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.

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