Download S i Section 4

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„
Population – group of interbreeding
individuals occupying the same habitat at
the same time
… Water
lilies in a particular lake
in New York City
… Humans
Section 4
S i Population ecology – study of what factors
affect population size and how these
factors change over space and time
„ Uses the tools of demography – birth
rates, death rates, age distributions, and
sizes of populations
„
Professor Donald McFarlane
Lecture 18 Ecology:
Population Growth
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„
Understanding populations
Quantifying population density
… Simple
visual count
methods to extrapolate captured
organism number to size of population
… Mark-recapture method
… Sampling
Density – number of organisms in a given
unit area
„ Population growth affects population
density
„ Knowledge can help us make decisions
about the management of species
„
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Dispersion patterns
… Clumped
Most common
Resources tend to be clustered in nature
„ Social behavior may promote this pattern
„
„
… Uniform
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Competition may cause this pattern
May also result from social interactions
… Random
Rarest
Resources are rarely randomly spaced
„ May occur where resources are common and
abundant
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Reproductive strategies
… Semelparity
– produce all offspring in single
reproductive event, individuals reproduce
once and
d di
die
… Iteroparity – reproduce in successive years or
breeding seasons
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„
Seasonal iteroparity – distinct breeding seasons
Continuous iteroparity – reproduce repeatedly at
any time of the year
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„
Age classes
strategy has a strong effect on
subsequent age classes of a population
… Semelparous organisms with same-aged
young called cohorts
… Iteroparous organisms have young of different
ages
… Expect a population increasing in size to have
many young and a decreasing population to
have few young
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Percent of trees
… Reproductive
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20
10
20
30
(a) Undisturbed forest
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50
60
10
20
Age of trees (years)
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40
50
60
70
(b) Overgrazed forest
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Life tables
… Data
on the number of individuals alive in a
p
particular
age
g class
… Males are usually not included
… North American beaver example
„
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Trappers provided mandibles
Teeth extracted for age classification
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Survivorship curve – plots numbers of
surviving individuals at each age
… Use
log scale to make it easier to examine
wide range of population sizes
… Beavers have a fairly uniform rate of death
over the life span
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3 patterns of survivorship curves
… Type
I – rate of loss of juveniles low and most
individuals lost later in life
… Type II – fairly uniform death rate
„
Beaver example
… Type
III – rate of loss for juveniles high and
then loss low for survivors
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Age-specific fertility rate, mx
… Proportion
of female offspring born to females
of reproductive age
… 100 females p
produce 75 female offspring
p g
mx=0.75
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„
R0 = net reproductive rate
… Overall
growth rate per generation
… Number of offspring born to females of all ages
Age-specific survivorship rate, lx
… Use
survivorship data to find proportion of
individuals alive at the start of any given age
class
„ lxmx
= contribution of each age class to
overall population growth
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How populations grow
„
To calculate future size of population,
multiply number of individuals in the
population
p
p
by
y the net reproductive
p
rate
„ For beaver example,
„
… Nt+1
„
= NtR0
… Exponential
growth – resources not limiting,
prodigious growth
… Logistic growth – resources limiting, limits to growth
= 1,000 x 1.1
= 1,100
If R0>1, population growing
If R0<1, population declining
„ If R0 = 1, population is at equilibrium
Life tables can provide accurate information
about how populations grow from generation to
generation
Simpler models can give insight to shorter time
periods
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… When
r>0, population increase is rapid
J-shaped curve
… Intrinsic rate of increase
increase, rmax = r at maximum
… Because population growth depends on the
value of N as well as the value of r, the
population increase is even greater as time
passes
… Reintroduction of a population to a habitat,
growth of introduced exotic species, and
global human population
in population size over any time
… Characteristic
period
p
births and deaths expressed per
individual
… 100 births to 1000 deer = 0.10
… 50 deaths in 1000 deer = 0.50
… Often
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400
600
Predicted abundance
Actual abundance
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Num
mber of animals
… Change
Exponential growth
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Per capita growth rate
Popu
ulation size
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300
200
100
200
100
0
0
1970
