Post on 21-Dec-2015
transcript
Population Ecology
AP Chap 53
• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size
Fig. 53-1
A population is a group of individuals of a single species living in the same general area
Every population has geographic boundaries.
• 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
Population density is often determined by sampling techniques
Population size can be estimated by
1. Direct counting
2. Random sampling based on sample plots (quadrats)
3. Indexes such as tracks, nests, burrows, fecal droppings, etc.
4. Mark and recapture method
Quadrat sampling
Mark-recapture Method
Mark-Recapture Formula for estimating population size
• Estimate of Total Population =
(total number recaptured) x (number marked)
(total number recaptured with mark)
In a mark-recapture study, an ecologist traps, marks and releases 25 voles in a small wooded area. A week later she resets her traps and captures 30 voles, 10 of which are marked. What is her estimate of the vole population in that area?
How does population density change?
• Addition – birth, immigration
• Removal – death, emigration
Fig. 53-3
Births
Births and immigrationadd individuals toa population.
Immigration
Deaths and emigrationremove individualsfrom a population.
Deaths
Emigration
Patterns of Dispersion
• Environmental and social factors influence spacing of individuals in a population• In a clumped dispersion, individuals aggregate in patches• A clumped dispersion may be influenced by resource availability and behavior
Fig. 53-4a
(a) Clumped
UNIFORM
• A uniform dispersion is one in which individuals are evenly distributed
• It may be influenced by social interactions such as territoriality
Fig. 53-4b
(b) Uniform
RANDOM
• In a random dispersion, the position of each individual is independent of other individuals
• It occurs in the absence of strong attractions or repulsions
Fig. 53-4c
(c) Random
Demographics• Demography is the study of the vital statistics (death and birth rates)
of a population and how they change over time• Death rates and birth rates are of particular interest to
demographers• A life table is an age-specific summary of the survival pattern of a
population• It is best made by following the fate of a cohort, a group of individuals
of the same age
Table 53-1
The life table of Belding’s ground squirrels reveals many things about this population
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 o
f 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
More parental care, better health care
Predation, accidents,disease at all levels
High mortalityof many offspring
What type of survivorship curve?
Type 2
Type 3
Type 1
Table 53-2
Reproductive tables focus on female reproductivity.
Life history traits are products of natural selection
• An organism’s life history comprises the traits that affect its schedule of reproduction and survival:– The age at which reproduction begins
– How often the organism reproduces
– How many offspring are produced during each reproductive cycle
• 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.
“Trade-offs” and Life Histories
• Organisms have finite resources, which may lead to trade-offs between survival and reproduction
Examples:
• brood size vs parental life span
• number of seeds and chance of gemination and growth
Fig. 53-8
MaleFemale
100
RESULTS
80
60
40
20
0Reduced
brood sizeNormal
brood sizeEnlarged
brood sizePar
ents
su
rviv
ing
th
e fo
llo
win
g w
inte
r (%
)
Fig. 53-9
(a) Dandelion
(b) Coconut palm
Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
In what way might high competition for limited resources in a predictable
environment influence the evolution of life history traits? Semelparity or
iteroparity
Selection would most likely favor iteroparity, with fewer, larger, better-provisioned or cared-for offspring.
How do we model population growth?
• By construction graphs and using mathematical formulas
• If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
• r = b - d
• 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
Exponential Growth
• Exponential population growth is population increase under idealized. unlimited conditions
• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase
• Equation of exponential population growth:
dNdt
rmaxN
Exponential population growth results in a J-shaped curve.
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
)
Fig. 53-11
8,000
6,000
4,000
2,000
01920 1940 1960 1980
Year
Ele
ph
ant
po
pu
lati
on
1900
Elephants in Kruger National Park in S. Africa after they were protectedfrom hunting.
The J-shaped curve of exponential growth characterizes some rebounding populations.
But, is this the normal state of population growth?
• 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
The Logistic Growth Model• In the logistic population growth model,
the per capita rate of increase declines as carrying capacity is reached
• We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N approaches K.
dNdt
(K N)Krmax N
Table 53-3
As N approachesK, rate nears“0”.
• The logistic model of population growth produces a sigmoid (S-shaped) curve
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
K
Fig. 53-13a
1,000
800
600
400
200
00 5 10 15
Time (days)
Nu
mb
er
of
Pa
ram
ec
ium
/mL
(a) A Paramecium population in the lab
These organisms are grown in a constant environment lacking predators and competitors
Some populations overshoot K before settling down to a relatively stable density
Fig. 53-13b
Nu
mb
er
of
Da
ph
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
Some populations fluctuate greatly and make it difficult to define K.
• Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small
The Logistic Model and Life HistoriesNatural selection shapes the final life history of
individual species. Some members of populations are subject to r-
selection and some to k-selection.When population size is low relative to K, r-selection favors r-strategies:• high fecundity (ability to reproduce), • small body size, • early maturity onset, • short generation time, and • the ability to disperse offspring widely.
Characteristics of r - Selected Opportunists
• Very high intrinsic rate of increase.
• Opportunistic
• Populations can expand rapidly to take advantage of temporarily favorable conditions
• Ex – Bacteria, some fungi, many insects, rodents, weeds, and annual plants.
• In environments that are relatively stable and populations tend to be near K, with minimal fluctuations in population size, K-selection favors K strategies: large body size, long life expectancy, and the production of fewer offspring that require extensive parental care until they mature.
