Population Biology
What is Population Biology?
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Population Biology asks the question:
Why is a population the size that it is?
Main Idea #1
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Given ideal conditions for growth and
reproduction, a population of
organisms will grow exponentially.
Exponential Growth Curve
Main Idea #2
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Most populations go through a series
of growth phases, which can be
represented by a logistic growth curve.
Logistic or Sigmoid Growth Curve
This Growth Curve Has 3 Phases
1. Lag Phase – little initial growth.
2. Rapid Growth Phase
3. Stable Phase – stabilizing factors
limit growth
Figure 35-6
Main Idea #3
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Together, all of the limiting factors
acting on a population determine how
many individuals can be supported.
Limiting Factors
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Abiotic factors (temperature, humidity, pH,
altitude, light/shade, pollution…)
Food supply
Interspecific Competition
Intraspecific Competition
Predation
Parasitism/Disease
Main Idea #4
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A population usually achieves a steady
state when it reaches the carrying
capacity of the environment.
Main Idea #5
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Humans have increased our own
carrying capacity through agricultural
and technological developments, but
we have only postponed the
environmental limits to our population
growth.
Population Size is Important For:
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Impact assessments, e.g. measuring
the effects of disturbance or pollution
Restoration ecology
Harvest limits (hunting/fishing)
How can population size be determined?
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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.
Sampling Techniques Include:
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The Quadrant Method
The Transect Method
Mark-Recapture Method
Quadrant Method
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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.
Limits to Accuracy
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Quadrant method assumes organisms
are distributed evenly
If populations are clumped, resultant
estimates to be too high/low
Distribution
Transect Method
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Describes distribution of species along
a straight line
Useful for identifying and describing
CHANGE in a habitat
Example: WSC Tree Survey
Mark-Recapture Method
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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.
Calculations:
N = nM/m
N = the population estimate
n = # marked & unmarked (2nd capture)
M = # individuals (1st capture)
m = # marked individuals (2nd capture)
Limits to Accuracy
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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.
Accuracy is Dependent Upon
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The number of samples takenaccuracy increases with increases in
the number of samples taken
The method of collecting samples
The proportion of the total population
sampled