Where N t + 1 is the population size in the next year or generation, N t is the population size in the current year or generation, R is the maximum rate of increase per year or generation, and K is the carrying capacity. The discrete form of the equation defines the population increment over an interval of time, such as a year, and is expressed as Where dN/dt is the instantaneous rate of change in N, N is the size of the population (number of individuals), r is the intrinsic rate of natural increase, and K is the carrying capacity, that is, the maximum population size that the environment can support as affected by resource abundance. Mechanisms may include competition for resources among members of the same species at high densities (Ginzburg, 1986 Berryman, 2003), complex social behaviors (Wynne-Edwards, 1965), and combinations of physiological responses to social cues (Wolff, 1997).ĭensity dependence can be seen most easily by examining the S-shaped curve of population size changing over time described by the logistic equation
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It is a general principle of ecology that populations do not continue to grow indefinitely, but the mechanisms of reduction in growth as densities increase are not always well understood (Flux, 2001). This chapter examines the changes in population processes of free-ranging equids due to density-dependent, density-independent, predation, and management factors. Population growth can also be affected by dispersal, immigration and emigration, and management factors, such as removal of animals from the range and contraception. Population size can be reduced by predation, and predator abundance is affected by prey abundance. Their numbers are not controlled by removals or contraception.įIGURE 3-1 Population processes, including density-independent and density-dependent controls. Climatic conditions also affect resource abundance, for example, through effects on forage production.ġ Unmanaged populations of horses and burros are not domestic animals, and they are not fed or given veterinary care. Natality and mortality can be affected by climatic conditions through direct effects on animals. Population processes are also altered by density-independent factors, particularly climatic conditions and variations. As density increases, population growth rate decreases because of increased competition for resources. That is, population processes-such as population growth rates, age-specific survival rates, natality, and age of bearing first offspring (primiparity)-must be density-dependent.
![download born to be wild 1995 world download born to be wild 1995 world](https://i.ytimg.com/vi/chDeyAHI4ck/hqdefault.jpg)
Indirect responses could include increased fertility, foal survival, or adult survival due to reduced competition for forage.įor self-limitation to occur, it is necessary for population processes to respond to population density ( Figure 3-1). Compensatory reproduction is defined as an increase in reproduction as a direct or indirect consequence of management reductions, including removals and contraception. The committee was also asked to assess whether there is compensatory reproduction as a result of population-size control, such as fertility control or removal from Herd Management Areas (HMAs). The underlying functions include changes in natality and survival in response to environmental variables that affect forage availability, such as weather and population density. Population growth and self-limitation are population processes in the sense that they involve a suite of underlying functions that lead to the result. The Bureau of Land Management (BLM) asked the committee to address the following questions as part of the discussion of potential rates of horse and burro population growth: Would free-ranging horse and burro populations self-limit if they were not controlled? If so, what indicators (such as rangeland condition, animal condition, and health) would be present at the point of self-limitation? To address those questions, it is necessary to review the factors that limit population growth in an unmanaged population 1 and that determine free-ranging horse and burro population growth and dynamics aside from management removals.