1980
1990
Year
(a) Tule elk
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2000
Survey year
(b) Black-footed ferrets
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Logistic growth
Logistic growth – pattern where growth
slows down as it approaches K
„ Model fits some populations but not others
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… For
most species, resources become limiting
as p
populations
p
g
grow
… Carrying capacity (K) or upper boundary for
population
… Logistic equation
… Variations
in nature change resource levels
that cause changes in carrying capacity
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Density-dependent factors
… Mortality
factor whose influence varies with the
density of the population
… Parasitism, predation, and competition
… Predators kill few prey when the prey population is
l
low,
th
they kill more prey when
h th
the population
l ti iis hi
higher
h
… Detected by plotting mortality against population
density and finding positive slope
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Density-independent factor
… Mortality
factor whose influence is not affected by
changes in population size or density
… Generally physical factors – weather, drought, flood,
fire
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„
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Life history strategies
… Continuum
… r-selected
species – high rate of per capita
population growth, r, but poor competitive
ability (weeds)
… K-selected species – more or less stable
populations adapted to exist at or near
carrying capacity, K
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Lower reproductive rate but better competitors
(trees)
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Human population growth
In 2006, the world’s population was
estimated to be increasing at the rate of
146 people every minute
„ 2 in developed nations and 144 in less
developed nations
„ Human growth fits an exponential pattern
„
… Low
until agriculture and animal
domestication
… Between 1750 and 1998, population surged
from 800 million to 6 billion
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Popula
ation (billions)
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6
2000
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5
4
3
2
1
1975
1.
Human populations can exist at equilibrium
densities in one of two ways
High birth and high death rates
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1950
1900
1800
2.
B f
Before
1750,
1750 thi
this was often
ft th
the case, with
ith high
hi h bi
birth
th
rates offset by deaths from wars, famines, and
epidemics
Low birth and low death rates
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In Europe, beginning in the 18th century, better
health and living conditions reduced the death rate
„ Eventually, social changes such as increasing
education for women and marriage at a later age
reduced the birth rate
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Stage 1
Stage 2
Stage 3
Stage 4
Relative
population
size
High birth rate
High death rate
Birth rate
decreases
Death rate
decreases
Death rate
Low
r
Birth rate
Low birth rate
Low death rate
Relative po
opulation size
Demographic transition
… Shift in birth and death rates with
development
… First stage - birth and death rates are both
high, and the population remains in
equilibrium
… Second stage - death rate declines first
first, while
the birth rate remains high - high rates of
population growth result
… Third stage - birth rates drop and death rates
stabilize, so that population growth slows
… Fourth stage - both birth and death rates are
low, and the population is again at equilibrium
Birth and death rates
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Relative population size
Increasing Maximal Decreasing
r
Low
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Age structure
… Relative
numbers of individuals in each
defined age
g g
group
p
… Commonly displayed as population pyramid
… Helps predict future population growth
„
Exact pace varies between countries depending
on culture, economics, politics, and religion
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Earth’s carrying capacity
… Many
and varied estimates
has a huge influence
… Total fertility rate – average number of live
births a woman has during her lifetime
… Lifestyle
Global TFR declined from 4.47 in 1970s to 2.59 in
2007
„ 2.3 needed for zero population growth
„ Differs considerably between geographic areas
„ In developed nations, population has stabilized
„ In developing countries, population is still
increasing dramatically
„
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6.7
4.7
North
America 5.0
Europe
5.0
2.3
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2.4
A i
Asia
Africa
Latin America
& Caribbean
1970–1975
1.5
3.2
2.3
Populatio
on (billions)
2.2
2.0 2.0
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TFR of 2.5 (high)
TFR of 2.0 (medium)
TFR of 1.5 (low)
10.8
9.2
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7.8
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Oceania
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2000 2005 2010 2015 2020 2025 2030 2035 2040 2045 2050
Year
2005
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Ecological footprint
… Aggregate
total of productive land needed for
survival in a sustainable world
… Average footprint size is about 3 hectares
(1ha=10,000 m2)
… Wide variation is found around the globe
„
7.5 for Canadians, 10 for Americans
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