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An ecosystem in flux
By Bob Berwyn
The 10-year study used electronic tags to monitor the seabirds at the remote site in the far South Atlantic Ocean near Antarctica. The macaroni penguin population on the South Atlantic island of South Georgia has declined by almost seventy per cent since the early 1980s.
“Penguins are facing rapid changes in their environment, but at South Georgia, in the southwest Atlantic, we found compelling evidence that predators are the most important factor influencing the survival of chicks as they leave the colony for the first time,” lead author Catharine Horswill said. “This is a big leap forward as we had no idea that predation could be such a strong driving force. Knowing what drives survival rates of penguins puts us in a much better place to predict how these populations may change in the future.”
“A census in the late 1970’s showed about 5.4 million pairs on South Georgia. A follow up census in early 2000’s showed this has declined to about 1 million pairs. Reductions have principally occurred at sites where numbers had previously been very large,” Horswill said via email. “The macaroni penguin is currently classified as a vulnerable species by the IUCN and has declined throughout its sub-Antarctic range,” she added.
Previous studies of penguin survival rates focused on environmental pressures. This study shows that macaroni penguins are influenced by both environmental and predation pressures. The scientists found penguins were particularly vulnerable to predation by other seabirds, giant petrels, which also nest on the island. Chicks were found to be particularly vulnerable with only a third surviving their fledgling year.
Together with findings from other studies, the new data on macaroni penguins suggest an ecosystem in flux, still recovering after the brutal whaling and seal hunting area, as well as facing pressure from climate change.
“For the 10 years considered in this study predation didn’t show a systematic increase but varied from year-to-year based on the breeding success of giant petrels,” Horswill said. “There has been a general increase in giant petrel numbers at South Georgia. This has been linked largely to the increased availability of carrion from fur seals that have increased rapidly in population size following the cessation of hunting during the sealing era.”Studies dating back to the early 1980s show that penguins have always been an important part of the petrels’ diet, she added.
The scientists also looked at other environmental factors, finding that, for this 10-year study period, sea surface temperature had the strongest effect, with survival positively influenced by localized warming. But Horswill cautioned that a majority of penguin studies show a negative response to climate change. That means the relationships between survival rates and environmental effects are non-linear,” she said.
“By this I mean that survival rates in macaroni penguins on South Georgia will not continue to increase indefinitely as sea surface temperatures continue to rise, but that a critical threshold may exist beyond which this population will become negatively affected. Determination of the point at which environmental conditions move from favourable to unfavourable would require a longer time series with greater variation in sea surface temperature,” she said.
Climate-driven changes are also a factor.
“During the early to mid 1980s a decline in krill at South Georgia was linked with reduced winter sea-ice extent at the Antarctic Peninsula. However there has been no systematic decline in krill abundance over the past 20 years. The fact that whales have shown signs of recovery and that fur seals are at high levels of abundance suggests there is still lots of krill, she concluded.