The title of the largest organism on Earth is often attributed to Armillaria ostoyae, a vast fungus located in Oregon’s Malheur National Forest. Commonly known as the “humongous fungus,” this organism spans an impressive area of 2,385 acres—nearly three times the size of Central Park—and weighs around 35,000 tons. This extraordinary size makes it not only the largest known organism in terms of mass but also a living, expansive network. According to Discover Magazine, while this fungal network is well-recognized, debates continue over its true size and the definition of an organism, leaving many questions unanswered.
The Expansive Growth of the Humongous Fungus
Unlike other life forms, the humongous fungus does not grow in a single, easily visible shape. While mushrooms are the fruiting bodies of this fungus, most of its mass resides underground in the form of mycelium, a dense network of threadlike structures.
These structures spread across vast distances, forming a complex system capable of breaking down dead plant matter and recycling nutrients back into the ecosystem. The fungus’ growth is indeterminate, meaning it can expand as long as the environmental conditions allow.
“You can think of the humongous fungus as this underground network of an organism; it doesn’t have a very defined shape other than its network of hairlike structures that spread over a large area. That characteristic, mycelial growth, is a feature of many fungi,” says Antonis Rokas, professor of biological sciences at Vanderbilt University.
“What makes this particular organism somewhat special is that it’s a big generalist, so it can grow on various substrates.”
This mycelial network is the key to its massive size. Its ability to grow over such a large area makes it an efficient decomposer, playing a significant role in maintaining ecological balance in its environment.
The fungus thrives in Oregon’s moist Pacific Northwestern climate, where decomposing wood and plant matter provide abundant food sources.
A Remarkable Decomposer
As Andrew Wilson, associate curator of mycology at the Denver Botanic Gardens, notes, fungi like the humongous fungus are
“The most efficient decomposers of terrestrial plant matter in the world. This helps them break down dead plants and recycle their nutrients back into the ecosystem so other organisms can access this energy.”
What Makes This Fungus Special?
The Armillaria ostoyae is remarkable not just for its size but also for its role in the environment. It is both a saprotroph, breaking down dead organic matter, and an infecting pathogen, capable of attacking living trees.
This dual nature allows the fungus to create its own food sources by killing partially dead trees, further expanding its ability to grow. Scientists believe the humongous fungus has existed for over 8,650 years, a testament to its resilience and the extent of its growth. Despite the challenges in fully mapping its boundaries, it’s widely accepted that this organism is the largest of its kind.
The Competition for the Title of Largest Organism
While the humongous fungus is a clear frontrunner, other organisms are often considered for the title of the world’s largest organism:
1. The Antarctic Blue Whale
At 85 feet on average, the Antarctic blue whale is the largest animal to ever live, surpassing even the humongous fungus in terms of sheer mass. However, it is not the largest living organism when considering its growth potential and longevity.
2. Pando – The Aspen Forest
In central Utah, an entire 106-acre forest of 47,000 trees, genetically identical and connected by a single underground root system, forms the organism known as Pando. While it covers a vast area, its total weight is far less than that of the fungus.
3. The Hybrid Seagrass Meadow
In Shark Bay, Western Australia, a 111-mile-wide meadow of hybridized eelgrass has been identified as another contender. Its genetic makeup traces back to a single plant that began cloning itself over 4,500 years ago.
This discovery in 2022 earned it recognition as the world’s largest organism by Guinness World Records.
The Philosophical Challenge of Defining an Organism
What defines a single organism is a philosophical and biological question that many experts are still debating. Organisms like the humongous fungus, Pando, and the seagrass meadow are all the result of clonal growth, where one organism can produce genetically identical offspring that remain physically connected.
“This gets down to the philosophical discussions about the nature of life,” says Wilson.
Organisms like the Antarctic blue whale, for example, are sentient beings with their own thoughts and emotions. But can a fungus or a plant clone share the same status?
In the case of the humongous fungus, some argue that its vast underground network of mycelium may represent a single entity despite its many visible mushrooms. This question continues to challenge our understanding of life and what it means to be “one” organism.
“The humongous fungus will always be a mystery because you can never really measure its full extent,” says Wilson.
“It’s the nature of fungus to be cryptic and to keep their secret – but that’s also what’s fun about them!”
