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  • Course Biology Global - for use with NGSS (NEW)
  • Unit 8 Ecology
  • Concept 8.5 Describing Populations

    • Describing Populations

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    What Are the Differences Between Type I, Type II, and Type III Survivorship Curves?

    Survivorship Curve

    A survivorship curveglossary term (opens in a new window) is a graph that shows rates of survival (numbers or percentage alive) for individuals at various ages in a populationglossary term (opens in a new window). Survivorship is a key factor that determines changes in population size. Reproductive rate, immigration, and emigration are other key factors. A population is characterized by one of three basic types of survivorship curves.

    A type I survivorship curve describes a population in which long life spans are typical. There are high survival rates among young and middle-aged individuals. The survivorship curve begins to decrease towards older ages as death rate increases. Organisms with a type I survivorship curve tend to have long life expectancy and a long juvenile period (the pre-reproductive stage). Organisms with a type I survivorship curve tend to produce fewer offspring and provide these offspring with greater parental care. Humans and other large mammals are examples of organisms with a type I survivorship curve.

    A type II survivorship curve is similar to a straight line, as death rates are fairly consistent among individuals of all age groups. These organisms are faced with a consistent risk of mortality throughout their lives. These organisms tend to produce a medium number of offspring and provide some parental care. Some invertebrates, rodents, and most birds are examples of organisms with a type II survivorship curve.

    A type III survivorship curve depicts populations in which mortality rates are significantly higher for young individuals than for individuals in older age groups. These organisms tend to produce many offspring and provide little parental care. Most insects and annual plants are examples of organisms with a type III survivorship curve.

    Teacher Note: Practice

    In these items students apply scientific reasoning, theory, and mathematical models to link evidence to a claim about the relationship between survivorship curves and reproductive strategy. To extend this activity, have small groups of students discuss age structure, using evidence to support their claims about the relationships among age structure, survivorship curves, and reproductive strategies.

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    Each type of survivorship curve has a distinct shape. Match each graph with the scenario it best describes.
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    Female grizzlies give birth to one or two cubs, who stay with their mother for two to three years. Grizzlies can live to be 25 years old in the wild.
    Many species of hawks lay between one and five eggs every year. They care for these young until they leave the nest.
    Sea turtles lay an average of 100 eggs in a nest, and nest up to 8 times a year. Many of these turtles die before reaching adulthood.
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