BLM Program Support

ESTIMATING HERD SIZE AND POPULATION MODELING

Estimating Herd Size and Population ModelingEstimating Herd Size and Population Modeling

Wild horse and burro herd size estimates are essential in management decisions. To make those estimates, the BLM prioritizes personnel safety, and using methods that have been scientifically validated as being accurate and precise. Today, simultaneous double-observer aerial surveys are the most commonly used method.

Population size can be projected based on estimates from surveys, estimates of annual growth rates, and records about any removals and fertility control applications.

Comparing the expected outcomes of different possible management alternatives can be informative. USGS has a new horse population and cost projection model, ‘PopEquus,’ which should be available in late 2022. An older horse population modeling software called ‘WinEquus’ is not as flexible or easy to use.

BLM Contact: Michelle Crabb, HQ-261 wild horse and burro population biologist. Michelle helps field, district, and state offices with herd management area & herd area survey planning, training, in-flight assistance, analyses, interpretation, and population projections and modeling.

General References

Natural Resources Council of the National Academies of Sciences (NAS). 2013. Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward. See Chapter 2: “Estimating population size and growth rates.” Pages 37-72.

Simultaneous Double-observer Method

Griffin, P. C., L.S. Ekernas, K.A. Schoenecker, and B. C. Lubow. 2020. Standard Operating Procedures for wild horse and burro double-observer aerial surveys. U.S. Geological Survey Techniques and Methods, book 2, chap. A16, 76 p., https://doi.org/10.3133/tm2A16. See “Standard Operating Procedure 1—Conducting Aerial Surveys with the Simultaneous Double-Observer Method.”

Ekernas, L. S., and B. C. Lubow. 2019. R script to analyze wild horse and burro double-observer aerial surveys. USGS Software Release.

Lubow, B. C., and J. I. Ransom. 2016. Practical bias correction in aerial surveys of large mammals: validation of hybrid double-observer with sightability method against known abundance of feral horse (Equus caballus) populations. PLoS-ONE 11(5):e0154902. doi:10.1371/journal.pone.0154902.

Hennig, J.D., K.A. Schoenecker, J.W. Cain, G.W. Roemer, and J.L. Laake. 2022. Accounting for residual heterogeneity in double-observer sightability models to decrease bias in feral burro abundance estimates. Journal of Wildlife Management 2022;e22239.

Photo Mark-resight Method

Lubow, B. C. and Jason I. Ransom. 2009. Validating aerial photographic mark-recapture for naturally marked feral horses. Journal of Wildlife Management 73(8):1420-1429.

Thermal Infrared Surveys

Schoenecker, K.A., P.F. Doherty, J.S. Hourt, and J.P. Romero, J.P. 2018. Testing infrared camera surveys and distance analyses to estimate feral horse abundance in a known population. Wildlife Society Bulletin 42:452-459.

Estimating Population Size from Fecal DNA

Schoenecker, K.A., S.R. King, L.S. Ekernas, and S.J. Oyler‐McCance. 2021. Using fecal DNA and closed‐capture models to estimate feral horse population size. Journal of Wildlife Management 85:1150-1161

SOPs for Projecting March 1 Herd Size Estimates

Annual projections made following the guidelines in SOP 7 of the reference below should reflect the approximate numbers of horses and burros present for the herd that uses each HMA, standardized to represent the expected abundance on March 1 of each year. In most cases, aerial surveys represent the best methods for estimation of abundance at the time of survey. In some cases, a gather operation after an aerial survey will provide a more recent approximation of the abundance, taking into account a survey-based estimate, the known numbers of animals removed, and the known numbers of animals returned to the range. However, several months or years may transpire between the time of those surveys or gathers and the annual March 1 reporting date. Abundance projections should account for the most reliable values for past abundance, estimates of local annual population growth rates, and recorded numbers of animals removed at different times. An “estimate” as the expected value of a given population size or growth rate based on the survey data and analysis – an estimate is not a guess.

Griffin, P. C., L.S. Ekernas, K.A. Schoenecker, and B. C. Lubow. 2020. Standard Operating Procedures for wild horse and burro double-observer aerial surveys. U.S. Geological Survey Techniques and Methods, book 2, chap. A16, 76 p., https://doi.org/10.3133/tm2A16. See “Standard Operating Procedure 7—Principles for Projecting Population Size.”

Modeling Different Management Alternatives

PopEquus is a predictive population modeling tool for wild horses from US Geological Survey (USGS). The model simulates how different management actions influence the size of horse populations, numbers of animals captured, treated, and removed, on-range costs, and off-range costs. Users can specify features of a horse population and select from among six management actions to be applied to the population, either alone or in combination with other actions. The model then projects how population size might be expected to change through time. PopEquus results can suggest relative costs and benefits of different alternatives. However, the model does not reflect any ecological conditions or local constraints and opportunities. As of January 2023, the model is only for wild horses; updates with wild burros and possibly including some marginal effects of predation are planned for later in 2023.

Folt, B. and K. Schoenecker. PopEquus 1.0.0. This is a population and cost projection tool that allows side-by-side comparisons of various alternatives, with combinations of removals, and fertility control treatments. The model also provides approximate on-range and off-range costs. Link to online access here.

There is also an older population model, called “WinEquus.” Alternatives are limited, and the user interface is not so intuitive. The software is not available online any more, but can be shared within BLM; Contact Paul Griffin or Michelle Crabb for a copy if needed. The reference for WinEquus is:

Jenkins, S.H. 1996. Wild Horse Population Model. Version 3.2 User’s Guide. University of Nevada Reno, Department of Biology.

General Reference on Population Dynamics

Natural Resources Council of the National Academies of Sciences (NAS). 2013. Using Science to Improve the BLM Wild Horse and Burro Program: A Way Forward. See Chapter 6: “Population models and evaluation of models.” Pages 201-222.