Thursday, March 13, 2014

Bison migrations, counter evidence?

Hornaday was convinced the bison migrated north-south in the Great Plains on a regular basis.

Still, there are doubts about how regular bison migrations would have been.

Hart in 2001 wrote a paper "Where the buffalo roamed--or did they?". In it, he revisits first-hand accounts of bison in the 18th and 19th centuries, questioning the assumption that bison had an annual north-south migration.

"...evidence from explorers' and other travelers' journals contradict...annual north-south migration...While bison moved, it does not appear they "migrated," if migration is defined as following a regular and predictable seasonal or annual route..."

Part of the debate becomes what negative evidence would have been necessary to refute migration? Certainly, bison responded to stochastic events, just like wildebeest do today. And some animals could be found to be resident year-round, just like non-migratory elk, which coexist next to migratory elk, are now. But, that doesn't mean that there wasn't a tendency for bison to follow similar patterns of long-distance movement.

The migrations that bison would have undertaken are on the order of 800 miles/1200 km a year in one direction.

That's a lot, but actually shorter than what many caribou migrate today. Individual caribou have been recorded to travel 5000 km in a year.

So, the distances estimated for bison to travel are reasonable. The spatio-temporal gradients in nutritional quality also would have been there to drive the migrations. Collected observations support the migration of animals in the 1800's.

We still need more information about what the potential weight gains of animals would be along the gradients in a typical year and the consequences for reproduction. In short, migration has to be associated with enhanced reproductive success to be evolutionarily stable. 

There is scattered evidence that bison reproduction is lower in the south than north, but those data have never been officially compiled.

At this point, we need better data on seasonal patterns of weight gain for bison in different parts of the ranges, which helps understand what can drive their migration better. 

Another tough thing is that bison diet is limited by protein concentrations in plants, not energy. Yet, most models of migration are based on energy balance. So, the driver of migrations is protein, but the currency is energy. That makes it hard to balance. New models are really needed that can work in protein and energy at the same time.

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.

Tuesday, March 11, 2014

Reading Hornaday

Reading William Hornaday's 1889 The Extermination of the American Bison../

It's a powerful book.

To me, it's like reading Weaver's account of the Great Plains, before, during, and after the Great Drought. Both transcribed the last vestiges in amazing detail.

Choice quotes below:

The first European description of bison:
"among which the greatest Rarity was the Mexican Bull; a wonderful composition of divers Animals. It has crooked Shoulders, with a Bunch on its Back like a Camel; its Flanks dry, its Tail large, and its Neck cover'd with Hair like a Lion. It is cloven footed, its Head armed like that of a Bull, which it resembles in Fierceness, with no less strength and Agility"

Sex segregation:
We were much surprised at sometimes meeting innumerable herds of bulls without a single cow,
and other herds of cows without bulls.”

"So long as the bison held undisputed possession of the great plains his migratory habits were as above—regular, general, and on a scale that was truly grand. The herds that wintered in Texas, the Indian Territory, and New Mexico probably spent their summers in Nebraska, southwestern Dakota, and Wyoming. The winter herds of northern Colorado, Wyoming, Nebraska, and southern Dakota went to northern Dakota and Montana, while the great Montana herds spent the summer on the Grand Coteau des Prairies lying between the Saskatchewan and the Missouri. "

On toughness:
"A buffalo can weather storms and outlive hunger and cold which would kill any domestic steer that ever lived."

The extermination:
"Such was the end of the great southern herd. In 1871 it contained certainly no fewer than three million buffaloes, and by the beginning of 1875 its existence as a herd had utterly ceased, and nothing but scattered, fugitive bands remained."