4/8: In-Depth Notes on Population Dynamics (REC)(DOC)

Problems with Previous Models

• Exponential Growth Model: Describes how populations grow in ideal conditions; assumes unlimited resources.

• Logistic Growth Model: Accounts for environmental limitations and carrying capacity but has shortcomings.

• Shortcomings of Both Models:

  • Do not consider predator-prey interactions and disease.

  • Assume all individuals are identical regarding reproduction, ignoring age and maturity differences.

Life Tables

• A life table summarizes the probabilities of survival and reproduction based on an individual's age.

• Cohort: A group of individuals born at the same time; e.g., baby squirrels born in the same breeding season.

• Age Classes: Categorized by age ranges, not necessarily corresponding to complete years.

  • Example: Age class 0-1 includes all individuals less than one year old.

• Terminology: Important to understand definitions clearly to answer related quiz or test questions.

Gray Squirrel Life Table Example

• Survivorship (l_X): The proportion of the original cohort surviving to age class X.

  • For age class 0-1: 100% (all born squirrels).

  • For age class 1-2: 30% might survive.

• Mortality (D_X): Proportion of the original cohort that dies in a specific age class.

  • Calculated as: D_X = l_X - l_X+1.

• Importance: Life tables help predict future population demographics based on past survival and mortality data.

Predicted Trends and Probabilities

• Using life tables, one can estimate:

  • Probability of an individual reaching a particular age class.

  • Probability of death in given age classes.

• These predictions help in understanding long-term population trends and can influence conservation efforts.

Survivorship Curves

• Three types of survivorship curves based on life history strategies:

  1. Type I: High survivorship until late in life (e.g., humans).

  2. Type II: Steady decline in survivorship throughout life (e.g., many birds).

  3. Type III: High mortality early in life, but survivors have high longevity (e.g., many fish species).

• Natural History Traits: Traits related to resource allocation in survival, growth, and reproduction.

Fecundity in Life Tables

• Fecundity: The average number of offspring produced over a lifetime, primarily concerning female offspring.

• Importance of fecundity: Determines potential growth rates of populations and influences conservation strategies.

• Survivorship multiplied by fecundity gives insight into age class contributions to population growth.

Understanding Population Dynamics

• Cohort analysis allows estimation of population changes and highlights critical points for population stability (e.g., ensuring third age class survival).

• To predict overall population size, especially in the female population, multiply fecundity numbers to assess future growth potentials.

  • Value interpretation: Values < 1 indicate decline, 1 indicates stabilization, and values > 1 signal growth.

Importance of All Three Models

• Each model (exponential, logistic, and life tables) provides different insights into population growth.

• They complement each other, allowing for a comprehensive understanding of population dynamics.

  • Integration: Compare models for their strengths and weaknesses to grasp the entire picture of population ecology.

Natural History and Life History Trade-offs

• Life History Traits: Show how organisms allocate resources to various life processes (e.g., growth vs. reproduction).

• Reproductive strategies vary based on environment; e.g., species may change reproduction age based on predation risk and resource availability (e.g., bluegill sunfish).

• Phenotypic Plasticity: Organisms adjusting life strategies based on environmental conditions (e.g., developmental changes in response to predator presence).

Conclusions

• The discussion on population dynamics is crucial for ecological understanding and biodiversity conservation.

• Integrating knowledge of population growth models informs practical efforts in managing wildlife and ecosystem health.

Demography

Definition: Demography is the study of factors affecting population size and structure over time.

Includes: age classes, sex ratio, immigration/emigration rates, survivorship, mortality, and fecundity.

Goal: Understanding population changes and predicting future changes.

Life Tables

Purpose: Summarizes survival and reproductive probabilities of age classes.

Cohort: Group of individuals born at the same time.

Construction: Track a cohort over years, or collect data on multiple age classes simultaneously.

Components:

x: Year or age class considered.

n: Number of the cohort remaining in the population.

n_x: Number remaining for age class x.

l_x: Survivorship percentage of the original cohort surviving to age x.

d_x: Mortality rate, calculated as l_x - l_{x+1}.

Survivorship Curves

Understanding Change: Important for grasping population dynamics over time.

Types:

Type I: High survivorship in early life, rapid decline in later years (e.g., humans).

Type II: Constant mortality rate throughout life (e.g., many birds).

Type III: Low survivorship early, high survivorship later (e.g., many plants/invertebrates).

Utility: Identifies vulnerable life stages and peak reproductive times for conservation efforts.

Fecundity

Definition: Total number of offspring produced per individual in a lifetime.

Important Note: Often refers to female offspring since females are the primary reproductive sex.

Calculation:

Incorporate age-specific fecundity into life table data to determine population growth rates.

Net Reproductive Rate: Calculated as Survivorship x Fecundity.

If total sum < 1.0: Population declining.

If total sum = 1: Stable population.

If total sum > 1: Population growing.

Application: Population tables provide insight into actual population dynamics.

Life History Traits

Cost of Reproduction: Higher offspring often linked to increased mortality.

Energy division between growth, reproduction, and maintenance.

Trade-offs: Balancing energy allocation creates life history strategies.

Selection for Fitness: Organisms aim to maximize fitness, optimizing offspring survival.

High fecundity examples: Low survivorship (e.g., mustard plant).

Low fecundity examples: Higher survivorship (e.g., coconut palm).

Shortcomings of the Logistics Model:

Carrying capacity is portrayed as constant

Says ‘K’ stays the same, but in reality, it can fluctuate due to environmental factors, resource availability, and interactions with other species.

Variability in Reproduction

Not all members of a population are the same age

Life Tables: Help us predict population growth by tracking age differences in reproduction and survivorship…

Age-specific survival and fecundity rates are analyzed to assess potential growth patterns, allowing ecologists to make informed predictions about future population trends. This information is crucial for understanding how different age groups contribute to population dynamics and how environmental factors may impact these rates.

Can track and shed light on life history differences between different age classes

(x) Age class: is always broken into years unless labeled otherwise

(nx) Individuals born in the ‘season’

(lx) Survivorship Percentage: tells us about life expectancy in that age class

(dx) Mortality rate

(qx) Age-specific mortality

It takes a long time to build a life table— many years

When there is a range in (x) it’s because not much changes within those years… “Like with humans… what is the real difference between 24 and 25? Not much.”

Dall Sheep - Alaska'

Horns grow a segment a year

Male horns are curved, female horns are straighter and shorter

Habitat: Dall Sheep are typically found in mountainous regions and tundra across Alaska, where they graze on grasses, sedges, and other vegetation.

(Graph the survivorship curve) What sticks out about it?

Drop off in survivorship is rapid

Once they get past the second age class, survivorship acts more like a Type I group, standing a better chance of growing to full maturity before dying

Stats Explained for the Life Table'

Multiplying the fecundity column by the survivorship percentage (Lx * fecundity = contribution), understanding the age classes’ contribution to population growth

Adding together the entire contribution column = how many offspring the individual produces before they die on average