Survivorship and life strategies

Survivorship curves

A survivorship curve plots the number of survivors (usually on a log scale) against age. There are three standard types:

• Type I: low death rate in early and middle life, then a sharp fall in old age. The curve is flat and then drops. Examples: humans, elephants and other large mammals that care for few young.
• Type II: a roughly constant death rate at all ages, giving a straight diagonal line. Examples: many birds, some lizards and small mammals.
• Type III: very high death rate among the young; the few that survive live long. The curve drops steeply at first, then flattens. Examples: oysters, many fish, frogs and most plants that release huge numbers of seeds.

r- and K-strategies

In population equations, r is the intrinsic rate of natural increase and K is the carrying capacity. Species can be grouped by which they “maximise”.

• r-strategists reproduce fast: many small offspring, little or no parental care, early maturity, short life, small body size. They suit unstable, changing environments and are good colonisers. Examples: insects, bacteria, weeds, mice. They often show Type III survivorship.
• K-strategists live near carrying capacity: few large offspring, much parental care, late maturity, long life, large body size. They suit stable environments and are good competitors. Examples: elephants, whales, great apes, humans. They often show Type I survivorship.

Many species fall in between; the two strategies are ends of a range.

Key points

• Type I = most die old (humans); Type II = constant risk (many birds); Type III = most die young (oysters, fish).
• r = rate of increase; K = carrying capacity.
• r-strategists: many offspring, no care, short life, unstable habitats.
• K-strategists: few offspring, much care, long life, stable habitats.
• K-strategists are often more vulnerable to extinction because they recover slowly.

Exam tips

Questions give a description (e.g. “lays millions of eggs, no parental care”) and ask for the curve type or strategy. Link: r-strategy ↔ Type III, K-strategy ↔ Type I.

Common mistakes

• Thinking Type I means high early death; it is the opposite.
• Assuming r-strategists are “weaker”; they are very successful in disturbed habitats.
• Confusing K (carrying capacity) with a growth rate.