A survivorship curve is a graph of the survival rate of a group of organisms. In making a survivorship graph, a cohort is used; in this case, a group of individuals who were born at roughly the same time. This cohort is then graphed with the number of surviving individuals on the y-axis and the age of the cohort on the x-axis.
Generally, a species will have one of three types of survivorship curves.
- Type I
- A type I curve signifies a high rate of survivorship early in life. It is not until later that organisms start to die. We find type I survivorship curves in large mammals. Type I organisms have small brood sizes and will take great care of their young. Species with type I curves have adapted to exploit specific aspects of their environment to increase the carrying capacity, thus, it is not surprising that many type I species are endangered. When graphed, a type I curve is concave down, there is a low rate of death early on, but a high rate later.
- Type II
- A type II curve shows an equal death rate throughout the life of an organism. Examples of species with type II curves are coral, certain small mammals, and many species of reptiles. Graphically, it looks like a straight line with a negative slope.
- Type III
- A type III curve represents a species which has a high death rate early in life. Large brood sizes, little to no parental care, and rapid population are all characteristic of species with type III survivorship curves. These species have adapted to reproduce and spread to new habitats very rapidly. Many species considered to be pests have type III curves. When a type III curve is plotted, it is concave up; the rate of death decreases over time.
Cecie Starr and Ralph Taggart. Biology: The Unity and Diversity of Life. United States of America: Brooks/Cole, 2001.
Survivorship Curves. http://bioinquiry.biol.vt.edu/bioinquiry/Cheetah/cheetahpaid/cheetahhtmls/popsurvivor.html. Arthur L. Buikema, Jr. and John Wiley and Sons, Inc. April 10, 2004.
Dr. John W. Kimball. Checks on Population Growth. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Populations2.html#r-Strategists_and_K-Strategists. John W. Kimball. April 10, 2004.