Are Bison Numbers in Yellowstone Sustainable?

The National Park Service recently released its environmental impact statement concerning the numbers of bison they will manage for in the future. Its “preferred alternative” calls for between 4,500 and 6,000 bison in Yellowstone National Park. When bison numbers reach 6,000, NPS will initiate control actions, mainly tribal and state-sponsored hunts, to keep the herds at that level.

But there’s a problem: the current numbers of bison, which NPS puts at 4,450 before bison calves were born in spring, are irreparably damaging the streams and riparian areas on the Northern Range to such an extent that they can no longer preform the “job” they evolved to do within the larger landscape. Basically, it boils down to having too many hooves in too small an area for too long a time.

Many people take streams for granted: they are always there, they move water downhill, fish live in them. In reality, it’s a little more complex. The job of a stream is to collect and distribute the most precious resource in the West: water. In so doing, they’re constantly adjusting their dimension (cross-section), pattern (crookedness) and profile (steepness) to reflect changing conditions in their watershed and achieve stability. When changes are small as in the case of tectonics and, until recently, climate, the stream reacts slowly. But the response is much quicker when the changes occur to the channel itself. There is always room for some change—most resources work on the concept of thresholds—if change occurs up to the threshold, systems remain stable. If the threshold is exceeded, however, things unravel quickly.

Federal land-management agencies practice something called “ecosystem management” with regards to their wild lands. The term most used to describe the target of EM is “function,” or whether a resource is performing the role it evolved for. In other words, is it doing its job? The idea of EM is to maintain all components of an ecosystem in a functioning condition, so the larger ecosystem remains functioning. Undisturbed ecosystems achieve a stability that tends to last a long time, and the ecosystem responds slowly to changes in climate and geology. In the 12,000 years or so since the end of the Pleistocene epoch, the streams of Yellowstone’s Northern Range have reached a level of stability within the landscape that enables them to perform their functions with efficiency.

Ask most people what their idea of what stream/riparian function is, and they will mention habitat for everything from fish to moose. While that is true, habitat is a secondary benefit of a functioning stream. Streams are physical systems first and must meet three physical conditions to achieve function: 1) move water and sediment through the system in the most efficient manner, 2) make use of their floodplain, and 3) recharge the local water table. All three are closely tied to the maintenance of riparian vegetation—mainly willows and sedges—which is necessary to hold the streams together by virtue of their dense root systems.

In the EIS and in “The Grazing Issue” of the NPS-published journal Yellowstone Science, park scientists explain that forage is not a limiting factor in determining bison numbers. There is indeed enough forage to support expanded levels of bison on the Northern Range, but that is totally irrelevant to the situation on the ground. For when all the resources are forced to interact on the same landscape, other limiting factors come into play.

In the case of the Northern Range, the limiting factor is the collateral damage done to streams/riparian areas as bison are using the available forage. It has nothing to do with “grazing” per se. It is the trampling of streambanks, and the subsequent widening of the channel, that leads to the downward spiral preventing streams from achieving function.

Of all the components of a stream—width, depth, slope, streamflow, sediment size, sediment load, sinuosity—stream width is the easiest to change. As a stream widens, the water spreads out and the velocity of the water drops. This drop in velocity prevents the stream from carrying its sediment load. In turn, gravel bars, or deposited sediment, form within the confines of the widened banks. Since it is always easier for a stream to erode its banks than to pick up instream sediment, the streamflow must bend around the gravel bars, thus causing more bank erosion and further widening. A wider channel has a larger cross-section, and it takes more water to fill the channel and spill over onto the floodplain.

As floodplain inundation becomes less frequent, the water table dries out and riparian vegetation, with its critical root systems, is replaced by upland species such as sagebrush and bunchgrass. Where a stream is on this continuum depends on how much disturbance it has received, but from what I have seen on the Northern Range, most streams are well on their way along the downward spiral that leads to non-function—some are there already—all because of bison widening the stream.

The point is that once the stream becomes over-widened, it is at the mercy of both the physics of sediment transport and of flowing water. The photos below show this. To see it in the field, compare Rose Creek in the Lamar Valley with Glen Creek on Swan Lake Flats.

Of course, the elephant in the room in all this is climate change. Most predictions are for a warmer and drier West. The best hedge against climate change is functioning streams and riparian areas. Riparian areas act as big sponges that collect and store water early in the season and then release it slowly later in the year. Water not stored because riparian areas are not functioning properly is lost to late-season downstream users such as irrigators and municipalities. The EIS makes no mention of this function of riparian areas.

So, what to do? I see this bison issue is analogous to the wolf project—perhaps the most successful thing Yellowstone National Park has ever done. There is a current crisis on the ground that requires out-of-the-box thinking to solve. The solutions may go against Park Service dogma, but the problem is too large to solve any other way.

The first thing to do is to maintain or increase bison removal to reduce herd size to a point that streambank trampling is no longer a problem. The so-called “hunt” is both unfortunate and necessary. It should be expanded. Once numbers are reduced, restoration of damaged streams and riparian zones could begin. I would line these channels with willows and sedge mats to the desired stream width. Then, after a few years, reintroduce beaver—another species whose function has been removed from the ecosystem—to the same streams.

Concurrently with restoration, the Park Service could be looking for places to introduce bison. One example would be the Gravelly/Snowcrest/Centennial complex on the Beaverhead National Forest. Abutting Greater Yellowstone to the west, the area is roughly 85 percent under federal management today. The habitat for bison is as good as the Northern Range, and there is about three times more of it. So, solutions exist. The wolf project was a bold decision that produced outstanding results. We need more of that kind of thinking.

For a federal flagship agency such as the National Park Service to do an EIS on any grazing issue and not include a discussion on streams and riparian areas is puzzling to say the least. There was no riparian expertise on the team that put the document together, but they merely had to approach one of the surrounding national forests and ask how they’ve dealt with riparian issues.

As it stands now, the “Effects” section of all the alternatives should read something like this: “Due to bison trampling streambanks, tributary streams to the Lamar River will cease to perform their hydrologic function because of stream over-widening, lack of bedload transport, a shift in riparian vegetation, and lack of floodplain access. More bank erosion will occur, and sediment loads to the Lamar River will increase. Water storage in the riparian areas will diminish, causing late-season flows to be affected. There will be a loss of both terrestrial and aquatic habitat for riparian-dependent species.” 

If the Park Service chooses to favor one resource—in this case bison—over another—in this case streams/riparian zones—that’s not Ecosystem Management. That’s playing God in Yellowstone.