Propagule pressure cannot always overcome biotic resistance: the role of density-dependent establishment in four invasive species

Successful invasions result from species functional traits interacting with the receiving community. Some have proposed that propagule pressure, or the size and number of introductions, can overcome high invasion resistance. However, few studies empirically investigate the relationship among functional traits, community composition, and propagule pressure. Here, we empirically examined how propagule pressure influenced the invasive success of four species in four resident communities (bare ground, grass-only, forb-only, and full resident community). The four species represented both small- and large-seeded grasses and forbs, and propagule sizes (the number of individuals in an introduction event) ranged from 16 to 160000 seeds m(-2). Invasion probability varied among species and communities and was strongly influenced by the propagule size. The large-seeded species had the highest invasion success in all communities, while the grasses did as well or better, respectively, in the grass-only and bare ground communities. However, contrary to existing assumptions, we found that the establishment probability of individual seeds decreased exponentially with increasing propagule size. Therefore, increased propagule pressure was subjected to severely diminishing returns on invasiveness. Thus, while propagule pressure played a role in determining invasion success, it was not always able to overcome communities with high invasion resistance, particularly for species that experienced strong density-dependent inhibition. The role of propagule pressure in invasion is more complex than previously thought and may present important trade-offs for bet-hedging colonisation strategies.