Establishment from seed is more important for exotic than for native plant species

Climate change has initiated the movement of both native and non-native
(exotic) species across the landscape.  Exotic species are
hypothesized to establish from seed more readily than comparable native
species.  We tested the hypothesis that seed limitation is more
important for exotic species than native grassland species.  We
compared seed limitation and invasion resistance over three growing
seasons between 18 native and 18 exotic species, grown in both
monocultures and mixtures in a field experiment.  Half of the
plots received a seed mix of the contrasting treatment (i.e. exotic
species were seeded into native plots, and native species were seeded into
exotic plots), and half served as controls. We found that 1) establishment
in this perennial grassland is seed limited, 2) establishment from seed is
greater in exotic than native species, and 3) community resistance to
seedling establishment was positively related diversity of extant species,
but only in native communities.  Native-exotic species diversity
and composition differences did not converge over time.  Our
results imply that native-to-exotic transformations occur when diversity
declines in native vegetation and exotic seeds arrive from adjacent sites,
suggesting that managing for high diversity will reduce transformations to
exotic dominance.