November 18, 2009 – Population Estimation

Previously, I have discussed the concept of Appropriate Management Level. This is a number that describes the desired population size for different wild horse herds in different areas. When the population size exceeds AML, there are frequently management actions that occur, such as gathers and removals. Before any of this can occur, though, the population size needs to be determined. Here in the Pryors, and in other similar herds, we closely monitor the population and are able to make population size estimates that are very close to the actual population size. However, this is unusual for wild horse herds. Thus, the topic of my post today: How are wild horse population sizes determined?

The widespread censusing of wild horses dates back to the passage of the Wild Free-Roaming Horses and Burros Act of 1971. Corresponding with the 1971 Act is 43 CFR Part 4700 – Protection, Management, and Control of Wild Free-Roaming Horses and Burros. This document has some more specifics on wild horse management. In Subpart 4710 – Management Considerations is a section (4710.2) on “inventory and monitoring.” Within this section is this line: The authorized officer shall maintain a record of the herd areas that existed in 1971, and a current inventory of the numbers of animals and their areas of use. (In this document, authorized officer is defined as “any employee of the Bureau of Land Management to whom has been delegated the authority to perform the duties described herein.”) So, basically the BLM was directed in 1971 to establish and maintain records of the herd areas created in 1971; and they were to continue to maintain current records on the population sizes of herds.

Well, the BLM was required to do their best in making population estimates in the 1970’s; but it is hard to say how accurate these estimates were given the knowledge that existed on estimation techniques at the time. Accuracy became a big question to all as wild horse management became a bigger issue, and this was a subject of the National Research Council’s 1982 report “Wild and Free-Roaming Horses and Burros.” Starting on page 40 of this document is a section entitled “How Many Horses/Burros Are There In The West?” This section describes how studies done had indicated a wide range of possible error in censusing techniques employed at the time; attempts at censusing were believed to have missed 7% to 60% of the actual population. They had no way of determining which censuses were only counting 93% of the herd and which were counting only 40% of the herd. From this, they recommended some techniques that could be performed every 2 to 3 years to obtain the type of population estimates needed by the BLM. Notice that the new emphasis here was on attempting to accurately estimate the population size as the study had found that attempting to count everything had a lot of potential for error.

Today, the science of counting wild horses is still a matter of debate and research. The BLM has stated that their goal is to obtain accurate population counts for all wild horse herds at least once per four years, but many managers seem to be making annual population counts. In an effort to develop more accurate population estimation techniques, the BLM has teamed up with researchers from the United States Geological Survey and Colorado State University. This group has done a lot of research, and they have concluded that there are four techniques that are potentially good candidates for estimation wild horse populations. These can be read about by clicking here. These techniques seek to combine field observations with statistical analysis so that accurate population size estimates can be consistently and efficiently determined.

A recent publication (“Validating Aerial Photographic Mark-Recapture for Naturally Marked Feral Horses” by Bruce Lubow and Jason Ransom, Journal of Wildlife Management, 2009) describes how testing was done to determine how accurate the mark-recapture method is. This study was done in three wild horse herds that people pay a lot of attention to – The Pryors, the McCullough Peaks, and the Little Book Cliffs. Because it was done in these areas, they could estimate the population sizes for these herds and then compare their estimates to the actually known population sizes. The best estimates in the study ranged from less than 8.6% to over 2.6% of the actual size. Recall that the aforementioned National Research Council report described that censusing back then could be off by 7% to 60%. The results of the Lubow and Ransom study are thus a lot more accurate, and so these researchers feel that this particular technique has the great potential to help managers estimate the population sizes of wild horse herds whose members have many unique and visible markings.

