Life History and Population Dynamics

Class Information

  • Date: February 24th

  • Topic: Life History (S1-S2)

  • Activities:

    • 2 Visual Classroom Exercises to calculate r and age structure.

    • Learning Strategies Instruction office offers appointments to learn new study/exam prep strategies (link provided).

  • Extra Credit Opportunity: Complete SimUText: Understanding Pop Growth (S1-3) worth 10 points for a full assignment completion including both reading and graded elements.

Life History Section 1: Life Cycles and Life Histories

Definition of Life History

  • Life History: Refers to the sequence, timing, and type of events in an organism’s life from birth to death.

  • Life-history strategies evolve through natural selection to navigate trade-offs associated with resource allocation.

Key Questions Addressed by Life History Strategies

  • How much to invest in growth and survival versus reproduction?

  • When to begin reproduction, and how long to remain reproductively active?

  • How many offspring to produce, and how much investment should each receive?

Life Cycle Description

Definition of Life Cycle

  • Life Cycle: The series of stages that individuals go through from being born or hatched to reproducing and eventually dying.

Variation in Life Cycles

Features of Complex Life Cycles

  • Complex life cycles may include:

    • Mobile and sessile stages.

    • Terrestrial and aquatic stages.

    • Extreme morphological transformations.

  • Maturation rates vary widely:

    • Some species reach reproductive maturity quickly, while others take years or decades.

  • Modes of reproduction:

    • Sexual, asexual, or both.

  • Most sexually reproducing species have separate males and females, though some are hermaphroditic.

Examples of Various Life Histories

  • Seaweed: Alternates generations.

  • Loggerhead Sea Turtle: High fertility, reaches sexual maturity at age 35.

  • Barnacles: Hermaphroditic with low fertility.

  • Sea Stars: Life cycle and reproductive strategies vary greatly.

  • Salmon: Exhibits semelparity with a 'big bang' reproduction strategy, often hermaphroditic.

Life History Traits

Definition of Life History Traits

  • Heritable traits that determine aspects of the life history of an organism or species, important for influencing fitness (evolutionary success through offspring).

Examples of Life History Traits

  • Age and size at maturity (first reproduction).

  • Number and size of offspring.

  • Longevity of the organism.

Impact of Life History Traits

  • Variation in these traits leads to diverse life history strategies in organisms.

Implications of Variation in Life-History Traits

Case Study: SimPloids vs. Wild Type

  • The mutant SimPloids have a simple life cycle and reach reproductive maturity at Stage I, hence reproducing sooner than the Wild Type.

  • SimPloids exhibit higher fecundity (which is the physiological ability to reproduce) compared to regular types.

Trade-offs in Life-History Evolution

Concept of Trade-offs

  • Investing energy and resources in one life-history trait to enhance fitness may detract from another.

Examples of Trade-offs

  • Early reproduction comes with fitness costs (e.g., offspring might be smaller, more vulnerable).

  • Increasing the number of offspring can lead to smaller sizes or vulnerabilities.

  • Caring for more offspring affects overall development speed and delays future reproduction.

Trade-off between Egg Size and Number

  • A balance must be struck between producing larger eggs (with higher investment) and numerous smaller eggs.

Resource Constraints in Life History Strategies

Resource Allocation Examples

  • A bird that allocates 40% of resources to reproduce one offspring invests significantly compared to a bird that distributes the same percentage to ten offspring, leading to less investment per individual.

Clutch Size Variation

Importance of Clutch Size

  • The number of offspring produced in a single reproductive event, known as clutch size, is critical to life history.

  • Optimal clutch size is context-specific; both high and low offspring counts can be successful strategies.

David Lack's Hypothesis on Clutch Size

Findings from Kestrel Experiments

  • Lack proposed that optimal clutch size reflects the maximum number of chicks parents can successfully rear to independence.

  • Evidence indicates that kestrels lay fewer eggs than Lack's hypothesis suggests; nests that produced extra eggs had higher survival rates.

