A classification of survivorship curves

Life tables provide a great deal of data on specific organisms. But ecologists search for generalities: patterns of life and death that we can see repeated in the lives of many species. A useful set of survivorship curves was developed long ago by Pearl (1928) whose three types generalize what we know about the way in which the risks of death are distributed through the lives of different organisms (Figure 4.8). Type I describes the situation in which mortality is concentrated toward the end of the maximum lifespan. It is perhaps most typical of humans in developed countries and their carefully tended zoo animals and pets. Type II is a straight line that describes a constant mortality rate from birth to maximum age. It describes, for instance, the survival of seeds buried in the soil. Type III indicates extensive early mortality, but a high rate of subsequent survival. This is typical of species that produce many offspring. Few survive initially, but once individuals reach a critical size, their risk of death remains low and more or less constant. This appears to be the most common survivorship curve among animals and plants in nature.

Survivorship Curves Oak Trees

Figure 4.7 Mortality and survivorship in the life cycle of Phlox drummondii. (a) The age-specific daily mortality rate (qx) and daily killing power (kx). (b) The survivorship curve: log10 lx plotted against age. (After Leverich & Levin, 1979.)

the logarithmic scale in survivorship curves

Figure 4.7 Mortality and survivorship in the life cycle of Phlox drummondii. (a) The age-specific daily mortality rate (qx) and daily killing power (kx). (b) The survivorship curve: log10 lx plotted against age. (After Leverich & Levin, 1979.)

Survivorship

Figure 4.8 A classification of survivorship curves. Type I (convex) - epitomized perhaps by humans in rich countries, cosseted animals in a zoo or leaves on a plant - describes the situation in which mortality is concentrated at the end of the maximum lifespan. Type II (straight) indicates that the probability of death remains constant with age, and may well apply to the buried seed banks of many plant populations. Type III (concave) indicates extensive early mortality, with those that remain having a high rate of survival subsequently. This is true, for example, of many marine fish, which produce millions of eggs of which very few survive to become adults. (After Pearl, 1928; Deevey, 1947.)

Figure 4.8 A classification of survivorship curves. Type I (convex) - epitomized perhaps by humans in rich countries, cosseted animals in a zoo or leaves on a plant - describes the situation in which mortality is concentrated at the end of the maximum lifespan. Type II (straight) indicates that the probability of death remains constant with age, and may well apply to the buried seed banks of many plant populations. Type III (concave) indicates extensive early mortality, with those that remain having a high rate of survival subsequently. This is true, for example, of many marine fish, which produce millions of eggs of which very few survive to become adults. (After Pearl, 1928; Deevey, 1947.)

These types of survivorship curve are useful generalizations, but in practice, patterns of survival are usually more complex. Thus, in a population of Erophila verna, a very short-lived annual plant inhabiting sand dunes, survival can follow a type I curve when the plants grow at low densities; a type II curve, at least until the end of the lifespan, at medium densities; and a type III curve in the early stages of life at the highest densities (Figure 4.9).

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Responses

  • matthew largo
    What is animal survivorship?
    2 years ago
  • gregory
    Why convex curve have high survival rate?
    2 years ago
  • hagos
    What type of survivorship curve do humans have?
    1 year ago
  • iggi
    What survivorship curve do plants follow?
    4 months ago

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