Plant invaders may gain an advantage over natives because only enemy-free species are likely to invade (the predator filter hypothesis), or because invaders lose enemies while moving to a new area (the predator escape hypothesis). Working at the Joker's Hill Field Station, near Newmarket, Ontario, I have combined site descriptions, surveys, and experiments to investigate the role of predators and pathogens in seed ecology, and to understand their role in the establishment and spread of exotic species.
Results indicate that seeds of both natives and exotics suffered significant losses to above-ground predators and below-ground pathogens. Losses varied among species and habitats; wetlands had particularly high levels of fungal mortality. Aliens and natives did not consistently differ in their susceptibility to predators and pathogens, even when analyzed using methods that controlled for phylogenetic biases. These results suggest natural enemies of seeds do not as a general rule determine invasive ability.