Population Biology

1
Population Biology
2
What is Population Biology?

• Population Biology asks the question
• Why is a population the size that it is?

3
Main Idea 1

• Given ideal conditions for growth and
  reproduction, a population of organisms will grow
  exponentially.

4
(No Transcript)
5
Exponential Growth Curve
6
Main Idea 2

• Most populations go through a series of growth
  phases, which can be represented by a logistic
  growth curve.

7
Logistic or Sigmoid Growth Curve
8
This Growth Curve Has 3 Phases

1. Lag Phase little initial growth.
2. Rapid Growth Phase
3. Stable Phase stabilizing factors limit growth

9
(No Transcript)
10
Main Idea 3

• Together, all of the limiting factors acting on a
  population determine how many individuals can be
  supported.

11
Limiting Factors

• Abiotic factors (temperature, humidity, pH,
  altitude, light/shade, pollution)
• Food supply
• Interspecific Competition
• Intraspecific Competition
• Predation
• Parasitism/Disease

12
Main Idea 4

• A population usually achieves a steady state when
  it reaches the carrying capacity of the
  environment.

13
(No Transcript)
14
Main Idea 5

• Humans have increased our own carrying capacity
  through agricultural and technological
  developments, but we have only postponed the
  environmental limits to our population growth.

15

16
Population Size is Important For

• Impact assessments, e.g. measuring the effects of
  disturbance or pollution
• Restoration ecology
• Harvest limits (hunting/fishing)

17
How can population size be determined?

• It is difficult or impossible to census all
  individuals in a target area.
• Consequently, population size must be ESTIMATED.
• Scientists use sampling techniques in order to
  estimate population size.

18
Sampling Techniques Include

• The Quadrant Method
• The Transect Method
• Mark-Recapture Method

19
Quadrant Method

• Used primarily for plant populations.
• Assumes quadrants are chosen randomly and
  organisms do not move from one quadrant to
  another.
• Number of individuals counted in each of the
  chosen quadrants.
• Yields of organisms per unit area.

20
Limits to Accuracy

• Quadrant method assumes organisms are distributed
  evenly
• If populations are clumped, resultant estimates
  to be too high/low

21
Distribution
22
Transect Method

• Describes distribution of species along a
  straight line
• Useful for identifying and describing CHANGE in a
  habitat
• Example WSC Tree Survey

23
Mark-Recapture Method

• Used for populations that move around.
• Organisms are captured, counted, marked, and
  released.
• After leaving time for dispersal, the population
  is recaptured and counted.
• The new count includes marked and unmarked
  individuals.

24
Calculations

• N nM/m
• N the population estimate
• n marked unmarked (2nd capture)
• M individuals (1st capture)
• m marked individuals (2nd capture)

25
Limits to Accuracy

• Assumes marked unmarked have the same chance of
  being caught in the 2nd capture.
• But once captured, some organisms may become more
  wary which could lead to overestimating the
  population.

26
Accuracy is Dependent Upon

• The number of samples taken-accuracy increases
  with increases in the number of samples taken
• The method of collecting samples
• The proportion of the total population sampled