The measure of recruitment is critically dependent on the life history stage at which death and/or emigration of individuals is assessed. To illustrate this important point, we will consider for example a population of a large mammal in a temperate ecosystem. The births are highly synchronized over a short yearly pulse, producing one new cohort each spring. The 'life cycle graph' (Figure 1) describes the different stages that individuals may experience from birth to death. In our example, individuals may survive from birth to weaning (summer survival of juveniles), from weaning to i year of age (winter survival of juveniles), from i to 2 years of age (yearling survival), then each year from 2 to 7 years of age (prime-aged adult survival), and finally each year from 8 to the maximum longevity (senescent adult survival). Each cohort is the sum of newborns produced by 2-year-olds (primiparous females), and prime-aged and
PAS ~ PAS Prime-aged adults
Figure 1 Life-cycle graph of a population of a temperate large mammal: (1) newborns, (2) weaned young, (3) yearlings, (4) 2-year-olds, (5) prime-aged adults, (6) senescent adults (older than 7 years). Straight lines indicate transitions (survival) from one age group to the next; curved lines indicate reproduction. These demographic rates describe the development of individuals through the life cycle. SSJ, summer survival of juveniles; WSJ, winter survival of juveniles; YS, yearling survival; PAS, prime-aged adult survival; SS, senescent survival; FY, fecundity of yearlings; F2, fecundity of 2-year-old females; FA, fecundity of prime-aged females; FS, fecundity of senescent females.
senescent females (mostly multiparous females). Recruitment can then be measured as the total number of newborns, the number of weaned juveniles (newborns that survive their first summer), the number of yearlings (newborns surviving to 1 year of age), or the number of 2-year-olds (newborns surviving to the age of primiparity). The choice of a given measure of recruitment will depend on the context of the study. For instance, in species with parental care, separating out costs of reproduction might be a relevant issue. Two traits determine the number of newborns produced by a given age class: the proportion of reproducing females and litter size. Let us consider a idealized population with neither emigration nor immigration. This population includes 100 adult females (i.e., potential breeders) of which 15 of 20 2-year-olds and all 80 older females give birth to an average of 1.8 newborns. Independent of maternal age, summer juvenile survival is 0.65, winter juvenile survival is 0.85, and yearling survival is 0.92. Those reproductive and survival rates would lead to the recruitment of 171 newborns, 111 weaned juveniles, 94 1-year-olds, and 87 2-year-olds. Consequently, our estimate of recruitment varies twofold depending on the stage at which it is assessed. It is therefore important to describe recruitment units (e.g., newborns surviving to a given number of months of age). In this example, the stage at which proportionally most mortality occurs is summer juvenile survival so that the weaning period is the dominant filter in the life cycle. As a rule of thumb, to assess the importance of recruitment for population dynamics, it should be measured at the end of the life cycle stage with the greatest mortality.
Variation in Recruitment: Causes and Magnitude
The definition of recruitment is critically important to assess not only which factors affect it but also the magnitude of its variability over space and time. In plants and marine organisms, dispersal plays a prominent role in recruitment. Even in marine ecosystems often considered as open systems in which recruitment is from outside the local population, a larger than expected proportion of juveniles may return to their natal population ('self-recruitment'). Obviously, sampling problems currently impede progress in the assessment of the impact of dispersing juveniles on population dynamics of marine organisms.
In most studies, independent of the taxon considered, recruitment has been reported to vary substantially in time and space in response to a large array of environmental and maternal factors. Thus, at birth, propagule (either larvae, eggs, seeds, or newborns) size strongly influences recruitment measured later on (e.g., at the end of the juvenile stage) in most studies, and, as a general rule at the intraspecific level, producing heavier/larger propagules is better. Similarly, between-cohort variation in the timing of births often accounts for variation in recruitment measured later on. While the relative advantage of being born early or late can be context specific, some general patterns can be identified. For instance, for most vertebrates of temperate ecosystems, those born early in the season are often more likely to recruit into the next life history stage than those born late.
Environmental conditions often drive variation in recruitment. In most case studies, the amount of predation, climatic conditions (e.g., drought for plants or terrestrial animals, temperature in marine organisms, reduced light for trees), physical and chemical properties of the ecosystem (e.g., exposition to chemical inducers that reduce growth and survival of juveniles), the prevalence of various diseases, population density, habitat quality, the availability of refuges, and the intensity of interspecific competition all markedly influence recruitment. Most of the time, several environmental factors interplay to shape recruitment, and their interaction can either increase or dampen recruitment variation. For instance, in flatfish, variation in habitat quality has been reported to lower rather than to increase recruitment variability. Besides these ecological sources of variation, recruitment can also vary as a direct consequence of the life history strategy. For instance, the high fecundity of weakly developed propagules leads to a random variation in mortality rates during early life history stages, and provides thereby the raw material for large fluctuations of recruitment in space and time, as illustrated by many exploited marine species. All empirical studies on annual plants, trees, marine invertebrates, and terrestrial or marine vertebrates have underlined that recruitment is a complex process involving the interaction ofbiotic and abiotic factors which operate at different temporal and spatial scales.
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