Evolution of an expanded niche via hybridization: an experimental comparison of the ecological amplitude of Spartina foliosa, Spartina alterniflora and their hybrids

Background/Question/Methods

Ecologists have been intrigued by the high incidence of hybrid taxa among invasive species, yet experiments showing a fitness advantage or expanded niche in hybrids as compared to parental species are rare. Spartina alterniflora x foliosa, the hybrid of native Spartina foliosa and introduced Spartina alterniflora, is an invasive weed in San Francisco Bay salt marshes. While representatives of the pure exotic parental species have nearly vanished from the Bay, introduced genes are sweeping through the native population, generating highly fit hybrids that appear to outcompete both parent species as well as colonize habitat beyond the range of either parent.  Hybrid Spartina is particularly interesting because of its apparent ability to grow both higher and lower in the intertidal than native Spartina, indicating that it has evolved greater tolerance to the very different environmental stresses that occur at either extreme of the intertidal. We planted hybrids and both parental species in a common garden experiment across a range of intertidal elevations to determine if hybrids are more fit and/or have an expanded niche compared to their progenitors. Plants were monitored for survivorship and vegetative growth.

Results/Conclusions

Hybrids survived at higher elevations and had higher relative fitness than either parental species from the mid to high intertidal. However, exotic S. alterniflora genets had the highest relative fitness at low elevations. Hybridization has generated genotypes with an expanded intertidal niche as compared to parent species. However, this advantage is only seen at high elevations, indicating that hybrids are better able to withstand the extreme salinity of the high marsh, but are less able to withstand the long duration of flooding in the low marsh as compared to S. alterniflora. All taxa grew best at mid elevations, and the fitness advantage of hybrids was most pronounced in optimal habitat. Results from this experiment indicate that hybrids are effective invaders both because they can better capitalize on optimal growing conditions as well as colonize new habitat.