Which Species Show a Type III Survivorship Curve?

Survivorship curves are fundamental tools in ecology for understanding how different organisms cope with mortality across their life spans. Among the three main types, the Type III survivorship curve stands out for its dramatic early-life mortality and is commonly associated with species that invest little in parental care but produce large numbers of offspring. This article explores the defining features of the Type III curve, which species best exemplify it, and why this life history strategy is so prevalent in certain environments.

Understanding Survivorship Curves

Survivorship curves graphically represent the proportion of individuals in a population that survive from birth to each subsequent age. There are three primary types:

• Type I: Low mortality in early life, high mortality in old age (e.g., humans, large mammals).

• Type II: Constant mortality rate throughout life (e.g., some birds, rodents).

• Type III: High mortality in early life, low mortality in adulthood (e.g., many fish, insects, plants).

These curves help ecologists predict population dynamics and understand the life strategies of different species.

What Is a Type III Survivorship Curve?

A Type III survivorship curve is characterized by a steep initial decline in the number of survivors, indicating very high mortality among juveniles. Those individuals that survive past this vulnerable period tend to live much longer, resulting in a flatter curve in later life stages.

Key Features

• High juvenile mortality: Most offspring die soon after birth or hatching.

• Low parental care: Little to no investment in protecting or nurturing young.

• High fecundity: Large numbers of offspring are produced to compensate for early losses.

• Life expectancy increases with age: Survivors of early life have a much higher chance of reaching adulthood.

Examples of Species with Type III Survivorship

Pelagic Fishes

Pelagic fishes, such as herring and anchovy, are classic examples of organisms with a Type III survivorship curve. These species release vast numbers of eggs into the open ocean, where most eggs and larvae fall victim to predation, starvation, or environmental challenges. Only a small fraction survive to adulthood, but those that do often live longer and reproduce multiple times.

Fruit Flies

Fruit flies (Drosophila) also exhibit a Type III survivorship curve. They produce large numbers of offspring, but many eggs and larvae do not survive to adulthood due to predation, competition, and environmental factors. Those that do survive can live relatively long lives for an insect and reproduce prolifically.

Other Notable Examples

• Oysters and shellfish: Release millions of eggs or larvae, with only a tiny fraction surviving to adulthood.

• Many insects: Such as butterflies and beetles, which lay hundreds or thousands of eggs, most of which perish early.

• Some plants: Especially those that produce numerous seeds with little parental investment.

Why Not Birds or Humans?

Birds generally exhibit a Type II survivorship curve, where mortality is relatively constant across all ages. Some birds may show features of Type III if juvenile mortality is particularly high, but this is not the norm.

Humans (and other large mammals) show a Type I curve, with low mortality in early life and high survival until old age, thanks to extensive parental care and low fecundity.

Ecological Significance of Type III Survivorship

The Type III curve is typical of r-selected species, which are adapted to environments where resources are unpredictable and competition is low. These species maximize reproductive output to compensate for high early mortality. This strategy allows populations to recover quickly from losses and adapt to changing conditions.

Adaptations for High Juvenile Mortality

• Massive reproductive output: Ensures that at least some offspring survive despite high mortality.

• Early maturity: Allows individuals to reproduce before they are likely to die.

• Low parental investment: Frees adults to invest energy in producing more offspring rather than caring for a few.

Population Dynamics and Type III Survivorship

Populations with Type III survivorship are highly sensitive to changes in the survival of juveniles. Environmental factors such as temperature, food availability, and predation pressure can dramatically affect population size from year to year. This variability is a hallmark of many marine and insect populations.

Real-World Implications

Understanding Type III survivorship is crucial for conservation and management. For example, protecting spawning grounds for pelagic fishes or ensuring habitat quality for insect pollinators can have a significant impact on population sustainability. Overexploitation or habitat destruction that affects early life stages can lead to rapid population declines.

Comparing Survivorship Curves

Case Studies

Pelagic Fishes: A Model of Type III Survivorship

Pelagic fishes, such as anchovies and sardines, release millions of eggs into the water column. The vast majority of these eggs and larvae are consumed by predators or succumb to environmental challenges. Only a small percentage survive to adulthood, but those that do can live for several years and reproduce multiple times, ensuring the continuation of the species.

Fruit Flies in the Lab and Field

Fruit flies are widely used in biological research due to their short generation times and high fecundity. In nature, fruit fly populations are subject to high juvenile mortality, but survivors can rapidly rebuild populations under favorable conditions.

Survivorship Curves and Evolutionary Strategies

The Type III survivorship curve reflects an evolutionary response to high and unpredictable juvenile mortality. By producing large numbers of offspring, species increase the chances that at least some will survive to adulthood, even in harsh or variable environments.

Conclusion

The Type III survivorship curve is characteristic of species that experience high mortality in early life, such as pelagic fishes and fruit flies. These organisms produce large numbers of offspring with little parental care, ensuring that at least some survive to adulthood. In contrast, birds typically show a Type II curve, and humans a Type I curve. Understanding these patterns is essential for predicting population dynamics, managing ecosystems, and conserving biodiversity.

Answer to the Question:
The characteristic survivorship curve III is shown by (1) Fruit flies and (2) Pelagic fishes. However, if only one correct option is allowed (as in multiple-choice exams), pelagic fishes are a classic and widely cited example of Type III survivorship, but fruit flies are also correct in many contexts. For a typical exam question with the options given, both are correct in different contexts, but since the question asks for a single “characteristic” example, and both are strong examples, the most ecologically general answer across most sources is pelagic fishes. However, if the options are strictly as given and only one can be selected, and you must pick from the list:

• (1) Fruit flies

• (2) Pelagic fishes

• (3) Birds

• (4) Humans

Both (1) Fruit flies and (2) Pelagic fishes are correct for Type III survivorship, but if you must choose only one, pelagic fishes are more universally recognized in ecological literature as a classic Type III example.
If the question allows multiple answers or is open-ended, both are correct. If strictly single-choice, pick (2) Pelagic fishes as the most characteristic.