Tuesday, October 28, 2025

Polar bears act as crucial providers for Arctic species

New study shows polar bears annually provide millions of kilograms of food, supporting a vast Arctic scavenger network

San Diego Zoo Wildlife Alliance

Polar bears providing carrion for vast network of arctic scavengers — image:
Two-year-old polar bears with bearded seal carcass and ivory gulls
Image Credit: Wayne Lynch
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Credit: Photo Credit: Wayne Lynch

Photos: here

SAN DIEGO (Oct. 28, 2025) – A new study published in the scientific journal Oikos reveals for the first time the critical role polar bears play as carrion providers for Arctic species. Researchers from University of Manitoba and San Diego Zoo Wildlife Alliance, alongside researchers from Environment and Climate Change Canada, and the University of Alberta, have estimated that polar bears leave behind approximately 7.6 million kilograms of their prey annually, creating a massive and vital food source for a wide network of arctic scavenger species.

This research demonstrates that these apex predators are a crucial link between the marine and terrestrial ecosystems. By hunting seals on the sea ice and abandoning the remains, polar bears transfer a substantial amount of energy from the ocean to the ice surface, making it accessible to other animals. The study identifies at least 11 vertebrate species known to benefit from this carrion, including Arctic foxes and ravens, with an additional eight potential scavenger species.

“Our findings quantify for the first time, the sheer scale of polar bears as a food provider to other species and the interconnectedness of their ecosystem,” says Holly Gamblin, lead author of the study and PhD Candidate in the Department of Biological Sciences at the University of Manitoba. “What is apparent from this review is that there is no other species that adequately replaces how a polar bear hunts, in which they drag their prey from the water to the sea ice and leave substantial remains for other species to access.”

Past research has emphasized that continued warming in the Arctic and the resulting loss of sea ice directly endanger polar bear populations. However, this new research highlights that a decline in polar bears would not only impact the species itself but the loss of the carrion they provide could have significant consequences for the entire Arctic ecosystem.

“Our research highlights the important role of polar bears as carrion providers,” says Dr. Nicholas Pilfold, Scientist in Population Sustainability at San Diego Zoo Wildlife Alliance. “The sea ice acts as a platform for many species to access scavenging resources provided by polar bears, and ultimately, declines in sea ice will reduce access to this energy source. Our findings indicate that documented declines in polar bear abundance in two subpopulations have already resulted in the loss of more than 300 tonnes of food resources for scavengers annually.”

These findings highlight the interdependence of arctic wildlife species and their shared vulnerabilities in the face of rapid environmental change. With polar bear populations continuing to decline, this research underscores the urgency of conservation efforts to protect them, not only for their own sake but for the species that rely on them.

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About San Diego Zoo Wildlife Alliance 
San Diego Zoo Wildlife Alliance, a nonprofit conservation leader, inspires passion for nature and collaboration for a healthier world. The Alliance supports innovative conservation science through global partnerships and groundbreaking efforts at the world-famous San Diego Zoo and San Diego Zoo Safari Park, both leading zoological institutions and accredited botanical gardens. Through wildlife care expertise, cutting-edge science and continued collaboration, more than 44 endangered species have been reintroduced to native habitats. The Alliance reaches over 1 billion people annually through its two conservation parks and media channels in 170 countries, including San Diego Zoo Wildlife Explorers television, available in children’s hospitals across 14 countries. Wildlife Allies—members, donors and guests—make success possible. 

About University of Manitoba
The University of Manitoba (UM) is recognized as Western Canada's first university. It is part of the U15, ranking among Canada’s top research-intensive universities and provides exceptional undergraduate and graduate liberal arts, science and professional programs of study.

UM campuses and research spaces are located on original lands of Anishinaabeg, Ininiwak, Anisininewuk, Dakota Oyate, Dene and Inuit, and on the National Homeland of the Red River Métis. UM recognizes that the Treaties signed on these lands are a lifelong, enduring relationship, and we are dedicated to upholding their spirit and intent. We acknowledge the harms and mistakes of the past and the present. With this understanding, we commit to supporting Indigenous excellence through active Reconciliation, meaningful change, and the creation of an environment where everyone can thrive. Our collaboration with Indigenous communities is grounded in respect and reciprocity and this guides how we move forward as an institution. For more information, please visit umanitoba.ca.

Link includes:

B-roll and photos of polar bears

Journal

Oikos

DOI

10.1002/oik.11628

Subject of Research

Animals

Article Title

Predators and scavengers: Polar bears as marine carrion providers

Article Publication Date

28-Oct-2025

All prey are not the same: marine predators face uneven nutritional payoffs

New study finds that the nutritional value of prey within a single species can widely vary, offering key insights for food web dynamics and ecosystem change

University of California - San Diego

Sea lions and pups — image:
Mother sea lions and pups at San Miguel Island.
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Credit: Anthony Orr, NOAA Fisheries; NMFS permits 782-1812, 16087, 22678

The hunt is on and a predator finally zeroes in on its prey. The animal consumes the nutritious meal and moves on to forage for its next target. But how much prey does a predator need to consume?

