Anthropogenic impacts on an intertidal seaweed: population density predicts decline in size of Mazzaella splendens over a 150-year period

Abstract

It is well established that ecosystems exhibit “threshold effects” whereby a small change in the intensity of a human disturbance produces large responses in the ecosystem. In a dataset spanning over 150 years of size data of the intertidal macroalga Mazzaella splendens obtained from herbarium specimens, we show that a statistically significant decrease in size occurred among specimens collected from two urban areas with the highest human population densities (San Francisco, Puget Sound) but not among specimens collected from areas with lower human population densities (San Juan, Monterey, San Luis Obispo, and Marin Counties). These results run counter to earlier work which suggested that small amounts of human disturbance were sufficient to produce substantial decreases in intertidal macroalgal size and that initial disturbances produced larger decreases in macroalgal size than subsequent disturbances.