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Lecture 8: Population Ecology 2

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Dynamics and Growth

How do populations grow?
What controls population growth?
What mathematical models are used to describe growth?

Many questions in ecology can be reduced to attempts to understand the distributions and abundances of organisms and the processes - birth, death and movement - that determine them.

N now = N then + B - D + I - E

Population Dynamics

  • Size and density of populations over space and time
  • Population dynamics includes
    • birth, death, survivorship
    • age distribution
    • dispersal
    • rates of population change
  • Need info on life history

Life History Data

  • Age of first reproduction
  • Number of young
  • Number of reproductive events
  • Life span
  • Mortality

Life Tables

  • Provide a picture of survival and mortality in populations
  • Used to explore population dynamics in context of
    • birth
    • death
    • survivorship
    • age distribution

Two General Types of Life Table

Life Tables

  • Consist of a series of columns which describe aspects of mortality and reproductive output for members of a population according to age.
  • Used to:
    • Analyze probabilities of survival of individuals in a population
    • Determine ages most vulnerable to mortality
    • Predict population growth

Life Tables - Survivorship

  • Three main methods of estimation:
    • Cohort life table
      • Identify individuals born at same time and keep records from birth to death (good for plants and sessile orgs.)
    • Static life table
      • Record age at death of individuals within a certain time period (good for mobile and long-lived orgs.)
    • Age distribution
      • Calculate difference in proportion of individuals in succeeding age classes
      • Assumes differences from mortality
      • Also produces a static life table

Life Table Parameters

lx = survival of newborn individuals to age x

bx = fecundity at age x

mx = proportion of individuals of age x dying by age x+1

sx = proportion of individuals of age x surviving to age x+1

ex = expectation of further life of individuals of age x

Two kinds of life table are useful

  1. Cohort (dynamic) life table – good for plants and other sessile organisms
  2. Static life table – good for mobile and long-lived organisms

Survivorship curve for Dall Sheep

Survivorship Curves

  • Some plants and most mammals have high survivorship of young
  • Some birds and amphibians have a more constant survivorship or mortality through life
  • Many plants, invertebrates, amphibians, and fish have very low survivorship as juveniles

Age Distribution

  • Age distribution of a population reflects:
    • History of survival (high and low periods)
    • Periods of successful reproduction
    • Growth potential
      • Are older individuals replacing themselves or not?

How does age distribution affect population growth?


Stable age distribution example

Stable age distribution results in steady growth

When age distributions are not stable, what happens to growth?

Effects of age distribution on plant populations:

Human population growth is one of the most important ecological phenomena to hit the planet.

The rate of change in the population is exponential.

If a population is growing exponentially, how is the rate

determined ?

dN/dt = r N

r = exponential growth rate

Growth is on a per individual basis.

What does this equation mean in English?

Why is it reasonable to assume the human population grows continuously?

Not all organisms reproduce continuously AND population growth rate can change yearly.

These populations grow Geometrically.

N(t+1) = N(t)λ

Comparing exponential and geometric growth

Intro to case study 2

What makes a species invasive?

Copyright 2008, by the Contributing Authors. Cite/attribute Resource . factpetersen. (2008, March 06). Lecture 8: Population Ecology 2. Retrieved January 07, 2011, from Free Online Course Materials — USU OpenCourseWare Web site: This work is licensed under a Creative Commons License Creative Commons License