Life-History Trade-offs and How Reproduction Is Allocated

Suppose two animals obtain different amounts of food. The animal with more food might both grow faster and produce more offspring. That observation compares resource acquisition as well as resource allocation.

An allocation trade-off asks a different question: what happens when an organism divides a fixed available supply? If growth and reproduction use that supply, increasing one allocation leaves less for the other. A positive association between growth and reproduction across well-fed and poorly fed individuals does not disprove that constraint.

Life-history comparisons also need reproductive returns. Earlier breeding avoids some risk of dying before reproduction. Waiting can allow growth that increases later output, but some individuals die before obtaining that benefit. A prediction about timing must compare both consequences. Low adult survival by itself does not establish brood size or the number of reproductive episodes.

A cohort contains individuals born in the same period. Survivorship records the number or proportion of that cohort still alive at each age. Subtract consecutive survivor counts to find deaths during an interval.

Type I describes relatively low early mortality followed by higher late mortality. Type III describes high early mortality followed by lower mortality among later survivors. Type II means a constant probability of death over equal age intervals. For example, counts of 64, 32, 16 and 8 halve each interval. The losses are 32, 16 and 8, so they are not equal numbers.

Constant-proportion survival gives a straight declining curve when the survivor axis is logarithmic. It does not give a straight line on an ordinary linear survivor axis. Always check the axes before interpreting shape. These patterns summarize survival; they do not establish parental care or identify causes of death. The historical r/K categories describe broad associations, not a rule that determines every trait from one curve.

Consider a hypothetical fixed budget of 30 units. A brood of five young receives six units per young, while a brood of fifteen receives two. This arithmetic establishes investment per young, not which brood leaves more survivors.

To compare expected survivors, multiply offspring number by each young's survival probability. If the probabilities are 0.8 for the five-young brood and 0.2 for the fifteen-young brood, their expectations are four and three survivors. These are averages over repeated comparable broods, not guaranteed counts in any one brood.

The result depends on the stated survival probabilities. A larger investment may improve survival, but that improvement must be large enough to compensate for producing fewer young. Neither a larger brood nor greater investment per young always gives the higher total. Other comparisons may also need the parent's survival and future reproduction.

Semelparity describes one reproductive episode during an individual's lifetime. Iteroparity describes repeated episodes. Chinook salmon spawn once and then die, while many birds and mammals can breed repeatedly. The Chinook example does not describe every salmon species, and a large brood does not define semelparity.

Repeated reproduction can make survival to another breeding opportunity valuable. Whether a single or repeated schedule is favoured depends on survival and the returns from reproductive effort.

Life tables record survival and reproduction by age. A rise in deaths after first breeding is consistent with a reproductive cost, but it does not prove the cause. Age, disease or seasonal hazards may change at the same time. A stronger test compares otherwise similar groups assigned different reproductive effort, using controls and repeated measurements. The conclusion must match the effort changed and the survival outcome measured.