Impotent pathogens and invasive success

Understanding why some non-native invasive species are so incredibly successful in foreign places is a major question in ecology.  Experimental studies find greater damage to invading plant species in their home vs. invaded ranges, but little is known about the biological mechanisms underlying this variation.  Isolates of the soil-borne Pythium pathogens are abundant, but isolates collected from soil surrounding the invasive tree Prunus serotina in its native USA range caused more root rot (≥151%) and mortality (≥64%) of seedlings than Pythium from the non-native European range.  These experimental results, along with demographic patterns of P. serotina density in nature, strongly indicate that Pythium associated with P. serotina are causing more mortality in native than non-native ranges.  Our results support the Enemy Release Hypothesis, which predicts plants that invade non-native regions experience less density regulation by enemies.  Understanding why invading species are less affected by resident enemies is important for developing strategies to predict and control exotic plants in their new ranges.