Life-History Trade-offs and How Reproduction Is Allocated
An organism cannot grow, survive and reproduce at full rate at once, so life histories are compromises: few large offspring or many small ones, early breeding or late. Where a species sits is predicted by how risky its environment is for juveniles and adults.
What a learner can do afterwards
- Explains a trade-off in terms of a shared limited budget rather than as a preference.
- Predicts the life history favoured when adult survival is unpredictable.
- Reads a survivorship curve as a statement about where mortality falls.
1 · Read
Every organism spends from one limited lifetime budget, so investing in current young means spending less on growth, survival, or future broods. That shared budget is what makes a trade-off real rather than a mere preference. Selection favors the schedule whose lifetime tally of surviving offspring runs largest.
A survivorship curve plots how many of a cohort remain alive at each age, and its shape names where death falls. A steep early drop means most young die while the few survivors live on. A steady diagonal means death strikes evenly, and a flat start with a late cliff means almost everyone survives until old age. Salmon pack everything into one giant burst then die, while most birds and mammals spread smaller efforts across many seasons.
Fecundity itself trades against offspring size and care: a fixed reproductive budget divided among more young leaves less per offspring. Life tables track survival and births age by age, giving the net reproductive rate and generation time. When deaths pile up right after first breeding, the cost of reproduction is written in the data.
Read the mortality pattern first, then predict the schedule. Where adult survival is chancy, breed early and often rather than saving for a future that may never come. Where adults are safe but young face fierce competition, hold back and invest heavily in a few young.
Life histories spend one limited budget, so risky adults favor early breeding while safe adults favor heavy investment in few young.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.