Download 4. Population Dynamics new1

Populations, Their changes
and Their measurement
IB syllabus: 2.1.6, 2.3.1, 2.3.2,
2.6.1-2.6.4, 2.7.2
AP syllabus
Ch 9
Syllabus Statements
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2.1.6: Define the terms species, population, habitat, niche,
community and ecosystem with reference to local examples
2.3.1: Construct simple keys and use published keys for the
identification of organisms
2.3.2: Describe and evaluate methods for estimating
abundance of organisms
2.6.1: Explain the concepts of limiting factors and carrying
capacity in the context of population growth
2.6.2: Describe and explain s and J population curves
2.6.3: Describe the role of density-dependent and densityindependent factors and internal and external factors, in the
regulation of population
2.6.4: Describe the principles associated with survivorship
curves including K and r-strategists
2.7.2: Describe and evaluate methods for measuring change in
abiotic and biotic components of an ecosystem due to a specific
human activity
Vocabulary
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Abiotic factor
Biotic factor
Carrying Capacity
Habitat
K-strategist
Population
r-strategist
Population
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A group of individuals of the same
species found in the same area
(habitat) at the same time
The gopher tortoises in scrub
habitats in Volusia county
The bottlenose dolphins of the Indian
River Lagoon
Sea Otters: A case study
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Sea otters keystone species in Pacific kelp forests
Daily consume 25% body weight in urchins &
molluscs
Population > 1 million before settlers arrived
1700’s hunted to near extinction – 1000 in the
Aleutians, AK only 20 off California
In 1971 A-bomb test in AK used sea otter
population to assess bomb’s power  1000’s died
1973 Endangered Species Act passes, 1976
Marine Mammal Conservation Act
1989 1000’s died in Exxon Valdez Oil spill
Otters recovering in most places after 1970’s
The spring 2008 survey found 2760 sea otters,
down 8.8-percent from the record 2007 spring
survey.
New Threats?
Pollution Effects
- Shellfish magnify
toxins
- Reduce disease
resistance
- Reduce fertility
Increased Predation
- Killer Whales
- Switch to otters
when other food
is scarce
Population characteristics
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Populations are dynamic – change in
response to environment
• Size (# of individuals)
• Density (# of individuals in a certain space)
• Dispersion (spatial pattern of individuals)
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Random, Uniform, Clumped  based on food
• Age distribution (proportion of each age)
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Changes called Population dynamics
• Respond to environmental stress & change
Common Dispersion Patterns
Clumped
(elephants)
Uniform
(creosote bush)
Random
(dandelions)
Clumped is most common because resources have a patchy
distribution.
Limiting Factors & Population
Growth
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4 variables govern changes in
population size
• Birth, Death, Immigration, emigration
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Variables are dependent on resource
availability & environmental
conditions
Population change = (Birth +
Immigration)– (Death + Emigration)
POPULATION SIZE
Growth factors
(biotic potential)
Abiotic
Favorable light
Favorable temperature
Favorable chemical environment
(optimal level of critical nutrients)
Biotic
High reproductive rate
Generalized niche
Adequate food supply
Suitable habitat
Ability to compete for resources
Ability to hide from or defend
against predators
Ability to resist diseases and parasites
Ability to migrate and live in other
habitats
Ability to adapt to environmental
change
© 2004 Brooks/Cole – Thomson Learning
Decrease factors
(environmental resistance)
Abiotic
Too much or too little light
Temperature too high or too low
Unfavorable chemical environment
(too much or too little of critical
nutrients)
Biotic
Low reproductive rate
Specialized niche
Inadequate food supply
Unsuitable or destroyed habitat
Too many competitors
Insufficient ability to hide from or defend
against predators
Inability to resist diseases and parasites
Inability to migrate and live in other
habitats
Inability to adapt to environmental
change
Capacity for Growth
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Capacity for growth = Biotic potential
Rate at which a population grows with
unlimited resources is intrinsic rate of
increase (r)
High (r)  (1)reproduce early in life,
(2)short generation time, (3)multiple
reproductive events, (4)many offspring each
time
BUT – no population can grow indefinitely
Always limits on population growth in nature
Carrying Capacity
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Environmental resistance = all
factors which limit the growth of
populations
Population size depends on
interaction between biotic potential
and environmental resistance
Carrying capacity (K) = # of
individuals of a given population
which can be sustained infinitely in a
given area
Limiting Factors
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Carrying capacity established by limited
resources in the environment
Only one resource needs to be limiting
even if there is an over abundance of
everything else
Ex. Space, food, water, soil nutrients,
sunlight, predators, competition, disease
A desert plant is limited by…
Birds nesting on an island are limited by…
Minimum Values
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(r) depends on having a certain
minimum population size MVP –
minimum viable pop.
