Abstract
Invasions by exotic plants may be more likely if exotics have low rates of attack by natural enemies, including post-dispersal seed predators (granivores). We investigated this idea with a field experiment conducted near Newmarket, Ontario, in which we experimentally excluded vertebrate and terrestrial insect seed predators from seeds of 43 native and exotic old-field plants. Protection from vertebrates significantly increased recovery of seeds: vertebrate exclusion produced higher recovery than controls for 30 of the experimental species, increasing overall seed recovery from 38.2% to 45.6%. Losses to vertebrates varied among species, significantly increasing with seed mass. In contrast, insect exclusion did not significantly improve seed recovery. There was no evidence that aliens benefitted from a reduced rate of post-dispersal seed predation. The impacts of seed predators did not differ significantly between natives and exotics, which instead showed very similar responses to predator exclusion treatments. These results indicate that while vertebrate granivores had important impacts, especially on large-seeded species, exotics did not generally benefit from reduced rates of seed predation. Instead, differences between natives and exotics were small compared with interspecific variation within these groups.