Survivorship curves are fundamental tools in ecology, serving as graphical representations that illustrate the proportion of individuals in a population surviving to different ages. These curves allow scientists to visualize the mortality patterns of a species over its lifespan. By plotting this data, ecologists gain insight into the life history strategies an organism employs, such as its reproductive output and the level of parental care it provides.
Understanding the Purpose and Axes of the Curve
The primary goal of a survivorship curve is to compare survival strategies across different species, revealing how the probability of death changes with age. The graph plots two specific variables. The horizontal axis (X-axis) represents the age of the individuals, often standardized as the percentage of the maximum life span for a species.
The vertical axis (Y-axis) plots the number of survivors from an initial group, known as a cohort. This axis typically employs a logarithmic scale to make proportional changes in survival clearer. Using a logarithmic scale allows a constant rate of death, such as that seen in Type II curves, to appear as a straight line, reflecting a constant mortality rate across all age groups.
Type I: High Survival in Early Life
The Type I survivorship curve is characterized by a convex shape, demonstrating that most individuals survive through their early and middle years. Mortality rates in the juvenile and young adult phases are extremely low. The steep decline in the curve occurs only late in life, concentrated in the oldest age groups, typically due to senescence.
This pattern is strongly associated with species that invest heavily in a small number of offspring, a strategy that often includes extensive parental care. This high investment ensures that the few young produced have a high probability of surviving to reproductive age and beyond. Classic examples include humans and large, slow-reproducing mammals such as elephants, chimpanzees, and gorillas.
This life history strategy directs energy toward protecting and nurturing the young. For instance, a human infant’s low mortality rate is a direct result of biological factors coupled with social and medical advancements that mitigate early-life risks.
Type II: Consistent Mortality Rates
The Type II survivorship curve appears as a straight, diagonal line sloping downward. This linear shape indicates a constant rate of mortality throughout the organism’s entire lifespan, meaning the risk of dying is independent of age. An individual is equally likely to die young, in middle age, or old age.
This constant risk often results from environmental factors like predation, accidents, or disease that affect all age groups equally. Species exhibiting this pattern do not show a period of high juvenile protection or a steep increase in death at old age. The energy investment in offspring is generally intermediate between the other two types.
Examples include many species of songbirds, certain small reptiles like lizards, and some rodents. In these populations, a consistent proportion of individuals are lost each year. This survival pattern demonstrates a life where external, random threats govern the likelihood of survival more than internal, age-related factors.
Type III: High Mortality in Early Life
The Type III survivorship curve has a concave shape, characterized by a sharp drop at the beginning of the graph that eventually flattens out. This initial decline signifies extremely high mortality rates among the youngest individuals in the population. The vast majority of offspring produced die early, often within days or weeks of birth or hatching.
This pattern is characteristic of species that produce a massive number of offspring but provide little to no parental care. The strategy is to produce sheer numbers, ensuring that even if only a tiny fraction survives, the species will persist. Those few individuals who survive the initial high-risk period tend to have a much higher probability of living to a relatively old age, causing the curve to flatten.
This survival profile is common in many marine invertebrates, such as oysters and clams, most species of fish, and many plants and insects. For example, a single oyster can release millions of eggs, yet most larvae are consumed by predators or fail to find a suitable habitat. The few individuals that successfully settle and form a protective shell, however, may live for decades.