In the first blog post of my management series, I discussed the geography of the Pryor Mountain Wild Horse Range so that it is easier to visualize that the wild horses do live in areas that are a certain size that can thus produce a certain volume of resources that can support a certain number of wild horses. (Please click here to read this blog post.) With that, let’s go through what should be the most math-heavy post of this blog series.
Today I’d like to focus on actual numbers; how does one define what the aforementioned “certain number of wild horses” is for a given area? This number is referred to as the Appropriate Management Level, and it is commonly referred to as AML. In the glossary of the BLM’s Pryor Mountain Wild Horse Range Evaluation (February 2008), there is this definition: Appropriate Management Level means the maximum number of wild horses or burros excluding the current year’s foal crop that can be maintained within an area without causing deterioration of rangeland resources. So the previous blog post walked though the concept of carrying capacity as related to the PMWHR, and all of this is basically the reasoning behind the definition of AML.
All herd’s have their own AML. To get a feeling for what typical AMLs are for different herds, I used the BLM’s 2008 wild horse census information; and I made a histogram showing the range of AMLs that exist. I only used data from herds that have wild horse AMLs. Thus, this isn’t about burro AMLs, though some herds with horse AMLs may also have burro AMLs; these aren’t the focus here though. This chart includes data from 171 Herd Management Areas, which is the number I counted with an AML of at least 1 wild horse. In the histogram, the X-axis shows different values of AML, and the Y-axis shows how many herds have that particular AML. So, for example, the first column shows us that there are 53 HMAs with AMLs of 50 or less wild horses and the second column shows us that there are 42 HMAs with AMLs between 51 and 100 wild horses. Here is the histogram (Please click on it and all following images for full-sized versions.):
So what is AML for the PMWHR? The first AML set for the Range occurred in 1984; it was determined through the 1984 Herd Management Area Plan and the 1984 Billings Resource Area Resource Management Plan. This AML was 121 adult wild horses, so from 1984 to 1992, the AML for the PMWHR was 121 adult wild horses.
AML was recalculated in 1992 with the revision of the 1984 HMAP. Recall from the previous post that there was a change in the size of the PMWHR that occurred due to the loss of National Park Service lands; the result was a smaller area. Because of this, AML went from 121 adult wild horses to 95 plus or minus 10% horses. Thus, starting in 1992, the AML was 95 to 105 wild horses. Due to concerns over the herd’s genetics, recommendations were made in 1994 to not bring the wild horse population down below 100 individuals one year of age and older plus that year’s foal crop. This is basically why the population was always floating around 140 individuals from 1992 onward into the early 2000’s. However, AML was still legally 95 to 105 wild horses. I learned of this information from an interesting discussion of Appropriate Management Level that was included in the BLM’s Environmental Assessment for the 2001 roundup.
The AML of 95 to 105 wild horses was still in existence until recently, when the BLM developed a new Herd Management Area Plan. This HMAP will reset AML at 90 to 120 individuals one year of age and older plus that year’s foal crop. The target population size is the high end of the range, 120 individuals plus that year’s foal crop.
So now we know what AML means and what it is for the PMWHR, but how is it determined? In the BLM’s Pryor Mountain Wild Horse Range Evaluation (February 2008), Staff Technical Recommendations section B as well as Appendices VII and VIII go through the actual calculations that led to the determination of the current AML. The formulae used in the following calculations come from BLM manuals on rangeland evaluation.
The low end of AML (90) was calculated with one particular formula that estimates carrying capacity. The equation works with ratios. It says that if you have a population of a certain size that are utilizing a certain percentage of resources, then how big of population size would you need to have a set amount of resources utilized? Put another way:
So the actual population size is measured each year, of course. The utilization, which is expressed as a percentage, is found through field observations on range conditions. There is an actual, objective process used by the BLM to quantify utilization. Basically, the utilization describes the percentage of forage consumed by the entire population each year. The goal in this calculation is to see 45% of forage consumed each year. Where did 45% come from? It is arbitrary, but it within the commonly used range of values for desired utilization. It basically is saying that the goal is for the population to be using just under half of the available forage. So reworking the formula with this information, we can represent the formula like this:
In the actual calculations that occurred, the BLM determined the “desired use,” which we can think of as the estimated carrying capacity, from 1995 to 2006. Let’s calculate the desired use for 1995 as an example. That year, there were 146 wild horses, and the measured utilization was 81%. Here’s how these numbers are used to calculate that the desired use for that year was 81 wild horses:
The average of the values from all those years was then taken, and this average is 92 horses. Thus, according to this calculation, from 1995 to 2006 there should have been an average of 92 horses on the PMWHR each year. This was rounded down to 90, and this is where the AML’s low end of 90 horses comes from. Remember, this was determined only from observations on the wild horse population and the observed forage utilization for the years 1995 to 2006.
The high end of AML (120) was calculated with another particular formula that also estimates carrying capacity. However, instead of only relying on wild horse population size and observed forage utilization, this formula brings in another factor: Precipitation. This formula modifies the observed utilization with precipitation data; it assumes that utilization is influenced by both wild horse grazing patterns and by precipitation. The first step in performing this calculation is to determine the modifying factor for each year. This factor is called the Yield Index. The Yield Index is based on the annual precipitation from October to September (Crop Year Precipitation) and the 30 year average of this aforementioned precipitation (Average Crop Year Precipitation). A constant factor ((1.23)-.23) is also in the formula. The Yield Index is determined from this formula:
Precipitation data from Lovell, Wyoming, was mostly used in calculating the Yield Indices for the years 1995 to 2006. This is because the Lovell, Wyoming, data are the most complete collection of precipitation data in the vicinity of the PMWHR. These Yield Indices were put into the formula described earlier to make this new formula:
This equation was used to calculate another value of “desired use”; but this value has been modified by the Yield Index to give a new set of values for desired use from 1995 to 2006. Let’s calculate the desired use for 1995 using this new equation. Remember from above, in 1995 there were 146 wild horses on the range, and the measured utilization was 81%. Also in 1995, the Crop Year Precipitation was 6.67 inches. The Average Crop Year Precipitation used in all calculations was 6.79 inches. Here is what the calculation for 1995 looks:
There isn’t a big difference in the results of the two calculations for 1995. This is because 1995 experienced just below average precipitation. Lower precipitation yields higher values for desired use. This is because lower precipitation leads to lower forage production. If there is lower forage production, then actual utilization is more heavily influenced by a combination of precipitation and wild horse grazing than wild horse grazing by itself. The year 2000’s low end desired use result was 95 wild horses. However, 2000 was also a year with below-average precipitation. Because of this, the high end desired use was calculated to be 151 wild horses.
When the average for these values was taken, it was determined to be 117. This was rounded up to 120, and this is where the AML’s high end of 120 horses comes from.
So that is what Appropriate Management Level is and how it relates to the PMWHR. With that, I think there some relevant questions:
When and how can AML be raised? After all, a larger herd size is a definite goal here and in many other herds.
How will AML be reached? We’ve seen how the 2009 roundup was used to move the herd size closer to 120, but roundups aren’t the only way to manage a wild horse population.
These are the topics of upcoming posts. This post and the previous one have worked toward establishing the scientific and legal reasoning behind management. Future posts will focus on actual methods of management. Thank you for taking the time to better understand AML!