Yellowstone bison--should we preserve artificial population substructure or rely on ecological processes?

Halbert et al. (2012) analyzed microsatellite genotypes collected from 661 Yellowstone bison sampled during winters from 1999 to 2003 and identified 2 genetically distinct subpopulations (central, northern) based on genotypic diversity and allelic distributions. On the basis of these findings, they raised concerns about the management and long-term conservation of Yellowstone bison because of disproportionate culling of the 2 subpopulations in some winters. The data and findings of Halbert et al. (2012) are significant and useful for managers charged with conserving these iconic wildlife. However, their article provides information regarding the behavior and management of Yellowstone bison that does not accurately portray historic or current conditions. This response clarifies those conditions and challenges some of their apparent deductions and recommendations. Halbert et al. (2012, p. 1) indicate that Yellowstone bison provide an opportunity to examine a “. . . natural population substructure, which could have important implications for the long-term evolution of these populations.” They assume “. . . the Yellowstone population was not subdivided before 1936” and that “these 2 subpopulations [central, northern] have differentiated in a relatively short period of approximately 8 generations [64 years]” (Halbert et al. 2012, p. 5, 7). However, these statements ignore that humans contributed to the observed population and genetic substructure in Yellowstone bison by nearly extirpating them in the late 19th century (except for approximately 23 bison that survived in central Yellowstone) and then by creating another breeding herd in northern Yellowstone at the turn of the 20th century from 21 bison of unrelated breeding descent and divergent genetic stock that were relocated from northern Montana and Texas (Meagher 1973). A few individuals from the endemic central herd were introduced into the northern herd in the early 1900s, whereas 71 bison from the northern herd were relocated to central Yellowstone during 1935–1936 (Cahalane 1944). The northern herd was not released from traditional livestock management practices and allowed to evolve natural patterns of distribution until the 1950s—which likely contributed to some geographic separation between the herds (Meagher 1973). Further, each herd was sporadically culled from the 1950s to present (Meagher 1973, White et al. 2011b). Thus, the history of Yellowstone bison suggests the population substructure and genetic differentiation was substantially influenced by a human-induced bottleneck in the late 1800s and the effects of human stewardship thereafter. As a result, there is evidence that the existing genetic substructure was artificially created. Halbert et al. (2012, p. 2,5) state that “Radiotelemetry data indicate the [central and northern] herds remain isolated during the summer breeding season” and “the number of migrants into and out of each subpopulation each generation is about 2 (Nm = 2.3) or approximately 1 every fourth year.” This statement and estimate may generally reflect conditions during the period of intense human stewardship (1900–1968) and subsequent increase in bison abundance and distribution during the period of ecological process management (Plumb et al. 2009). However, extensive monitoring of the movements and productivity of radio-collared bison since 2005, when the population reached an abundance of approximately 5000 bison, suggests that emigration and gene flow is now much higher. Since 2007 (one half of one generation), biologists have detected 17 radio-collared bison emigrating between the central and northern herds and remaining through one or more breeding seasons (see Supplementary Table 1 online). Female bison rarely travel alone, so dispersal by these marked females likely represents emigration in groups of 25–40 bison each time, which increases the probability that gene flow occurred. Eleven of these 17 radio-collared bison produced calves on their new range (e.g., northern) that were conceived on the range they left (e.g., central). At least 23 calves were produced by these dispersing bison through mating and calving on their new JHEREDOUPJournal Of HeredityJHERED0022-15031465-7333OUPUS