AP Syllabus focus:
‘A survivorship curve shows the relative survival rates of a cohort (same-age group) from birth to the maximum age reached by cohort members.’
Survivorship curves are a key ecological tool for describing how long organisms tend to live and when deaths are most likely to occur.
Idealized survivorship curves (Types I, II, and III) plotted against age show how mortality can be concentrated late in life (Type I), remain roughly constant across ages (Type II), or be heaviest in early life (Type III). The log-scaled survivorship axis makes proportional declines comparable across life stages, so the curve’s shape highlights when losses occur rather than just the raw number dying. Source
They visualise survival across the lifespan, helping compare populations and infer life-history pressures.
What a survivorship curve is showing
Survivorship curve: A graph that displays the relative survival of individuals in a cohort from birth to the maximum age reached by members of that cohort.
A survivorship curve answers the question: “Of the individuals born at the same time, what fraction is still alive at each age?” It does this by tracking survival through time rather than counting births alone.
The cohort being tracked
A survivorship curve is built for a cohort, meaning the curve follows a single, same-age group as it ages.
Cohort: A group of individuals of the same species born (or starting life) during the same time interval and followed through time.
Between birth and the oldest age reached, the cohort shrinks as individuals die; the curve displays that decline.
How to read the graph
Axes and “relative” survival
Most survivorship curves use:
x-axis: Age (or time since birth/hatching/germination)
y-axis: Number surviving or proportion surviving (often starting at 1.0 or 100%)
“Relative survival” means the curve commonly shows survival as a fraction of the original cohort, not the absolute number of organisms in the ecosystem. This makes curves comparable even when starting cohort sizes differ.
What the slope communicates
The steepness of the curve at any age indicates the mortality rate at that life stage:
A survivorship curve plots survivorship (often from a life table) versus age, commonly on a logarithmic scale, which makes constant proportional mortality appear as a straight line. The three labeled curve classes summarize different mortality schedules: late-life mortality (Type I), age-independent mortality (Type II), and high juvenile mortality with later survivorship stabilizing (Type III). Source
A steeper downward segment means higher mortality during that period.
A flatter segment means higher survivorship (lower mortality) during that period.
A curve can change slope across the lifespan, indicating that survival challenges (predation, disease, harsh weather, starvation) are not constant for an organism.
What information a survivorship curve provides (and what it does not)
What it provides
A survivorship curve helps you identify:
Whether mortality is concentrated early, middle, or late in life (without needing to name specific curve “types” here)
The typical longevity pattern of a population (how quickly survival drops with age)
The life stages where conservation or management might be most effective (e.g., protecting juveniles vs. adults), based strictly on where survivorship declines most sharply
It also supports comparisons:
Across species living under different ecological pressures
Across populations of the same species in different habitats
Across time, if repeated surveys show the curve shifting due to environmental change
What it does not provide by itself
On its own, a survivorship curve does not directly show:
Population growth rate (it tracks survival of a cohort, not births in later years)
Causes of death (it shows when deaths occur, not why)
Future conditions (it reflects the cohort’s experienced conditions, which may not persist)
How survivorship curves are constructed in practice (conceptually)
To build the curve, ecologists estimate survivorship at successive ages by:
Identifying/estimating the starting cohort size
Recording or estimating how many individuals remain alive at each age interval
Plotting survivors (or proportion surviving) against age until the maximum age observed for that cohort
Common data sources include long-term monitoring, tagged individuals, or age-at-death records; the key requirement is that the data represent the same cohort through time.
Why survivorship curves matter in environmental science
Survivorship curves connect environmental conditions to biological outcomes because survival is shaped by:
Resource availability (food, water, shelter)
Exposure to hazards (pollutants, extreme temperatures, storms)
Biotic interactions (predation, disease, competition)
When survival declines sharply at particular ages, it often signals a life stage that is especially sensitive to environmental stressors, making survivorship curves useful for interpreting population vulnerability and guiding targeted protection.
FAQ
A log scale makes proportional losses easier to compare across ages.
It can visually separate early-life mortality from later-life mortality when numbers drop quickly.
Methods include growth rings (trees, fish otoliths), body size classes, tooth wear, or known-age marking.
Choice depends on species biology and measurement error.
Right-censoring occurs when individuals are still alive when the study ends.
If ignored, survivorship can be underestimated at older ages.
Yes. If individuals leave the study area, they may be recorded as “dead”.
Researchers try to reduce this using tracking, larger study boundaries, or correction models.
If juveniles are harder to detect than adults, early mortality may be overestimated.
If carcasses are rarely found, deaths may be missed, inflating survivorship estimates.
Practice Questions
Define a survivorship curve and state what group it tracks. (2 marks)
Defines survivorship curve as showing relative survival across ages from birth to maximum age (1)
Identifies that it tracks a cohort / same-age group (1)
A population study produces a survivorship curve where survivorship remains high for many age intervals and then declines rapidly near the maximum age. Explain what this indicates about mortality across the lifespan and give two environmental or ecological factors that could influence the observed pattern. (5 marks)
Describes low mortality/high survivorship through early and middle ages (1)
Describes higher mortality concentrated at older ages/near maximum age (1)
Links the curve’s slope/shape to changing mortality risk with age (1)
Gives one plausible factor (e.g., disease, predation, resource limitation, harsh weather, pollutants) that could affect survival (1)
Gives a second distinct plausible factor affecting survival (1)
