Furthermore, as discussed in Chapters 2 and 3, the management strategy of removing free-ranging horses and burros from the range leaves the animals that remain on the range unaffected by density-dependent population processes. Thus, population growth is not regulated by self-limiting pressures, such as lack of water or forage, and this allows horse, and possibly burro, populations to grow at an annual rate of 15-20 percent. Such successful herd productivity hampers BLM’s ability to keep population sizes within AMLs and affects the agency’s ability to maintain rangeland health.


Fortunately, tools that could help BLM to tackle many of those challenges already exist. Available improvements of common management practices on the range have been reviewed in this report and, if broadly and completely implemented, could address concerns about animal welfare and program expense. More immediately, they could help BLM to respond to two chief criticisms of the Wild Horse and Burro Program: unsubstantiated estimates for Herd Management Area (HMA) populations and of the population as a whole and lack of evidence that management decisions are informed by science. Addressing those issues could help increase public confidence in the agency.

Improving Population Estimates and Informing Management Actions with Science

Consistently conducted surveys of horse and burro populations that use scientifically sound methods of population estimation would substantially increase the credibility of the numbers reported by the Wild Horse and Burro Program. Improving the methods of horse and burro surveys was also called for by the National Research Council Committee on Wild and Free-Roaming Horses and Burros in its 1980 and 1982 reports. BLM has already taken a step in that direction through its collaboration with the U.S. Geological Survey (USGS). This cooperative work has demonstrated that survey methods available to BLM can increase the accuracy and quantify the uncertainty of population estimates.

Statistically rigorous and scientifically defensible estimates of demographic parameters and population sizes of horses and burros constitute essential data for any model that could project the outcome of different management decisions. As reviewed in Chapters 3 and 6, the absence of such data limits the applicability of modeled outcomes projected by WinEquus because the input parameters used in the model are most likely based on default datasets available within WinEquus rather than on the specific population being modeled. It is unknown whether the default datasets are representative of other horse herds or even of the populations studied, given that the default parameters were estimated from data collected more than 2 decades ago. There are no representative population data on burros. Inaccurate data on demographic and management parameters and population size and structure undermine the relevance of modeling effects of management decisions. Similarly, it undercuts efforts to develop forage production estimates made using forage utilization data as recommended in the Wild Horses and Burros Management Handbook.

In addition to more accurate demographic and population-size data, the Wild Horse and Burro Program would benefit from a more comprehensive model or suite of models. WinEquus can capture effects of contracepting mares, changing the sex ratio, or removing animals from the range, but it cannot model the implications of contracepting males, forecast the effects of management decisions on genetic diversity, or link the effect of climatic variability on forage availability with survival and reproductive success. It also lacks

The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement