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51
Introduction to Ecology: Population Ecology
Lecture Outline
I.
Populations possess characteristic features
A. Population ecology includes the study of the numbers of individuals of a
particular species at a given time and location, and the interactions of that
population with other populations and the environment
B. Density and dispersion are important features of populations
1. Population size, expressed in terms of density, is most useful
a) Density is the number of individuals of a species per unit space
2. Dispersion describes the spacing of individuals
a) Uniform dispersion occurs when individuals are more evenly
distributed than would be expected randomly
(1) Uniform dispersion is often a result of agonistic behavior such as
territoriality
b) Random dispersion results from lack of interaction between
individuals or a homogeneous environment
c) Clumped dispersion is the most common type of distribution, and
results from patchy distribution of resources or by asexual
reproduction
(1) Clumped distribution may include social behavior in animals
II.
Mathematical models describe population growth
A. Population numbers change due to natality and mortality
1. ∆N / ∆t = b – d
a) ∆N is the change in numbers in the population
b) ∆t is the change in time
c) b is the natality rate
d) d is the mortality
2. r = b – d
a) r is the growth rate of the population
b) If r = 0, the population is stable; if r > 1, the population is increasing in
size; if r < 1, the population is declining
3. These equations can be expressed as dN / dt = rN
B. Dispersal affects the growth rate in some populations
1. Dispersal is the movement of individuals among populations
2. Immigration (i) is the migration of individuals into the population
3. Emigration (e) is the migration of individuals out of the population
4. r = (b – d) + (i – e)
C.
Each population has a characteristic intrinsic rate of increase
1. The intrinsic rate of increase is the maximum rate of increase under ideal
conditions for that population
2. Life history characteristics determine the biotic potential for a species
3. Typically, larger animals have a greater biotic potential
D. No population can increase exponentially indefinitely
1. Exponential growth may exist for a period of time until limiting factors
become important
2. Environmental resistance prevents continued exponential growth
a) As N increases, environmental resistance increases
3. The carrying capacity is the largest population that can be sustained for an
indefinite period of time
4. Growth regulated by environmental resistance shows an S shaped growth
curve
5. This growth may be described by dN / dt = rN [(K – N) / K]; the logistic
equation
a) When N is small, the expression [(K – N) / K] has a value of almost 1
and minimally limits growth
b) When N is large, the expression [(K – N) / K] has a value that
approaches 0 and greatly limits growth
6. Populations do not always approach and stabilize at K, but may overshoot
K, followed by a population crash
III.
Factors influencing population size
E. Density-dependent factors regulate population size
1. Density-dependent factors are those that increase in proportional effect
with increasing population size
a) Predation and disease are examples of density-dependent factors
2. Competition is an important density-dependent factor
a) Intraspecific competition exists between members of the same species
(1) In scramble competition, members of the population share the
resource equally
(2) In contest competition, some members of the population obtain a
"lion’s share" of the resource
b) Interspecific competition exists between members of different species
3. The effects of different density-dependent factors are difficult to assess in
nature
a) Many populations are regulated by multiple density-dependent
factors
F. Density-independent factors limit population size
1. Environmental factors that affect population regardless of size are densityindependent factors
2. Many climatic factors are density-independent
3.
Most density-independent factors have some relationship to population
density
IV.
Life history traits
G. Species that expend their energy in a single, immense reproductive effort are
semelparous; iteroparous species exhibit repeated reproductive cycles
H. Based on life history strategies, organisms may be classified as r-strategists
and K-strategists
I. r-strategists have a high r, small size, large numbers of offspring, and may live in
unpredictable habitats
1. Many weeds or pest species are r-strategists
2. Many r-strategists reproduce asexually
J. K-strategists have a population size near K, are larger in size, have fewer
offspring, and may care for their young
1. Many endangered species are K-strategists
2. K-strategists typically live in stable environments
K. Survivorship is related to r and K selection
1. Survivorship curves may be classified as type I, type II, or type III
a) Humans exhibit type I survivorship, which is typical of K-selected
organisms
(1) Type I curves are characterized by high mortality late in life
b) Some birds and lizards have a type II survivorship, characterized by
constant mortality over time
c) Type III survivorship is characterized by high mortality early in life, as
seen in r-selected organisms
V.
Metapopulations
L. Many species are not distributed as one large population across the landscape
1. Many species exist as a series of local populations distributed in distinct
habitat patches
2. Such a population is called a metapopulation
M. Good habitats are called sources and are areas where local reproductive
success is greater than local mortality
N. Sinks are areas where local reproductive success is less than local mortality
VI.
Human populations
O. Thomas Malthus recognized that the human population cannot continue in
exponential growth indefinitely
1. Human population growth has been marked by a large decrease in the
death rate
2. Human population size will increase to unknown proportions due to
uncertainty of the carrying capacity for our species
P. Not all countries have the same growth rate
1. Demographics describe population statistics
2.
The growth rate is higher in developing countries, which have a lower
degree of industrialization and prosperity than the developed countries
a) Moderately developed countries include most countries of South
America, as well as Mexico and Turkey
b) Less developed countries have the highest growth rates, and include
many African and Southeast Asian countries
3. The doubling time is the time needed for a population to double in size
a) Doubling time is less than 20 years in some developing countries, and
hundreds of years in some developed countries
4. Replacement-level fertility is the number of children a couple must have to
replace themselves
a) Replacement-level fertility is typically 2.1 in developed countries, and
2.7 children in developing countries
5. The total fertility rate is the average number of children born to a woman
during her lifetime
Q. The age structure of a country can be used to predict future population
growth
1. Age structure shows the percentages of the population at different ages
2. A broader base to the age structure indicates a growing population; a
narrower base indicates a stable or shrinking population
R. Environmental degradation is related to population growth and resource
consumption
1. Developed nations require more resources per capita and cause more
environmental problems than developing nations
2. People overpopulation is a result of too many people living in a certain
area; consumption overpopulation is a result of people over-utilizing
resources
Research and Discussion Topics

One of the major concerns of biologists who concentrate on preserving tropical
diversity is the continued growth of the populations of these tropical countries.
Look up the growth rates of several tropical countries, and describe the potential
effects of this growth on deforestation. Consider the impact of increasing
populations in these developing countries on deforestation, as well as the
exploitation by developed countries.

Are conservation biologists typically concerned with r- or K- selected species? If you
were in charge of attempts to repopulate an endangered species, what characteristics
of the life history of that organism might hamper your efforts?