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18 May 2023 Exploring Wild Horse Population Dynamics on US Public Rangelands Using a Simple Systems Simulation Model
Scott Crozier, John Worthington, Makenzie Wright, Lane Michna, Benjamin L. Turner
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Public rangeland managers face mounting pressure to resolve wild horse (Equus ferus caballus) population growth in the western United States, whose escalated ecological impact has alarmed numerous stakeholders. Unfortunately, long-term solutions have remained elusive as approved control methods, limited in breadth and scope as a result of legislative changes, hamstring managers and provide only limited, short-term reduction potential (e.g., drylot confinement, adoption, short-term fertility treatments). Management flexibility is further constrained due to costs of confinement (> 50% of the Bureau of Land Management's wild horse and burro program budget), constraints on per-person horse adoptions, and labor intensity of fertility treatments given required follow-up vaccination, which all place significant burdens on federal budgets and staff. To better understand the wild horse population problem, a system dynamics model was constructed linking population levels with current and historical control strategies. For the population to reach balance with appropriate management levels (AMLs) and assuming horse harvest bans and confinement capacities do not change, annual adoption rates of ≈15 000 head or annual short-term fertility control of 40% of mature horses would be required for 38–50 yr, while grazing lease buyouts would only alleviate internal pressure on desired removals. Combinations of increased adoptions and fertility controls balanced the population with AMLs between 16 and 38 yr. Should horse harvest bans be lifted and permanent fertility control become feasible, balance with AML could be achieved in 9–25 yr depending on time of implementation and rate of use. Recognizing mental models underlying various policy priorities will help improve coalition building. Finding ways to reward management creativity that accelerates ecosystem health improvement efforts within the various constraints on public agencies and stakeholders will shorten time delays between implementation of long-term resolutions and observing the results of such effort.

Scott Crozier, John Worthington, Makenzie Wright, Lane Michna, and Benjamin L. Turner "Exploring Wild Horse Population Dynamics on US Public Rangelands Using a Simple Systems Simulation Model," Rangeland Ecology and Management 88(1), 47-61, (18 May 2023).
Received: 21 June 2022; Accepted: 11 February 2023; Published: 18 May 2023
Dynamic modeling
mental models
public land
systems thinking
wild horses
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