Transcript Population Ecology

Population
Ecology
Chapter 46
ECOLOGY
• Ecology: the study of relationships of organisms to one
another and to their environment
– Abiotic factors: physical factors, such as climate,
nutrient availability
– Biotic/biological factors: competitors, predators,
parasites, prey
POPULATIONS & THEIR
PROPERTIES
46.1 A population consists of all the individuals of a
given species that live and reproduce in a
particular place.
• A population is characterized by:
– Size: number of individuals at a particular time and
place
– Range: total area in which a population lives
– Density: population size divided by range
• Population size estimates: sampling & mark-andrecapture techniques
POPULATION
Size
The size of
the Antarctic
population
of krill is
estimated to
be 800
trillion
animals.
Range
Density
Mean no. krill/km2
<2
32-64
2-4
64-128
4-8
128-256
8-16
256-512
16-32
>512
Their range is
approximately
19 million
square
kilometers in
the Southern
Ocean.
800 trillion krill
over 19 million
square kilometers
of the Southern
Ocean means the
population
density is 42
million animals
per square
kilometer on
average.
Key Features of a Population
Size
The size of
the Antarctic
population
of krill is
estimated to
be 800
trillion
animals.
Range
Density
Mean no. krill/km2
<2
32-64
2-4
64-128
4-8
128-256
8-16
256-512
16-32
>512
Their range is
approximately
19 million
square
kilometers in
the Southern
Ocean.
800 trillion krill
over 19 million
square kilometers
of the Southern
Ocean means the
population
density is 42
million animals
per square
kilometer on
average.
a.
Estimating
Population
Size
The distributions of individual
trees or other organisms can
appear to be random, with no
clear pattern to where they
occur.
b.
If resources are clustered or
spatial proximity to other
individuals enhances fitness,
populations may be clustered.
c.
When resources are limited
or predators target a single
species, an individual might
be better off if it is as far from
others as possible, producing
a uniform pattern of
distribution.
Mark-and-Recapture
Organisms are captured and released on 2 successive days
Population Size (N)
Butterflies caught on the 1st day (M)
Marked and Unmarked on 2nd Day (C)
Recaptures (R)
N = C/R x M
POPULATION GROWTH AND DECLINE
46.2 Population size can increase or decrease
over time
• Population size influenced by:
– Births
– Deaths
– Immigration
– emigration
Factors
Affecting
Population Size
Birth
Birth and
immigration
both contribute
to population
increase.
Emigration
Population
size
Immigration
Mortality
(death) and
emigration
both contribute
to population
decrease.
Mortality
Population Size Changes
N = N2 – N1 = (B – D) + (I – E)
Where
N = change in population size
And N1 number of individuals at time 1
N2 number of individuals at time 2
A population starts with 80 individuals and after 2
years, has grown to 120.
40 individuals in 2 years = 20 individuals/year
Per Capita Growth Rate
• Interested in proportional increase/decrease over time
• Per capita growth rate: rate of population growth per
individual; symbolized as r
r=(
N/
t)/N1
• In our example, r = 20 individuals/year divided by 80
individuals
• Per capita growth rate = 0.25 per year
Quick Check
• If a population triples in size in a year, what is
the per capita growth rate?
Answer
The per capita growth rate is
r = ( N/ t)/N1
t = 1 year, so r = N/N1
If starting population is x, at triple the size it
is 3x
Therefore N = 3x – x = 2x
R = 2x/x = 2
Population sizes
In exponential growth, the
per capita growth rate is
constant, but the actual
number of individuals
added in a given interval of
time increases as the
population grows because
new individuals reproduce.
Exponential
Growth
Time
Q: If the per capita rate of increase we calculated in the previous exam
• We can answer this question using the following formula:
Nt = N1 (1 + r)t
• Solving: in 10 years the population size would be 745 (80 times 1 plus
0.25, raised to the 10th power).
• After 15 years, the population would grow to 2273.
Population
sizes
In exponential growth, the per
capita growth rate is constant,
but the actual number of
individuals added in a given
interval of time increases as the
population grows because new
individuals reproduce.
Time
Exponential
Growth
• A prominent feature of
exponential growth is that the
number of individuals added
to the population in any time
interval is proportional to the
size of the population at the
start of the interval.
• i.e. While the per capita
growth rate remains constant,
the actual number of
individuals added per time
unit increases as the
population grows (new
individuals reproduce).
• The per capita growth rate, r, is commonly called the intrinsic growth rate of a
population……the maximum rate of growth when no environmental factors limit
population increase.
• Intraspecies (within) competition: results in natural selection
• Interspecies (between) competition: increase or decrease in population size
Carrying Capacity of Environment
• Initially growth rates are exponential….the number of
individuals added to population increases over time
• But population density limits population growth : due to
decreasing resources
• Called the carrying capacity of the environment
– Symbolized as K
• Logistic growth: growth potential slows down as
population size reaches K, its maximum sustainable size
1
2
Population size (N)
At first, growth rate
approximates r and the
number of individuals
increases rapidly.
As the population size N
approaches the carrying capacity K,
growth rate slows due to crowding
and competition for resources.
Logistic
Growth in a
Population
Carrying capacity of
environment (K)
(K–N)/K = 0
Fraction of carrying
capacity available
for further growth
(K–N)/K = 0.5
Time
(K–N)/K = 1
The number of
territories usually
declines
following dry
years, indicated
by arrows.
