What is a Survivorship Curve?
A survivorship curve is a graph showing the proportion of individuals from a group, or cohort, that are still alive at each age. It visually represents a species' pattern of survival over its entire lifespan, from birth to the maximum age attained by any member.
Section 2: The Three Main Types of Curves
There are three generalized types of survivorship curves. A Type I curve shows low mortality in early and middle life and a rapid decline in old age (e.g., humans). A Type II curve shows a constant mortality rate throughout the entire lifespan (e.g., birds, rodents). A Type III curve features very high mortality for the young, followed by a flattening curve for those few individuals who survive to old age (e.g., oysters, trees).
Section 3: A Practical Example
Consider a sea turtle, which represents a Type III curve. A female may lay 100 eggs, but most hatchlings will be eaten by predators on their way to the sea. The graph would show a massive drop in survivors almost immediately. However, the few turtles that reach adulthood have a much higher chance of living for many years, causing the curve to level off.
Section 4: Importance in Ecology
Survivorship curves are a critical tool for ecologists to understand and compare the life history strategies of different species. They provide insights into parental care, reproductive strategies, and environmental pressures. This data is vital for conservation, managing wildlife populations, and modeling population dynamics.