Exam 3 Notes

4/8: Population Dynamics

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

• (n_x) Individuals born in the ‘season’

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

• (d_x) Mortality rate

• (q_x) 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