Submitted to: Diversity and Distributions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2010
Publication Date: 7/29/2010
Citation: Santos, M.J., Anderson, L.W., Ustin, S.L. 2010. Effects of invasive species on plant communities: an example using submersed aquatic plants at the regional scale. Biological Invasions. DOI 10.1007/s10530-010-9840-6. Interpretive Summary: The Sacramento-San Joaquin Delta in California is a critical water resource for domestic (potable), industrial and irrigation uses, and for sustaining natural aquatic and wetland communities of plants and animals. Aquatic invasive plants interfere with this resource, but the interacions of multiple species are poorly understood. Remote sensing combined with ground-truth sampling in 2007 and 2008 showed that the most abundant invasive aquatic weed is Brazilian egeria and that native plants and other invasive plants co-exist with this non-native plant, although their abundance appears to be greatly limited by it. This information will help resource managers develop and evaluate actions aimed at reducing impacts from exotic invasive aquatic plants.
Technical Abstract: Submerged aquatic plants have a key role in maintaining functioning aquatic ecosystems through their effects in the hydrological regime, sedimentation, nutrient cycling and habitats of associated fauna. Modifications of aquatic plant communities, as for example through the introduction of invasive species, can irreversibly alter these functions. Brazilian waterweed (Egeria densa) is a major invasive species in the Sacramento-San Joaquin River Delta. E.densa has two seasonal peaks of growth (summer and fall), which we suggest influence its persistence, relative abundance and distribution. We assessed community composition and distribution during the fall of 2007 and summer of 2008, and measured biomass, temperature, pH, salinity, and turbidity. In the fall, submersed aquatic plants covered a much higher proportion of the waterways (60.7%) than in the summer (37.4%), with a high overlap between the distribution of natives and exotics (74.3% in the fall and 38.2% in the summer). Most patches were monospecific (>50% in fall and summer), and multispecies patches had significantly higher dominance by E. dense (X2=61.65, p=0.001), especially in the fall. As species richness of exotic plants increased there was a significant decrease in richness of natives (F=3.19, p=0.05), and of native biomass (X2=8.14, p=0.04). Egeria densa co-occurred with most species, especially with Ceratophyllum demersum. Sustained E. densa summer biomass negatively affected the likelihood of presence of Myriophyllum spicatum, Potamogeton crispus, and Elodea canadensis; however, it did not affect their biomass within patches. Depth, temperature and salinity were associated with biomass; however, the direction of the effect was species specific. Our results suggest that despite native and exotic submerged species sharing available niches in the Delta, E. densa affects aquatic plant community structure and composition by facilitating persistence of some species and reducing the likelihood of establishment of other species.