We have studied the long- and short-term periods of seabird influence on
coastal vegetation. In the Aleutian Islands during the Holocene,
terrestrial predators were virtually 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. The vegetation of
several Aleutian Islands has been reconstructed; however, only the
vegetation on Carlisle Island had noticeable impact from the seabird
guano. For more detailed investigation of bird influence, we conducted
pollen analysis to reconstruct the 9,300-year-old vegetation dynamics of
the coast of Shemya Island. From earlier studies of nitrogen isotopes in
peat, we discovered that a large seabird colony existed on Shemya from
4600 to 2400 years ago, and birds also influenced coastal ecosystems
between 1470–1160 and 810–360 years ago. In these sequences, the
tundra dominated by Ericaceae dwarf shrubs initially spread on the
coast. During a period of at least 2200-years nitrogen enrichment led to
the development of herb meadows with a high presence of Apiaceae. After
a noticeable reduction in seabird colonies due to human hunting,
grass-meadows spread. During the late Holocene several hundred years of
seabird impact led to an increase in abundance of indicator taxa, ferns
and umbelliferous species, as well as in total pollen concentration, but
this did not result in a radical change of dominants. In recent decades,
due to the extinction of the bird colonies, heather communities have
begun to spread on the Shemya coast. Also large ash emissions in the
Aleutian Islands can lead to a decrease in pollen concentration even in
peat located far from an eruption.
Ornithogenic vegetation: How significant has the seabird influence been on the Aleutian Island vegetation during the Holocene?
