1 Chapter 53 Population Ecology • Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size processes influence population density, dispersion, and demographics •A population is a group of individuals of a single species living in the same general area Density and Dispersion • Density is the number of individuals per unit area or volume • Dispersion is the pattern of spacing among individuals within the boundaries of the population Density: A Dynamic Perspective • In most cases, it is impractical or impossible to count all individuals in a population • Sampling techniques can be used to estimate densities and total population sizes • Population size can be estimated by either extrapolation from small samples, an index of population size, or the mark- recapture method • Density is the result of an interplay between processes that add individuals to a population and those that remove individuals • Immigration is the influx of new individuals from other areas • Emigration is the movement of individuals out of a population
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1
Chapter 53
Population Ecology
• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size
Concept 53.1: Dynamic biological processes influence population
density, dispersion, and demographics• A population is a group of individuals of
a single species living in the same general area
Density and Dispersion
• Density is the number of individuals per unit area or volume
• Dispersion is the pattern of spacing among individuals within the boundaries of the population
Density: A Dynamic Perspective
• In most cases, it is impractical or impossible to count all individuals in a population
• Sampling techniques can be used to estimate densities and total population sizes
• Population size can be estimated by either extrapolation from small samples, an index of population size, or the mark-
recapture method
• Density is the result of an interplay between processes that add individuals to a population and those that remove individuals
• Immigration is the influx of new individuals from other areas
• Emigration is the movement of individuals out of a population
2
Fig. 53-3
Births
Births and immigrationadd individuals toa population.
Immigration
Deaths and emigrationremove individualsfrom a population.
Deaths
Emigration
Demographics
• Demography is the study of the vital statistics of a population and how they change over time
• Death rates and birth rates are of particular interest to demographers
Survivorship Curves
• A survivorship curve is a graphic way of representing the data in a life table
• The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate
Fig. 53-5
Age (years)
20 4 86
10
101
1,000
100
Nu
mb
er
of
su
rviv
ors
(lo
g s
cale
)
Males
Females
• Survivorship curves can be classified into three general types:
– Type I: low death rates during early and middle life, then an increase among older age groups
– Type II: the death rate is constant over the organism’s life span
– Type III: high death rates for the young, then a slower death rate for survivors
Fig. 53-6
1,000
100
10
10 50 100
II
III
Percentage of maximum life span
Nu
mb
er
of
su
rviv
ors
(lo
g s
ca
le)
I
3
Reproductive Rates
• For species with sexual reproduction, demographers often concentrate on females in a population
• A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population
• It describes reproductive patterns of a population
• Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
• In animals, parental care of smaller broods may facilitate survival of offspring
• Zero population growth occurs when the birth rate equals the death rate
• Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:
∆N∆t
= rN
where N = population size, t = time, and r = per capita rate of increase = birth – death
Exponential Growth
• Exponential population growth is population increase under idealized conditions
• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase
Fig. 53-10
Number of generations
0 5 10 150
500
1,000
1,500
2,000
1.0N=dN
dt
0.5N=dN
dt
Po
pu
lati
on
siz
e (
N)
4
Fig. 53-11
8,000
6,000
4,000
2,000
01920 1940 1960 1980
Year
Ele
ph
an
t p
op
ula
tio
n
1900
Concept 53.4: The logistic model describes how a population grows more slowly as it nears its carrying capacity
• Exponential growth cannot be sustained for long in any population
• A more realistic population model limits growth by incorporating carrying capacity
• Carrying capacity (K) is the maximum population size the environment can support
Fig. 53-12
2,000
1,500
1,000
500
00 5 10 15
Number of generations
Po
pu
lati
on
siz
e (
N)
Exponentialgrowth
1.0N=dN
dt
1.0N=dN
dt
K = 1,500
Logistic growth
1,500 – N
1,500
The Logistic Model and Real Populations
• The growth of laboratory populations of paramecia fits an S-shaped curve
• These organisms are grown in a constant environment lacking predators and competitors
Fig. 53-13
1,000
800
600
400
200
0
0 5 10 15
Time (days)
Nu
mb
er
of
Para
me
ciu
m/m
L
Nu
mb
er
of
Da
ph
nia
/50 m
L
0
30
60
90
180
150
120
0 20 40 60 80 100 120 140 160
Time (days)
(b) A Daphnia population in the lab(a) A Paramecium population in the lab
Population Dynamics
• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size
5
Stability and Fluctuation
• Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time
• Weather can affect population size over time
Fig. 53-18
2,100
1,900
1,700
1,500
1,300
1,100
900
700
500
01955 1965 1975 1985 1995 2005
Year
Nu
mb
er
of
sh
eep
• Changes in predation pressure can drive population fluctuations
Fig. 53-19
Wolves Moose
2,500
2,000
1,500
1,000
500
Nu
mb
er
of
mo
ose
0
Nu
mb
er
of
wo
lve
s50
40
30
20
10
01955 1965 1975 1985 1995 2005
Year
Population Cycles: Scientific Inquiry
• Some populations undergo regular boom-and-bust cycles
• Lynx populations follow the 10 year boom-and-bust cycle of hare populations
• Three hypotheses have been proposed to explain the hare’s 10-year interval
Fig. 53-20
Snowshoe hare
Lynx
Nu
mb
er
of
lyn
x(t
ho
usa
nd
s)
Nu
mb
er
of
ha
res
(th
ou
sa
nd
s)
160
120
80
40
01850 1875 1900 1925
Year
9
6
3
0
6
The Global Human Population
• The human population increased relatively slowly until about 1650 and then began to grow exponentially
Fig. 53-22
8000B.C.E.
