The influence of nutrient loading and sediment disturbance on native-invasive species dynamics

Our understanding of the effects of anthropogenic nutrient loading on the success of biological invasions is best developed in terrestrial systems, where studies have generally shown that increased nutrients facilitate the spread of invasive plants. To date, these same relationships have remained largely unexamined in nearshore benthic marine systems. In addition to excess nutrients, small-scale disturbances caused by invasive species behaviors affect ecosystem processes in mudflat communities and negatively affect the native species, potentially creating a feedback loop that facilitates invasion. I conducted a suite of lab and field studies to examine the interaction of nutrient loading and invasion-related disturbance produced by the invasive Atlantic mud snail, Ilyanassa obsoleta, on the native California mud snail Cerithidea californica. In San Francisco Bay, CA, where both species coexist, the invasive snail can affect the microalgal community by burrowing (bioturbation) as well as grazing. In addition, Ilyanassa has been documented as competitively displacing the native snail through behavioral interactions. I experimentally manipulated both densities of Cerithidea and Ilyanassa (crossing disturbance and resource competition) as well as nutrient levels. Results indicate that nutrient additions shift the microalgal community, which then affects trophic interactions between the snails. Interestingly, the documented behavioral interference between the species was not observed in either field or lab experiments. This response suggests plasticity among the snail populations or a possible interaction with nutrients. My results suggest that the nutrient loading can interact with disturbance to facilitate species invasions, and provides further justification for reducing nutrient loading in coastal estuaries.