Imagine you're in the final stretch of studying for a test that you have tomorrow morning. You may want to focus on the subjects that you’re the least familiar with. You assess how much time you have and how much sleep you need to get that night to perform well on the test. You decide you are going to focus on studying population dynamics.
In this imaginary situation, you made a choice about how you partition your time and your energy. Organisms go through a similar process in how they grow and reproduce, investing their energy into the parts that they believe are important. When organisms do this, we call it a life history strategy!
Life history theory
To understand life history strategies and theory, we first need to identify the life history of an organism and what characteristics affect an organism’s life history. An individual organism’s life history is the events that characterize its life cycle, such as birth, growth, maturation, reproduction, and death. Looking at these characteristics can tell us important information about how that organism survives and reproduces.
The life history of an organism is defined by the events that characterize its life such as birth, death, growth, and reproduction.
Life history theory was born from the idea that natural selection and evolutionary factors (i.e., competition for resources) influence the life history of an organism. Therefore, life history theory looks at how events related to reproduction and survival are shaped by interactions between species and between individuals and their environment.
Why is it critical to study life history theory? Life history theory seeks to understand how adaptations that organisms have or develop increase their fitness, otherwise known as their survival and reproduction. Many organisms have developed various life history strategies, so life history theory also seeks to study the plethora of life history strategies.
The definition of a life history strategy
The definition of life history strategy is how an organism decides to invest its energy and resources. An organism has trade-offs between its individual survival and reproduction (the survival of the next generation). These trade-offs, which are influenced by the abiotic and biotic components of its environment, lead the organism to develop a life history strategy.
Ecologists measure these life history strategies by looking at the life history traits of groups of organisms.
Life history strategies are different strategies or ways of approaching survival and reproduction, in which different organisms use their energy differently.
Life history traits
When trying to understand the events that influence the life history strategies, scientists have turned to observe organisms' life history traits. Life history traits are influenced by an organism’s interactions with its environment and when observed, they can be used to understand the life history strategy of an organism.
Life history traits include:
size and growth rate of organisms,
age at first reproduction,
length of time when reproduction is possible (number of reproductive events),
number of offspring produced at each reproductive event,
and the average length of lifespan.
Horseshoe crabs are like living fossils because they have gone almost unchanged for about 400 million years, and their life-history traits have been well-tracked. Horseshoe crabs take almost ten years to reach sexual maturity and when they do they can reproduce for a number of years, living up to 20 years old! Every year a female can lay up to 88,000 eggs, which are independent and receive no care from the parents. Many eggs do not make it because of predators (birds, turtles, etc.) that find the eggs an important food source, so it's important that the female horseshoe crabs lay so many.1
Figure 1: Horseshoe crabs amplexing (mating), preparing for a season of egg-laying. Source: pixabay.com.
Life tables
Life history traits are the demographic events that scientists use to create life tables. Life tables look at particular cohorts or groups of organisms born in the same time frame (kind of like your class year in school).
Life tables include information such as the age class (age of organisms at a certain point) or stages in their life cycle (See Table 1). Scientists can then look at the number of females each female organism produces at each age class or life stage, also known as fecundity.
Fecundity is often used as a general term to represent the reproductive success of the organism.
Scientists can also look at how many individuals have died, or mortality. By looking at mortality, scientists know the number of survivors from the cohort at each life stage, also known by the term survivorship (Table 1).
Using survivorship and looking at it through time, gives us our next topic: survivorship curves!
Table 1: A hypothetical life table for an insect showing the life stages, number surviving at each stage, and mortality rates.
Life stage | Number of individuals | Number that will survive | Mortality rate (number died/number of individuals in that life stage) | Total percent survived |
Egg | 500 | 250 | 0.5 | 50 |
Larva | 250 | 50 | 0.8 | 10 |
Pupa | 50 | 15 | 0.7 | 3 |
Adult | 15 | X | X | X |
Life history examples: using survivorship curves
Survivorship curves are the graphic representation of the data that a life table helps track. The survivorship curves show the number of individual organisms surviving at each stage in the organism's life cycle or each age, they are a great way to represent life history examples. There are three types of survivorship curves that allow scientists to observe three different life history strategies: types 1, 2, and 3.
Type 1
Type 1 survivorship curves show a low mortality rate earlier in life and tend to decline rapidly when organisms reach an older age. This curve represents organisms that have an overall low fecundity (offspring produced) but parents give their offspring a high amount of care.
Humans, whales, and other big mammal species display this survivorship curve
Type 2
Type 2 survivorship curves are linear, meaning that an organism faces a similar chance of survival (or mortality) at any point in their lives.
Organisms that often show this survivorship type include birds, rodents, and reptiles.
Type 3
Type 3 survivorship curves show organisms with high mortality rates at early life stages, but have a higher chance of survival once they make it past the early life stages. These organisms usually have many offspring (high fecundity) and provide little parental care.
Examples of organisms that display type 3 survivorships include annual plants and insects.
Life history strategies in ecology
Survivorship curves typically show life history strategies in ecology and allow ecologists to see trends that may characterize different types of life history strategies graphically. By studying how different organisms use their energy between reproduction and individual survival, ecologists have identified two life history strategies that organisms may use. These strategies include organisms that reproduce once with one large brood of offspring (r-strategists) and those that may reproduce more than once but typically have fewer offspring (k-strategists).
R-strategists versus k-strategists: example of life history strategies
R-strategists tend to be organisms that reproduce lots of offspring, be smaller, and focus less on maternal care. They are often also semelparous, which means that they reproduce only once throughout their lives.
K-strategists will invest more in parental care and are slower to mature to their reproductive age. They often have more than one reproductive event throughout their lifetimes, making them iteroparous. Below, Table 2 compares the life history traits of r-strategists versus k-strategists.
Table 2: Comparison of life history traits between r-strategists and k-strategists.
Type of life history strategy | R-strategists | K-strategists |
Reproductive events | Usually only one reproductive event (semelparity). | Usually more than one reproductive event (iteroparity). |
Parental care | Low amount or none at all. | High investment in parental care. |
Number of offspring usually | Usually many offspring (hence little parental care). | Usually, a few number of offspring allows for more parental care. |
Size of organism | Smaller organisms | Bigger organisms |
Rate of maturation | Fast to mature | Slow to mature |
Example | Insects, fish, mice | Elephants, humans, whales |
Figure 3: Examples of an r-strategist (grasshopper) and k-strategists (whale). Source: Images from pixabay.com, edited.
Life History Strategies - Key takeaways
- A life history strategy is an organism’s way of deciding where its energy should go. Particularly, life history strategies often result from trade-offs between reproduction and survival.
- Life history traits are influenced by an organism's interaction with its environment. Examples of life history traits are age at first reproduction, number of offspring, and number of reproductive events.
- Three types of survival curves represent organisms that face high mortality early in life (type 1), a constant rate of mortality (type 2), and high mortality later in life (type 3).
- R-strategists are often semelparous, have many offspring, are smaller, and focus less on maternal care.
- K-strategists tend to be larger, reproduce more than once (iteroparous), have fewer offspring, and put a lot of energy into maternal care.
References
- "Horseshoe Crab Life History," Maryland Department of Natural Resources.
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