Here in the Pryors, we work hard to maintain accurate population estimates; this is one of the reasons I spend so much time on the PMWHR observing the horses. This is something many people before me have invested many, many hours into as well; I just continue to build on their work. We have a Memorandum of Understanding with the BLM, and part of this MOU regards a relationship in which we openly share our data with the BLM. We do this because we want to do as much as we can to assist the BLM in their decision making processes. My population estimates are ranges. The low end estimate reflects my known population size. The high end estimate reflects my known population size plus the number of horses that I have listed as being missing. However, given the frequency of sightings here, the probability that missing horses are actually alive decreases over time to the point where they are counted as deceased one year after going missing. Thus, my estimate range can actually be quite small; and I can even sometimes say with some confidence what I feel the exact population size is. For example, my last population estimate before the 2009 gather here was on August 31. On that day I estimated the population size to be 189 horses ages one year and up. At the same time, I had 7 horses on my missing list. Of these, one was an old bachelor who hadn’t been seen in eleven months. Two others were middle-aged mares who were found missing this spring. The remaining four were horses born last year that went missing late last year or early this year. Thus, it was probable that these 7 missing horses were actually deceased. This is why I have stated that the pregather population in the Pryors was 189 horses. The largest the population could have possibly been was 196 horses. The largest error I could have had was to underestimate the population by about 4%.

Notice that the Lubow and Ransom study discussed above concerned a method of wild horse population estimation that could estimate the population size to within -8.6% to 2.6%. This isn’t too far off from what we can come up with here in the Pryors. Our field observations here also provide us information on births, interchange, relationships, and the like. However, if it is simply a population size estimate that is needed, then it is likely that the mark-recapture method of counting horses has a lot of potential in allow for accurate estimates to occur. I think that it should be understood that past wild horse population size estimates were likely off by a significant margin. As we now know, population size is a very big factor in the wild horse management decision making process. Thus, it is very important for managers to know herd sizes as accurately as possible; and so it is paramount that accurate techniques, such as those described above, be developed and implemented.

November 6, 2009 – Appropriate Management Level

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!

October 27, 2009 – The Pryor Mountain Wild Horse Range

Any sentence containing the words “management” and “Pryor Mountain Wild Horses” is typically unpopular. Popular or not, this is something that I feel is very important to understand. Thus begins our journey into understanding the management of the Pryor Mountain Wild Horses. Please remain patient and open-minded as you read this post and subsequent posts.

I always feel it is important to first understand why management must occur. To understand the reasoning behind management, we must first understand where the Pryor Mountain Wild Horses live. I think that it is important to know that the Pryor Mountain Wild Horse Range is fenced. There is a barrier around the wild horse range that is meant to keep the wild horses from leaving. Some of this barrier is actual fence while other parts of it are natural (steep canyons). More importantly, perhaps, is that this is a legal boundary; the law states that wild horses must be within the boundary of the PMWHR. Let’s stop here and understand what the PMWHR looks like today. I say today because it has changed over time. Below are some maps I drew based on BLM documents and reports; these maps show the evolution of the PMWHR. These maps are approximations, but they are pretty close to reality.

When looking at these maps (except for the first location map), what you’ll really want to look at is the bold, dark red line. This is the PMWHR boundary. As far as the color of the land areas goes, green is National Forest, yellow of BLM, purple is National Park Service, orange is tribal land, white is private land, and light blue is state land. Streams, rivers, and the like are blue. The Wyoming-Montana border is the dashed black line. This is all overlaid on a topographical model of the area. To get a feeling for scale, there is a section grid on the map; each of those squares is a mile by a mile in size. North is always “up” on the maps. Feel free to click on these maps to see larger versions of them.

So let’s start with understanding where in the country we are actually looking. The area I will be describing is on the Montana-Wyoming border on East Pryor Mountain (the southeastern area of the Pryor Mountains) just north of Lovell, Wyoming. See that little red polygon under the words “Pryor Mountain Wild Horse Range”? That is the PMWHR shown at approximately its actual size compared to the Western states.

Now let’s go back in time to the PMWHR in late 1968. This was when the range was first established through an order by Secretary of the Interior Stewart Udall.

This original PMWHR was about 33,600 acres. It covered only BLM land and portions of the National Park Service’s Bighorn Canyon National Recreation Area. Remember, we’re in 1968 now. There is no such thing as the Wild Free-Roaming Horses and Burros Act, and so the aren’t a whole lot of rules and regulations concerning wild horses compared to those of today.

