Post on 09-Jul-2020
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Bio 1101 Lecture 19
Chapter 19: Population Ecology
• Click here for video
intro
What is population ecology?
• Major questions of population ecology:
– What influences the population size?
– What influences the population’s growth rate
(change in population size)?
– What influences population density (#
individuals per unit area or volume)
– What are some of the features of the
population structure? (numbers of individuals
of different sexes or age classes)
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• Population = a group of individuals of same species
living in a given area at a given time
• The “area” may be arbitrary – depends on the question
you are asking
• Click here for audio:
• Population Density
– Number (N) of individuals per area or volume
– Usually cannot be directly counted • Usually not possible to see every individual, and/or the area
may be too large to physically survey every individual
– May be estimated from counts on multiple,
randomly selected sample plots • The more sample plots, the more accurate the estimate
• Example: your grades… would it be accurate to assess your
performance in class based on just one exam? Or to
estimate the average height of a student at OSU by only
sampling four individuals from the men’s basketball team?
• Random samples and a large sample size increase accuracy
of estimates
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• Once density is known, you can multiply
the number of individuals per unit area by
the total area to obtain an estimate of N
• For example to estimate the number of
daisies in a meadow, you could estimate
the density per m2 using a sample of
randomly selected plots
• Video on using quadrat sampling to
estimate density and population size:
• https://www.youtube.com/watch?v=RhMO
CxXcDrQ
– Note: we are just concerned with the first part
of the video (about the first three minutes)
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– Mark-recapture method
• Trap animals in an area
• Mark the captured
animals
• Release the marked
animals
• Later, set traps again
• How many of the
animals you catch are
marked?
• Click here for audio:
• The number of animals you initially
capture and mark (m1) is some fraction of
the overall population size (N):
m1
N
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• The animals in your second capture that
are marked (in other words, your re-
captures, m2) are some fraction of the total
number you catch the second time(n2):
m2
n2
• In a well-designed mark-recapture study,
these two fractions should be equal
m1 / N = m2 / n2
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• Population size (N) calculated by solving
for N:
– N =(m1 X n2)/m2
– Where m1 = # of individuals caught and
marked the first time; n2= total # of individuals
caught the second time; and m2 = # of
individuals in the second capture that were
marked
• Mark-recapture makes the following
assumptions:
– Marked and unmarked individuals have equal
probability of being caught the second time
• No trap-happy or trap-shy individuals
• Click here for audio:
– Marked and unmarked individuals are equally
dispersed in the area
• You don’t have your traps in an area where most
(or hardly any) of the marked individuals are
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• Being able to determine
the size of a population
is important for most
ecological and
conservation studies
• Scientists must be able
to determine how
population size or
density responds to
various changes in
environment
Sample Mark-Recapture Problem
• You are attempting to estimate the size of a population
of chickadees in a forest. You set up “mist nets” to catch
the birds flying through the forest, and you catch a
sample of 14 chickadees, which you mark with plastic
bands on their legs. You release those birds, and then a
week later set up your mist nests again. This time you
catch 12 chickadees, and of those, 6 are marked. What
is you estimate of population size?
• m1 = 14
• n2 = 12
• m2 = 6
• So N = (14 X 12)/6 = 28
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Activity Quiz!
Log in to Carmen Canvas and take the activity quiz entitled “Population Ecology: Mark Recapture” by the end of the day on Monday, April 20th.
This 2-question quiz is worth 4 activity points
Population Growth Models
• Most organisms have the potential to
generate many more offspring than their
habitats can support in a short time
What limits population growth?
factors like availability of food, water, and space can limit the
population’s growth
diseases and predators can increase mortality and thus
decrease population growth
natural disasters like floods or hurricanes can kill large
numbers of individuals, and thus decrease population growth
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• In an ideal environment, organisms grow
exponentially:
• In Exponential Growth, a population multiplies by
a constant factor during constant time intervals
– i.e. each bacterial cell divides every 20 minutes, no
matter how many bacterial cells there are
• Rate of growth of the population (“G”) depends
on:
– the organism’s intrinsic rate of growth (“r”): an
organism’s inherent capacity to reproduce under ideal
conditions
– the number of individuals in the population (“N”)
– G = rN
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– Rabbit populations may temporarily
experience exponential growth, but no natural
environment can sustain exponential growth
indefinitely
• In nature, populations
may grow
exponentially for a
short time, but then
something limits their
growth, and the
growth curve levels
off
• This is the Logistic
Growth Model
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• One or more population-limiting factors
prevent growth from exceeding the
“carrying capacity” (K) of the population
• The carrying capacity (K) of a population is
the number of individuals of a population
the environment can maintain with no net
increase or decrease
• What regulates population growth? (refer
back to our list of factors that limit growth)
• Density-Dependent Factors: a population-
limiting factor whose effects intensify as
population sizes increase
– Examples? (food, space, disease, predators)
• Density-Independent Factors: a
population-limiting factor whose effects are
independent of population size
– Examples? (natural disasters)
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– Seal population on St. Paul Island off coast of Alaska
demonstrate logistic growth
– Populations were low prior to 1925 due to uncontrolled
hunting
– After 1925, hunting was regulated, and populations
increased to about 10,000, the carrying capacity for this
species on the island
• Many populations of organisms go through
“boom and bust” cycles
• Example: snow shoe hares and lynx
• Click here for audio:
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Technical and
cultural advances Agricultural advances
Industrial
and medical
advances
7
6
5
4
3
2
1
0
2003
1999
1987
1975
1960
1930
1830
12,000 11,000 10,000 9000 8000 7000 6000 5000 4000 3000 2000 1000 B.C./A.D. 1000 2000 B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. A.D. A.D.
