Early bird catches the worm: germination as a critical step in plant invasion

The germination behavior of a plant influences its fitness, persistence, and evolutionary potential, as well as its biotic environment. This can have major effects on the invasive potential of a species. We review the findings of four types of experimental studies comparing basic germination characteristics of invasive versus non-invasive congeners, in their non-native or native distribution range; invasive alien versus native species; and invasive species in their native versus non-native distribution range. Early and/or rapid germination is typical of invasive species rather than their noninvasive congeners, and represents a pre-adaptation from which many invasive and naturalized species benefit. It also occurs more often in invasive than native species, suggesting that competition mitigation or avoidance in the early stages of a plant's life, via the exploitation of vacant germination niches, might be more useful than a superior competitive ability in novel environments. This is further supported by a tendency of invasive species to germinate earlier and/or faster and have broader germination cues in their non-native than in their native range. It is also supported by broader germination requirements being reported for invasive species than their non-invasive or native congeners. In contrast, high percentage germination is not a consistent predictor of invasiveness, suggesting that the incorporation of a larger fraction of seed production into the soil seed bank rather than high germination is a better (or safer) strategy in novel environments. These patterns indicate that differences in the germination behavior of alien and native species contribute to the invasiveness of many species, although evidence under natural conditions is needed. The role of such differences in the establishment and spread of invasive species in novel environments and their long-term impact on community dynamics requires further study.