Invasions by alien plants provide a unique opportunity to examine competitive interactions among plants. While resource competition has long been regarded as a major mechanism responsible for successful invasions, given a well-known capacity for many invaders to become dominant and reduce plant diversity in the invaded communities, few studies have measured resource competition directly or have assessed its importance relative to that of other mechanisms, at different stages of an invasion process. Here, we review evidence comparing the competitive ability of invasive species vs. that of co-occurring native plants, along a range of environmental gradients, showing that many invasive species have a superior competitive ability over native species, although invasive congeners are not necessarily competitively superior over native congeners, nor are alien dominants are better competitors than native dominants. We discuss how the outcomes of competition depend on a number of factors, such as the heterogeneous distribution of resources, the stage of the invasion process, as well as phenotypic plasticity and evolutionary adaptation, which may result in increased or decreased competitive ability in both invasive and native species. Competitive advantages of invasive species over natives are often transient and only important at the early stages of an invasion process. It remains unclear how important resource competition is relative to other mechanisms (competition avoidance via phenological differences, niche differentiation in space associated with phylogenetic distance, recruitment and dispersal limitation, indirect competition, and allelopathy). Finally, we identify the conceptual and methodological issues characterizing competition studies in plant invasions, and we discuss future research needs, including examination of resource competition dynamics and the impact of global environmental change on competitive interactions between invasive and native species.

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 non-invasive 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.

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