# Survivorship Curves

By: Emily Rauschert (Department of Crop and Soil Sciences, The Pennsylvania State University) © 2010 Nature Education
Citation: Rauschert, E. (2010) Survivorship Curves. Nature Education Knowledge 3(10):18
How long do we live? How long do individuals in other species live? Do most individuals die young or live to ripe old ages? Survivorship curves visually answer these demographic questions.

## Introduction

Imagine a population of 1,000 individuals born at the same time in the same place. As time progresses, some individuals die, so there are fewer and fewer individuals present each year. But when do most individuals die? Do most individuals live to old age or do many individuals die at young ages? Ecologists use survivorship curves to visualize how the number of individuals in a population drops off with time. In order to measure a population, ecologists identify a cohort, which is a group of individuals of the same species, in the same population, born at the same time. Data is then collected on when each individual in a population dies. Survivorship curves can be used to compare generations, populations, or even different species. Survivorship curves actually describe the survivorship in a cohort: If cohorts are similar through time, they can be considered to describe the survivorship of a population. Because survivorship can be drastically different in different environments, this metric is not usually considered to be a property of a species. Besides the constraint of the general life history strategy of a species, the shape of survivorship curves can be affected by both biotic and abiotic factors, such as competition and temperature.

By plotting the number of survivors per 1,000 individuals on a log scale versus time, three basic patterns emerge (Pearl 1944, Deevey 1947; Figure 1). Individuals with Type I survivorship exhibit high survivorship throughout their life cycle. Populations with Type II survivorship have a constant proportion of individuals dying over time. Populations with Type III survivorship have very high mortality at young ages. Most real populations are some mix of these three types. For example, survivorship of juveniles for some species is Type III, but is followed by type II survivorship for the long-lived adults.

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

Examples of populations with Type I survivorship include humans in developed countries and animals in zoos. A lot of effort is invested in each individual, resulting in high survivorship throughout the life cycle: Most individuals die of old age. In general, this is more typical of K-selected species, which tend to grow in stable environments where intense competition between individuals is experienced. The heavy parental investment improves competitive ability and makes it more likely that individuals will survive to reproduction. Figure 3 shows actual data from a population of Dall sheep (Ovis dalii), which exhibit Type I survivorship.

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

Demography is the study of characteristics of human populations such as births, deaths, and growth rates: Survivorship patterns are also an important part of this. Humans in developed countries have more of a Type I survivorship. By comparing cohorts from different time periods, the effects of wars or disease outbreaks can be clearly seen (Flood & Horn 1991). For example, Figure 6 shows the different curves of cohorts of males born in central Pennsylvania. These data were gathered from cemeteries by the Centre County Genealogical Society. There is a general trend for survivorship to be higher in cohorts born later, perhaps due to advances in medicine. In addition, the cohort born in 1810–1819 experienced more mortality in the American Civil War. Survivorship curves in humans may also be strongly different in different regions or areas of the world.

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.

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