Life Histories

A life history encompasses all events affecting an organism's survival and reproduction, including birth, development, maturation, reproduction, and death. The pattern of these events varies significantly across species, even among organisms sharing the same habitat. For example, small snails mature quickly and produce thousands of eggs, while howler monkeys mature slowly and have few offspring over long lifespans.

R-strategists produce many offspring with minimal parental care, relying on high reproduction rates to ensure some survive. K-strategists produce fewer offspring but invest substantial parental care to maximize survival chances. R-strategists thrive in unpredictable environments and mature early, while K-strategists prosper in stable environments. Most organisms lie somewhere on a continuum between these strategies rather than being strict r- or K-strategists.

A Type III survivorship curve, characteristic of r-strategists, shows high mortality early in life but long lifespans for survivors. Species like small snails and marine invertebrates display this pattern. Individuals that survive early developmental stages have much higher chances of reaching adulthood, reflecting the trade-off between producing many offspring and providing minimal parental care.

Type I curves, shown by K-strategists like primates and humans, display high survival during early and middle life with mortality concentrated in old age. Type II curves show equal survival and mortality probability throughout an organism's lifespan, forming a steady diagonal line. Songbirds exemplify Type II, facing similar threats from predators and disease at every age, unlike Type III's early-life bottleneck.

Organisms have limited energy and resources, requiring compromise between producing many offspring versus investing heavily in each one's survival. This fundamental constraint shapes reproductive strategies: r-strategists maximize offspring number with minimal care, while K-strategists reduce offspring number but increase parental support. These trade-offs directly influence survivorship patterns and population growth potential.

Survivorship curves display the percentage of a population surviving at different age intervals, revealing mortality patterns across lifespans. Combined with age structure diagrams showing population proportions at each age, these tools predict whether populations will grow, remain stable, or shrink. Populations with more young females and favorable survivorship curves are expected to grow more rapidly.

Most organisms are not strict r- or K-strategists but lie on a continuum of these traits. Sea turtles exemplify this flexibility, possessing K-strategy traits like long lifespans and strong competitiveness alongside r-strategy traits like producing many offspring with minimal parental care. Species may also display different survivorship patterns across life stages, such as Type III juvenile survivorship transitioning to Type II adult survivorship.