Elsewhere on this website,
the role of human hunting in past evolution of modern
bison is discussed. At the end of the ice age, natural
selection by large and dangerous predators was replaced
by human predation. Evolution responded by altering the
predator-accommodation strategy of North American bison.
Once large bison, with large and/or aggressively shaped
horns and a predator-evasion strategy of “stand
and defend”, were replaced by smaller bison with
a strategy based on great mobility. Our modern bison species
developed from this process. When attacked, bison herds
would travel great distances, beyond the ability of pedestrian
predators to follow.
Consequently, mobility is a
basic characteristic of modern bison. Notably, our bison
are smaller than their ice-age ancestors. But all other
characteristics of bison anatomy, physiology and behavior
must be compatible with, or supportive of, great mobility.
Presumably, bison stamina based on skeletal, muscular
and cardio-vascular anatomy and physiology, have developed
to enhance mobility. Once established as a predator-evasion
strategy, great mobility also influenced how bison have
evolved to effectively utilize a large and diverse environment.
Bison appear to learn and remember characteristics of
a large environment; and their social behavior is designed
to pass that information across generations. A wild bison
herd, established on a large range with topographic and
vegetative diversity can range widely to best exploit
its environment in response to seasonal and among-years
variation in weather, vegetation and availability of water.
In the evolutionary past of
bison, animals that best developed these characteristics
achieved higher rates of survival and reproduction –
enhancing or maintaining the genetic composition of the
herd across generations. However, south of Canada, few
of today’s bison live on large ranges.
In a survey of 44 conservation
herds of plains bison south of Canada (Bailey, 2013),
only 1 herd had a range larger than 110 square miles.
Bison could cross a range of 110 square miles in less
than a day. Only 6 herds had ranges of 51-110 square miles.
Worse, 37 of 44 bison conservation herds had ranges of
50 or fewer square miles.
Thus, great mobility is neither
useful nor necessary for almost all of today’s conservation
herds. Natural selection to maintain characteristics associated
with great mobility has ended for these bison. It is a
major part of the gradual process of domestication –
simplifying and disorganizing the wild genome.
To maintain wild bison, the
Coalition recommends at least 1000 bison on at least 100
square miles of diverse habitat. Larger herds on much
larger ranges would be much better. But anything less
than these minimums would contribute little or nothing
to forestalling the evolutionary decline of wild bison.
We already have many small conservation herds on small
ranges.
Bailey, J. A. 2013. American
Plains Bison: Rewilding an Icon. Sweetgrass Books,
Helena, MT.