Life Histories

Constrained by limited energy and resources, organisms must compromise between offspring quantity and parental investment. This trade-off is represented by two primary reproductive strategies; K-strategists produce few offspring but provide substantial parental support, whereas r-strategists produce much progeny that receives little care. These strategies are related to an organism’s survival likelihood across its lifespan, which is represented by a survivorship curve. Three general types of survivorship curves are exhibited by organisms that: tend to live long lives (Type I, K-strategists); are equally likely to die at all ages (Type II); or have high early mortality rates, but long lifespans if they survive into adulthood (Type III, r-strategists).

Life History Strategies Reflect a Trade-Off between Offspring “Quantity” and “Quality”

An organism’s life history includes all the events occurring across its lifespan, including birth, development, sexual maturation, reproduction, and death. Trade-offs involving the patterns and timing of life history events (notably survival and reproduction) across different ages and developmental stages represent different life history strategies. R-strategists and K-strategists make distinct reproductive compromises between the number of offspring and level of parental care, or offspring quantity versus quality. R-strategists (r for rate) tend to have high fecundity (reproductive potential) and produce many offspring that are widely dispersed, receive little parental care, and have low early survival rates. R-strategists typically thrive in unpredictable environments, mature early and have small bodies. K-strategists (K for Kapazitätsgrenze, or capacity-limit in German) produce fewer offspring and devote more parental care than r-strategists, and their progeny thus have higher early survival rates. K-strategists prosper in stable environments. Importantly, most organisms are not strict r- or K- strategists, but lie somewhere on a continuum of these traits. For example, sea turtles have long lifespans and strong competitiveness (K traits), yet produce many offspring that receive little parental care (r traits).

Population Survival and Growth Can Be Visualized on Survivorship Curves and Age Structure Diagrams

A survivorship curve displays the percentage of a population surviving (y-axis) at different age intervals (x-axis). Of the three general types of survivorship curves, two correspond to K- and r- strategists. K-strategists, including humans and most primates and zoo animals, exhibit Type I survivorship and tend to die when elderly. Species displaying Type II survivorship have an equal chance of survival at each age interval. Type II survivorship is highly theoretical, with few real-world examples. Hydras, gulls, and American robins exhibit fairly linear survivorship curves, and rodents, rabbits, many adult birds, and some turtles exhibit a sigmoid or concave curve that approaches linearity. R-strategists, including trees, marine invertebrates, fish, and some insects, display Type III survivorship. These organisms often die young, but those that survive to adulthood generally live long lives. Most species display some combination of these patterns. For example, a species displaying Type III juvenile survivorship may exhibit Type II adult survivorship. On survivorship curves, the number of surviving individuals (y-axis) is often plotted on a logarithmic scale to better display per capita (proportion) effects and allow comparisons with idealized Types I, II, and III.

An age structure diagram shows the proportion of a population (x-axis) at each age interval (y-axis) and can be used to predict whether a population will shrink or grow. For example, greater growth would be expected in a population with more females and young individuals. Different diagram shapes represent distinct patterns of likely population changes (growing, stable, or shrinking).