Getting Started
Population ecology examines how groups of organisms change over time. A key factor influencing a population's growth or decline is its survival pattern. By tracking a group of individuals from birth to death, we can visualize a species' life history and understand the evolutionary strategies that shape its interactions with the environment.
What You Should Be Able to Do
After completing this section, you will be able to:
Define a survivorship curve and explain the information presented on its axes.
Compare and contrast the characteristics of Type I, Type II, and Type III survivorship curves.
Provide examples of organisms that exhibit each type of survivorship pattern.
Connect a species' survivorship curve to its reproductive strategy (K-selected vs. r-selected).
Key Concepts & Mechanisms
Survivorship curves are powerful tools for comparing the life history strategies of different species. A survivorship curve is a graph that plots the proportion of a cohort—a group of individuals born at the same time—that remains alive at each age. The y-axis typically represents the percentage of survivors (on a logarithmic scale), and the x-axis represents age (as a percentage of maximum life span). This allows us to compare species with vastly different lifespans, such as a fruit fly and an elephant, on the same conceptual graph.
The shape of the curve reveals critical information about mortality risk at different life stages, which is closely tied to a species' reproductive strategy and the level of parental care it provides. We can classify these patterns into three idealized types: Type I, Type II, and Type III.
| Feature | Type I Curve | Type II Curve | Type III Curve |
|---|---|---|---|
| Shape of Curve | Starts flat, then drops steeply at old age. (Convex) | A straight, diagonal line. (Linear) | Drops steeply at the start, then flattens out. (Concave) |
| Early-Life Survival | Very high. Most individuals survive infancy and youth. | Constant. The chance of dying is the same at any age. | Very low. Most individuals die as larvae, eggs, or juveniles. |
| Late-Life Survival | Very low. Mortality rate increases sharply as individuals reach maximum lifespan. | Constant. The chance of dying in old age is the same as in youth. | High for the few individuals who survive the initial "die-off" period. |
| Parental Care | High. Parents invest significant energy and time in protecting and nurturing a small number of offspring. | Moderate. Some parental care may be present, but mortality is often due to consistent external factors. | Very low to none. Parents produce a massive number of offspring and provide little or no individual care. |
| Reproductive Strategy | Typically K-selected species. They produce few, large offspring and live in stable environments near carrying capacity. | Can be found in both K-selected and some r-selected species. | Typically r-selected species. They produce many, small offspring and often live in unstable environments. |
| Example Organisms | Humans, elephants, whales, and other large mammals. | Birds (like robins), small mammals (like squirrels), some lizards, and perennial plants. | Oysters, sea turtles, fish, insects, and most annual plants (e.g., dandelions). |
Key Models & Diagrams
The three survivorship curves are best understood as a single graphical model. This matrix summarizes the key features of that model.
| Survivorship Curve Type | Key Characteristics | Associated Reproductive Strategy | Example Organisms |
|---|---|---|---|
| Type I | High survival through early and middle ages; sharp increase in mortality in old age. | K-selected: Few offspring, high parental investment. | Humans, Elephants |
| Type II | Constant mortality rate regardless of age. | Intermediate; consistent predation or environmental risk. | Squirrels, Songbirds |
| Type III | Extremely high mortality for the very young; high survival for the few who reach maturity. | r-selected: Many offspring, low parental investment. | Oysters, Dandelions |
Conceptual Graph of Survivorship Curves
Imagine a graph with "Percentage of Maximum Lifespan" on the x-axis and "Number of Survivors (log scale)" on the y-axis.
Type I would look like a cliff: flat across the top and then dropping off sharply on the right.
Type II would be a straight line slanting downwards from the top left to the bottom right.
Type III would look like a slide: dropping almost vertically at the start and then leveling out close to the bottom.
Key Components & Evidence
Cohort: A group of individuals of the same species, born in the same year or season, who are tracked for demographic studies like survivorship.
K-selected species: Species that are adapted to stable, predictable environments. They tend to have longer lifespans, fewer offspring, and provide significant parental care (e.g., grizzly bears, humans). Their populations often hover near the environment's carrying capacity (K). They typically exhibit Type I or Type II curves.
r-selected species: Species that are adapted to unstable, changing environments. They tend to have short lifespans and produce large numbers of offspring with little to no parental care (e.g., mosquitoes, weeds). Their populations can grow exponentially (high intrinsic rate of growth, r). They almost always exhibit a Type III curve.
