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Population Ecology
Population ecology (or autecology) is a sub-field of ecology that deals with
1) the dynamics of species populations
2) how these populations interact with the environment
3) how the population sizes of species change over time and geographic location
Population Ecology Facts You Should Know:
1. The term population ecology is often used interchangeably with population biology or
population dynamics.
2. The development of population ecology owes much to demography and actuarial life
tables.
3. Population ecology is important in conservation biology, especially in the development
of population viability analysis (PVA) which makes it possible to predict the long-term
probability of a species persisting in a given habitat patch.
4. Although population ecology is a subfield of biology, it provides interesting problems for
mathematicians and statisticians who work in population dynamics.
The human population is growing at an exponential rate and is affecting the populations of
other species in return.
1. Examples
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chemical pollution
deforestation
irrigation
desertification
waste
resource depletion
This principle in population ecology provides the basis for formulating predictive theories
Population density
Dispersion pattern - The way individuals are spaced within an area
1. Clumped dispersion – grouped in patches
2. Uniform dispersion
3. Random dispersion
Dispersion pattern details
1. Clumped dispersion pattern
 grouped in patches
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most common type of dispersion
Is often the result of an unequal distribution of resources
Examples:
1. Humans
2. Sea stars group where food is abundant
3. Mushrooms grow where rich soil is
2. Uniform dispersion pattern – evenly spread out over a given area
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Is often the result individual interactions of a population
Examples:
1. People in a lecture hall tend to spread out
2. Some plants secrete chemicals and inhibit germination and growth of
nearby plants that would be in competition for resources
3. Territorial behavior
3. Random dispersion pattern – spread out in an unpredictable way
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Is often due to…
1. Social interactions
2. Varying habitat conditions
Examples:
1. Plants that grow from wind-blown seeds
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Dandelions
Life Tables and Survivorship Curves
Life tables
 Life tables track survivorship
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Survivorship is the probability (or chance) of an individual to survive (or live
past) a certain age.
Another way to look at it is that it is a table which shows, for each age, what
the probability is that a person of that age will die before his or her next
birthday ("probability of death"). In other words, it represents
the survivorship of people from a certain population
Survivorship curves
How do we make a survivorship curve? Plot the log of the percentage of survivors by the
percent maximum lifespan to get the “survivorship curves”.
There are 3 types of survivorship curves
1. Type I
2. Type II
3. Type III
1. Type I survivorship curve
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For a Species that exhibits a type I survivorship curve – Most of the individuals of
that species will survive until they reach “old age”
Examples
1. Humans
2. Large mammals
2. Type II survivorship curve
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For a Species that exhibits a type Ii survivorship curve – the survivorship of that
species remains relatively constant over the lifespan.
Examples:
1. Hydra
2. Invertebrates
3. Lizards
4. Rodents
3. Type III survivorship curve
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For a Species that exhibits a type IIi survivorship curve – Most of the individuals of
that species will die young, but the ones that live on to adulthood will live a relatively
long time.
Animals who have large numbers of offspring, but provide little care for them
Examples:
1. Fish
2. Oysters
Idealized models
Idealized models give Predictions of population growth
In biology, population growth is the increase in the number of individuals in a population.
A. Notes about the human population
1. Global human population growth amounts to around 75 million
annually, or 1.1% per year.
2. The global population has grown from 1 billion in 1800 to 7 billion
in 2012.
3. The human population is expected to keep growing.
4. estimates have put the total population at 8.4 billion by mid-2030
5. estimates have put the total population at 9.6 billion by mid-2050.
6. The nations with rapid population growth generally have low
standards of living, whereas those nations with low rates of
population growth have high standards of living.
Two idealized patterns of population growth:
1. Exponential (or “geometric” for discrete version)
2. Logistic (s-shaped)
Population growth models
Exponential Growth Model
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In the exponential model of population growth, you do not see the effect of limiting factors.
Population undergo rapid and explosive growth of population.
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This equation expresses rate of change
the intrinsic rate of increase “r” is assumed to be constant
•When a population is introduced to a new environment, it will often display
Exponential Growth.
•After some time, this will change to a logistic growth model.
Logistic Growth Model
•Carrying capacity (K) (the limitations of that environment)
Limiting factors that contribute to the logistical growth model are …
A. Predation
B. Parasites
C. Food sources
D. Illness
E. Change in environment
F. Predation
G. Territory
H. Illness
I. Change in environment
J. Intraspecific competition – when members of the same species
compete for limited resources. This leads to a reduction in
fitness for both individuals.
K. Interspecific competition - when members of different species
compete for a shared resource.
“Boom-and-bust” population cycles
- These are dramatic fluxuations in population
Boom-and-Bust cycles occur when the population growth of one species is closely tied to a
limiting factor that may be expended.
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The predator populations increase and decrease as the prey numbers change.
