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Introduction
Note that survivorship curves must be plotted on a log scale to compare with idealized Type I, II, and III curves; they will look different on a linear scale. The use of a log scale better allows a focus on per capita effects rather than the actual number of individuals dying. For example, the type II curve has a constant proportion of individuals dying each time period. Starting with 1,000 individuals, in the first time period if 40% survive, then only 400 will be left. In the second time period, 40% of the remaining 600 will be left: 160. Plotting this on a linear scale, these three points are not a straight line: The biggest drop occurs when 60% of the original 1,000 die in the first time period. Nonetheless, the same proportion of individuals died both times. On a log scale, the relationship of survivorship with time is linear; this scale highlights that the same proportion dies in the second time period as in the first (Figure 2).
Examples
For populations with Type II survivorship, the mortality of an individual does not depend on its age. Commonly listed examples of this include rodents, adult birds, and certain turtle species. Figure 4 shows actual data from a population of the slider turtle (Pseudemys scripta), which exhibits Type II survivorship from ages one to fifteen years.
Most individuals in populations with Type III survivorship produce many thousands of individuals, most of whom die right away: Once this initial period is over, survivorship is relatively constant. Examples of this include fishes, seeds, and marine larvae. Relatively little effort or parental care is invested in each individual. In general, this is more typical of r-selected species. R-selected species experience a frequent disturbance or uncertainty in their environments. Producing a large number of offspring makes it more likely that at least a few will land in favorable areas. Figure 5 shows actual data of a population of the invasive cheatgrass, Bromus tectorum, which has Type III survivorship under certain conditions. In the same study, other populations of B. tectorum had survivorship more like Type II or even Type I, further demonstrating that survivorship curves depend on the particular time and place a cohort is in.
Human Survivorship Patterns
References and Recommended Reading
Flood, N. & Horn, C. Cemetery
Demography. EcoEd Digital Library, 1991.
Deevey E. S., Jr. Life tables for natural populations of animals. Quarterly Review of Biology 22, 283–314 (1947).
Gibbons, J. W. & Semlitsch, R. D. Survivorship and longevity of a long-lived vertebrate species — how long to turtles live. Journal of Animal Ecology 51, 523–527 (1982).
Mack, R. N. & Pyke, D. A. The demography of Bromus tectorum: variation in time and space. Journal of Ecology 71, 69–93 (1983).
Murie, A. The wolves of Mount McKinley. Fauna of the National Parks of the United States (Fauna Series 5). Washington, DC: United States Government, 1944.
Pearl, R. The Rate of Living. New York, NY: Knopf, 1928.
Additional cemetery data available at http://academics.hamilton.edu/biology/ewilliam/cemetery/#datasets.