Once the ice diminishes too far, there will be no polar bears left
En el archipiélago de Svalbard, donde el Ártico se derrite más rápido que en casi cualquier otro lugar del planeta, los osos polares han encontrado una respuesta inesperada a la desaparición del hielo: cambiar lo que comen. Un estudio publicado en Scientific Reports revela que, lejos de sucumbir al colapso predicho, algunos osos han mejorado su condición corporal adaptándose a nuevas presas terrestres y marinas. Es una historia de resiliencia provisional que nos recuerda que la naturaleza puede sorprendernos, aunque no necesariamente salvarnos.
- El Mar de Barents perdió cuatro días de cobertura de hielo por año entre 1979 y 2014, el doble que otras regiones árticas, poniendo a los osos polares ante una amenaza que parecía inevitable.
- Contra todo pronóstico, los osos de Svalbard comenzaron a ganar peso a partir del año 2000, desafiando las predicciones que los habían convertido en símbolo del desastre climático.
- La clave está en un cambio de dieta: los osos han aprendido a cazar renos en tierra firme y a aprovechar poblaciones recuperadas de morsas y focas que se concentran en los últimos parches de hielo.
- Otras poblaciones árticas, como las de la Bahía de Hudson, siguen deteriorándose, lo que sugiere que la adaptación de Svalbard es una excepción local, no una solución global.
- Los propios investigadores advierten que este margen de supervivencia es frágil: si las presas alternativas escasean o el hielo sigue retrocediendo, el colapso que se ha pospuesto acabará llegando.
En Svalbard, uno de los rincones del Ártico donde el hielo desaparece más deprisa, los osos polares están engordando. Es una paradoja que ha sorprendido a los propios científicos: el Mar de Barents perdió cobertura helada al doble de velocidad que otras regiones entre 1979 y 2014, y sin embargo, un estudio del Instituto Polar Noruego encontró que la condición corporal de 770 osos adultos mejoró a partir del año 2000, justo cuando el deterioro del hábitat se aceleraba.
La explicación no está en el entorno, sino en el comportamiento. Los osos de Svalbard han ampliado su dieta: cazan renos en tierra firme y aprovechan las poblaciones de morsas y focas anilladas que, recuperadas de décadas de caza excesiva, se concentran ahora en los últimos retazos de hielo. Jon Aars y Andrew Derocher, del Instituto Polar Noruego, señalan que esta flexibilidad no es nueva en la especie, pero sí lo es la oportunidad que la ha desencadenado.
Sin embargo, los investigadores se niegan a leer estos datos como una buena noticia. En otras regiones como la Bahía de Hudson, los osos siguen perdiendo peso y viabilidad reproductiva. La investigadora Sarah Cubaynes advierte que Svalbard contradice el patrón global, no lo refuta. La abundancia local de presas alternativas es una circunstancia particular, no una garantía.
Derocher es directo: si el hielo continúa retrocediendo más allá de cierto umbral, los osos polares dejarán de existir como especie en el Ártico. Los osos de Svalbard han ganado tiempo gracias a su capacidad de adaptación, pero en un Ártico que se calienta sin pausa, el tiempo es un recurso que también se agota.
In the Svalbard archipelago, where the Arctic is melting faster than almost anywhere else on Earth, something unexpected is happening. Polar bears are getting fatter, even as the sea ice they depend on vanishes at twice the rate of other Arctic regions. Between 1979 and 2014, the Barents Sea lost four days of ice coverage per year—a pace that should have spelled disaster for predators that hunt from frozen platforms. Yet a study published in Scientific Reports found that bears in this region actually improved their body condition over the past two decades, defying the grim predictions that have made polar bears the poster animal for climate catastrophe.
For decades, the story seemed straightforward. Polar bears are marine mammals that use sea ice as a hunting platform to catch seals and walruses, and as a place to build the dens where their cubs are born. As the ice retreats, the theory went, bears would starve. The Svalbard population offered a perfect test case—one of the Arctic's most severely affected regions, where researchers had been monitoring bears since the 1970s. When Jon Aars and his team at the Norwegian Polar Institute analyzed the body condition index of 770 adult bears between 1995 and 2019, they found something that surprised even them. After an initial decline through 2000, the bears began to gain weight. The trend continued even during periods of dramatic ice loss.
The explanation, it turns out, lies not in some miraculous change to the environment, but in a shift in what the bears eat. The Svalbard population has increasingly turned to hunting reindeer on land and capturing walruses and ringed seals that now congregate in smaller, more accessible patches of remaining ice. These prey species have recovered from decades of overhunting, making them available in numbers not seen in generations. Andrew Derocher, a coauthor of the study, explains that once a bear discovers it can successfully hunt reindeer, it returns to that location and keeps trying. The behavior is not new—bears have always been opportunistic—but the opportunity itself is new.
The data tells a story of adaptation under pressure. The Barents Sea population, estimated between 1,900 and 3,600 individuals in 2004, showed signs of growth even as its habitat contracted. Bears that might have starved under the old paradigm found alternative pathways to survival. For a moment, it looked as though nature might absorb the shock of rapid climate change through behavioral flexibility.
But the researchers are careful not to celebrate. This phenomenon appears to be local and temporary, not a global trend. In Hudson Bay and other Arctic regions, polar bears have experienced the opposite—declining body condition and reduced survival rates as ice disappears. Sarah Cubaynes, a researcher at the French research center CEFE, notes that Svalbard's results contradict the broader pattern documented elsewhere. The difference may lie in the particular abundance of alternative prey in the Arctic north, a bounty that may not last.
Aars and Derocher warn that weight gain, however encouraging it appears, offers no guarantee of long-term survival. The changes they documented depend entirely on the continued availability of both sea ice and terrestrial prey. If reindeer populations decline, if walruses disperse, if the remaining ice becomes too fragmented to support seal colonies, the bears will face the crisis that climate models have long predicted. Derocher is blunt about the timeline: once the ice diminishes beyond a certain threshold, polar bears as a species in the Arctic will cease to exist. The Svalbard bears have bought themselves time through dietary innovation, but time, in a warming Arctic, is a finite resource.
Citações Notáveis
The increase in bear body condition during a period of significant sea ice loss was a surprise— Jon Aars, Norwegian Polar Institute
Once the ice diminishes too much, we will not have polar bears— Andrew Derocher, study coauthor
A Conversa do Hearth Outra perspectiva sobre a história
So these bears are actually thriving while their habitat disappears. How is that possible?
They've switched what they hunt. Instead of waiting on ice for seals, they're going ashore for reindeer, and they're catching walruses in the smaller ice patches that remain. It's a real shift in behavior, not a lucky accident.
But if this is working in Svalbard, why isn't it happening everywhere in the Arctic?
Because Svalbard happens to have recovering populations of walrus and reindeer. Other regions like Hudson Bay don't have that same abundance. The bears there are actually getting thinner as ice vanishes.
So this is a local success story, then?
It looks like one right now, but the researchers are cautious. They're saying this only works as long as the prey stays available. If reindeer numbers drop or walruses move elsewhere, the bears lose their backup plan.
What's the real threat, then?
Eventually, there won't be enough ice left for anything—not for seals to breed, not for bears to rest and den. The alternative prey can only sustain them for so long. Once the ice is gone, the whole system collapses.
So this study is actually a warning dressed up as good news?
Exactly. It shows adaptation is possible, but it also shows how fragile that adaptation is. These bears have found a workaround, but it's not a solution.