Life History: Videos & Practice Problems

Life history in organisms involves strategic allocation of limited energy, resources, and time, impacting traits like survivorship, fecundity, and growth. Survivorship refers to the proportion of individuals surviving to a certain age, while fecundity indicates reproductive capacity, often measured by viable offspring produced. A key trade-off exists between high survivorship and low fecundity, exemplified by fruit flies and African bush elephants. Additionally, reproductive strategies are categorized into semelparity, with one massive reproductive event, and iteroparity, involving multiple reproductive events throughout an organism's life.

Life History

Life history is a crucial concept in understanding how living organisms allocate their limited energy, resources, and time throughout their lives. This strategic allocation leads to various fitness trade-offs that impact an organism's survivorship, fecundity, and growth. Essentially, life history encompasses the entire life story of an organism, including individual traits and strategies that influence its survival and reproductive success.

Survivorship refers to the proportion of individuals in a population that survive to a specific age, contrasting with mortality, which indicates the proportion of individuals that die at a given age. Fecundity, on the other hand, is the ability of an organism to reproduce, often measured as the average number of viable offspring produced per reproductive event or over a lifetime.

A key takeaway is the trade-off between survivorship and fecundity. Organisms that exhibit high survivorship typically have lower fecundity, and vice versa. For instance, fruit flies demonstrate high fecundity, with a single female capable of producing between 400 to 1,000 offspring in her lifetime. However, this high reproductive output comes at the cost of low survivorship, as only a small fraction of these offspring are expected to reach adulthood, with the average lifespan of fruit flies being about one month.

In contrast, African bush elephants showcase high survivorship, with a significant proportion of their offspring surviving to adulthood and living long lives, averaging around 70 years. However, they produce only about 4 to 6 offspring throughout their lifetimes, illustrating the trade-off between high survivorship and low fecundity.

This relationship can be visualized in a graph where most organisms fall along a trend line indicating the inverse relationship between fecundity and survivorship. Organisms that achieve both high survivorship and high fecundity are exceedingly rare. Understanding these dynamics is essential for studying population ecology and the evolutionary strategies of different species.

Life History Example 1

In ecological studies, understanding the relationship between fecundity and survivorship is crucial. Fecundity refers to the reproductive capacity of an organism, specifically the number of offspring it can produce, while survivorship indicates the proportion of offspring that survive to adulthood. Typically, organisms face a trade-off between these two factors: high fecundity often results in lower survivorship, and vice versa.

When evaluating potential scenarios in nature, it is essential to analyze the given options based on this trade-off. For instance, an organism that produces hundreds of offspring per year and lives up to 80 years old may seem plausible at first. However, this scenario does not provide information about survivorship, as it only addresses lifespan. Therefore, this option is not the least likely to occur in nature.

Another option describes an organism that produces fewer than 10 offspring per year and has a lifespan of up to 20 years. Again, this option does not reflect the trade-off between fecundity and survivorship, making it a possible scenario in nature.

Focusing on the remaining options, one describes an organism with low fecundity (fewer than 5 offspring per year) but high survivorship (90% survival to adulthood). This scenario aligns with natural expectations, as organisms with lower reproductive rates often have higher survival rates among their offspring.

Conversely, the last option presents an organism that produces hundreds of offspring per year, with more than 50% surviving to adulthood. This combination of high fecundity and high survivorship is unexpected in nature, as it contradicts the typical trade-off observed. Thus, this scenario is the least likely to occur.

In conclusion, the correct answer to the problem is the option that describes an organism producing hundreds of offspring per year with a high survival rate, as this situation is not commonly found in natural ecosystems.

Life History: Population Reproductive Strategies

Population reproductive strategies are essential for understanding how different species adapt to their environments through their reproductive behaviors. Two primary strategies are semelparity and iteroparity, each characterized by distinct reproductive patterns.

Semelparity, derived from the Latin roots "semel" meaning once and "parity" meaning to produce, refers to organisms that reproduce only once in their lifetime. This strategy often involves a single, massive reproductive event where the organism produces a large number of offspring, sometimes in the hundreds or thousands. A classic example of semelparity is the century plant, which spends most of its life without reproducing and culminates its life cycle in a single reproductive event before dying shortly thereafter. In graphical representations, the lifetime of the organism is plotted on the x-axis, while the number of offspring produced is shown on the y-axis, illustrating the dramatic spike in offspring production at the end of the organism's life.

