Survivorship Curves

Patterns of mortality observed in a particular species provide insight into its evolutionary life history. If one plots the logarithm of lx, the cumulative survival across lifespan (where at birth an individual is considered to have 100% survival and the oldest age possible will have 0% survival), three hypothetical curves exist (Figure 1). Species that exhibit type I survivorship curves (sometimes referred to as K-selected species) experience low mortality during the early stages of their life and higher mortality late in life. These species tend to have longer maturation times, longer life spans, low pre-reproductive mortality, few offspring, and large amounts of parental care. Humans exemplify type I species. Species that exhibit type II survivorship curves experience constant mortality rates across all age classes, a pattern observed in some species of birds and reptiles. Species that exhibit type III survivorship curves

Selected Species Examples
Figure 1 Theoretical survivorship curves in nature where cumulative survival is plotted against relative age of the species.

(sometimes referred to as an r-selected species) experience high mortality in the early stages of life and lower mortality late in life. These species tend to have short maturation times, short life spans, high pre-reproductive mortality, larger litters or clutch sizes, and little parental care. Many species of insects exemplify type III curves. Unfortunately, these theoretical relationships are not always clear in nature. For example, a hybrid of the three curves might exist where a species has low survival in the first age classes, then has a phase of fairly constant mortality, and then exhibit rapid senescence later in life.

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