In its most general sense, 'recruitment' refers to the process of adding new individuals to a population or to a specific segment of that population (e.g., breeding or harvestable individuals) through reproduction, immigration, and stocking. Recruitment can be measured in numbers, biomass, or rate. In the context of population dynamics, recruitment includes both processes that add individuals to a given population: the birth and the immigration. Recruitment is consequently opposed by the two processes that measure loss of individuals from a population: the mortality and the emigration. The difference between additions and losses determines 'net recruitment'.
For a birth pulse population censused just before births, we can thus simply write the transition from a population of size Nt in year t to one of size Nt +i in year t + i as
which is equivalent to
- emigrati°n[t t+ij = Nt+i [1'] which is equivalent to
Equation [i"] can be arranged to demonstrate that the net recruitment rate in a given year (net recruitment^ + i]/Nt +1) is equal to the 'per capita change' (A — i) where A (or Nt + i/Nt) is the 'natural rate of increase' of the population. Finally, note that the natural log of A corresponds to r, the 'exponential growth rate' of the population or 'Malthusian growth rate'. In most studies ofpopulation dynamics, the 'per capita change' (A — i), the natural rate of increase (A), and the 'exponential growth rate' (r) are all used to measure the 'population growth rate'. Recruitment is thus central to population dynamics.
Most studies of population dynamics, however, use a more restricted definition of recruitment, the number of births that occur within a pulse minus the number of deaths and emigrations of those individuals before the next birth pulse. The estimate of recruitment thus crucially depends on the life cycle stage at which death and emigration are measured, so that some standardization is required when performing comparative analyses across populations or species. As the number of births within a given pulse corresponds to a 'cohort', the recruitment rate of a closed population (i.e., without immigration or emigration) corresponds to the proportion of individuals from a given cohort that are added to the population at some specific stage.
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