Survivorship curve

Definition

A graphical representation of the proportion or absolute number of individuals from a cohort that survive to successive ages or life stages. The curve plots survivorship (lx) on the vertical axis against age or stage on the horizontal axis, revealing patterns of mortality risk across the life span. Ecologists recognize three idealized types: Type I (low early mortality, high late-life mortality, convex curve—typical of large-bodied species with parental care), Type II (constant mortality risk at all ages, diagonal line), and Type III (high early mortality, low late-life mortality, concave curve—common in rhabditiform nematodes, many insects, and other organisms producing numerous small offspring with little investment).

Etymology

From 'survivor' (one who remains alive) + '-ship' (state or condition) + 'curve' (line representing a variable quantity); introduced to ecology through life-table analysis in the early 20th century.

Example

A field study of the grasshopper *Chorthippus brunneus* might reveal a Type III survivorship curve: thousands of eggs laid in soil suffer heavy predation and desiccation, so only a few nymphs emerge and survive to adulthood, after which adult mortality drops sharply.

Synonyms

• survival curve
• lx curve

Related Terms

• Life table
• cohort
• fecundity schedule
• age-specific mortality
• life history strategy
• r/K selection theory
• Euler-Lotka equation

Usage Notes

Survivorship curves describe cohorts (same-age groups), not populations with continuous recruitment; mixing cohorts produces misleading composite curves. The three 'types' are idealized extremes—real species often show intermediate or stage-specific shifts (e.g., insects with high egg and pupal mortality but lower larval mortality). In entomology, laboratory-reared cohorts under constant conditions may yield different curves than wild cohorts facing variable predation, parasitism, and weather. Logarithmic scaling of the y-axis is standard to visualize mortality rates clearly. Not to be confused with 'survival analysis' in medical statistics, though the underlying mathematics (Kaplan-Meier estimators) are related.