Chapter 52
• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size.
• Demographics refers to the study of vital statistics especially birth and death rates.
Fig. 53-14
Population, Density and Dispersion• A population is a group of individuals
of a single species living in the same general area
• Density is the number of individuals per unit area or volume
Ex: the number of squirrels per square kilometer or
The number of esterichia coli bacteria per milliliter in a test tube.
Determining Density• In most cases, it is impractical or impossible to
count all individuals in a population
Researchers use different methods to estimate:• One way is to count the number of individuals in
a series of randomly located plots, calculate the average density in the samples, and extrapolate to estimate the population size in the entire area.
• Such estimates are accurate when there are many sample plots and a homogeneous (evenly dispersed) habitat.
The mark-recapture methodCommonly used to estimate wildlife populations.
Individuals are trapped and captured, marked with a tag, recorded, and then released.
•After a period of time has elapsed, traps are set again, and individuals are captured and identified.
•The second capture yields both marked and unmarked individuals.
•From this data, researchers estimate the total number of individuals in the population.
Fig. 53-3
Births
Births and immigrationadd individuals toa population.
Immigration
Deaths and emigrationremove individualsfrom a population.
Deaths
Emigration
Density in a population depends on:
•Immigration is the influx of new individuals from other areas•Emigration is the movement of individuals out of a population
• Dispersion is the pattern of spacing among individuals within the boundaries of the population.. They may be – Clumped
– Uniform
– random
Patterns of Dispersion• Dispersion is clumped when individuals
aggregate in patches.• Plants and fungi are often clumped where soil
conditions favor germination and growth.• Animals may clump in favorable
microenvironments (such as isopods under a fallen log) or to facilitate mating interactions.
• Group living may increase the effectiveness of certain predators, such as a wolf pack.
Fig. 53-4a
(a) Clumped dispersion in the intertidal zone
Video: Flapping Geese (Clumped)Video: Flapping Geese (Clumped)
CLUMPED DISPERSION in a forest
• Dispersion is uniform when individuals are evenly spaced.
• For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors.
• Animals often exhibit uniform dispersion as a result of territoriality, the defense of a bounded space against encroachment by others.
Fig. 53-4b
(b) Uniform
• In random dispersion, the position of each individual is independent of the others, and spacing is unpredictable.
• Random dispersion occurs where there is neither a strong attraction or repulsion among individuals in a population, or when key physical or chemical environmental factors are relatively homogeneously (evenly) distributed.
• For example, plants may grow where windblown seeds land.
Fig. 53-4c
(c) Random
Video: Prokaryotic Flagella (Video: Prokaryotic Flagella (Salmonella typhimuriumSalmonella typhimurium) (Random)) (Random)
Demography• Demography is the study of the vital
statistics of populations and how they change over time.
• Of particular interest are– birth rates and how they vary among
individuals (specifically females)– death rates.
• A life table is an age-specific summary of the survival pattern of a population.
Fig. 53-5
Age (years)20 4 86
10
101
1,000
100
Nu
mb
er o
f su
rviv
ors
(lo
g s
cale
)
Males
Females
A graphic way of representing the data in a life table is a survivorship curve.
A graphic way of representing the data in a life table is a survivorship curve.
• Survivorship curves can be classified into three general types:
• A Type I curve is relatively flat at the start, reflecting a low death rate in early and middle life, and drops steeply as death rates increase among older age groups.
• Humans and many other large mammals exhibit Type I survivorship curves.
• The Type II curve is intermediate, with constant mortality over an organism’s life span.
• Many species of rodent, various invertebrates, and some annual plants show Type II survivorship curves.
• Prey species that are subject to predation.
• A Type III curve drops off at the start, reflecting very high death rates early in life, then flattens out as death rates decline for the few individuals that survive to a critical age.
• Type III are usually organisms that produce large numbers of offspring but provide little or no parental care.
• Examples are many fishes, long-lived plants, and marine invertebrates.
• Also young that are subject to predation and severe environmental conditions
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
Idealized survivorship curves:
Types I, II, and III
Life history traits • Natural selection favors traits that improve an
organism’s chances of survival and reproductive success.
• In every species, there are trade-offs between survival and traits such as
1. The age at which reproduction begins
2. How often the organism reproduces
3. How many offspring are produced during each reproductive cycle
• The traits that affect an organism’s schedule of reproduction and survival make up its life history.
