This article examines how BLM estimates the number of burros on the range and how many to remove. BLM has never disclosed the formula used to determine that less than 3000 wild burros is all that the western U.S. can sustain.
At a time when burro populations worldwide are plummeting due to the (often) illegal trade in hides to fuel the Ejaio industry, BLM needs to move out of the dark ages, use a bit of science and common sense and actually begin to protect our treasured long ears. (More on the worldwide crisis HERE)
Fuzzy Math (or sometimes an advocate just needs to vent about the absurdity of BLM “growth rate” calculations) Article by Laurie Ford, WHE Subject Matter Expert
On March 1, 2023, the Bureau of Land Management (BLM) will publish their annual herd statistics that will lead to a certain number of burros being removed from a very uncertain population size existing in the few remaining Herd Management Areas (HMA) and Herd Areas (HA) where they now live. These annual “statistics reports” are the key BLM uses in projecting the future population sizes that then determine where removals will take place and how many will be removed.
These numbers are derived from two sources – the analysis of aerial survey data and the application of annual growth rates that predetermine the number of foals that will be born through February 28 of the following year.
Extra: You can read more in our analysis of the 2022 report
How it works (or doesn’t)
On March 1, the designated birthday for every burro (and horse) on the range, the total number of adults, including the previous years’ foals who are now considered an adult if weaned, are multiplied by a standardized past growth rates of 15-20% (burros) to provide the current estimated population.
For example, if a burro is born in September, 2022, they will be aged at one years old on March 1, 2023, and included in the annual growth rate equation from that point forward despite not actually being capable of reproducing until September, 2025. Even at age three there is about a 20-25% chance of conceiving and giving birth – increasing to a 42-26% chance the following year.
Population sizes are also adjusted after a roundup takes place when the specialist can simply subtract the number of adults and foals removed from the pre-gather population estimate to find the estimated number of animals that remain.
Yet, despite having been included as a foal “yet to be born” in the pre- gather estimate those foals born in holding after the roundup are never accounted for even though they were removed from the same population as the foals who were rounded up and subtracted from the pre-gather number.
To obtain more accurate population numbers the USGS study recommended that the BLM consider the age and sex distribution, and age-specific reproductive and survival rates because they may reflect the most realistic expectations of population change from year to year.
Instead, the BLM continues to focus on projecting foal births – targeting foals for removal before they are even born – and insisting that 95% survive to yearling age – a percentage shown to be incorrect by scientific literature and questionable through simple observation. (The ratio of foals to yearlings on the range is not equal and while observing roundups I have seen far fewer yearlings despite their tendency to stay with their mothers’ “herd” after weaning.)
While the annual birth rate may fluctuate around 20% of the total HMA population size, if only 50% of foals survive the growth would drop immediately to 10%. This growth rate would drop even further if the BLM didn’t ignore those foals born during the year in holding – their numbers often equal to, or greater than, those captured during a roundup. After the recent Black Mountain Burro roundup more foals were born in holding than captured.
After a roundup these foal births in holding are rarely documented nor is there much effort to “age” any of the captured burros properly. Yes, they receive minimal veterinarian care but that does not include close examinations to accurately determine age. A recent Freedom of Information Act (FOIA) attempting to obtain records of burros between the ages of 8 months and 2 years removed during the Blue Wing Complex roundup received the response that, out of 800 burros, “none meets these criteria”. (Active litigation at Blue Wing)
Arizona seems to be the exception. FOIA found most of the Black Mountain burros gathered this past year were younger – few surpassing 12 years old – which brings light to another BLM fallacy that many wild burros live well into their 20s and 30s. A 2020 FOIA response from Utah, which did include an age specific inventory, showed only 7.8% of 895 burros as being over the age of 12 with only a handful upwards into their 20s.
Sale authority burros must be “three strikers” or age 10 or older, yet I have obtained such burros – all recently rounded up and documented as being over 15 – who were all later aged by a veterinarian to be under 8 years old. (Burros are not horses; it appears that BLM does not address these differences.)
According to the Standard Operating Procedures for Wild Horse and Burro Double-Observer Aerial Surveys (SOP), not only does the BLM have an incomplete understanding of the average expected annual growth rate for most HMAs and HAs but little knowledge regarding how annual growth rates respond to such factors as annual variation in climate, the relative density of horses and burros, and other stochastic (random) factors. Also dismissed are the disturbances created by the increasing multiple use of resources within the HMAs which can impact and alter herd growth from year to year. Wild burro habitat is often located in regions with unregulated accelerating year-round outdoor recreation that not only effects the burros but the land they are scapegoated for destroying.
As the BLM continues to justify removals to reach AML by claiming “climate change and extreme drought continue to impact the west” it appears that neither of these have any effect on birth or survival rates in their calculations.
After analyzing the data from a November, 2021, aerial survey of the Black Mountain Burro HMA the BLM increased the “count” from the previous estimate of 2012 to 2976 – almost a 30% increase. The previous 2014 aerial survey had only photographed 72 burros over nine days yet flight notes recorded 1378 as being counted without any additional supporting images.
It is a dangerous balancing act the BLM is performing to keep their unscientific 10-year gather plans alive and well and fully funded. The annual herd numbers must be high enough to support the concept of overpopulation but must also show that progress is being made towards reaching AML. It requires a bit of tweaking now and then – modifications in aerial survey analysis where as much as possible, BLM is moving toward obtaining estimates of abundance from surveys, by using survey methods that account for and estimate the number of horses present, but not seen by any observer (SOP) and, rather than using a standard growth rate, inserting undefined parameter values in the population growth model that often result in biologically impossible growth – sometimes over 200% – but is allowed to play a role in future projections indefinitely.
While The BLM continues to ignore a key point found in their own current directives that accurate and repeatable population surveys are vital to inform the BLM of herd size and to evaluate the outcomes of management actions they also, simultaneously, make the unsubstantiated assertion that burro numbers persist in being underestimated.
It could be the numbers on the spreadsheet are the only thing increasing – not those actually on the range.
In 2020 the BLM Advisory Board recommended that the differences in horses and burros be identified and that the BLM should expand the programs capabilities to manage burro populations humanely and appropriately based on these differences.
Not only do these differences need to be acknowledged in population modeling, but in all aspects of burro management including how roundups are being conducted and fertility control might be administered in the future.
This issue, in conjunction with a diminishing gene pool that continues to weaken the immune systems and antibody levels of our burros – affecting their ability to respond to outside pathogen exposures after capture – endangers the health and survival of our wild burros both on and off the range.
More: Burros and the diminishing U.S. populations amid a worldwide crisis
Standard Operating Procedures for Wild Horse and Burro Double-Observer Aerial Surveys (2020) Standard Operating Procedure 7—Principles for Projecting Population Size
National Academy of Science Using Science to Improve the BLM Wild Horse and Burro Program chapter 6 Population Models and Evaluation of Models
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