What Beavers Need: Re-examining MacDonald’s 1956 Study on Beaver Forage and Habitat Capacity
In 1956, a wildlife biologist named Duncan MacDonald completed a thesis that still deserves our attention nearly 70 years later. His study, “Beaver Carrying Capacity of Certain Mountain Streams in North Park, Colorado”, was a straightforward, boots-on-the-ground effort to answer one deceptively simple question: How many beavers can a stream support based on the food that’s actually there?
For those of us working today to support beaver recolonization in high desert or working landscapes, this study is gold. MacDonald wasn’t looking to harness beavers as a tool. He was observing them as animals with agency, each with their own needs, behaviors, and limitations. He approached the beaver not as an instrument for humans to use, but as a native species worth understanding and supporting.
A Study Grounded in Reality
The research took place in North Park, a high mountain valley in northern Colorado, where streams meander through open rangeland and willow thickets. MacDonald selected several stream segments with current or past beaver activity and took careful stock of what beavers had available to eat, what they actually used, and how much they could realistically harvest in a year.
His field methods were detailed and data-rich: measuring stream length, identifying woody species along the banks, calculating tree sizes, and tallying cut stumps to quantify past use. This was not a desktop exercise—it was a yearlong immersion in beaver habitat, with attention to forage availability down to the twig.
What Did He Find?
1. Beavers Eat a Lot, and They Waste a Lot (By Necessity)
An adult beaver needs to consume roughly 2.5 to 5 pounds of digestible dry matter per day, translating into about 1,000 to 2,200 pounds of woody material per year, depending on conditions and availability.
But not all of a tree is edible or usable. In fact, the bigger the tree, the more waste beavers generate. They prefer young, small-diameter wood, like the terminal ends of willows and aspens. Large stems and trunks are often left to rot or used only for dam construction. From a food perspective, that biomass is mostly “waste.”
This has direct implications: planting or protecting large, mature trees will not feed beavers. What they need is an abundant supply of small-diameter willow, aspen, and cottonwood, ideally within close range of their lodge or bank burrow.
2. Tree Size Matters for Nutritional Yield
MacDonald classified tree forage by size:
- Small (<1 inch diameter): ~80% of the biomass is digestible forage, with minimal waste.
- Medium (1–3 inches): ~40% digestible forage, with 60% waste.
- Large (>3 inches): Only ~15% of biomass is usable. The rest? Waste.
In other words, as trees grow, their nutritional efficiency drops sharply. So while larger trees may seem like “more food,” beavers don’t see it that way.
3. A Beaver Needs About 250–300 Small Trees Each Year
MacDonald estimated that a single adult beaver requires 250 to 300 small trees annually. A family group of five could need upwards of 1,500 trees per year to support them through all seasons, including the long Rocky Mountain winter.
And this doesn’t account for regeneration lag—if you don’t have fast-regrowing willow or aspen nearby, you’re quickly on the path to local overuse and habitat abandonment.
4. Beavers Won’t Stay Where Food Isn’t Accessible
Another critical insight: beavers are tied to water not just for survival, but for food transport and safety. If the best forage is more than 100 feet from water, it’s largely unusable. Dragging trees further increases energy cost and exposure to predators.
MacDonald observed that even where high-volume forage existed, if it was too far from water, it might as well not exist—beavers wouldn't stay.
