Population ecology studies how populations change over time and space, relying on tools to categorize species’ life histories. A species’ life history strategy—the pattern of survival and reproduction—is shaped by natural selection and defines the dynamics of its population. One of the most insightful graphical tools in this field is the survivorship curve, which visually represents how mortality rates vary across an organism’s lifespan.
Understanding Survivorship Curves
A survivorship curve is a graph that plots the proportion of individuals from a specific cohort—a group of organisms born at the same time—that are still alive at various ages. The graph typically places the number or proportion of survivors on the vertical axis against the age of the organism on the horizontal axis. By analyzing the curve’s shape, ecologists can classify a species’ mortality pattern into three generalized types.
Type I survivorship curves are convex, showing a high probability of survival throughout early and middle life, with a steep decline in survival only in old age. This pattern is characteristic of species that produce few offspring but invest heavily in parental care, such as humans and many large mammals. Type II survivorship curves are diagonal, indicating a roughly constant mortality rate across all age groups, a pattern sometimes seen in certain birds and some lizards.
Type III survivorship curves are concave, characterized by extremely high mortality rates in the earliest life stages. For those few individuals that manage to survive this vulnerable juvenile period, the death rate drops considerably, and they have a relatively high probability of living to an older age. This pattern is typical of species that employ a strategy of producing a massive number of offspring with minimal or no parental investment in each one. The specific survivorship curve a species exhibits is fundamentally linked to its overall reproductive strategy.
Defining r-Selection
The concept of r-selection describes a life history strategy that prioritizes a high population growth rate, designated by the variable ‘r’. Species employing this strategy, known as r-strategists, are adapted to unstable, unpredictable, or disturbed environments where resources may be plentiful but competition is low. Their success relies on their ability to rapidly colonize an area and reproduce quickly before conditions change.
R-selected species exhibit a suite of traits designed to maximize reproductive output and speed. These organisms tend to have small body sizes, short lifespans, and reach sexual maturity quickly. The defining characteristic is high fecundity, meaning they produce a large number of offspring in one or a few reproductive events.
The trade-off for this high quantity of offspring is a minimal or absent parental investment in the young. The sheer number of progeny increases the chance that at least a few will survive to reproductive age, especially in a fluctuating environment. This strategy is often contrasted with K-selection, which favors traits that are advantageous at high population densities, involving fewer, higher-quality offspring and extensive parental care.
The Type III Curve and r-Selection
The strategy of r-selection directly results in the Type III survivorship curve. The massive initial drop characteristic of this curve is a biological consequence of producing large quantities of unprotected offspring. Since no energy is invested in caring for or guarding the young, they are highly susceptible to predation, disease, and environmental fluctuations like temperature changes or lack of food.
This steep, concave curve at the beginning of the graph reflects the reality that the vast majority of the cohort dies very early in life. For instance, a fish may lay thousands of eggs, but only a handful will hatch, avoid predators, and find enough nourishment to survive past the larval stage. This high juvenile mortality is the evolutionary cost of the r-strategy’s focus on maximizing the growth rate.
However, the curve flattens out for the few individuals who successfully navigate this initial bottleneck. These survivors avoid the common causes of early death and often live out their short adult lives with a lower risk of mortality. The Type III curve, therefore, is the graphical signature of a species that sacrifices the survival of most individuals for the rapid, large-scale production of the next generation.
Real-World Examples of r-Selected Species
Many insects are classic examples of r-selected species that display a Type III survivorship pattern. A female mosquito, for example, can lay hundreds of eggs in a small pool of water, with little expectation that more than a few will complete the metamorphosis to adulthood. Similarly, fruit flies, with their short generation times and prolific reproduction, experience high mortality in their larval and pupal stages.
In the marine environment, many invertebrates and fish utilize this strategy, producing millions of gametes or larvae in a single season. Oysters are a prime illustration, releasing millions of eggs into the water column, where the majority are eaten or fail to settle in a suitable habitat. The few that successfully attach to a substrate and develop a hard shell are the ones that survive for years.
Annual plants also embody the r-strategy, as they must complete their entire life cycle and set seed within a single growing season. A dandelion, for instance, produces thousands of wind-dispersed seeds, most of which fall on unsuitable ground. The few that land in a disturbed, open patch can quickly germinate and establish a new population. This rapid, high-volume reproduction is a successful adaptation for exploiting temporary resources in unpredictable habitats.