Following a period of massive starvation among animals living along the California coast, University of California San Diego scientists began asking questions about predator sustenance and the value of prey consumption. Their investigations found that all prey are not the same, and this can have significant impacts on predator foraging. Even prey of the same size and weight within the same species can drastically vary in terms of the sustenance they provide, the study showed.

The new findings carry implications for understanding ocean food resources and the impacts that changes in these resources can have on animals that depend on them.

Former UC San Diego graduate student Stephanie Nehasil, who earned her PhD in the School of Biological Sciences, and Professor Carolyn Kurle, along with coauthors affiliated with the Southwest Fisheries Science Center (NOAA) and UC Santa Cruz, were initially interested in a mid-2010s marine heat wave that caused marine mammal and seabird starvation across the California Current Ecosystem, a rich ocean area along the West Coast that stretches for thousands of miles and supports a wide variety of sea life. These questions eventually led to a multi-year investigation of ocean food sources and the daily nourishment required for marine predators. To explore these questions, the team focused on one of the species most affected by the heat wave: the California sea lion.

The study’s results, published in the Journal of Animal Ecology, revealed that members of the same species of anchovy, sardine and squid can widely vary in their energy density, which is the standard by which prey are measured in terms of assessing how much nutritional value they provide for predator subsistence. Using multiple measures of prey quality, the researchers discovered that relying on lower-quality prey can force predators to double their intake, despite being the same size as more nutritious prey.

“We were surprised by how much variation there was within a single species,” said Nehasil, now a postdoctoral researcher at Stony Brook University. “You could have two fish side-by-side that are the same size but have a huge range of variation in the amount of energy they provide. In some cases, predators would need to consume tens of thousands of the smaller, lower-quality fish to survive, and that’s just not possible.”

The new findings offer more precision for data featured in “bioenergetics” models, which are used to understand ecosystems and make key assessments of the state of marine animal populations. Previously, prey animals of the same species and similar size and weight factored the same in terms of their value to daily consumption by predators.

“This is another piece of information to help inform our models to get a better understanding of how these ecosystems work, so we can recognize subtle dynamics that might otherwise be overlooked and respond more thoughtfully to environmental shifts,” said Kurle, a faculty member in the Department of Ecology, Behavior and Evolution. “This information will help us understand the complexity of these ecosystems that so many care about, especially the fishers and others who use the ecosystem.”

In 2014 an immense heat wave in the North Pacific Ocean caused a significant disruption of the normal structure of the marine food web. The event disrupted the typical upwelling flow of nutrients from the deep ocean to the surface, thus curtailing the source of food for zooplankton, which in turn feed the fish that serve as prey and are commercially valuable for humans. A massive wave of starvation resulted that rippled through communities of marine mammals, seabirds and other species.

“We saw a lot of mortality, which led us to question whether prey abundance or availability had changed during the event,” said Nehasil. “Looking more closely at shifts at the base of the food web, we began to wonder whether it wasn’t just the amount of food that had changed, but the quality as well.”

In particular, the researchers witnessed emaciated sea lions and their pups, indicating that mother sea lions were not able to adequately feed their offspring and themselves, which initiated the researchers’ search for the underlying mechanisms.

Nehasil spent several painstaking years collecting data and measuring the energy density of specimens from a multitude of sources, including NOAA fish surveys, bait barges and samples from the California Department of Fish and Wildlife. In collaboration with Ocean Discovery Institute, a nonprofit that engages youth from the underserved community of City Heights in San Diego, she also involved local students in the research, providing hands-on science experience for the next generation.

To assess the true nutritional value of northern anchovy, Pacific sardine and market squid, rather than simply measuring length and weight, she used a time-tested instrument known as a bomb calorimeter that measures the energy content of a sample. The greater the temperature change as a specimen burns, the more energy and nutritional value it holds.

The measurement data were fed into a model that estimated the energy density and content of prey collected across different oceanographic regions, seasons, sizes and maturity stages. These estimates were then used to calculate how much prey would need to be consumed to meet sea lion energetic demands, revealing vast bioenergetic differences.

"You have to use the currency of energy value to inform bioenergetics models," said Nehasil. "To help us predict what will happen as our climate and oceans change we need this baseline ecosystem data, especially to understand prey dynamics and how predators are going to respond."

Fish within the same species can vary widely in energy value, depending on regional ocean productivity, seasonal nutrient supply, life stage and size. While regional and seasonal differences reflect environmental conditions, variation by size and maturity is driven by how energy is allocated — either toward growth or reproduction.

“We all want to have healthy ecosystems and there are many stakeholders who want these systems to succeed. We want everyone to have the best data possible to make important analyses and estimations,” said Kurle. “Anytime we can provide a little more understanding of ecosystem complexity it’s valuable and important.”

Stephanie Nehasil collects sea lion scat samples in the field at San Nicolas Island. The research study was prompted by changes observed in sea lion foraging and a mass starvation event caused by a 2014-2016 marine heatwave.

Credit

Greg Sanders, US Navy (retired); NMFS permit #19091-01

The researchers collaborated with Ocean Discovery Institute as part of the study.

At Ocean Discovery's Living Lab in City Heights, students in an after-school program gained hands-on research experience with fish measurements that directly contributed to the study’s results.

Credit

Ocean Discovery Institute