Parental Investment: The time and energy parents expend for their offspring's benefit. High parental investment is a primary driver of the high early-life survival seen in Type I curves.
Human Survivorship: Modern humans in developed nations are a prime example of a Type I curve, thanks to advances in medicine, sanitation, and nutrition that have dramatically reduced infant and child mortality.
Oak Trees: An oak tree produces thousands of acorns in its lifetime (an r-selected trait), but very few will survive to become mature trees, making it a classic example of a Type III survivorship pattern.
Sea Turtles: A female sea turtle lays over a hundred eggs, but most hatchlings are consumed by predators on their journey to the sea. The few that survive to adulthood have a high probability of living for many decades, demonstrating a Type III curve.
Skill Snapshots
Causation
Cause: A species invests high levels of parental care, including feeding and protection.
Effect: The offspring have a very high probability of surviving to adulthood, resulting in a Type I curve.
Cause: An organism produces thousands of eggs or seeds and provides no further care.
Effect: The vast majority of offspring are consumed by predators or fail to find resources, leading to the steep initial drop of a Type III curve.
Cause: A songbird faces a relatively constant threat of predation and disease throughout its life.
Effect: Its mortality rate does not change significantly with age, producing a linear Type II curve.
Comparison
Type I vs. Type III: Type I species have high survivorship in their youth, while Type III species have extremely low survivorship in their youth.
K-selected vs. r-selected: K-selected species (e.g., elephants) typically have few, large offspring and long lifespans, whereas r-selected species (e.g., dandelions) have many, small offspring and short lifespans.
Humans vs. Fish: Humans exhibit a Type I curve due to extensive parental care and medicine, while a species like cod exhibits a Type III curve by releasing millions of eggs with the expectation that only a few will survive to maturity.
Change and Continuity Over Time (of a Cohort)
Baseline: A cohort of 1,000 oysters is released as larvae into the ocean.
Change 1: Within the first few weeks, predation and environmental hazards eliminate 99% of the cohort, leaving only 10 individuals. This represents the steep initial drop of the Type III curve.
Change 2: The surviving 10 oysters grow large, hard shells, making them much less vulnerable to predators. Their mortality rate flattens out, and they have a high probability of surviving for many more years.
Continuity: Throughout the process, the fundamental life history strategy of the oyster—producing a massive number of offspring with no parental care—remains the defining feature of its population dynamics.
Common Misconceptions & Clarifications
Misconception: The y-axis on a survivorship curve shows the absolute number of individuals.
Clarification: The y-axis typically uses a logarithmic scale to show the proportion or percentage of the original cohort that is still alive. This allows for standardized comparisons between populations of different initial sizes.
Misconception: All individuals in a Type I species live to old age.
Clarification: A Type I curve indicates a high probability of surviving to old age, not a guarantee. Accidents, disease, and predation can still cause death in younger individuals, but it is much less common than in the other curve types.
Misconception: A species must fit perfectly into one of the three curve types.
Clarification: The three curves are idealized models. Many species exhibit patterns that are intermediate between these types or that change at different life stages (e.g., a crab might have a Type III curve as a larva and a Type II curve as an adult).
Misconception: Humans have always had a Type I survivorship curve.
Clarification: For most of human history, high infant and child mortality rates made our survivorship curve much closer to a Type II. The strong Type I curve seen in many countries today is a relatively recent phenomenon resulting from public health, medicine, and improved nutrition.
One-Paragraph Summary
Survivorship curves are graphical representations that illustrate the survival patterns of a cohort over its lifespan, providing insight into a species' life history. There are three main types: Type I, characterized by high early-life survival and late-life mortality, is common in K-selected species like humans that provide extensive parental care. Type III curves show the opposite pattern, with extremely high juvenile mortality followed by high survival for the few adults, typical of r-selected species like oysters that produce many offspring with no parental care. Type II curves display a constant rate of mortality at all ages, as seen in species like songbirds that face consistent environmental pressures. Ultimately, a species' survivorship curve is a reflection of the evolutionary trade-offs between the number of offspring and the energy invested in their survival.