In contrast, iteroparity, from the Latin "itero" meaning to repeat, describes organisms that engage in multiple reproductive events throughout their lives. This strategy can be further divided into seasonal and continuous iteroparity. Seasonal iteroparity, exemplified by the American Robin, involves distinct breeding seasons where reproduction occurs at specific times, leading to evenly spaced reproductive events. The number of offspring produced in each event is generally lower compared to semelparous organisms. Continuous iteroparity, as seen in the red howler monkey, allows for reproduction at any time once the organism reaches reproductive maturity, resulting in irregularly spaced reproductive events.

Understanding these reproductive strategies provides insight into the life history traits of various species and their evolutionary adaptations to environmental pressures.

Life History Example 2

In the study of reproductive strategies, two primary types are semelparity and iteroparity, each with distinct characteristics and implications for offspring survival. Semelparity refers to organisms that reproduce only once in their lifetime, typically producing a large number of offspring, often ranging from 100 to 1,000. This strategy is exemplified by cicadas, which invest all their reproductive effort into a single event. However, due to the lack of parental care, most of these offspring do not survive to adulthood.

In contrast, iteroparity describes organisms that reproduce multiple times throughout their lives. These organisms generally produce fewer offspring per reproductive event compared to semelparous species. A notable example of iteroparity is seen in elephants, which often provide parental care, increasing the likelihood that their offspring will survive to adulthood. This care can vary by species, but it is a common trait among iteroparous organisms, enhancing the survival rates of their young.

Understanding these reproductive strategies is crucial for studying population dynamics and the ecological roles of different species. The differences in parental investment and offspring survival rates highlight the adaptive strategies organisms employ in response to their environments.

Which of the following is an example of semelparity?

• A. An agave plant which stores energy throughout life, then releases many seeds before dying.
• B. Fruit flies, who can lay hundreds of eggs per week once they reach maturity.
• C. A female labrador who has 3 puppies one year, then another 5 puppies the next year.
• D. None of the above.

Answer: (A)

Here’s what students ask on this topic:

What is the difference between survivorship and fecundity in life history?

Survivorship refers to the proportion of individuals in a population that survive to a given age, essentially the opposite of mortality. It is not the same as lifespan, which is the expected duration of an organism's life. Fecundity, on the other hand, is the capacity or ability of an organism to reproduce, often measured by the average number of viable offspring produced per reproductive event or over a lifetime. A key trade-off in life history is that high survivorship often comes at the cost of low fecundity and vice versa. For example, fruit flies have high fecundity but low survivorship, while African bush elephants have high survivorship but low fecundity.

What are semelparity and iteroparity in population reproductive strategies?

Semelparity and iteroparity are two types of reproductive strategies in populations. Semelparity involves a single, massive reproductive event after which the organism typically dies. An example is the century plant, which produces thousands of seeds in one event before dying. Iteroparity, in contrast, involves multiple reproductive events throughout an organism's lifetime. Iteroparity can be seasonal, with reproduction occurring during specific breeding seasons, or continuous, where reproduction can occur at any time. The American Robin is an example of seasonal iteroparity, while the red howler monkey exemplifies continuous iteroparity.

How do organisms allocate energy, resources, and time in life history strategies?

Organisms must strategically allocate their limited energy, resources, and time to maximize their fitness. This involves making trade-offs between key life history traits such as survivorship, fecundity, and growth. For instance, an organism may allocate more energy to reproduction (high fecundity) at the cost of lower survivorship, as seen in fruit flies. Conversely, an organism like the African bush elephant may allocate more resources to ensuring high survivorship, resulting in lower fecundity. These trade-offs are essential for understanding how different species adapt to their environments and optimize their life history strategies.

Why is there a trade-off between survivorship and fecundity in life history?

The trade-off between survivorship and fecundity arises because organisms have limited energy, resources, and time. Allocating more resources to one aspect often means less is available for another. High fecundity, or the ability to produce many offspring, typically comes at the cost of lower survivorship, as seen in species like fruit flies. Conversely, high survivorship, where a large proportion of offspring survive to adulthood, often results in lower fecundity, as seen in African bush elephants. This trade-off is a fundamental aspect of life history strategies, influencing how species evolve and adapt to their environments.

What are some examples of organisms with high fecundity and low survivorship?

Examples of organisms with high fecundity and low survivorship include fruit flies and many species of fish and amphibians. Fruit flies, for instance, can produce between 400 and 900 offspring in their lifetime, but only a small fraction of these offspring survive to adulthood. Similarly, many fish and amphibians lay hundreds or thousands of eggs, but only a few of the hatched young survive to maturity. These organisms invest heavily in producing a large number of offspring, but due to high mortality rates, only a few reach adulthood.