Bears and bird flu? Exploring interspecies connections and infectious disease

Photo: Jamie Scarrow
Though large in stature, bears are susceptible to threats from much smaller species. The deaths of three Montana grizzlies due to Avian Flu show this interspecies complexity.

By Kayla Heinze, Communications Specialist

Movement is the defining theme of the season. As the Northern Hemisphere awakens under the lengthening spring sun, the first grizzlies are making their way back onto the landscape after winter’s hibernation, the sidewalks and trails near our headquarters in Missoula, MT, are filling up with warmth-seeking humans, and across the continent billions of birds are embarking on their annual spring migrations.

At the end of March, a small group of our staff and supporters visited Freezeout Lake to witness the staggering numbers of snow geese and tundra swans that stop there before continuing their journey northward. Near Vital Ground project sites on the Rocky Mountain Front, the heavily-trafficked lake is a stunning sight to behold and one of many treasures found in the region. 

It also puts on display the sheer abundance of wild birds moving through the Northern Rockies at this time of year, which offers a chance, once again, to focus our binoculars on the intersection of avian and mammalian conservation.  

From Birds to Bears 

Snow geese at Freezeout Lake in central Montana. Migrating birds can spread disease to new areas. (Photo: Rocky Mountain Research Station)

Last fall three grizzlies in Montana were infected with Avian Flu and eventually euthanized by wildlife managers with Montana Fish, Wildlife, and Parks (FWP). Also known as Highly Pathogenic Avian Influenza (HPAI), the virus originates in bird species and can occasionally spread to other animals. FWP officials inferred that the infected grizzlies likely contracted the virus from eating diseased birds. 

These bears account for some of the small number of mammalian casualties resulting from the widespread Avian Flu outbreak, which ravaged poultry flocks across the country last year and has spread to wild bird populations. While the virus has not shown to be a particularly infectious zoonotic disease (the term for those that transmit between other species and humans), it has significantly affected wildlife — with detections of the virus in at least 126 bird species and in mammals ranging from foxes to mountain lions to seals.

In Montana’s grizzlies the virus presented itself with symptoms of partial blindness, disorientation, and other neurological issues, according to FWP. In all animals, but especially certain groups of birds — including waterfowl, raptors and domestic poultry — the disease can be fatal. 

Parts of both the Central and Pacific flyways pass through the Northern Rockies, meaning the pathways of millions of migratory birds cross through habitat that grizzlies, resident birds and many other wild creatures call home. This overlap has useful conservation implications. Many sites considered high-priority habitat for grizzlies are also crucial spots for at-risk birds, meaning our land protection projects benefit a wide array of vulnerable species. 

Broadly, biodiversity has also been found to correlate with the resiliency of an ecosystem, making it a good measurement of health and something to be celebrated. But, as these cases of Avian Flu in grizzlies reveal, there are many types of interspecies interactions and not all of them contribute to individual well-being. 

Grizzlies and Disease 

Due to genetic isolation, recovering grizzly bear populations are vulnerable to disease outbreaks. (Photo: Jacob W. Frank/ Yellowstone National Park)

Disease is a normal phenomenon in any population. While tragic for the individual, it is a natural part of life on Earth, meaning a few sick bears is generally nothing to worry about when it comes to long-term species health.

For bears, Avian Flu is an incredibly rare threat. Other illnesses, such as those caused by bacterial infections and parasitic hookworms, are more prevalent. But even these ailments are often fairly mundane and harmless, both for the individual and in the grand scheme of species conservation.

However, since the Lower 48’s remaining grizzly populations are already threatened, wildlife biologists keep an extra close eye on the emergence of infectious diseases. Their concern is that the “perfect storm” of circumstances could allow for a widespread epidemic that would prevent the species from continuing its trajectory toward recovery. In cases such as chytrid fungal outbreaks in frogs and white-nose syndrome in bats, disease has decimated wildlife populations to the point of endangerment or extinction.  

A few factors make grizzlies vulnerable. As omnivores that forage in both wild and urban settings, they are exposed to a wide variety of sources of potential infection, such as domestic poultry populations teeming with Avian Flu. In our highly developed, modern landscapes, grizzlies are limited to small, mostly isolated populations, with relatively low levels of genetic diversity. Especially in areas like the Cabinet-Yaak and Selkirk ecosystems, these realities make it more likely that a disease would rapidly spread through the entire community. It also means they would have limited possibilities for adaptation or for migrant bears to revitalize their numbers following an outbreak.

But these factors also contribute to the grizzly’s tenacity: Their diverse diet means that if one of their favored food sources were to be wiped out they could likely pivot to another. And as threatening as it is to species-level health, habitat fragmentation also offers natural buffers that can prevent disease spread. 

As we build landscape connectivity to boost genetic diversity and population numbers so grizzlies have a better chance of surviving myriad threats, it is important to remember that nature does not typically stick to simple binaries. Working with the complexity of ecological systems requires us to understand the tradeoffs that can come with increases in connectivity and biodiversity. 

Supporting Species Resiliency

Donovan Creek wetland habitat
Habitat protection sites, like this one at Donovan Creek, help build resilient grizzly populations in Montana and beyond. (Photo: Mitch Doherty)

Knowing the nuanced reality of ecology, we can be smart about how we work for a better future for bears and other animals. It is difficult to control an outbreak once it occurs, but proactive steps can help a species avoid devastating levels of disease. 

As we’ve explored before on the blog, the solution is not as straightforward as increasing sheer numbers, though that does help. Ensuring a species is diverse in terms of genetics, age, geographic distribution and other demographic factors can increase the odds that at least a subset of the larger population will be able to endure an outbreak. 

The existence of zoonotic diseases demonstrates how closely tied our fates are to those of wildlife. In very direct ways, infections can spread between species and impact us all. And healthy bear populations bring ample benefits to our shared ecosystems, revealing the less obvious but equally important ways we are connected as neighbors and fellow mammals. 

Being invested in the well-being of grizzlies and other species requires us to navigate the intricacies inherent in natural systems. At Vital Ground, we rely on relationships with biologists, wildlife managers and a variety of community partners to ensure our work on the ground is informed by the latest scientific research and grassroots ecological knowledge. 

With our community of committed conservationists, we are advancing habitat protection projects aimed at increasing connectivity, genetic diversity, and other factors that will allow the mighty grizzly to continue enduring even under tough circumstances. Disease will always be a reality on our shared landscapes, which is why we are investing in a future of resiliency for bears and other species so outbreaks do not send them spiraling out of existence.

For now, with birds migrating and bears awakening, we cherish the opportunity to experience a breadth of cross-species interactions and relish in all the complexity they provide. 

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