Population ecology studies how populations change over time, focusing on processes that govern the size and structure of species groups. Ecologists analyze an organism’s life history strategy, which includes traits like age at first reproduction, number of offspring, and lifespan. Tracking individual survival rates is fundamental to this analysis. The survivorship curve is a standardized tool used to visually represent the pattern of mortality within a specific population.
Defining the Survivorship Curve System
A survivorship curve is a graph plotting the proportion of individuals from a starting group (a cohort) that survive to each successive age or life stage. The horizontal axis (x-axis) represents age, often scaled as a percentage of maximum lifespan. The vertical axis (y-axis) plots the number or logarithm of survivors. Using a logarithmic scale on the y-axis is common because it clearly categorizes the three basic mortality patterns by the line’s shape. Analyzing the curve’s shape allows ecologists to classify a population’s general life history strategy into three idealized types: Type I, Type II, and Type III.
The Characteristics of a Type II Curve
The Type II survivorship curve is defined by a straight, diagonal line sloping downward on a logarithmic plot. This linear shape signifies a constant rate of mortality throughout the organism’s entire lifespan. The probability of dying is independent of the individual’s age; a young juvenile is just as likely to perish as an adult.
This constant decline means the age-specific death rate remains uniform across all life stages. Forces causing death, such as predation, disease, or accidents, are nondiscriminatory in the age they affect. Every age class loses approximately the same proportion of its members within a given time interval, resulting in the characteristic straight-line pattern. For example, if a population loses 10% of its individuals each year, it will continue to lose 10% of the remaining individuals in subsequent years, irrespective of their age.
Real-World Examples of Type II Populations
While the Type II pattern is an idealized model, several groups of organisms closely approximate this constant rate of mortality. Many species of small mammals, such as squirrels, exhibit this curve. For these animals, the risk of predation, accidental death, or disease remains consistently high whether they are juveniles or experienced adults.
Numerous bird species, particularly songbirds, are classic Type II examples. A young bird is vulnerable to ground predators, while an adult faces aerial hunters and environmental hazards. The ecological pressures do not significantly shift in severity as they age, leading to a steady attrition of the cohort. Some reptiles, such as specific species of lizards, also demonstrate this intermediate survival strategy.
Visualizing the Three Types
The Type II curve occupies an intermediate position between the two other primary survivorship models, Type I and Type III. The Type I curve is convex, staying high and flat through early and middle life before plunging steeply in older age. This pattern, seen in humans and many large mammals, indicates high survival for the young and concentrated mortality among the elderly.
In contrast, the Type III curve is concave, showing a dramatic drop in survival immediately after birth. The curve flattens for the few individuals who survive the initial high-mortality period. This is typical of organisms that produce many offspring with little parental care, such as most marine invertebrates and many plants. The Type II curve’s linear shape stands out as the only model where the survival probability decreases at a uniform rate.