Below MVP
• 1 – some individuals may not find mates
• 2 – genetically related individuals reproduce
producing weak or deformed offspring
• 3 – genetic diversity may drop too low to
enable adaptation to environmental changes
–bottleneck effect
Forms of Growth
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Exponential growth  starts slow and
proceeds with increasing speed
• J curve results
• Occurs with few or no resource limitations
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Logistic growth  (1) exponential
growth, (2) slower growth (3) then
plateau at carrying capacity
• S curve results
• Population will fluctuate around carrying
capacity
Population Growth Curves Ideal
© 2004 Brooks/Cole – Thomson Learning
Population size (N)
Population size (N)
K
Time (t)
Exponential Growth
Time (t)
Logistic Growth
Carrying capacity alterations
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In rapid growth population may
overshoot carrying capacity
• Consumes resource base
• Reproduction must slow, Death must
increase
• Leads to crash or dieback
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Carrying capacity is not fixed, affected
by:
• Seasonal changes, natural & human
catastrophes, immigration & emigration
Density Effects
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Density Independent Factors: effects
regardless of population density
Mostly regulates r-strategists
• Floods, fires, weather, habitat destruction,
pollution
• Weather is most important factor
Density Effects
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Density dependent Factors: effects based on
amount of individuals in an area
Operate as negative feedback mechanisms
leading to stability or regulation of population
External Factors
• Competition, predation, parasitism
• Disease – most epidemics spread in cramped
conditions
Internal Factors
• Reproductive effects  Density dependent fertility,
Breeding territory size
Natural Cycles: Predation
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Over longer time spans populations
cycle
Canadian lynx & Snowshoe hare - 10
year cycles
Once thought that predators controlled
prey #’s  Top down control
Now see a negative feedback
mechanism in place  community
equilibrium
Population size (thousands)
160
140
Hare
120
Lynx
100
80
60
40
20
0
1845
1855
1865
1875
1885
1895
Year
1905
1915
1925
1935
5,000
Moose population
Wolf population
3,000
100
90
80
2,000
70
60
50
40
1,000
30
20
500
10
0
1900 1910
1930
1950
Year
1970
1990
2000
1999
Number of wolves
Number of moose
4,000
Reproduction Strategies effect
Survival
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Asexual reproduction
• Produce clones of parents
• Common in constant environments
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Sexual reproduction
• Mating has costs – time, injury, parental
investment, genetic errors
• Improves genetic diversity  survive
environmental change
• Different male & female roles in
parental care
MacArthur – Wilson Models
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Two idealized categories for reproductive patterns
but really it’s a continuum
r-selected & K-selected species depending on
position on sigmoid population curve
r-selected species: (opportunists) reproduce early,
many young few survive
• Common after disturbance, but poor competitors
K-selected species: (competitors) reproduce late,
few young most survive
• Common in stable areas, strong competitors
Carrying capacity
K
Number of individuals
K species;
experience
K selection
r species;
experience
r selection
Time
r-Selected Species
cockroach
dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species
K-Selected Species
elephant
saguaro
Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close
to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species
r versus K
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Most organisms somewhere in the
middle
Agriculture  crops = r-selected,
livestock = K-selected
Reproductive patterns give
temporary advantage
Resource availability determines
ultimate population size
Survivorship curves
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Different life expectancies for different
species
Survivorship curve: shows age structure
of population
Late loss curve: K-selected species with
few young cared for until reproductive
age
Early loss curve: r-selected species
many die early but high survivorship
after certain age
Constant loss curve: intermediate
steady mortality
Percentage surviving (log scale)
100
10
1
0
Age
Humans Impact Natural
Populations
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Fragmenting & degrading habitats
Simplifying natural ecosystems
Using or destroying world primary
productivity which supports all consumers
Strengthening pest and disease populations
Eliminating predators
Introducing exotic species
Overharvesting renewable resources
Interfering with natural chemical cycling
and energy flow
Environmental Stress
Organism Level
Population Level
Ecosystem Level
Physiological changes
Psychological changes
Behavior changes
Fewer or no offspring
Genetic defects
Birth defects
Cancers
Death
Change in population size
Change in age structure
(old, young, and weak may die)
Survival of strains genetically
resistant to stress
Loss of genetic diversity
and adaptability
Extinction
Disruption of energy flow through
Disruption
of biogeochemical
food chains
and webs
cycles