  • Kestrels do not maximize fitness by merely increasing the number of chicks fledged.

Reasons for Evolving Small Clutch Sizes

Three Hypotheses Explaining Small Clutch Sizes

  1. Parental Cost Hypothesis: Smaller clutches increase parent survival likelihood for future reproduction, enhancing lifetime fitness.

  2. Predation Risk Hypothesis: Smaller clutches lower foraging effort and the noise from hungry nestlings, reducing predation risk.

  3. Variable Food Supply Hypothesis: Fewer offspring raise the chances of raising them successfully during scarce food times.

Evidence for Smaller Clutch Benefits

Experimental Results

  • Studies show that smaller clutches correlate with higher adult survival rates and increased lifetime fitness by reducing parental costs.

Life History Strategies and Trade-offs

Distinguishing Between "r" and "K" Species

  • "r" species: Traits favoring high birth rates and less parental care leading to higher offspring numbers but smaller sizes.

  • "K" species: Traits favoring significant investment in fewer offspring leading to larger offspring.

Section Summary: Key Points on Life History

  • The life cycle encapsulates the stages from conception through death.

  • Life histories include sequences of reproductive and growth events.

  • Life-history strategy is shaped by resource allocation responses to evolutionary pressures.

  • Trade-offs underscore the complexities of evolutionary fitness and resource investment across life history traits.

  • Evolving strategies adapt to real-world constraints, reflecting inherent trade-offs in lives of organisms.

Life History Section 2: Life-History Parameters

Implications of Life History Strategies

  • Life-history strategies significantly influence fitness and population dynamics (size changes over time).

Objectives of Upcoming Exercises

  • To calculate various measures including population growth rates and age structures.

Understanding Demographics

Introduction to Key Demographic Parameters

  • Important metrics include birth rate, death rate, and growth rate (r), defined as follows:

    • r = b - d

Definitions of Terms

  • Per Capita: From Latin, meaning "for each head", usually indicating statistics per individual.

  • Fecundity: Average number of offspring produced per female during her reproductive age.

Estimation Examples

Growth Rate (r)

  • The growth rate can be estimated by observing births and deaths over a sufficient time to average yearly variations:

  • For example, dolphin population growth is characterized by low birth rate with high parental investment.

Estimating Dolphin Growth Rates

  • Average growth rate calculated using:
    r ext{ (for dolphins)} = rac{391 - 300}{20 imes 355} ext{ and results in approximately: } rac{ 0.013 }

Estimating Barnacle Growth Rates

  • Barnacles have significantly different life strategies; their growth rate can be estimated similarly:
    r ext{ (for barnacles)} = rac{3759 - 3665}{20 imes 340} ext{ which results in approximately: } rac{0.0138}{1}

Visual Classroom Exercise Example

Lemma: Estimating Salamander Growth Rates

  • Given initial population and subsequent births/deaths, calculate the annual growth rate (r).

Population Age Structures

Definition and Effectiveness

  • Age structure informs us on demographics, revealing how a population's distribution across different ages can impact survival and reproductive patterns.

  • The significance of age distribution illuminates life history strategies employed by different species.

Differences in Age Structure of Species

  • Example:

    • Barnacle strategies center around high birth rates and low investment (leading to short lifespans).

    • Dolphin strategies reflect slower reproduction, higher care, and longer lifespans.

Environmental Influence on Age Structures

Population Responses to Environmental Changes

  • Barnacles can capitalize on favorable conditions to increase their population rapidly, unlike dolphins who have a slower response.

Human Population Age Structure Insights

U.S. Census Age Pyramid

  • Charts reflect demographic shifts over time, showing trends such as baby booms and decreasing growth rates.

Final Summary on Population Dynamics

  • Successful life history leads to stable or growing populations, utility of demographic parameters for predictions, and indicators on population health.

  • Diversity in life histories shows adaptability to various environmental conditions. Age structure dynamics give insight into historical and forthcoming demographic patterns.