Type Iii Survivorship Curves Are Typical Of Species That Exhibit

The type III survivorship curve paints a stark picture of life for many species, one characterized by high mortality rates early in life. These curves, a fundamental concept in ecology, are invaluable tools for understanding population dynamics and life history strategies. Understanding which species exhibit this type of curve gives us deep insights into the evolutionary pressures shaping their survival and reproduction.

Understanding Survivorship Curves

Survivorship curves graphically represent the number of individuals in a population surviving at different ages. Generally, they plot the logarithm of the number of survivors against their age. This visualization helps ecologists understand mortality patterns within a species. There are three primary types of survivorship curves:

Type I: Characterized by high survival rates throughout most of the lifespan, with mortality rates increasing sharply in old age. This pattern is typical of species that invest heavily in parental care, producing few offspring with a high probability of surviving to adulthood. Humans and many large mammals are examples.
Type II: Depicts a relatively constant mortality rate throughout the lifespan. This means that an individual's chance of dying is independent of its age. Some birds, small mammals, and certain reptiles exhibit this pattern.
Type III: Marked by extremely high mortality rates early in life, followed by relatively high survival rates for the remaining individuals who make it past this initial critical period. This is the focus of our discussion.

Characteristics of Type III Survivorship Curves

Type III survivorship curves have a distinctive shape. The curve plunges dramatically at the beginning, reflecting the massive die-off of young individuals. After this initial period of high mortality, the curve flattens out, indicating that the survivors have a significantly better chance of reaching older ages. This pattern suggests that if an individual can overcome the challenges of early life, its prospects for long-term survival improve considerably.

Species exhibiting Type III survivorship curves typically share several key characteristics:

High Fecundity: To compensate for the enormous losses in early life, these species produce a vast number of offspring. This reproductive strategy is often referred to as r-selection, where the focus is on maximizing reproductive rate rather than investing heavily in individual offspring.
Minimal Parental Care: Given the sheer number of offspring produced, providing extensive parental care becomes impractical. The young are often left to fend for themselves shortly after birth or hatching.
Small Size: Many organisms following a Type III curve are relatively small, making them vulnerable to a wide range of predators and environmental hazards.
Rapid Maturation: While not always the case, some species with Type III curves mature quickly, allowing them to reproduce early and contribute to the next generation before they succumb to mortality.
External Fertilization: Many aquatic species exhibiting Type III survivorship employ external fertilization, releasing eggs and sperm into the water. This method results in a large number of zygotes, but also leaves them highly exposed to predation and environmental stressors.

Species That Exhibit Type III Survivorship Curves

Numerous species across diverse taxa demonstrate Type III survivorship. Here are some prominent examples:

Marine Invertebrates

Many marine invertebrates exemplify Type III survivorship. These organisms typically release vast quantities of eggs and sperm into the water, relying on external fertilization.

Oysters: A single oyster can release millions of eggs in a spawning event. The resulting larvae are planktonic, drifting in the ocean and vulnerable to predation, starvation, and unfavorable environmental conditions. Only a tiny fraction of these larvae survive to settle and mature into adult oysters.
Sea Urchins: Similar to oysters, sea urchins release millions of eggs and sperm. The resulting larvae are subject to high mortality rates due to predation and environmental factors.
Starfish: Starfish also produce numerous eggs and larvae that face significant mortality challenges in their early stages.

Fish

Several fish species also exhibit Type III survivorship curves.

Cod: Cod are prolific spawners, releasing millions of eggs into the ocean. However, the vast majority of these eggs and larvae do not survive to adulthood due to predation and environmental pressures.
Herring: Like cod, herring are broadcast spawners, producing large numbers of eggs that are subject to high mortality rates.
Salmon: While salmon exhibit a complex life cycle, their early life stages in freshwater streams are characterized by high mortality rates due to predation and habitat limitations.

Insects

Many insects follow a Type III survivorship pattern.

Aphids: Aphids reproduce rapidly, often through parthenogenesis (asexual reproduction). They produce a large number of offspring, but these young aphids are vulnerable to predation and environmental stressors.
Mosquitoes: Mosquitoes lay hundreds of eggs in water. The larvae are susceptible to predation and require specific environmental conditions to survive.
Butterflies: Butterflies lay numerous eggs on host plants. The resulting caterpillars are vulnerable to predation by birds, insects, and other animals.

Plants

While survivorship curves are typically associated with animals, some plants also exhibit Type III patterns.

Dandelions: Dandelions produce a large number of seeds that are dispersed by wind. However, only a small percentage of these seeds germinate and survive to become mature plants.
Weeds: Many weedy plant species produce copious amounts of seeds with a low probability of survival.

Evolutionary Advantages and Disadvantages

The Type III survivorship strategy offers certain evolutionary advantages:

Rapid Population Growth: The high reproductive rate allows for rapid population growth when conditions are favorable. This can be advantageous in exploiting newly available resources or recovering from population declines.
Wide Dispersal: Producing a large number of offspring increases the likelihood that at least some individuals will disperse to new habitats, reducing the risk of extinction due to localized environmental changes.