What about human populations?
• They’re getting bigger… a lot bigger!
• Why? What the heck happened???
• Check out these links:
• https://www.census.gov/popclock/
• https://www.youtube.com/watch?v=evgdu8yHXww
Some Sobering Statistics
• There are over 7.6 Billion
people on earth today
• At current growth rate,
there will be 8.6 Billion by
2030
• Every 20 minutes, the
human population grows by
~3000 individuals
• Every 20 minutes, a
species of plant or animal
goes extinct
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• Can the planet support all these people?
• What is the carrying capacity of the planet for
people?
• Not all people use the same amount of
resources
• Wealthier nations have a larger “ecological
footprint” per person
– An estimate of the land and water area required to
provide the resources a individual consumes, and to
absorb the waste generated
• Countries with a larger population (like China)
can have a bigger total ecological footprint
• As of 2012, the average ecological footprint of the
Earth’s human population was 1.5X the capacity of the
planet
• We would need 4 Earths to support everyone living at
the average American standard of living
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Biodiversity Crisis – Biological diversity consists of all the species
of organisms on earth and their genetic diversity
– Human activities have caused (and continue to cause) the decline and extinction of many species
– At risk of extinction: • 13% of birds
• 21% of fish
• 25% of mammals
• 41% of amphibians
• The United States has
the highest number of
extinct species, and the
second-highest number
of threatened species
– Source: Mongabay.com
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• Most extinctions are a result of habitat
destruction
• Also pollution, climate change, spread of
exotic species, etc.
• Every year, an area of
tropical rainforest the size of
West Virginia is destroyed
– Rainforests are
biodiversity “hotspots”
with very high levels of
endemism
– Half of all species live in
tropical rainforests
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– Globally, we have lost 50% of rainforest cover
– At current rates of habitat destruction, 5-10% of
tropical rainforest species will go extinct each
decade for the next 50 years
– Primary cause of destruction: conversion to
agriculture
• 53% of the wetlands in the United States have been lost since the 1780s
– Historically viewed as “wastelands”
– Drained for agriculture and other development
– Wetlands provide important functions, such as flood control, aquifer recharge, and wildlife habitat
– Ohio has lost 90% of its wetlands (second only to California, which has lost 91%)
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Exotic Invasive Species
– Each species evolves in relation to its particular environment (climate, prey, predators, etc.)
– When a species is introduced to a new area, if it is able to survive, it may cause ecological problems
• Are there any predators now to keep its population in check?
• Will it be better able to obtain resources in the new location than the native species, thus out-competing and replacing them?
• Will it be poisonous to native species that may try to consume it?
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• Examples of exotic invasive species
• Kudzu: an invasive vine that was introduced in the
southern/eastern US to control erosion
• Amur Honeysuckle: an invasive shrub that grows in the
understory of forests, out-competing native flowers and
shrubs for light and soil moisture
• Invasion of southern Florida by Burmese Python
• Problem for endangered island species, Key
Largo Woodrat – Pythons were introduced to Florida when people released unwanted
pet pythons
– These pythons damage ecosystems by eating many small mammals
– Pythons have been found in Key Largo, which surprised scientists
because they didn’t think they would be able to swim from the mainland
out to the Keys
– If they aren’t controlled, the pythons could easily lead to the extinction
of the Key Largo Woodrat
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Aliens in Paradise…
Many of Hawaii’s plants and animals are endemic
Endemic = native to, and restricted to, a particular geographic region
89% of flowering plants
50% of all plants and animals
Evolved in the absence of grazing and browsing mammals and many terrestrial predators
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• People began introducing exotic species
to Hawaii as early as 400 A.D.
– Dogs, pigs, rats, etc.
– Led to extinction of many species, including
flightless birds
• Settled by Europeans in 1700s, and
thousands of species were introduced
• Fire tree
– Evergreen shrub native to
Madagascar
– Planted to control erosion
– Fruits and seeds dispersed
by birds and mammals
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• Brown Tree Snake
– Major problem for birds in Guam
– A recent effort to control their population: parachuting
dead, Tylenol-laced mice into the forests of Guam!
– Video:
https://www.youtube.com/watch?v=7Sl7in3oV3w
• Invasion of Asian Carp – Originally introduced from Southeast Asia to help keep
waste water treatment ponds clean
– Flooding allowed them to escape and they invaded
Mississippi River system
– Compete with native species for food
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• Video: Asian Carp Attacks CNN Reporter!
http://www.youtube.com/watch?v=ZpZ7smai2p8
• 50,000 introduced species in the U.S.
• Not all are harmful…
• …But those that are do much damage to
natural communities
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Lots of bad news…
• What can be done?
– Nobody can do everything, but everyone can do something!
– Support a conservation organization, recycle, landscape with native plants, etc.
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• All for today, except…
• Random Animal of the Day!
• Pallas’s Cat! – A small wild cat with an unusual face! Pupils form circles
rather than slits (as in other cats)
– Lives in rocky steppes and grasslands of central Asia
– Listed as “nearly threatened” due to habitat loss,
fragmentation, and disease