Salt Marsh-Upland Ecotones in Central California: Vulnerability to Invasions and Anthropogenic Stressors

Abstract

Ecotones, zones of abrupt biological transition typically reflecting strong physical gradients, may be particularly sensitive to changes in environmental conditions. Our characterization of the ecotone between salt marshes and uplands in the Elkhorn Slough watershed in central California revealed that extent of appropriate habitat for native high marsh species endemic to this zone is extremely limited. The ecotone is highly invaded, with non-native upland weeds accounting for a significant proportion of cover. We investigated responses to two anthropogenic landscape management strategies, restriction of tidal exchange through water control structures and cattle grazing. Moderate tidal restriction resulting in muted tidal exchange dramatically decreased ecotone width, native marsh plant richness, and cover by native ecotone specialists. Even stronger tidal restriction resulting in very low tidal exchange led to a seaward shift of the ecotone into the area formerly occupied by mid-marsh vegetation; upland plants now occupy the former ecotone zone so net loss of wetland habitat has occurred. Cattle grazing led to a very substantial increase in bare ground, a significant decrease in native marsh plant richness and a significant increase in non-native plant cover. Thus, both of these management regimes can have significant negative impacts on rare salt marsh ecotone extent and biodiversity.