“Terrestrialization” of coastal wetland ecosystems by a riparian invader

Invasions by introduced plants are currently one of the most serious threats to biodiversity.  In addition to altering ecosystem structure, invasive plant species may also be capable of changing the pace and/or direction of succession, especially within ecotones such as wetlands.  One of the most problematic invaders in the western United States has been salt cedar, Tamarix spp.; the impacts of this species in riparian and desert ecosystems have been well documented.   Large stands of different invasive genotypes of tamarisk now reside in the salt marshes in Tijuana River National Estuarine Research Reserve (TRNERR).  Salt marshes are a habitat not previously considered vulnerable to invasion by tamarisk.  We hypothesized that the tamarisk invasion into TRNERR would “terrestrialize” the salt marsh habitat; in other words, the invasion would speed natural succession towards a more terrestrial environment. 

Using mensurative comparisons in a paired block design, we described the impact of tamarisk invasion on abiotic and biotic properties of the sediment ecosystem in three zones along an elevation and salinity gradient.  In the low and high marshes, changes in physical and biological parameters between tamarisk-invaded and natural plots were minimal.  In the middle marsh zone, the physical environment was significantly drier, less humid with lower temperatures and lower light levels.  These physical changes were associated with increased microalgal biomass, an increased abundance of mites and insects, and a decreased density of marine oligochaetes and epifunal gastropods as compared to native treatment plots.  This research emphasizes the habitat-altering consequences of tamarisk’s first foray into the marine realm while also revealing that each marsh habitat subject to tamarisk invasion will have to be managed with different eradication strategies.