Describing Populations
Population Ecology
POPULATION
• Individuals of the same species living in a
particular area
• Population size, density, distribution and age
structure can help the ecologist understand how
a population may grow or decline
POPULATION SIZE
• The overall health of a population can often be
monitored by tracking how its size changes
• POPULATION SIZE– Describes the number of
individual organisms present in a given
population at a given time
• Population size may increase, decrease, undergo
cyclic change, or remain the same over time.
The Decline of the Passenger Pigeon
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•
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Shows extremes of population size
Once most abundant bird in North America
Huge flocks darkened skies.
In 1800’s 2 billion formed a near solid mass 240
miles long and took 5 hours to fly overhead…sounded
like a tornado!
• Passenger pigeons nested
and bred in forests.
• Forests cut down and
hunted. Shipped to
market as food
1898 Photograph of a Passenger Pigeon.
The last wild Passenger Pigeon was shot by a 14-year-old boy in Ohio in
1900, while the last known individual of the passenger pigeon species,
named "Martha" after Martha Washington, died at 1 p.m. on the 1st of
September 1914 in the Cincinnati Zoological Garden. She was 29.
Passenger Pigeons, Continued
• By end of 1800s
the population
was so small the
pigeons could
not form needed
colonies to
breed.
• Last passenger
pigeon on Earth
died in the
Cincinatti Zoo in
1914
DETERMINING POPULATION SIZE
• Were 2 billion pigeons actually “counted?”
• Probably not—sometimes not possible
• Population size is estimated using sampling
techniques
• We will be using a sampling technique later
today in our lab!
POPULATION DENSITY
• Measure of “how crowded” an area is
• Number of individuals within a population per
unit time
• Example: 1500 golden toads counted in 1987
within 4 square kilometers
• Population density = 1500toads/4 square km
= 375 toads/square km
• In general, larger organisms like lions have
lower population densities because they require
more resources and room to survive
Different Densities
• High population density can make it easier for
organisms to group together and find mates but
can also lead to conflict/competition
• Overcrowded = more vulnerable to predators
= increase transmission of disease
• Low density = may benefit from more space and
resources, but harder to mate
DENSITY AND THE HARLEQUIN FROG
• Lived in very specific locations called “splash
zones” in Costa Rica
• Spash zone= area alongside rivers and streams
that receive spray from waterfalls and rapids
• 1980s and ‘90s Montverde region of Costa
Rica got warmer and drier water flow
decreased and streams dried up
• Few splash zones left
• Frog densities up to 4.4x higher than normal
around remaining splash zones
DENSITY AND THE HARLEQUIN FROG
• Overcrowding= disease transmission
= predator attack
= assault from parasitic flies
• Harlequin frog disappeared from Monteverde
region but a few
exist elsewhere
POPULATION DISTRIBUTION
• How individuals are arranged in an area
• Sometimes called population DISPERSION
• Three types: random, uniform and clumped
RANDOM
DISTRIBUTION
• Individuals are arranged within a space in no
particular pattern
• Found where resources needed are found
throughout an area
UNIFORM DISTRIBUTION
• Individuals evenly spaced throughout an area
• When individuals hold territory or compete for
space
CLUMPED
DISTRIBUTION
• Individual organisms arrange themselves
according to the availability of resources needed
to survive
• Most common in nature
• Examples: Desert animals live in patches
around isolated sources of water
• Harlequin Frogs around splash zones
• Humans by urban centers
• Ants living in colonies
AGE STRUCTURE
• Describes relative numbers of organisms of each
age in a population (also known as age
distribution)
• Age pyramids/age structure diagrams– visual
tools used to show age structure of population
• Can be useful for predicting population growth
SEX RATIOS
• Proportion of males to females
• Age structure diagrams can give this information
• 50:50 ratio ideal, otherwise harder for any
individual to reproduce and pass on genes