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 year BP), sedge-heather tundra grew in the studied area. A seabird colony existed on Shemya from 4,600 to 2,400 cal year 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 year BP to present), there was a shift toward less productive sedge-tundra communities. However, the significant enrichment of guano affected only the coastal vegetation and did not alter the inland Shemya Island.

Material culture across the Aleutian archipelago.

The material evidence from sites across the Aleutian Islands reflects colonization events, subsequent adaptations, and influxes of ideas and/or people from the east. The occurrence in the eastern Aleutians of bifacial technology around 7000 BP, of artifacts similar to the Arctic Small Tool tradition between 4000 and 3500 BP, and of slate and jet objects around 1000 BP reflects repeated surges of influence or movement of peoples from further east into the eastern end of the chain. In the central and western Aleutians, influence or perhaps colonization from east of the Aleutians is also marked by the occurrence of bifacial technology about 6500 BP and the appearance of slate artifacts after 1000 BP, suggesting the movement of ideas or people from further east. Basic trends across the archipelago include a decrease in formal chipped-stone tools, an increase in the use and the complexity of bone technology, and the increase in use and variety of ground-stone tools. In addition, increasing village site sizes and denser midden deposits are seen later in time throughout the archipelago. The similarity in sites and assemblages, albeit with regional variations, reflects trends that are seen across the chain and indicates that these island communities were not isolated from one another or from mainland Alaska.