American Bison - Buffalo Field Campaign

Figure 7.4 Representation of plains and wood bison conservation herds in original ranges and major habitat types in North America. Habitat types were
based on Sanderson et al. (2008).
than 5,000 acres and less than 50,000 acres); large areas (more
than 200 km 2 and less than 2,000 km 2 ; more than 5,000 acres
and less than 500,000 acres); and very large areas (more than
2,000 km 2 ; more than 500,000 acres). About half of plains bison
conservation herds occur on small ranges and only 10% of
herds are on very large ranges (Figure 7.5). In contrast, 37% of
wood bison herds occur on very large ranges and none occur on
small ranges.
7.4 Population Size Distribution
Using a simulation model, Gross and Wang (2005) demonstrated
that a minimum population of about 400 animals was needed
to retain 90% of selectively neutral variation with a 90%
probability for 200 years. Allelic diversity was more sensitive
to management treatments than average heterozygosity. On
average, a high proportion of alleles with an initial frequency
of less than 0.05 were lost when herds had fewer than 400
animals. Differences in generation time accounted for about
75% of variation in retained heterozygosity for populations of
200-800 bison. As population size approached 1,000, the effects
of population management on genetic variation were small.
Therefore, we considered populations exceeding 1,000 to be
more resilient than smaller populations.
Sanderson et al. (2008) defined the following size classes for
ranking contributions of bison herds to ecological restoration:
small contribution, fewer than 400 animals; modest contribution,
400-1,000 animals; large contribution, 1,000-5,000 animals;
exceptional contribution, more than 5,000 animals. The
frequency distribution of conservation population size (Figures
7.1 and 7.6) illustrates that small populations (fewer than 400
animals) are the most common population size class among
both plains and wood bison (74% and 55%, respectively). Five
plains bison and three wood bison herds exceed 1,000 animals.
Only two populations have encompassed 5,000 animals within
their recent range of size variability (Greater Yellowstone Area
and Greater Wood Buffalo Park area).
American Bison: Status Survey and Conservation Guidelines 2010 59