Let’s fast forward a few years to 1971 now. The Wild Free-Roaming Horses and Burros Act has passed, and now wild horse herds across the West receive protection. As far as the areas that wild horses will be protected in, the 1971 Act has a couple lines that stand out. The last sentence of the first paragraph says this: It is the policy of Congress that wild free-roaming horses and burros shall be protected from capture, branding, harassment, or death; and to accomplish this they are to be considered in the area where presently found, as an integral part of the natural system of public lands. To reiterate the relevant point of this sentence, consider this line: they are to be considered in the area where presently found. This has been interpreted to mean that wild horses are to be managed on the areas in which they were found in 1971. I have also heard it interpreted to be as specific as where they were in December of 1971. The 1971 Act goes on a little later with a follow-up to this. In the Definitions area of the act, there is this sentence: “range” means the amount of land necessary to sustain an existing herd or herds of wild free-roaming horses and burros, which does not exceed their known territorial limits, and which is devoted principally but not necessarily exclusively to their welfare in keeping with the multiple-use management concept for the public lands. So the relevant line here is “which does not exceed their known territorial limits.” This seems to be interpreted as saying that new land can’t really be added onto existing wild horse areas unless there were horses found in those areas in 1971. (Just as a side note, the ROAM Act would basically strike out these two aforementioned lines.)

So what did all of this mean for the PMWHR? Well, the Pryor horses weren’t just living on the plot of land that was created for them in 1968; they had spread to adjacent areas as well. Because of this, the PMWHR grew in size with the passage of the 1971 Act. The major areas of expansion took place on the north end of the range. Land was also added along the southern end of the range. This new PMWHR covered over 40,000 acres on Forest Service, BLM, and National Park Service lands. Though the act passed in 1971, it took a few years to get everything done. By 1974, the PMWHR looked like this:

In 1984, the BLM closed some southern portions of the PMWHR to the wild horses. Around the same time, the National Park Service added new land to the PMWHR; and so there wasn’t really a significant net change in acreage. However, in 1990, the National Park Service closed this added land to the wild horses; and so there was a decrease in the size of the PMWHR. This is essentially the same area that is out there today. Today’s PMWHR is about 38,000 acres. It covers land varying in elevation from approximately 3600 feet to 8700 feet. It looks like this:

Up until this point, I’ve shown you some maps with boundary lines drawn on them; and I am telling you that the Pryor horses are supposed to stay within this boundary. So what’s keeping them in there? The following map shows the different parts that make up this boundary. Each different color of line corresponds to a different boundary which is described with the label near it.

Thus, in the PMWHR, and all other wild horse areas, there is a plot of land of a certain size that wild horses live on. Because the land is a certain size, 38,000 acres, then it produces a certain amount of resources. This certain amount of resources can support a certain number of wild horses. If there are too many wild horses on it, then the land may start to experience damage. Because the lands of the PMWHR were historically overgrazed by livestock (like most Western lands); and because this area gets very little precipitation, any damage to the land can take a long time to recover, especially if the Pryor horses are applying some small but constant grazing pressure. If the land is damaged and slowly recovering, it is producing fewer resources than it once had. If this is the case, then there will be insufficient resources to support the wild horse population present on the land. At this point, the population must be provided resources; or some of the wild horses will get sick or die due to poor nutrition. To prevent these situations from happening, it is thus necessary to manage the wild horse population. The same could be said of populations of other wildlife in the West. This is a reality of the West in 2009. As you read through this post and upcoming posts, please do keep in mind that we do live in 2009; and so there are some economic, geographic, political, and cultural realities that we just have to deal with.

I think that the Humane Society of the United States summarizes all of this well in their pamphlet entitled “America’s Wild Horses: Managing for the Future.” In this pamphlet the HSUS explains why management is necessary through this statement: Given the West’s current state, wild horses left totally unmanaged would suffer periodic mass starvation and in some spots further damage the fragile landscape.

With that, start thinking about the ways in which wild horses can be managed. This will be the topic of future posts.