Wednesday, March 12, 2014

Reconstructing bison migrations

Redrawing of Hornaday's extent of bison map.

The Serengeti is held up as a model for migratory grazers and the North American Great Plains is often compared to this.

But the Serengeti is pretty unique.

First, it's compact. It's actually smaller than the Sandhills of Nebraska. If we had a million bison moving around the Sandhills, it would seem like a small eddy.

Second, there are strong gradients in soil type and precipitation that pump the animals in the Serengeti. The Great Plains doesn't have a strong E-W or N-S gradient in soils like the Serengeti does. Precipitation gradients are broad in the Great Plains. The Serengeti precipitation gradient is about 600 mm from wet to dry. That's the precipitation gradient along the whole width of the Great Plains. Fort Collins to Iowa.

Thinking about how bison might have migrated on the Great Plains can use the Serengeti as a model, but it will have worked differently.

Here's what we know:

1) Hornaday states that bison regularly migrated north to south in the Great Plains. This is along a temperature gradient, not a precipitation gradient.
2) Strontium isotope evidence from bones and historic observations do not support E-W migrations.
3) The migration appeared to have limits of about 800 miles/1200 km.
4) Today, resident bison gain more weight in the north than south.
5) The N-S gradient is not a productivity gradient. Dietary quality is higher in the north than the south.
6) Green up dates are about 20 d later when you compare to a location 800 miles to the north.
7) Calving is typically in May, so that sets a terminus on the spring migration.

When you take all of that, you have the pump that drives the migrations.

Animals that start in the south and head north get nutritious grass earlier and can follow the green wave north to even greener grass.

Here's what we don't know.

1) Why 800 miles? The further north you go, the better the grass. Why stop? What sets the upper limit here?

2) What are the costs vs. benefits for being resident vs. migratory? How much more weight do you gain or how much more likely are you to reproduce if you migrate? Put another way, what are the costs to bison for shutting off the migration?

I'll admit that thinking of the Great Plains with annual migrations of bison is a bit different than my previous thoughts...

Building a working model of the Great Plains migrations has never been done. That should be an interesting challenge.

No comments:

Post a Comment