Anthropogenic disturbances have led to vast invasion of exotic annual species into previously perennial dominated grasslands (more than 9,000,000 hectares in California only). Nowadays many efforts are dedicated for restoring these perennial grasslands. However, it is not clear when perennials can bounce back by simply removing disturbance (succession) and when stochastic processes such as the order of species arrival can dictate dominance (priority effects).
Historically, dominance by annual plants has been considered a brief early stage of ecological succession preceding inevitable dominance by competitive perennials. A more recent, alternative view suggests that interactions between annuals and perennials can result in priority effects, causing annual dominance to persist if they are initially more common. However, the conditions under which these priority effects occur remain unknown.
Results/Conclusions
Using a simple simulation model, we show that long-term (500 years) priority effects are possible as long as the plants have low fecundity and show an establishment-longevity tradeoff, with annuals having competitive advantage over perennial seedlings. These results contrast previous models where the same trade-off has led to coexistence (stablizing trade-off) rather than priority effects (destablizing trade-off). We also show that short-term (up to 50 years) priority effects arise solely due to low fitness difference in cases where perennials dominate in the long term.
Our results provide a new theoretical basis for achieving successful restoration of native grassland ecosystems that were invaded by exotic annual species. They do so by indicating the conditions under which it is not enough to simply restore historical habitat conditions, but it is also necessary to actively remove exotic annuals and reintroduce native perennials