Using ecological restoration to constrain biological invasion

1. Biological invasion can permanently alter ecosystem structure and function. Invasive species are difficult to eradicate, so methods for constraining invasions would be ecologically valuable. We examined the potential of ecological restoration to constrain invasion of an old field by Agropyron cristatum, an introduced C-3 grass. 2. A field experiment was conducted in the northern Great Plains of North America. One-hundred and forty restored plots were planted in 1994-96 with a mixture of C-3 and C-4 native grass seed, while 100 unrestored plots were not. Vegetation on the plots was measured periodically between 1994 and 2002. 3. Agropyron cristatum invaded the old field between 1994 and 2002, occurring in 5% of plots in 1994 and 66% of plots in 2002, and increasing in mean cover from 0.2% in 1994 to 17.1% in 2002. However, A. cristatum invaded one-third fewer restored than unrestored plots between 1997 and 2002, suggesting that restoration constrained invasion. Further, A. cristatum cover in restored plots decreased with increasing planted grass cover. Stepwise regression indicated that A. cristatum cover was more strongly correlated with planted grass cover than with distance from the A. cristatum source, species richness, percentage bare ground or percentage litter. 4. The strength of the negative relationship between A. cristatum and planted native grasses varied among functional groups: the correlation was stronger with species with phenology and physiology similar to A. cristatum (i.e. C-3 grasses) than with dissimilar species (C-4 grasses). 5. Richness and cover of naturally establishing native species decreased with increasing A. cristatum cover. In contrast, restoration had little effect on the establishment and colonization of naturally establishing native species. Thus, A. cristatum hindered colonization by native species while planted native grasses did not. 6. Synthesis and applications. To our knowledge, this study provides the first indication that restoration can act as a filter, constraining invasive species while allowing colonization by native species. These results suggest that resistance to invasion depends on the identity of species in the community and that restoration seed mixes might be tailored to constrain selected invaders. Restoring areas before invasive species become established can reduce the magnitude of biological invasion.