However, this strategy also has disadvantages:

High Vulnerability: The lack of parental care and the small size of the offspring make them highly vulnerable to predation, disease, and environmental fluctuations.
Inefficient Resource Use: A significant proportion of the energy invested in reproduction is wasted, as most offspring do not survive to reproduce themselves.

Factors Influencing Type III Survivorship

Several factors contribute to the high mortality rates observed in species with Type III survivorship curves:

Predation: Young, small organisms are often highly vulnerable to predation by a wide range of predators.
Environmental Conditions: Harsh environmental conditions, such as extreme temperatures, drought, or flooding, can significantly increase mortality rates in young individuals.
Competition: Competition for resources, such as food and shelter, can be intense among young organisms, leading to starvation and increased mortality.
Disease: Young organisms are often more susceptible to disease due to their underdeveloped immune systems.
Habitat Loss: Destruction or degradation of habitat can reduce the availability of suitable breeding sites and increase mortality rates.

Implications for Conservation

Understanding survivorship curves is crucial for conservation efforts. Species with Type III survivorship curves can be particularly vulnerable to population declines due to the high mortality rates in early life. Conservation strategies must address the factors contributing to this mortality.

Habitat Protection: Protecting and restoring critical habitats, such as spawning grounds for fish and breeding sites for insects, is essential for ensuring the survival of young individuals.
Predator Control: In some cases, controlling predator populations may be necessary to reduce mortality rates in vulnerable species. However, this must be done carefully to avoid disrupting the ecosystem.
Pollution Reduction: Reducing pollution can improve water quality and reduce the incidence of disease, increasing the survival rates of young organisms.
Sustainable Harvesting: Implementing sustainable harvesting practices for commercially important species, such as fish and shellfish, is crucial for preventing overexploitation and maintaining healthy populations.

Examples in Detail

Let's delve into more detailed examples to illustrate the nuances of Type III survivorship:

Oysters: A Classic Case

Oysters are a quintessential example of a species with a Type III survivorship curve. A single female oyster can release millions of eggs during a spawning event. These eggs are fertilized externally and develop into free-swimming larvae called veligers. The veligers drift in the ocean for several weeks, feeding on phytoplankton and attempting to avoid predation.

The mortality rate during this larval stage is extremely high. Veligers are consumed by a wide variety of predators, including fish, crustaceans, and other invertebrates. They are also susceptible to starvation if food is scarce. Unfavorable environmental conditions, such as changes in salinity or temperature, can also increase mortality rates.

Only a tiny fraction of the veligers survive to settle on a suitable substrate, such as a rock or another oyster shell, and transform into juvenile oysters, or spat. The spat are still vulnerable to predation, but their survival rate is significantly higher than that of the veligers. Once they reach adulthood, oysters can live for many years, filtering water and contributing to the health of the ecosystem.

Dandelions: A Terrestrial Example

Dandelions, though seemingly simple, also exhibit Type III survivorship. A single dandelion plant can produce hundreds of seeds, each equipped with a parachute-like structure that allows it to be dispersed by wind. The seeds are scattered far and wide, landing in a variety of habitats.

The vast majority of these seeds fail to germinate or survive to become mature plants. They may land in unsuitable locations, such as on bare rock or in dense vegetation. They may be eaten by birds or other animals. They may succumb to drought or disease.

Only a small percentage of the seeds germinate and establish themselves as seedlings. These seedlings are vulnerable to competition from other plants, as well as to herbivory by insects and other animals. Those that survive these early challenges have a good chance of growing into mature dandelions, producing their own seeds, and continuing the cycle.

Sea Turtles: A Conservation Challenge

Sea turtles present a compelling, and concerning, example. Female sea turtles lay large clutches of eggs on sandy beaches. These eggs are vulnerable to a variety of threats, including predation by mammals, birds, and reptiles, as well as inundation by high tides and erosion.

Even after hatching, the tiny sea turtle hatchlings face a perilous journey to the ocean, where they are immediately vulnerable to predation by seabirds and fish. This "lost year" in the open ocean results in extremely high mortality rates.

Those that survive this initial gauntlet and find refuge in seaweed rafts or other protective habitats have a better chance of reaching adulthood. However, they continue to face threats from entanglement in fishing gear, plastic pollution, and habitat destruction.

The low survival rate of sea turtle hatchlings is a major concern for conservationists. Efforts to protect nesting beaches, reduce bycatch in fisheries, and mitigate pollution are crucial for increasing the survival rates of these iconic creatures.

Type III Survivorship in Human Contexts

While Type III survivorship is most commonly associated with the natural world, it can also be observed in certain human contexts. Historically, in societies with limited access to healthcare and sanitation, infant mortality rates were very high, resembling a Type III curve. In modern times, this pattern may be evident in regions experiencing famine, disease outbreaks, or conflict, where children are disproportionately affected by mortality.

Conclusion

The Type III survivorship curve is a testament to the diverse strategies that species employ to survive and reproduce. While it represents a high-risk, high-reward approach, it has proven successful for a vast array of organisms, from marine invertebrates to insects to plants. Understanding the factors that influence Type III survivorship is crucial for conservation efforts and for gaining a deeper appreciation of the intricate web of life on Earth. By studying these patterns, we can better protect vulnerable species and ensure the health and resilience of our ecosystems.