• These populations are strong competitors.
• They are specialists rather than colonists and may become extinct if their normal way of life is destroyed.
Characteristics of K - Selected Species
Population responds slowly, usually with negative feedback control so that constancy is the rule.
Their numbers are controlled by the availability of resources. In other words, they are a density dependent species
Most birds
Most predators
Elephants
Whales
Oaks
Chestnuts
Apple
Coconut
r or k-selected?• Nature is more complex though and most populations
lie somewhere in between these two extremes. • Ex- Gymnosperms and angiosperms are typically
classified as K-strategists but they release many seeds.
• Cod fish are large fish but release large numbers of gametes into the sea with no parental investment. So, cod are considered r-strategists.
K or r-selected ? When a farmer abandons a field, it is
quickly colonized by fast-growing weeds. Are these species more likely to be K-selected or r-selected species?
r
What about the bluegill fish?
Bluegill exhibit one of the most social and complex mating systems in nature. Parental males delay maturation and compete to construct nests in colonies, court females, and provide sole parental care for the young within their nest.
Many factors that regulate population growth are density dependent
• There are two general questions about regulation of population growth:
– What environmental factors stop a population from growing indefinitely?
– Why do some populations show radical fluctuations in size over time, while others remain stable?
Population Change and Population Density
• In density-independent populations, birth rate and death rate do not change with population density
• In density-dependent populations, birth rates fall and death rates rise with population density
Fig. 53-15
(a) Both birth rate and death rate vary.
Population density
Density-dependentbirth rate
Equilibriumdensity
Density-dependentdeath rate
Bir
th o
r d
eath
ra
tep
er c
ap
ita
(b) Birth rate varies; death rate is constant.
Population density
Density-dependentbirth rate
Equilibriumdensity
Density-independentdeath rate
(c) Death rate varies; birth rate is constant.
Population density
Density-dependentdeath rate
Equilibriumdensity
Density-independentbirth rate
Bir
th o
r d
eath
ra
tep
er c
ap
ita
So, to determine if the environmental factor is density dependent or
independent….• Density-independent factors may affect
all individuals in a population equally – rainfall, temperature, humidity, acidity,
salinity, catastrophic events
• Density-dependent factors have a greater affect when the population density is higher.
Food supply, disease, parasites, competition, predation
Density-Dependent Population Regulation
• Density-dependent birth and death rates are an example of negative feedback that regulates population growth
• They are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors
Fig. 53-17a
(a) Cheetah marking its territory
In many vertebrates and some invertebrates, competition for territory may limit densityCheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries
Fig. 53-17b
(b) Gannets
•Oceanic birds exhibit territoriality in nesting behavior
Disease
• Population density can influence the health and survival of organisms
• In dense populations, pathogens can spread more rapidly
Predation
• As a prey population builds up, predators may feed preferentially on that species
Toxic Wastes
• Accumulation of toxic wastes can contribute to density-dependent regulation of population size
Intrinsic Factors
• For some populations, intrinsic (physiological) factors appear to regulate population size
Population Dynamics
• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size
• 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 o
f sh
eep
Fig. 53-19
Wolves Moose
2,500
2,000
1,500
1,000
500
Nu
mb
er o
f m
oo
se
0
Nu
mb
er o
f w
olv
es
50
40
30
20
10
01955 1965 1975 1985 1995 2005
YearChanges in predation pressure can drive population fluctuations
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
- winter food supply - predators * - sunspot activity (quality of food) * * affected cycles
Fig. 53-20
Snowshoe hare
Lynx
Nu
mb
er
of
lyn
x(t
ho
us
an
ds
)
Nu
mb
er
of
ha
res
(th
ou
sa
nd
s)
160
120
80
40
01850 1875 1900 1925
Year
9
6
3
0
The human population is no longer growing exponentially but is still increasing rapidly
• No population can grow indefinitely, and humans are no exception
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 human population increased relatively slowly until about 1650 and then began to grow exponentially
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
Though the global population is still growing, the rate of growth began to slow during the 1960s
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
Fig. 53-24
1750 1800 1900 1950 2000 2050
Year
1850
Sweden MexicoBirth rate Birth rate
Death rateDeath rate0
10
20
30
40
50B
irth
or
dea
th r
ate
per
1,0
00 p
eop
le
The demographic transition in Sweden took about 150 years, from 1810 to 1960. It will take about the same length of time for Mexico.
• The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women
• Most of the current global population growth is concentrated in developing countries
Age Structure
• 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• Age structure diagrams can predict a population’s growth
trends• They can illuminate social conditions and help us plan for the
future
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
Infant Mortality and Life Expectancy
• Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition
Fig. 53-26
Less indus-trialized
countries
Indus-trialized
countries
60
50
40
30
20
10
0 0
20
40
80
Lif
e ex
pec
tan
cy (
year
s)
Infa
nt
mo
rtal
ity
(dea
ths
per
1,0
00 b
irth
s)
Less indus-trialized
countries
Indus-trialized
countries
60
Global Carrying Capacity
• How many humans can the biosphere support?• The carrying capacity of Earth for humans is uncertain• The average estimate is 10–15 billion
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
Fig. 53-27
Log (g carbon/year)
13.49.85.8
Not analyzed
Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes