relational database that is accessible to and used by all field offices for recording all pertinent population survey data.

FINDING: On the basis of the information provided to the committee, the statistics on the national population size cannot be considered scientifically rigorous.

The links between the statistics on the national population size and actual population surveys, which are the foundational data of all estimates, are obscure. The procedures used for developing annual HMA population-size estimates from counts are not standardized and often not documented. Therefore, it seems that the national statistics are the product of hundreds of subjective, probably independent, judgments and assumptions by range managers and administrators about the proportion of animals counted during surveys, population growth rates, effects of management interventions, and potential animal movements between HMAs.

Development and use of a uniform and centralized relational database, which captures all inventory and removal data generated at the level of the field offices and animal processing and holding facilities, to generate annual program-wide statistics would provide a clear connection between the data collected and the reported statistics. The committee also suggests that the survey data at the HMA level and procedures used to modify the survey data to generate population estimates be made readily available to the public to improve transparency and public trust in the management program.

In the committee’s judgment, the reported annual population statistics are probably underestimates of the actual number of equids on the range inasmuch as most of the individual HMA population estimates are based on the assumption that all animals are detected and counted in population surveys. A large body of scientific literature on techniques for inventorying horses and other large mammals clearly refutes that assumption and suggests that the proportion of animals missed on surveys ranges from 10 to 50 percent. An earlier National Research Council committee and the Government Accountability Office also concluded that reported statistics were underestimates.

FINDING: Horse populations are growing at 15-20 percent a year.

The committee concluded that the age-structure data on horses removed from the range can provide a reasonable assessment of the general growth rate of the free-ranging horse populations in the western United States. The population growth rate index derived from those data is generally consistent with the herd-specific population growth rates reported in the literature. On the basis of the published literature and the additional management data reviewed by the committee, the committee concluded that most free-ranging horse populations managed by BLM are probably growing at 15-20 percent a year.

FINDING: Management practices are facilitating high horse population growth rates.

Free-ranging horse populations are growing at high rates because their numbers are held below levels affected by food limitation and density dependence. In population ecology, density dependence refers to the influence of density on such population processes as population growth, age-specific survival, and natality. Effects of increased population density are manifested through such changes as reductions in pregnancy, fecundity, percentage of females lactating, young-to-female ratios, and survival rates. Regularly removing horses



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