Carrying Capacity and the Bison of Yellowstone National Park The following adapted from an article in Yellowstone Science, a joumal published by the National Park Service 2 3 4 Animportant part of the lowstone National Park ecosystem bison are the largest and mammals in North America Yellowstone bison historically occupied approximately 7,720 square miles (20.000 km) in the headwaters of The lowstone and Madison rivers Today this range is restricted to primarily Yellowstone National Park and some adjacent areas of Montana The bison population is subdivided into the central and northem breeding herds One of the key issues related to the Yellowstone bison population is how it should be managed. From the beginning of the twentieth century through 1966 bison numbers were kept in check through regular culting. This ling was stopped in 1969. Officials decided to allow bison numbers to fluctuate according to such factors as weather, the availability of resources, and predators. The result was a dramatic increase in the numbers of lowstone bison Figure shows population numbers for the central and northem herds for 1870-2010 Center Pend Northe Hed As the number of lowstone bison increased more bison began migrating to areas outside of the Yellowstone Park boundaries during the winter months in the 1940s Yellowstone bison were concentrated int two main areas comprising a relatively small portion of the park. By 2007, the range of Yellowstone bison was greatly expanded Bison were found throughout Yellowstone Park and even beyond its boundaries Bison are migratory. They move to lower elevation ranges during the winter months in response to snow accumulation and snowpack development. They also change their diets slightly during the printed in lowland meadows where the growth of sedges and grasses is concentrated. The movement of the bison beyond park boundaries, however, raised questions about whether their numbers were too high and the forage inside the [Distribution In 2011, Dr. P.J. White and others published a review paper addressing the question of whether Yellowstone bison were moving outside of the park because they had exceeded their carrying capacity (2011) As population increases in size, the finite amount of water, food, and shelter in an area must sustain and support a large number of individuals. Eventually numbers may increase to the point where this is no longer possible th point, the carrying capacity has been exceeded. One study the researchers referenced was an earlier study by one of the study's coauthors (Coughenour, 2005). This study used an ecosystem model to evaluate whether bison had exceeded their carrying capacity inside Yellowstone National Park. The study focused on food availability as the limiting factor. The model developed was named the SANNA model. The model integrates data from several etmed variables, including predator populations, the production of plant biomass, physical and features, and water valability Figure 2 shows a simplified version of the model and some of the interelated variable used in the model. For example, hunting and culling as well as predation can reduce the size of the bison population. The SAVANNA model also uses weather data from various sites within the park to simulate winter snow accumulation as we wet Pesaron Pulation Figure 1-Data for bison observed during aerial surveys conducted between July 1970 and August 2010 Source: NPS Part (mather Population Part C Figure 2-Selected essential components of the SAVANNA ecosystem model Modified from White (2011) The model is the populations of the two main bison breeding hands in Yellowstone National Park as well as nine plant groups. Eight simulations spanning a period of 50 years were completed using the model to determine whether Yellowstone bison had exceeded their food-limited carrying capacity inside the park. The simulations did not allow Yellowstone bison to expand their range beyond the park bounderies. The simulations predicted the size of the two main herds by estimating the mean size of each herd, the median size of each herd, and the possible range in the size of each herd. These data were then compared to actual 2005 population data for Yellowstone bison The following question has two parts. First, answer Part A Then, answer Part B. Part A Which statement is a valid analysis about bison movement and the carrying capacity of Yellowstone National Park? O O. The fact that bison only migrate in years of deep snow indicates that they have exceeded the park's carrying capacity O oooo The decrease in the northern herd's population is explained by its inability to migrate and the park's decreasing carrying capacity The movement of bison beyond park boundaries is not due to the bison population being too large for the park's carrying capacity Part B: Which selection from the text provides evidence that directly supports this analysis? O. The carrying capacity for other animals found in Yellowstone has increased due to the bison's migration outside of the park O They move to lower elevation ranges during the winter months in response to snow accumulation and snowpack development O. The model integrates data from several ecosystem-related variables, including predator populations, the production of plant biomass, physical land features, and water availability The actual population of the northern herd in 2005 was significantly smaller than the smallest population predicted using the model, as well as smaller than the median and mean predictions Scientists have suggested several possible explanations for the movement of Yellowstone bison outside of the park even though they have not exceeded their food-limited carrying capacity