In Antarctica, each marine mammal species has adapted perfectly to the extreme conditions and found its ecological niche. The highly specialised animals occupy the various habitats in such a way that interspecific competition is kept to a minimum. For example, while Weddell seals prefer living close to the fast ice that is attached to the continent where they rest and breed, crabeater seals are associated with the pack ice. The diet of the two seal species also differs significantly: crabeater seals feed mainly on Antarctic krill, Weddell seals are less picky and eat krill, fish and squid. A new study by scientists from the University of Canterbury, New Zealand, has investigated how these different strategies will affect the lives and futures of the two species in times of climate change.
The goal of the study, which was published in September in the journal Global Change Biology, was to create a dataset as a monitoring tool for the remote and largely ice-covered marine reserves in the Southern Ocean. With the help of thousands of citizen scientists from around the world who searched for crabeater and Weddell seals on satellite imagery, the research team found that both seal species breed near food sources.
According to Dr. Mia Wege, a recent lecturer at the University of Pretoria, South Africa and lead author of the study, it is particularly interesting to see how climate change will affect the seals’ breeding grounds differently. “Because of climate change, in addition to having less food, crabeater seals will be increasingly challenged to find a place to rest and raise their young. Surprisingly, Weddell seals are expected to be minimally affected, which is the opposite of what is happening elsewhere around Antarctica,” Wege says.
Dr Michelle LaRue, Antarctic researcher at the University of Canterbury’s Gateway Antarctica and leader of the study, adds that “different species will be exposed to impacts from climate change differently.”
“Our takeaway message is that if we want to mitigate population declines for ice-loving seals as the climate continues to warm, we need to be working now to set aside marine protected areas to ensure longevity of these species and their ecosystems,” LaRue said.
The results shed light on how the two seal species respond differently to climate change because of their unique ecology, Wege said. “Most importantly, our study again shows how these species can provide information about the entire ecosystem, and how valuable they are as sentinel species, especially when we are thinking of ways to monitor the effectiveness of marine protected areas. They certainly are more than just cute faces.”
The study showed that crabeater seals are particularly vulnerable to the effects of climate change. On the one hand, they use the seasonal and rather short-lived pack ice as a breeding platform. If the pack ice decreases in the course of climate change, the area available to the crabeater seals as a breeding ground will also decrease. Weddell seals, in contrast, have a more stable base in the form of fast ice.
On the other hand, crabeater seals are food specialists – Antarctic krill makes up more than 90 percent of their diet. So when the availability of krill changes, they cannot simply switch to other prey. Thus, the high specialization of crabeater seals may not bode well for their future.
The Weddell Sea is not yet as strongly affected by climate change as the Antarctic Peninsula. Nevertheless, recent studies show that even there, crabeater seals are already in danger of losing breeding sites and feeding habitats. Weddell seals are less affected by warming thanks to their greater flexibility, and the Weddell Sea probably still has sufficient resources to sustain their current population.
Dr. Leo Salas, an ecologist with Point Blue Conservation in California and co-author of the study, says, “Clearly, any planning to protect the Southern Ocean ecosystem should take into consideration these differences. There are no easy answers.”
In October, international leaders discussed the protection of over 2.2 million square kilometers of the Weddell Sea at the annual meeting of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). A decision has not yet been made.
The study was funded by Pew Charitable Trust.
Julia Hager, PolarJournal
Link to the study: Wege, M., et al. (2021) Ice matters: life-history strategies of two Antarctic seals dictate climate change eventualities in the Weddell Sea. Global Change Biology. doi.org/10.1111/gcb.15828.
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