Transcript Population Density

Population Dynamics
Population dynamics - the study of the long term
changes in population sizes and the factors that cause a
change
 Current focus is on human populations (ageing
studies, population booms or declines), but biologists
use population dynamics to study competing species
and predator-prey relationships as well
Examples
Examples
Population Dynamics
 220 year-old discipline
 Thomas Malthus proposed
the 1st equation to
characterize human
population growth
 This led to equations for
exponential and logistic
growth which we will look at
later
Factors affecting population growth
 There are 4 factors affecting population growth:
1) Births (natality)
2) Deaths (mortality)
3) Immigration
4) Emigration
 They can all be put into a neat little equation…
N1 = N0 + (B – D) + (I – E)
NX = population size in
generation X
 If you simply want to look at the change in population from one
time interval to the next simply look at:
(B – D) + (I – E)
Population Dispersion
 Population Dispersion is how individuals are arranged in their
habitat.
 There are 3 types of population dispersion:
1.
Uniform dispersion
(usually due to competition between individuals)
Population Dispersion
 Population Dispersion is how individuals are arranged in their
habitat.
 There are 3 types of population dispersion:
2. Clumped dispersion
(usually due to uneven distribution of resources)
Population Dispersion
 Population Dispersion is how individuals are arranged in their
habitat.
 There are 3 types of population dispersion:
3. Random dispersion
(usually due to an even distribution of resources)
Population Density
Population Density - a measure of how many individuals of a
species are found in a given area
 Two types of density:
1. Crude density

# of individuals divided by the total area
Ecological density
2.

# of individuals divided by the total useable area
D = N/S
D = density
N = population size
S = area
Density Sample Questions
1)
A backyard measuring 3.0 m by 4.0 m contains 215 dandelions.
Determine the population density of the plants.
2)
A small field having an area of 1.5 ha contains a pond with a surface
area of 0.3 ha and is home to 300 field mice. Calculate their
ecological density.
3)
Researchers want to relocate some nuisance black bears into a forest
in Northern Quebec that is 750 km2. If each bear requires 40 km2 of
forest to live successfully, how many bears can be successfully
relocated here?
Population Density
 Density calculations are done
differently if the species being
studied is mobile or stationary
 Quadrat studies are used for
non-mobile populations like
plants
 Mark-Recapture studies are
used for mobile populations like
animals
Quadrat Studies
 Used to study non-mobile
populations
Quadrat - a small frame of known
area that is placed, at random, in a
larger ecosystem
 All of the species in question that
are found in that quadrat are
counted
 That number is scaled up
according to the ecosystem’s true
size
 More practical than counting
every individual in a given
ecosystem!
Quadrat Studies
 Estimated Population Density (EPD)
EPD
=
Total # of
sampled individuals
Sampled area
(basically a density equation)
Quadrat Study Example
4)
Scientists are studying the distribution of Trilliums in a section of
Lemoine Point covering 100m by 100m. They place four 1.0m X 1.0m
quadrats randomly in this area and count the number of trilliums in
each to be 5, 2, 1 and 3

What is the estimated trillium population density?

What is the estimated trillium population size?

What is one source of error in this method?
Mark-Recapture Studies
 Used for mobile
populations
 Individuals are
trapped/captured, marked
with a ‘tag’ and returned to
the environment
A Longworth trap: standard small mammal
trap
Mark-Recapture Studies
 Over time, more
individuals are
trapped/captured in
subsequent rounds
 Every round it’s noted how
many individuals are
tagged
 This data can be plugged
into an equation to figure
out population density
Mark-Recapture Study
M/N = m/n
M = # of individuals marked in first capture
N = estimated population size
m = # of marked individuals recaptured in 2nd round
n = # of individuals recaptured in 2nd round
Mark-Recapture Example
5) On day one, 20 warblers are captured in mist nets. The
birds are then marked with leg bands and released. One
week later, the nets are reset and 50 warblers are
captured. Of these 50, 10 of them were banded from the
week before. Estimate the warbler population size.
Transect Studies
 A transect is a straight line (or rectangle) chosen by researchers and
along which they will travel, counting the species being monitored
 Both mobile and stationary species can be counted this way
 Size of the transect depends on the species monitored
 Works best when the expected
density is low or when the
individuals are very large
(eg. Douglas Firs)