarly, seedling survival was reduced by 7-13%. Both root and shoot biomass decreased in the nonsterilized soil treatments compared with sterilized soil, by as much as 81% after 6 wk of growth. A comparison of the introduced and native ranges failed to show a demographic release from natural enemies; biomass effects of pathogens after 6 and 12 wk of growth were nearly equal between studies in California and The Netherlands. Identification of pathogens and inoculation experiments with fungi from the nonsterilized soil revealed several common fungal pathogens, whereas virtually no pathogenic nematodes were found. Therefore, we found a partial escape from enemies (i.e., pathogenic nematodes were absent in the introduced range), although there was no demographic release. These findings suggest that to fully understand the success and impact of A. arenaria in North America, one must look beyond escape from natural enemies.