Population ecology investigates how organisms change over time and interact with their environments. This field analyzes life history strategies, which are adaptations influencing an organism’s survival and reproduction. Scientists use tools like the life table, a systematic summary of survival and reproduction rates, to understand these strategies. The survivorship curve is a graphical representation derived from the life table, illustrating the pattern of survival within a group of individuals, or cohort, from birth to the maximum age reached.
Defining the Survivorship Curve
The survivorship curve is a standardized way to visualize the mortality pattern of a species across its lifespan. This visualization allows ecologists to compare the diverse life history strategies employed by different organisms. The graph is structured with two primary axes to convey this information clearly.
The X-axis represents age, often scaled as a percentage of the maximum lifespan for meaningful comparison between species. The Y-axis plots the number or proportion of individuals from the original cohort that are still alive at each age interval. This Y-axis is typically presented on a logarithmic scale.
The logarithmic scale converts the rate of mortality into the curve’s slope. A steep downward slope indicates a high death rate during that age period, while a flatter slope shows a lower rate of death. The overall shape of the curve reveals the characteristic pattern of mortality for the species being studied.
Type I: High Survival in Early Life
The Type I survivorship curve exhibits a convex shape, characterized by a line that stays relatively flat throughout early and middle life before plunging steeply downward in older age. This pattern demonstrates that the majority of individuals survive through their younger years. The risk of death is low for most of the lifespan, increasing dramatically only as individuals approach their maximum biological age.
This curve is commonly associated with species that practice high levels of parental investment for their few offspring. The high survival of juveniles means the population produces fewer young, but those young have a high probability of reaching reproductive maturity. This reproductive strategy often involves reproducing multiple times over a long lifespan, sometimes referred to as K-selection.
Classic examples include humans, especially those with access to modern medicine, and large mammals such as elephants and mountain sheep. The extended period of low mortality allows individuals to benefit from learned behaviors and social structures. Senescence, or biological aging, ultimately leads to the sharp increase in death rates concentrated in the oldest age classes.
Type II: Constant Mortality Risk
The Type II survivorship curve is distinguished by a linear, downward-sloping line when plotted on the standard semi-logarithmic graph. This straight-line pattern indicates that the rate of mortality remains constant throughout the organism’s entire life. An individual has an equal probability of dying regardless of its age.
This constant risk suggests that external environmental pressures, rather than age itself, are the main cause of mortality. Factors such as consistent predation, accidental death, or disease affect all age groups uniformly. There is no period of life where the organism is significantly more or less vulnerable than any other.
This pattern is characteristic of certain groups, including many species of birds, some reptiles like lizards, and small mammals such as squirrels. The life history strategy involves a moderate level of parental investment and offspring number. This balances the risks that are continuously present in the environment.
Type III: High Mortality in Early Life
The Type III survivorship curve has a concave shape, starting with a steep, nearly vertical drop early in life, followed by a flattening out for the few survivors. This shape is the opposite of the Type I curve, demonstrating that a massive proportion of the population dies very young. Mortality rates are extraordinarily high for juveniles and larvae, but those who survive this bottleneck have a much higher chance of reaching an old age.
This pattern is a hallmark of species that employ a reproductive strategy involving the production of vast numbers of offspring with minimal to no parental investment. The sheer quantity of young produced is a mechanism to overcome the extremely high death rate in the earliest life stages. This is often termed an r-selection strategy, prioritizing quantity over the survival quality of individual offspring.
Organisms exhibiting this curve include most marine invertebrates, such as oysters and sea urchins, which broadcast thousands or millions of eggs and sperm. Many fish species, most insects, and annual plants also follow this pattern. The few individuals that persist are robust enough to survive, leading to the long, shallow slope that represents high survival for the adults.