High abundance of introduced plants on ancient Native American middens

Past land uses by humans can have long-term effects on natural communities. To test the hypothesis that past land use can alter patterns of biological invasion for extended times, we predicted that middens abandoned by Native Americans over a century ago would have high abundances of introduced plants. We measured cover of vascular species and soil characteristics on and off the eight remaining middens in grasslands along 20 km of the northern coast of California. Relative cover of introduced plants was about 20% higher on than off middens, and cover of introduced annuals was about 110% higher. Concentration of inorganic N in the upper soil was twice as high on as off middens and explained about 40% of the variation in cover of introduced annuals. Abundance of shells on middens was correlated with inorganic N, suggesting that present invasion was linked to intensity of past land use. Results confirm that ancient land use by Native Americans is associated with present-day invasion by introduced plants and suggest that persistent elevation of soil N is partly responsible. Land use that elevates nutrient levels may promote invasion far into the future.