Wild horse overpopulation myth debunked

Nevada mustang © Carl Mrozek

Nevada mustang © Carl Mrozek



Research Collaboration by

Kathleen Gregg Environmental Researcher

Lisa LeBlanc Environmental Researcher

Jesica Johnston Environmental Scientist April 25, 2014



The recent National Academy of Science (NAS) report on the Wild Horse and Burro Program determined that the Bureau of Land Management (BLM) has no evidence of excess wild horses and burros; because the BLM has failed to use scientifically sound methods to estimate the populations (NAS, 2013). The NAS cited two chief criticisms of the Wild Horse and Burro Program: unsubstantiated estimates in herd management areas (HMA), and management decisions that are not based in science (NAS, 2013).

Effective wild horse and burro management is dependent on accurate population counts and defensible assumptions. The Bureau of Land Management (BLM) routinely uses the assumption that wild horse and burro herds increase annually at an average rate of 20%. However, our review of available scientific literature combined with an analysis of BLM data for 5,859 wild horses found that approximately 50% of the foals survived to the age of 1 year, which indicates a 10% population growth rate based on yearling survival rates.


The data and analysis is based on the BLM’s wild horse and burro removal and processing documents acquired under the Freedom of Information Act. The data sets were evaluated separately, and then combined to total 5,859 wild horses, captured, aged, and branded by BLM. This data is the basis for the analysis in this report and the accompanying chart in table 1 below.

Burro data was also calculated for foal and yearling survival. That data indicated a 7% population growth rate for burros based on yearling survival, but that data is not included here as burros are not present in all of the HMAs.

The data was collected from 4 herds captured by BLM in Nevada and California in 2010 and 2011. The data below in table 1 shows the individual herds and accumulated age structure data which supports the overall conclusion. Wild horse foals and yearlings were tallied for population increases and in all four samples, recorded a combined foaling rate of less than 20%, but only half or 50% survived to the age of 1 year (see table 1 below).

Table 1 Age Structure Yearling Survival Rate

PM Population growth



This research does not include or reflect the additional adult mortality rates due to the complexity of population dynamics, but does raise serious questions about the validity of the BLM’s assumed 20% annual herd population growth rate. Furthermore, the BLMs assumption fails to consider that wild horse populations are dynamic due to isolation and have varied rates of reproduction and survival due to changing climates, forage, competition, disturbance and environmental conditions. All these are factors that can lead to varied herd growth rates and each herd should be evaluated separately.

This research paper is supported by previous studies using age structure data completed by Michael L. Wolfe, Jr. in 1980 titled “Feral Horse Demography: A Preliminary Report”. Mr. Wolfe cited observations in 12 HMAs, over a period of 2 to 5 years, and covered a much broader range over six Western states. He questioned the annual rate increase of 20%, and found that first-year survival rates to range between 50% and 70% (Wolfe, 1980).

Other supporting research includes The National Academy of Science National Wild and Free-Roaming Horse and Burro report of 1982, which states, “…several biases in the (BLM) census data, cited or calculated rates of increase based on a number of published values for reproduction and survival rates, as well as sex and age ratios, and concluded annual rates of increase of ten percent or less” (NAS, 1982).

The NAS 2013 report also used age structure data to estimate population growth. However, the report used foaling rates to draw conclusions about the population growth; rather than first year survival rates (NAS, pg.51-52 2013). This and other studies challenge the assumption that the 20% foaling rate provides an adequate measure of population growth.

The BLM bases their management decisions on environmental assessments that cite inflated population estimates. As shown in this study and previous research, the BLM’s assumption of a 20% annual wild horse population growth rate is not based in science; leading to unsubstantiated population estimates with no evidence of excess wild horses.



National Academy of Science 2013, “Using Science to Improve the BLM Wild Horse and Burro Program – A Way Forward”

Johnston, J. (2011). California’s, Wild Horses and Burros: Twin Peaks HMA.

http://csusdspace.calstate.edu/xmlui/bitstream/handle/10211.9/1492/WHB_Thesis_Final%2011.30.11.pdf?seq uence=1

“Feral Horse Demography: A Preliminary Report”, Michael L. Wolfe, Jr.

http://www.jstor.org/discover/10.2307/3897882?uid=3739560&uid=2&uid=4&uid=3739256&sid=2110368888 4451

National Academy of Science 1982, “Wild and Free-Roaming Horses and Burros”


U.S. Department of the Interior, Bureau of Land Management Freedom Of Information Act (2012). FOIA BLM FY12-011 1278.

U.S. Department of the Interior, Bureau of Land Management Freedom Of Information Act (2012). FOIA BLM- 2012-00934.

U.S. Department of the Interior, Bureau of Land Management Freedom Of Information Act (2012). FOIA BLM 2012-01046.

