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6 Population Models and Evaluation of Models
Pages 175-194

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From page 175...
... UTILITY OF POPULATION MODELS Models of population dynamics (hereafter, referred to as population models) are useful tools for understanding, explaining, and predicting the dynamics and persistence of biological populations.
From page 176...
... could backfire because populations of species characterized by early maturity and high fecundity may experience rapid growth after harvest or removal as a result of density-dependent overcompensation. Other examples of the application of population models include assessing the influences of culling and fertility control on the population dynamics of an overabundant elk population (Bradford and Hobbs, 2008)
From page 177...
... Gross concluded that management strategies based on removal and fertility control were most effective in achieving management goals but advocated strategies that rely less on removal and more on fertility control; he also highlighted the importance of management actions to delay age at first reproduction and increase generation length to reduce population growth. The model was somewhat unique in that it tracked individual animals throughout their lives and simulated breeding and genetic diversity.
From page 178...
... They concluded that the continued use of the current fertility-control strategy would further reduce the population growth rate, cause a major shift in age structure in favor of older animals, and lead to a low percentage of females that have reproductive opportunities. VORTEX was not developed specifically for horses, but it has many features that could be useful for modeling free-ranging horse population dynamics.
From page 179...
... , matrix models do not permit explicit consideration of such factors as allelic diversity, mating system, individual variation, and behavioral inter­ actions, which can affect free-ranging horse population dynamics. POPULATION-MODELING FRAMEWORK USED BY THE BUREAU OF LAND MANAGEMENT The committee was asked to evaluate the strengths and limitations of the population model used by BLM and the types of decisions that could be appropriately supported by the model.
From page 180...
... Under the assumptions of the model and given appropri ate data, WinEquus can adequately simulate horse population dynamics under alternative management actions (no treatment, removal, female fertility control, and both removal and female fertility control)
From page 181...
... , failed to provide any information regarding input parameter values or WinEquus options. The lack of relevant information regarding input or management parameters in gather plans or environmental assessments has attracted public attention.
From page 182...
... , of descriptions of the realized population growth rate during the simulation period, and of the numbers of horses gathered, removed, and treated with a fertility-­ control agent under alternative management actions. Most gather plans and environmental assessments, however, simply copied and pasted WinEquus output and gave no explanation or interpretation of the results being reported.
From page 183...
... data and relevant information regarding input parameters and WinEquus options, results of population-modeling exercises would be difficult for a critical reader to accept as pertinent and meaningful. Nonetheless, given appropriate data, WinEquus can be used to simulate free-ranging horse population dynamics without management interventions or under alternative management regimes that are available in the program (removal only, female fertility control only, and both removal and female fertility control)
From page 184...
... If BLM plans to use a population model for short-term population projection and to evaluate potential effects of the management alternatives (female fertility control, removal, or a combination of the two) , WinEquus is probably sufficient to support current needs.
From page 185...
... As described in Chapter 4, male fertility control, perhaps via such minimally invasive methods as chemical vasectomy, remains a viable management option. Fertility control that targets both males and females may be more effective in reducing population growth than a strategy that targets only one sex.
From page 186...
... Such a model could • Elucidate the basic processes operating in the Wild Horse and Burro Program and help to address BLM's current programmatic challenges. • Project the changes in the numbers of horses maintained in short-term and long term holding facilities and the budgets that would be required under current and potential future management alternatives.
From page 187...
... Ultimately, equid population dynamics are driven by forage abundance and forage dynamics. Forage abundance affects forage intake, which affects animal body condition, which then affects survival and foaling rates.
From page 188...
... A suitable modeling framework, or suite of models, would have to simulate life history; social behavior; mating system and genetics; forage limitation; use of forage, water, and space; and effects of alternative management actions throughout horse or burro life spans to meet the challenges outlined in the preceding paragraphs and to incorporate appropriately the factors and processes that influence free-ranging equid population dynamics. Possibly, different models could be used to address different aspects of the overall problem.
From page 189...
... When the same default datasets are used to model population dynamics of most or all HMAs or HMA complexes, results will necessarily be similar (give or take the effect of environmental stochasticity and initial age and sex structure)
From page 190...
... Virtually all population-modeling efforts under the auspices of BLM have been focused on HMAs or HMA complexes; a modeling study evaluating the entire free-ranging horse population on the range and in holding facilities was not available at the time the report was prepared. A comprehensive modeling study that evaluates population dynamics of horses in the western rangelands and in holding facilities and the costs and consequences of management alternatives, including those not currently available to BLM, would help in evaluating whether and to what extent stated management objectives are achievable under the current or projected funding situations and regulatory restrictions.
From page 191...
... 2007. Economic benefit of fertility control in wild horse populations.
From page 192...
... 1991. Feral horse fertility control: Potential and limitations.
From page 193...
... 1995. Population dynamics of a reintroduced Asiatic wild ass (Equus hemionus)


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