Disruption of biogeochemical
Habitat
cyclesloss & degradation
Lower species
species diversity
diversity
Lower
Less
complex
food
webs
Habitat loss or degradation
Lowercomplex
stabilityfood webs
Less
Ecosystem
collapse
Lower
stability
Ecosystem collapse
© 2004 Brooks/Cole – Thomson Learning
Sampling populations
Step 1: Identify the organism
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Use dichotomous keys, field guides,
observe a museum collection, or consult
an expert
http://www.earthlife.net/insects/orderskey.html#key
Sample key for insect ID
http://people.virginia.edu/~sosiwla/Stream-Study/Key/Key1.HTML
Macroinvertebrate key
Construct you Own Dichotomous Key
Mark & Recapture Method
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Used for fish & wildlife populations
Traps placed within boundaries of study area
Captured animals are marked with tags, collars,
bands or spots of dye & then immediately released
After a few days or weeks, enough time for the
marked animals to mix randomly with the others in
the population, traps are set again
The proportion of marked (recaptured) animals in
the second trapping is assumed equal to the
proportion of marked animals in the whole
population
Repeat the recapture as many times as possible to
ensure accuracy of results
Marking method should not affect the survival or
fitness of the organism
Mark & Recapture Calculation
# of recaptures in second catch
Total # in second catch
=
# marked in the first catch
Total population (N)
Assuming no births, deaths, immigration, or emigration
 population size is estimated as follows (Lincoln
Index)
N
=
(# marked in first catch) (Total # in second catch)
# of Recaptures in second catch
MEMORIZE THIS EQUATION
Example
50 snowshoe hares are captured in box
traps, marked with ear tags and released.
Two weeks later, 100 hares are captured
and checked for ear tags. If 10 hares in
the second catch are already marked
(10%), provide an estimate of N
N = (50 hares x 100 hares) / 10 = 5000 /
10
= 500 hares
**Realize for accuracy that you would
recapture multiple times and take an
average**
Quadrat Method
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Used for plants or sessile organisms
Mark out a gridline along two edges of an area
Use a calculator or tables to generate two random
numbers to use as coordinates and place a
quadrat on the ground with its corner at these
coordinates
Count how many individuals of your study
population are inside the quadrat
Repeat steps 2 & 3 as many times as possible
Measure the total size of the area occupied by the
population in square meters
Calculate the mean number of plants per quadrat.
Then calculate the population size with the
following equation
Quadrat Method
N = (Mean # per quadrat) (total area)
Area of each quadrat
This estimates the population size in an
area
Ex. If you count an average of 10 live oak trees per
square hectare in a given area, and there are 100
square hectares in your area, then
N = (10 X 100 hectare2) / 1 hectare2 = 1000 trees in
the 100 hectare2
In addition to population size we can
measure…
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Density = # of individuals per unit area
• Good measure of overall numbers
Frequency = the proportion of quadrats sampled
that contain your species
• Assessment of patchiness of distribution
% Cover = space within the quadrat occupied by
each species
• Distinguishes the larger and smaller species
How can changes in these
populations be measured?
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Necessary because populations may
change over time through processes
like succession
But also because human activities
may impact a population and we
want to know how
• Impacts include  toxins from mining,
landfills, eutrophication, effluent, oil
spills, overexploitation
Measuring changes cont.
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Can still use CMR or quadrat method
Just do it repeatedly over time
Also could use satellite images taken over
time
1. Do pre and post impact assessments
in one area
2. Measure comparable areas – one
impacted, one not at a given time
Overexploitation, Agricultural use, Global Warming have
Caused a decrease in Lake Chad’s area over last 50 years
Lake
Chad
Satellite
Images
Capture – Mark Recapture
Practice Problems
Question 1
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In a mark – recapture study of lake
trout populations, 40 fish were
captured, marked and released. In a
second capture 45 fish were caught;
9 of these were marked. What is the
estimated number of individuals in
the lake trout population
Question 2
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Woodlice are terrestrial crustaceans
that live under logs and stones in damp
soils. To assess the population of
woodlice in an area, students collected
as many of the animals as they could
find, and marked each with a drop of
fluorescent paint. A total of 303 were
marked. 24 hours later, woodlice were
collected again in the same place. This
time 297 were found, of which 99 were
seen to be already marked from the
first time. What approximately, is the
estimated population of woodlice in this
Review points
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Dispersion patterns
Carrying capacity and limiting
factors
r and K selection
Natural population cycles
Human effects
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http://www.otterproject.org