28.3
Life Histories
Constrained by limited energy and resources, organisms must compromise between offspring quantity and parental investment. This trade-off is represent…
All the events that affect an organism's survival and reproduction make up its life history. The pattern of these events varies across species, even among organisms living in the same habitat.
For example, a small snail matures quickly and produces thousands of eggs.
Most of these offspring die early in life, so producing many eggs increases the chance that a few survive to adulthood.
This pattern reflects an r-selected strategy, a term used to describe species that produce many offspring and provide little parental care.
Species with this strategy often show a Type III survivorship curve, characterized by high mortality early in life. Individuals that survive the early stages have a much higher chance of reaching adulthood.
On the other hand, howler monkeys follow a K-selected strategy. They mature slowly, have only a few offspring over a long lifespan, and provide years of parental care to help ensure their survival.
These primates show a Type I survivorship curve, characterized by high survival rates during early and middle life and high mortality in old age.
The final survivorship curve, Type II, shows an equal probability of survival and mortality throughout an organism's life cycle.
For example, many songbirds face similar threats from predators, disease, and environmental stress at every age. Because of that, their survivorship curve forms a steady diagonal line.
These survivorship curves show how different species adopt different strategies to balance their survival and reproduction.
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Q1: What is a life history and how does it vary among species?
A life history encompasses all events affecting an organism's survival and reproduction, including birth, development, maturation, reproduction, and death. The pattern of these events varies significantly across species, even among organisms sharing the same habitat. For example, small snails mature quickly and produce thousands of eggs, while howler monkeys mature slowly and have few offspring over long lifespans.
Q2: How do r-selected and K-selected strategies differ in reproductive approach?
R-strategists produce many offspring with minimal parental care, relying on high reproduction rates to ensure some survive. K-strategists produce fewer offspring but invest substantial parental care to maximize survival chances. R-strategists thrive in unpredictable environments and mature early, while K-strategists prosper in stable environments. Most organisms lie somewhere on a continuum between these strategies rather than being strict r- or K-strategists.
Q3: What does a Type III survivorship curve indicate about a species?
A Type III survivorship curve, characteristic of r-strategists, shows high mortality early in life but long lifespans for survivors. Species like small snails and marine invertebrates display this pattern. Individuals that survive early developmental stages have much higher chances of reaching adulthood, reflecting the trade-off between producing many offspring and providing minimal parental care.
Q4: How do Type I and Type II survivorship curves differ from Type III?
Type I curves, shown by K-strategists like primates and humans, display high survival during early and middle life with mortality concentrated in old age. Type II curves show equal survival and mortality probability throughout an organism's lifespan, forming a steady diagonal line. Songbirds exemplify Type II, facing similar threats from predators and disease at every age, unlike Type III's early-life bottleneck.
Q5: Why do organisms face trade-offs between offspring quantity and parental investment?
Organisms have limited energy and resources, requiring compromise between producing many offspring versus investing heavily in each one's survival. This fundamental constraint shapes reproductive strategies: r-strategists maximize offspring number with minimal care, while K-strategists reduce offspring number but increase parental support. These trade-offs directly influence survivorship patterns and population growth potential.
Q6: How can survivorship curves predict population changes?
Survivorship curves display the percentage of a population surviving at different age intervals, revealing mortality patterns across lifespans. Combined with age structure diagrams showing population proportions at each age, these tools predict whether populations will grow, remain stable, or shrink. Populations with more young females and favorable survivorship curves are expected to grow more rapidly.
Q7: Can organisms display characteristics of both r and K strategies?
Most organisms are not strict r- or K-strategists but lie on a continuum of these traits. Sea turtles exemplify this flexibility, possessing K-strategy traits like long lifespans and strong competitiveness alongside r-strategy traits like producing many offspring with minimal parental care. Species may also display different survivorship patterns across life stages, such as Type III juvenile survivorship transitioning to Type II adult survivorship.