Comparing bison weight gain

Locations and size classes of bison conservation herd in North America. Historic ranges shown, too. From Gates et al. 2010 IUCN report on bison.

Before European settlement, bison were distributed across North America. From the Atlantic coast almost to the Pacific and from northern Mexico to the Arctic Ocean. But where would have been the best place to be a bison?

There might not be an easy answer to the best metric for determining where the best place was to be a bison. Especially since we can't go back in time. Yet, bison have been reestablished across North America, which gives us some ability to begin to compare populations.

For bison, as with any animal, fecundity is the ultimate metric of fitness. It is almost axiomatic that when we compare individuals, the best metric of fitness is the number of offspring that an animal or population has. Yet, fecundity is density dependent. Fecund populations become dense, which lowers their fecundity. Population density could serve as another metric, depending on how important disease or predation become in limiting population size.

Systematic comparisons of the geographic ecology of bison have not been attempted. Yet, here's an interesting comparison. The latest IUCN report on bison "American Bison: Status Survey and Conservation Guidelines 2010" included a graph on the weights of bison at Wind Cave in South Dakota.  Weights were averaged for males and females by age. Wind Cave bison are considered some of the purest Plains bison and western South Dakota and its short grasses is thought of as prime bison habitat.
By comparison, Konza Prairie in Kansas is tallgrass. And tallgrass is sourgrass to some--not the best habitat for bison. The calving rates of bison at Konza can be pretty low. In some years only 50% of the adult female bison calve, which might not indicate the best nutrition.

Yet, with the data from Wind Cave, we can compare the weights of bison at Wind Cave and Konza. Based on what we know of the habitats and the calving rates, we might expect that Konza bison would weigh less than Wind Cave bison.

Not the case.

Closed symbols are females. Open symbols males. Circles are Konza. Squares are Wind Cave.

The two sites are right on top of one another. 

Either the two sites are equally good for bison or the weight of animals isn't the best metric for habitat quality.