Precipitation
(cm)
Song Sparrow
density (number of
territories)
Density-Dependent
Effects
Year
• At low density, food and other resources do not limit growth, but as density increases,
they exert more and more influence on population growth, spurring competition for
available resources.
• At high population density, individuals may be more vulnerable to predation or to
infection.
• Factors such as resources and predation are called density-dependent factors.
The number of
territories usually
declines following dry
years, indicated by
arrows.
Precipitation
(cm)
Song Sparrow
density (number
of territories)
DensityIndependent Effects
Year
• D
ensity-independent factors influence population size without regard for the
population’s density.
• Factors such as severe drought or a prolonged cold period, either of which can
cause widespread mortality independent of population density.
AGE-STRUCTURED POPULATION GROWTH
46.3 The age structure of a population helps
ecologists understand past changes and predict
future changes in population size.
• Age structure of a population: number of individuals of
each age group
• Usually shown as histogram
• Growing population: pyramid-shaped age distribution
• youngest are more abundant than older
• Stable population: even distribution
The number of males and females in
France is uniform from childhood
through the reproductive years,
showing that younger individuals
replace older individuals through time
and so ensure the stability of the
population.
France
Females
India
Males
Females
Age
Males
The number of males and females in
India is much greater in younger age
classes than in older classes. The
population will continue to grow as
individuals in the younger age classes
mature and reproduce.
Millions of
individuals
Millions of
individuals
Age Structure Data
Fishing removes most of the fish
older than 10 years, the ages
capable of producing the most
eggs and young.
Unfished
Age (years)
Age of reproductive
maturity
Fished
Relative abundance (males and females)
This change
in age
structure
severely
reduces the
growth rate
of rockfish
populations,
and
therefore
their ability
to sustain
fishing
pressure.
Dangers at Each Growth Stage
In one study, parasites
and other factors were
observed to remove 32%
of the eggs.
Predators of adults
Parasites of pupae
Adult Lime
Swallowtail
Young larva
Pupa
Older larva
Parasites and other factors remove 55% of the
pupae. Because pupae are the relatively few
survivors of egg and larval mortality, the impact of
pupal losses on population growth is magni ed.
Egg
Parasites of eggs
Predators of larvae
Number of individuals
alive (survivorship)
250 eggs
laid
170 eggs
become
larvae
Egg
Larva
The survivorship curve for a cohort
of 250 butterfly eggs shows high
mortality at the egg and pupal
stages.
158 larvae
become pupae
71 pupae
change to
adults
Pupa
Stage
Adult
Most humans and other large mammals survive
their early years and show a steep decline in
the last part of their life-span.
Survivorship
Type I
Survivorship
Curves
Humans
(U.S.A.)
Age in years
• Different organisms exhibit different
survivorship curves.
• Type I survivorship: most mortality is
late in life.
• Large animals such as humans,
elephants, and whales, show Type I
curves.
Survivorship
Type II
Small mammals and birds
tend to lose numbers steadily
throughout their lives.
European
Blackbird
Survivorship
Curves
Song Thrush
American
Robin
Age in years
• Type II survivorship:
Consistent levels of mortality
thru life
• Birds, small
mammals
Many Turtle Grass seedlings and those of most other
herbaceous plants do not survive the earliest period of life,
but those that do survive show only slow additional declines
over time.
Survivorship
Type III
Survivorship
Curves
Turtle Grass
Age in years
• Type III survivorship: high
mortality early in life
• Small plants
• Small animals such as mice,
insects
PATTERNS OF REPRODUCTION
• R-strategists: species that live in unpredictable
environments, produce large numbers of offspring but
provide little resources for their support
• K-strategists: species that live where resources are
predictable, produce few young but invest a lot of
resources in their support
• R and K-strategists: extremes on a continuum of
reproductive patterns
Reproductive Patterns
In one study, parasites
and other factors were
observed to remove 32%
of the eggs.
Predators of adults
Parasites of pupae
Adult Lime
Swallowtail
Young larva
Pupa
Older larva
Parasites and other factors remove 55% of the
pupae. Because pupae are the relatively few
survivors of egg and larval mortality, the impact of
pupal losses on population growth is magni ed.
Egg
Parasites of eggs
Predators of larvae
METAPOPULATION DYNAMICS
46.4 The dynamics of populations are influenced by the
colonization and extinction of smaller, interconnected
populations that make up a metapopulation.
• Metapopulation is a large population made up of smaller
populations (patches) linked by occasional movements of
individuals between them
Metapopulations
Groves of trees can support
populations of nuthatches,
even though not all may be
currently occupied.
Individual nuthatches can move
between populations in different
groves, indicated by corridors.
Population C
Corridor
Corridor
Corridor
Population B
Corridor Population D
Population A
The
independent
populations,
connected by
migrants,
constitute a
metapopulation
of nuthatches.
Habitat Patches
Control patch
An average of
17 species was
found on 20
untouched
patches of
moss.
Small,
isolated
patches
20 similar patches
were cut into isolated
spots of moss. A
year later, on these
fragmented patches,
an average of 7
species had become
extinct.
Small patches
connected by
corridors
Fewer species became
extinct in patches of moss
that were connected by
narrow corridors. An
average of only 2 species
became extinct.
Habitat
Fragmentation