Study tracks sudden drop in numbers at important breeding site in Southern Ocean
The long-term study found cyclical fluctuations in breeding success, showing that the birds defer breeding in less than optimal conditions. But the drop in numbers the past 10 to 15 years is alarming, according to the findings published in the journal Polar Biology online.
Whole-island counts show that, following an increase in breeding numbers from the mid 1980s to the mid 2000s, the population has halved from over 5,800 to around 2,600 nesting birds since 2005.
Southern giant petrels are large, colonially-nesting seabirds with wingspans of over 2 m capable of long-distance ocean travel. They feed on prey ranging from crustaceans (including Antarctic krill), squid and fish at sea, to penguin and seal remains on land. Most birds breed for the first time only when they are between six and ten years of age, and they produce at most one chick per year.
“In the fifty year study we found that, although the population fluctuated periodically over this time, in the last 10 years, both numbers and breeding success have declined, said British Antarctic Survey scientists and lead author Mike Dunn. “Since the South Orkney Islands, of which Signy is part, represent nearly 10 percent of the global population of this species, continuation of such a decline both at Signy and elsewhere in this island group would be of conservation concern,” Dunn said.
Scientists have studied the giant petrels breeding at Signy Island – in the South Orkney Islands (60°S) – since 1968. Twenty years ago, a structured monitoring program was introduced to keep a closer track of numbers of breeding birds and how many chicks fledged each season.
These surveys – the most detailed on this species in the South Orkneys – have now revealed that breeding success has declined substantially, from 60 percent in 1996 to 40 percent in 2015 in selected study colonies. Whole-island counts show that following an increase in breeding numbers from the mid 1980s to the mid 2000s, the population has halved from over 5800 to around 2600 nesting birds since 2005.
“The results are surprising because this species seemed to be doing well on Signy,” said BAS seabird ecologist and co-author Dr Richard Phillips. “We really don’t know what’s causing this decline. It could be a reduction in sea ice or other factors affecting food availability and we don’t know if it’s affecting the species regionally or more widely.
Phillips said the study highlights the need for ongoing wildlife monitoring because the conservation situation of a species can change in only a few years. Giant petrels were listed as a vulnerable species for a time, then relisted as a species of least concern on the IUCN Red List in the late 2000s because positive trends at some sites counteracted negative trends elsewhere.
“However, if breeding numbers and success at Signy do not improve, and populations elsewhere in the Antarctic have continued to decline, then the IUCN status of this species will need to be re-assessed,” Phillips said.
The challenge is to understand which climatic or other environmental factors are driving these big changes for this important Antarctic predator, he concluded