Status, prediction and prevention of introduced cordgrass Spartina spp. invasions in Pacific estuaries, USA
Along the Pacific coast of North America, four introduced cordgrass species (Spartina alterniflora, S. anglica, S. patens and S. densiflora) have thus far invaded five isolated estuaries. Dense growth of introduced Spartina spp. reduces open mud feeding habitats of shorebirds, while in the upper intertidal, introduced Spartina spp. compete with native salt marsh vegetation. Prediction of Spartina invasions is facilitated by the remarkable restriction of these species to distinct estuarine habitats which generally lack interspecific competitors and herbivores. We used physical characteristics to identify 31 specific sites along the US Pacific coast that are vulnerable to future Spartina invasions and then used species characteristics, like native latitudinal range and past invasion success, to predict which Spartina species will be likely to invade these sites in the future. All 31 sites were predicted to be vulnerable to S. alterniflora, while the other invasive Spartina spp. may be restricted to a subset of the vulnerable sites. At a finer scale, within a vulnerable site, the mean tidal range can be used to predict the extent of spatial spread of a Spartina sp. after colonization. These prediction techniques might be used to identify and prioritize sites for protection against future invasions. We suggest that a cost-effective way to prevent the transformation of unique North American Pacific mudflat and saltmarsh communities into introduced Spartina-dominated marshes is to survey the vulnerable sites frequently and eliminate introduced Spartina spp. propagules before they spread.