Layers of ancient bird poo preserved in peatlands have given scientists a window into 8,000 years of seabird history.
An international research team, led by the Swedish University of Agricultural Sciences and University of Bern in collaboration with the University of Exeter and British Antarctic Survey amongst others, were collecting peat to reconstruct past wind speeds on the sub-Antarctic island of Bird Island when they made an unexpected discovery.
They found that by analysing the Mercury content in the poo (or guano) trapped in peatlands below the seabirds' nesting site they could reliably indicate the size of past seabird populations.
Seabirds are top predators, and the mercury from the fish they eat accumulates in their bodies and passes into their poo.
That mercury then becomes trapped in successive layers of peat, creating a chemical record of how many birds were nesting above.
Using this method the researchers have shown that seabird colonies were established on Bird Island between 6,800 and 6,100 years ago, more than 1000 years earlier than previously thought.
They also found four distinct periods since then with major increases in seabird populations, and that these periods coincided with less intense Southern Hemisphere westerly winds.
"Our findings lead us to believe that the present-day increase in westerly wind intensity may bring about further declines in seabird populations in the Southern Ocean," said lead author Dr Chuxian Li.
As top predators that move across vast regions - with some species migrating across entire oceans - seabirds provide early-warning signals of largescale changes in ecosystems.
But how these seabird populations have varied in size in the past before the Anthropocene (modern era) has been largely unknown, which in turn limits our ability to predict the future consequences of climate change.
"The oceans are all connected - changes in one place ripple to others. This is why changes like the dramatic decline of globally monitored seabird populations - down by up to 70 per cent since the 1950s - is concerning for all of us", said Dr Li.
The researchers' new method could now be used to conduct similar studies at other locations around the world where there are suitable archives of peat layers or sediments.
"Mercury analyses, coupled with other techniques such as environmental DNA (eDNA), will provide new opportunities to reconstruct historical ecosystems. This can help us understand how seabirds respond to today's climate changes and how declining populations can affect ecosystems in the long term," said Dr Li.
