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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