4000B.C.E.
3000B.C.E.
2000B.C.E.
1000B.C.E.
0 1000C.E.
2000C.E.
0
1
2
3
4
5
6
The Plague
Hu
ma
n p
op
ula
tio
n (
bil
lio
ns)
7
• Though the global population is still growing, the rate of growth began to slow during the 1960s
Fig. 53-23
2005
Projecteddata
An
nu
al p
erc
en
t in
cre
ase
Year
1950 1975 2000 2025 2050
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
Regional Patterns of Population
Change
• To maintain population stability, a regional human population can exist in one of two configurations:
– Zero population growth = High birth rate – High death rate
– Zero population growth =Low birth rate – Low death rate
• The demographic transition is the move from the first state toward the second state
Limits on Human Population Size
• The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation
• It is one measure of how close we are to the carrying capacity of Earth
• Countries vary greatly in footprint size and available ecological capacity
7
Fig. 53-27
Log (g carbon/year)
13.4
9.85.8
Not analyzed
• Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes
Food From the SeaFood From the Sea
•• What types of organisms are harvested?What types of organisms are harvested?
–– Finfish (about 90% of worldwide harvest)Finfish (about 90% of worldwide harvest)
–– ShellfishShellfish
–– Other species such as jellyfish, sea Other species such as jellyfish, sea
cucumbers, polychaetes and seaweedcucumbers, polychaetes and seaweed
–– While seafood represents only about 1% of While seafood represents only about 1% of
the food consumed each year, it represents the food consumed each year, it represents
about 30% of total animal protein consumedabout 30% of total animal protein consumed
Worldwide Worldwide
Marine Catch Marine Catch
and Maricultureand Mariculture
Atlantic bluefin tuna Thunnusthynnus
• Can grow >300 cm; 680 kg
• Extremely streamlined, one of the ocean’s fastest swimmers, endothermic
Bluefin as food
• 2001 440 pound tuna sold for $220,000 ($500/pound)
• Farm in oceanic pens
• Spotter planes and electric harpoons
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Optimal Yield and OverfishingOptimal Yield and Overfishing
•• SeaSea--life species are renewable resourceslife species are renewable resources
•• However, for a fishery to last longHowever, for a fishery to last long--term, it term, it
must be fished in a sustainable waymust be fished in a sustainable way
•• The sustainable yield is the amount that The sustainable yield is the amount that
can be caught and just maintain a can be caught and just maintain a constant population sizeconstant population size
Collapse of a FisheryCollapse of a Fishery
•• A fishery is regarded as collapsed if A fishery is regarded as collapsed if
numbers fall to 10% of historic highsnumbers fall to 10% of historic highs
•• It is estimated that oneIt is estimated that one--third of fisheries third of fisheries
are already collapsedare already collapsed
•• A 2006 study indicates that all major A 2006 study indicates that all major fisheries will collapse by 2050 if protective fisheries will collapse by 2050 if protective
measure are not taken to better manage measure are not taken to better manage and protect these resourcesand protect these resources
Managing the ResourcesManaging the Resources
•• Management can be difficult for many Management can be difficult for many
reasons:reasons:
–– Maximum sustainable yield is difficult to Maximum sustainable yield is difficult to
calculatecalculate
–– Harvested species may compete with other Harvested species may compete with other
species and fishing pressure may affect species and fishing pressure may affect
competitive balancecompetitive balance
–– Real fisheries are more complex than modelsReal fisheries are more complex than models
–– High seas are High seas are ““common propertycommon property””