Survivorship Curves

An oyster releases millions of eggs and almost all of them die within days. A human baby in a country with modern medicine is very likely to reach old age. These are two different survival strategies, and a survivorship curve is the graph that makes the difference visible. You plot the number of survivors, starting from a cohort of the same size, on a logarithmic y-axis against age on the x-axis. Logarithmic scale matters because mortality often changes by orders of magnitude, and a normal scale would flatten the early collapse of a Type III species into an invisible sliver.

Three idealized shapes come out of this. Type I stays high and flat through youth and middle age, then drops sharply late in life — large mammals like elephants and humans, species with few offspring and heavy parental care. Type II slopes down at a constant rate the whole way, meaning the risk of death is roughly the same at any age — many birds and some rodents fit this. Type III starts with a steep plunge in the first days or weeks of life, then flattens out, because the few survivors who make it past infancy face good odds afterward — oysters, fish, insects, and most plants. A species where 900 of 1000 hatchlings die in year one, with the remaining 100 living on comfortably, is a textbook Type III.

None of this is a verdict on a species' success. A Type III curve pairs with an r-selected strategy: produce enormous numbers of cheap offspring, invest little per individual, and let sheer volume guarantee enough survivors. A Type I curve pairs with K-selected strategy: produce few offspring, invest heavily, and protect each one. Both are stable long-term solutions, just tuned to different environments and risks.