A particular fecundity pattern evolves as an adaptation to the specific constraints imposed by the niche exploited by a species. For example, the high fecundity of parasites and many marine organisms is thought to have evolved because the probability that any particular individual will establish itself and reproduce successfully is low. As such, life-history theory predicts that the degree to which parents place themselves or their offspring at risk of mortality when threatened with predation depends on the offspring number and survival probability of parents. This type of tradeoff will also vary between the sexes in relation to the degree of investment in the offspring. A species with low adult survival and high fecundity should tolerate greater risk from predation given the low probability of surviving to reproduce in the future. In contrast, species with high adult survival and lower fecundity should tolerate less risk even at a cost to their offspring. This fundamental tradeoff essentially expresses an organism's capacity to maximize its potential lifetime reproductive success by assessing the best strategy to achieve the highest potential fecundity. Thus, tradeoffs are the benefits accrued from one life-history process that are purchased at the expense of another. For example, pre-breeding migrations requiring the use of stored resources can result in less energy available for the production of offspring. Other processes that present an individual with additional energy costs can also affect reproductive output, including exposure to antigens or parasites that increase the costs associated with mounting an immune reaction.
This introduces the concept of 'reproductive allocation' (or 'reproductive effort') which is the proportion of an individual's acquired resources that is allocated to reproduction over a defined interval of time. Because resources are finite, reproduction itself can incur a cost in that it can reduce the survival probability or growth rate of the individual. As such, the rules governing reproductive allocation are viewed in a benefit-cost framework. Dimensionless ratios of the benefits and costs of reproduction, such as the ratio of the life span spent in reproduction (E) to the length of the pre-reproductive period (a), and the ratio of the fraction of adult mass (m) devoted to reproduction (R/m) to the inverse of the life span spent in reproduction (E_1), provide useful indices to classify life histories because they tend to be invariant within major taxonomic groups.
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