Ornithogenic vegetation: How significant has the seabird influence been on the Aleutian Island vegetation during the Holocene?

In the Aleutian Islands during the Holocene, terrestrial predators were actually absent; as a result, large seabird colonies thrived along the coasts or across entire islands. Bird guano enriches the soil with nitrogen, which can lead to the formation of highly modified ornithogenic (bird-formed) ecosystems. For a more detailed investigation of avian influence, we reconstructed more than 10,000-year-old vegetation dynamics of the coast of Shemya Island (Near Islands) by pollen analysis. At the initial stages of vegetation development (10,000-4,600 cal yr BP), sedge-heather tundra grew in the studied area. A seabird colony existed on Shemya from 4,600 to 2,400 cal yr BP according to stable isotope analysis. During a period of at least 2,200-years nitrogen enrichment led to the development of ornithogenic herb meadows with a high presence of Apiaceae. A long-term increase in δ15N above 9-10‰ led to radical shifts in vegetation. Noticeable reduction of seabird colonies due to human hunting led to grass-meadows spreading. After a prolonged decrease δ15N below 9-10‰ (2,400 cal yr BP to present), there was a shift towards less productive sedge-tundra communities. However, the significant enrichment of guano affected only the coastal vegetation and did not alter the inland Shemya Island.