Predation may be an important cause of density-dependent mortality for some prey.
Boom-and-bust cycles: Prey populations rapidly increase.
The Boom-and-Bust cycle has two-phases:
1. Boom – rapid population increase is followed by a…
2. BUST – population falls back to minimal levels
Example of Boom-and-Bust cycle - Lynx and snowshoe hare population cycles
1. Lynx and snowshoe hare population cycles have a period of rapid population
increase followed by a sudden precipitous decline in numbers.
A. The increase is prey is then followed by an increase in the predator
population.
B. As predators eat the prey, the prey population goes down.
C. As the predator’s food source becomes scarce, the predator population
decreases.
D. As the predator population decreases, The prey population can increase
again
E. —the cycle repeats; for example, the snowshoe hare and lynx have a 10year cycle.
Evolution shapes life histories
There are different kinds of evolutionary selectivity
1. r-selection
2. K-selection
A. r-selection is…
1. r-selection is a density-independent selection –
a. This means the selection is due to density-independent factors that
operate when an increase in population density lowers the survival odds
for an individual, such as
i. predation
ii. parasites
iii. disease
iv. competition for resources.
2. Rapid reproduction
3. Takes advantage of new or open environments
4. High biotic potential
B. K-selection is
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K.
L.
M.
A. Density-dependent selection –
a. This means the selection is due to density-dependent factors that exist
regardless of the population density.
B. These factors may cause more deaths or a lower birth rate or both.
C. Example of density-independent factors include temperature, precipitation,
and natural disturbances.
D. Efficient use of limited resources
E. Subject to competition
F. lower rates of reproduction
G. low biotic potential
r-selection and K-selection
I. Organisms are subject to each of these at varying degrees.
J. We usually do not see a species which is solely R-Selection or solely K-selection.
Life history traits
Life history characteristics are traits that have been shaped through natural selection. Traits
that are more desirable or advantageous, are more likely to persevere.
life history traits include:
N. Reproduction
O. development
P. Behavior
Q. life span
R. Post-reproductive behavior
The two evolutionary "strategies" are termed r-selection, for those species that produce
many "cheap" offspring and live in unstable environments and K-selection for those species
that produce few "expensive" offspring and live in stable environments.
Of course, the animal or plant is not thinking: "How do I change my characteristics?"
Natural selection is the force for change, not the individual's conscious decision. But,
natural selection has produced a gradation of strategies, with extreme r-selection at
one end of the spectrum and extreme K-selection at the other end.
The following table compares some characteristics of organisms which are extreme r
or K strategists:
r
K
Unstable environment, density
independent
Stable environment, density dependent
interactions
small size of organism
large size of organism
energy used to make each individual is low
energy used to make each individual is high
many offspring are produced
few offspring are produced
early maturity
late maturity, often after a prolonged period of
parental care
short life expectancy
long life expectancy
each individual reproduces only once
individuals can reproduce more than once in their
lifetime
type III survivorship pattern
in which most of the individuals die within a
short time
but a few live much longer
type I or II survivorship pattern
in which most individuals live to near the maximum
life span
The terms "r-selected" and "K-selected" come from a description of the population growth
regimes of the two types of organisms.
If you are in an unstable environment, you are unlikely to ever have population
growth to the point where density dependent factors come into play. The population
is still at low values relative to the carrying capacity of the environment and thus is
growing exponentially with intrinsic reproductive rate r (when it is not subject to
environmental perturbations.), hence the name r-strategist.
An extreme K-strategist lives in a stable environment which is not seriously affected
by sudden, unpredictable effects. Thus the population of a K-strategist is near the
carrying capacity K.
Surviorship curves give us additional insight into r and K-selected strategies. Notice that the
vertical axis of the survivorship plots is on a log scale and that horizontal axis is scaled to
the maximum lifetime for each species.
One of the interesting differences between r and K strategists is in the shape of the
survivorship curve. We can generate a survivorship curve by ploting the log of the
fraction of organisms surviving vs. the age of the organism. To compare different
species, we normalize the age axis by stretching or shrinking the curve in the
horizontal direction so that all curves end at the same point, the maximum life span
for individuals of that species. Notice that the vertical axis is on a log scale, dropping
from 1.0 (100%) to 0.1 (10%) to 0.01 (1%) to 0.001 (0.1%) in equally spaced intervals.
Extreme r-strategists, such as the oyster, lose most of the individuals very quickly,
relative to the maximum life span for the species. But, a very few individuals do
survive much longer than the rest. But, for extreme K-strategists, such as man, most
individuals live to old age (again relative to the maximum life span for the species).
These survivorship data are very valuable when studying the ecology of various
organisms. Two components are involved in reproduction: 1) How many females
survive to each age and 2) the average number of female offspring produced by
females at each age. By using these data, we can compute the intrinsic rate of
reproduction, r, a key parameter in models of population growth.