Friday, October 10, 2014

How large a bison herd?

The idea of the buffalo commons provoked many people to think in the late 80's and early 90's. With the commons, a large herd of bison--maybe millions--would roam the open expanses of the West.

Today, bison reintroductions continue, but most of the herds are small. Maybe a few hundred. By comparison, the largest public herd--Yellowstone--is roughly at 4000 animals.

So, realistically, how much larger could we get? How big a herd is possible?

There are a few constraints on figuring out how large a bison herd is realistic.

If you don't cull the bison, then you have to rely on predation. Unfortunately, wolves just are not a significant check on bison population. Yellowstone National Park, which has the most active wolf packs interacting with bison, still relies on culling animals when they leave the park to hit their target population size, which is actually about 1000 animals less than what are out there. Wolves don't keep them in check.

If predation isn't a check, then the upper limit becomes food and/or disease. This is an effective regulator, but that means periodic mass starvation and/or disease epidemics. 

I'm not sure people in North America have the stomach for that yet. 

Even Oostvaardersplassen hasn't quite made it to that level of hands off.

In reality, periodic culling is going to be necessary to manage a large bison herd.

If so, how big a herd is realistic?

Assuming that land area is not a limitation, a couple key numbers here to work off of.

First, is the intrinsic growth rate of a bison herd. This is going to vary, but a 25% growth rate is reasonable for most herds.**

**This is going to vary with sex ratios and other factors, but it's a good start.

Second, is how many animals a work crew could process. Bison have to be rounded up, worked individually, and then sorted. Some of those animals go back to the herd. Some are sorted off to go to market.

In general, a single crew can work about a bison a minute. 400 bison in a day is a good estimate of how many bison can be worked in a day. That translates to 2000 bison in a week. If you dedicate a month to working animals, then that would be approximately 8000 bison.

With a growth rate of 25% and 8000 bison that you can work in a month, that means a winter herd size of 6400 and culling off of 1600 animals.

So, if you dedicate one work crew and corral system for one month to processing animals, then your maximum herd size is 8000.

8000 animals in a corral at one time, is not a small corral. And it would take a lot of hay to keep them fed while they are being processed.

What if we relax the assumption that some animals are returned?

What if just the first 8000 get shipped off?

This would mean calves, yearlings, males, females. Whoever is caught in the "net" goes.

If you remove the assumption that 75% of the animals are returned to the herd, then roughly that would let you have a herd size of about 32,000 animals. This would roughly be stable if every year 8000 animals are shipped off and growth rate is 25%.

Assume natural mortality regardless of how ugly: no limit on herd size.
Assume you don't have to round up animals, but could just shoot them in place: no limit as long as you feel comfortable shooting thousands of animals.
Assume have to round up with one crew for a month, but don't sort the animals: 32,000.
Assume have to round up with on crew for a month, but do sort the animals: 8,000.

Can you add corrals, or sort longer than one month? Yes, but there is no precedent for this.

What about field harvests? With field harvests, animals are often shot in the field and processed in trailers on-site. Yet, the typical rate for a crew is 8 animals per day. Over a 6 month period, that's still just a 1000 animals per day. Harvesting bison sustainably this way would never permit a herd size of more than a few thousand at best.

So what are the likely prospects for a bison megaherd?

Assuming you can get the land, it's going to be hard to have more than 8000 animals in a herd.

So, how big an area is that? Depending where you are in the world, it might be about 100,000 - 200,000 ha (40-80k ha). Put in perspective, that's a square about 10-20 miles on a side.

That's really nothing. Maybe $20-$100 million to buy the land for that (depending on a lot).

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