Evolution and Life History Diversity
• Life histories are very diverse• Species that exhibit semelparity, or big-bang
reproduction, reproduce once and die• Species that exhibit iteroparity, or repeated
reproduction, produce offspring repeatedly• Highly variable or unpredictable environments
likely favor big-bang reproduction, while dependable environments may favor repeated reproduction
Fig. 53-7
Century Plants grow in arid climates with unpredictable rainfall. After several years it sends up a large flowering stalk, reproduces and then dies.
semelparity
• Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
Dandelion
• Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
Coconut palm
how much does an individual gain in reproductive success through one pattern
versus the other?• The critical factor is survival rate of the offspring.• In highly variable or unpredictable environments,
when the survival of offspring is low, semelparity (big-bang) reproduction is favored.
• In dependable environments where competition for resources is intense, iteroparity (repeated reproduction ) is favored.
Per Capita Rate of Increase
• If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
• 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:
Nt
rN
where N = population size, t = time, and r = per capita rate of increase = birth – death
Population Growth
Exponential population growth is apopulation increase under idealized conditions• rate of reproduction is at its maximum,
called the intrinsic rate of increase.• Any species regardless of life history is
capable of exponential growth if resources are unlimited
Fig. 53-10
Number of generations
0 5 10 150
500
1,000
1,500
2,000
1.0N =dNdt
0.5N =dN
dt
Po
pu
lati
on
siz
e (N
)
J shaped curve
Exponential curve
Fig. 53-11
8,000
6,000
4,000
2,000
01920 1940 1960 1980
Year
Ele
ph
ant
po
pu
lati
on
1900
The J-shaped curve of exponential growth characterizes some rebounding populations such as the African Elephant
The Logistic Growth Model (logical)
• Exponential growth cannot be sustained for long in any population.
• Limiting resources, food, water, space eventually limit population growth.
• Carrying capacity (K) is the maximum population size the environment can support.
• In the logistic growth model, per capita rate of increase slows down or declines as carrying capacity is reached.
Fig. 53-13
1,000
800
600
400
200
00 5 10 15
Time (days)
Nu
mb
er o
f P
aram
eciu
m/m
L
Nu
mb
er o
f D
aph
nia
/50
mL
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
Figure 53.13 How well do these populations fit the logistic growth model?
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 growth1,500 – N
1,500
S shaped curve
J shaped curve
The Logistic Model and Life Histories
• Life history traits favored by natural selection may vary with population density and environmental conditions.
• K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density
• r-selection, or density-independent selection, selects for life history traits that maximize reproduction
Density- Dependent factors that regulate population growth
Factors that reduce birth rate or increase death rates are density dependent.
• Competition for resources such as food, space, water or essential nutrients intensifies as populations increase.
• Territoriality- available space for territory or nesting may be limited.
• Disease- Increasing densities allow for easier transmission of disease.– Swine flu is a perfect example
• Predation- As prey populations increase, predators may find prey more easily.
Density-dependent regulation provides a negative feedback system that helps reduce birth rates and increase death rates or a population would grow exponentially!
Density Independent Factors
When a birth or death rate does not change with regard to population density it is said to be density independent.
• Natural Disasters will cause an increase in death rate regardless of density.
• Weather and climate such as floods or
Drought are density independent factors.
The human population
Global human populations have grown almost continuously throughout history.
But skyrocketed after the industrial revolution.
Note the dip resulting from the plague in Europe during the 1300 s
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
man
po
pu
lati
on
(b
illio
ns)
7
The Global Human Population
• The human population increased relatively slowly until about 1650 and then began to grow exponentially
• 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
ent
incr
ease
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
Famine in china 1960s about 60 million died
Age Structure Pyramids
• One important demographic factor in present and future growth trends is a country’s age structure
• Age structure is the relative number of individuals at each age
• Each horizontal bar represents a specific age group dividing male and female sides
Fig. 53-25
Rapid growthAfghanistan
Male Female Age AgeMale Female
Slow growthUnited States
Male Female
No growthItaly
85+80–8475–7970–74
60–6465–69
55–5950–5445–4940–4435–3930–3425–2920–2415–19
0–45–9
10–14
85+80–8475–7970–74
60–6465–69
55–5950–5445–4940–4435–3930–3425–2920–2415–19
0–45–9
10–14
10 10 8 866 4 422 0Percent of population Percent of population Percent of population
66 4 422 08 8 66 4 422 08 8
Wide bottom small top Developing nations rapid growth
Columnar structure industrialized nation slow growth
Small bottom wider in the middle stable population
Estimates of Carrying Capacity• The carrying capacity of Earth for humans is
uncertain• The average estimate is 10–15 billion people• A concept termed ecological footprint
examines the total land and water area needed for all resources a person consumes in a population.
• Currently 1.7 hectares (app 4.2 acres) per person is considered sustainable.
• A typical person in the United States has a footprint of 10 hectares (almost 25 acres)
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