U.S. Department of the Interior, Bureau of Land Management Freedom Of Information Act (2012). FOIA BLM 2012-00250.

Download the paper here: PM Population Growth 4.25.14 FINAL

8 thoughts on “Wild horse overpopulation myth debunked

  1. Thanks for calling attention to actual facts to debunk the BLM claim regarding wild horse overpopulation and yearly increases. Also sad to see how many die in our “humane” holding facilities. How much better for them to have remained in the wild with their family structures and how far less costly to we the taxpayers.

  2. Will we ever know the original motivation that initiated the BLM to start taking away our horses from our lands? Whether it was greed, ignorance or just plain stupidity, I don’t know. All I know now is that the red tape and b.s. has got to stop. Our horses should be returned to the wild areas they were rounded up from and they should all be left the hell alone. They lived without the BLM for hundreds of years and they’ll do just fine without them interfering once again.

  3. When in doubt, just double it” is apparently BLM’s motto & MO. In fact, a senior BLM official admitted as much at an agricultural conference in CA last spring when explaining to an ag & cattle audience how they came up with their wild horse & burro population numbers. The official actually admitted that they typically multiply by two to offset the presumption of a chronic undercount -referring to actual head counts. Hence, to know how many horses/burros BLM has actually counted or even extrapolaed recently, you really do need to divide by two. This works perfectly for wild burros.
    The last time I talled the total of all wild burro herds listed on BLM’s website the total was in the 3300 range. However, every time BLM publicly pronounces how many burros are left in the wild it is in excess of 6600, along with 34-35,000 horses. Independent analyses of how many horses are left on BLM lands, -often relying heavily on BLM’s own published numbers come out around half that number, ie. in the 17-18k range, or exactly half of BLM’s officially pronounced population estimate in the 34-35k range.
    One way to view this is that BLM always fudges the numbers and that there is no way of knowing how many horses or burros are left on BLM HMAs. Another way to look at it is to take them at their word, i.e. that they double whatever numbers they come up with on the ground -which themselves are not necessarily based on actual head counts. The point is that there seems to be a consistent pattern of BLM doubling whatever “hard numbers” they have -whatever they’re actually based on, whenever they publicly proclaim how many horses and burros are left in HMAs.
    The key thing to keep in mind is that BLM is often factoring in population increases of 20%/ year for each herd -regardless of what the true rate may be, whether 15, 10, 8 % or less. Hence even their hard numbers may be greatly inflated in many cases, particularly if they’ve mathematically increased herd populations by 20% year since the last roundup and actual head count of some sort. Hence, even their “hard numbers” may be inflated by 50% or more.
    The rule of thumb then is to divide by half whatever aggregate HMA numbers BLM provides, for starters, Then bear in mind that they’ve probably increased the last actual known population count of each herd by 20% year since it was last ‘gathered’. In other words even the 17-18k horses 35k divided by half) may still be as optimistic & unrealistic a number as 3300+ is for wild burros( 6600/ 2= 3300). In all probability then we’re probably looking at less than 20k wild horses and burros combined on BLM HMAs after correcting for BLM’s self-admitted multiplier and annual population increase formula. If I haven’t confused the issue, then you’ll finally be able to see the logic and consistency in the BLM’s ‘numbers game’ and also to come up with a more plausible population estimate for individual HMAs and also for all wild horses and wild burro herds combined.

  4. If anyone is dumb enough to think that a few thousand wild horses are destroying our public lands and that the millions of welfare cattle/sheep are not, then they’re just plain stupid. I’m in this for the long haul, and will not stop fighting until our wild horses & burros REMAIN on their rightful legal lands and ALL welfare cattle/sheep, mining & fracking are removed. I will not compromise with eco-sanctuaries and PZP.

  5. The BLM should be eliminated. Allow the growing number of environmental groups, rescue groups and societies receive the proper funding to preserve our wildlife and wild lands, including the Mustangs. Our government agencies are no longer trustworthy.

  6. It is maddening so to see that we as advocates have to take this time to produce reliable data from unreliable sources. Going back through history one finds multiple sources of scientific study of wild horses on the range. Unfortunately, we spend time addressing the invalidity of the information produced and picked up by the media to creaTe the publicity cloud over the public’s right to know the truth.

  7. The only over-population of Wild Horses and Burros is in the BLM HOLDING PENS!!!!!!! SET THE FREE!

  8. The BLM naturally skews the numbers because they have their own agenda for this land. If the horses are not lining someones pocket with dirty money then they need to go. This document needs to be shown to everyone’s congressmen and let them know that the public is sick of the BLM’s lies. We are tired of being ignored when the plain truth is staring us in the face.

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