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Chapter 8
Population Change
Lecture Outline:
I. Principles of Population Ecology
A. Population ecology is the branch of biology that deals with the numbers of a
particular species found in an area and how and why those numbers change
over time
B. Population density
i. Population density is the number of individuals of a species per unit of
area or volume at a given time
ii. A population is a group of individuals of the same species that live in
the same geographic area at the same time
iii. Population size is meaningful only when the boundaries of the
population are defined
C. How do populations change in size?
i. On a global scale, change is due to the birth rate (b - rate at which
individuals produce offspring) and the death rate (d - rate at which
organism die); therefore the natural increase in human populations (r growth rate) is equal to r = b – d
ii. On a local scale, movement from one region or country to another
(dispersal) affects population change
1. Immigration (i) – individuals enter a population
2. Emigration (e) – individuals leave a population
3. Therefore, r = (b - d) + (i - e)
D. Maximum population growth
i. The maximum rate that a population could increase under ideal
conditions is its intrinsic rate of increase (biotic potential)
1. Several factors influence biotic potential
a. Age that reproduction begins
b. Fraction of the life span during which an individual can
reproduce
c. Number of reproductive periods per lifetime
d. Number of offspring produced during each period of
reproduction
2. These factors, called life history characteristics, determine
whether a particular species has a large or small biotic potential
ii. When a population grows exponentially, the larger the population gets,
the faster it grows (exponential population growth)
1. The graph has a J shape characteristic
2. Populations will always increase exponentially as long as their
growth rates remain constant
E. Environmental resistance and carrying capacity
i. Organisms cannot reproduce indefinitely at their intrinsic rates of
increase because the environment sets limits (environmental
resistance)
1. Environmental resistance includes environmental conditions
that limit availability of food, water and shelter; as well as
limits imposed by disease, predation and competition
2. Environmental resistance is an excellent example of a negative
feedback mechanism
3. When a population affected by environmental resistance is
graphed over a long period, the curve has the characteristic S
shape of logistic population growth
ii. The maximum number of individuals of a given species that a
particular environment can support for an indefinite period, assuming
there are no changes in the environment, is termed carrying capacity
(K)
1. In nature, the carrying capacity is dynamic and changes in
response to environmental changes
2. Sometimes a population that overshoots K will experience a
population crash
II. Factors That Affect Population Size
A. Density-dependent factors
i. Density-dependent factors (predation, disease, competition)are
environmental factors whose effects on a population change as
population density changes (ex., predation has a greater influence on a
population when its population is greater
ii. As population density increases, density-dependent factors tend to
slow population growth by causing an increase in death and/or a
decrease in birth rate
B. Density-independent factors
i. Density-independent factors are environmental factors that affect the
size of a population but are not influenced by changes in population
density
ii. Severe weather conditions are an example of density-independent
factors
III. Reproductive Strategies
A. Each species has its own life history strategy – its own reproductive
characteristics, body size, habitat requirements, migration patterns, and
behaviors – that represents a series of tradeoffs reflecting an energy
compromise between survival and reproduction
i. r strategists (r-selected species) have traits that contribute to a high
population growth rate (i.e., small body size, early maturity, short life
span, large broods, little/no parental care)
ii. K strategists (K-selected species) maximize the chance of surviving in
an environment where the number of individuals (N) is near K of the
environment
1. They have characteristically long life spans with slow
development, late reproduction, large body size, and low
reproductive rates
2. K strategists are found in relatively constant or stable
environments, where they have high competitive ability
B. Survivorship
i. Ecologists construct life tables for plants and animals that show the
likelihood of survival for individuals at different times during their
lives
ii. Survivorship is the proportion of newborn individuals that are alive at
a given age
1. Type I – young and those at reproductive age have a high
probability of living, the probability of survival decreases more
rapidly with increasing age, death is concentrated later in life
(humans, elephants)
2. Type II – the probability of survival does not change with age
(lizards)
3. Type III – the probability of death is greatest early in life, those
that avoid early death subsequently have a high probability of
survival (fishes, oysters)
IV. The Human Population
A. Demography is the science of population structure and growth; the application
of population statistics is called demographics
B. Current population numbers
i. In 2006, the world population was 6.6 billion
ii. While the world birth rate (b) has declined during the last 200 years,
there has been a dramatic decline in the death rate (d), leading to an
overall increase in population
C. Projecting future population numbers
i. The world growth rate (r) has declined over the past several years,
from a peak of 2.2% per year in the 1960s to 1.2% in 2006; it is
projected that a zero population growth will occur at the end of the 21st
century
ii. The main unknown factor in any population growth scenario is Earth’s
carrying capacity (K)
V. Demographics of Countries
A. Population differences occur from country to country
i. Highly developed countries (HDCs - i.e., United States, Canada,
France, Germany, Japan, Australia, etc.) have low rates of population
growth, are highly industrialized, have low birth rates, and low infant
mortality rates
ii. Moderately developed countries (MDCs - i.e., Mexico, Thailand,
Turkey, etc.) have relatively higher birth rates and infant mortality
rates than HDCs, medium level of industrialization, and their average
per capita GNI PPPs are lower than HDCs
iii. Less developed countries (LDCs – i.e., Bangladesh, Ethiopia,
Cambodia, etc.) have the highest birth rates, the highest infant
mortality rates, the shortest life expectancies, and the lowest average
per capita GNI PPPs in the world
B. One way to express the population growth of a country is to determine its
doubling time (td)
C. Replacement-level fertility is the number of children a couple must produce to
“replace” themselves (=2.1); the current worldwide total fertility rate is
currently 2.7
D. Demographic stages
i. The demographic transition consists of four demographic stages
through which a population progresses as its society becomes
industrialized
ii. The four stages are preindustrial, transitional, industrial, and
postindustrial
E. Age structure of countries
i. To predict the future growth of a population, you must know its
distribution of people by age (age structure)
ii. Age structure diagrams depict expanding populations, stable
populations and declining populations
iii. A population growth momentum explains how the present age
distribution affects the future growth of a population
VI. Demographics of the United States
A. The United States has the largest population of all HDCs, and continues to
grow significantly due to immigration
i. International immigration is responsible for more than 50% of U.S.
population growth
ii. The U.S. has one of the highest rates of population increase of all the
HDCs
B. History of immigration in the United States
i. Pre 1875, the U.S. had no immigration laws
ii. 1875 – no convicts and prostitutes allowed in U.S.
iii. 1882 – Chinese Exclusion Act passed
iv. 1891 –the Bureau of Immigration was established which led to an
official policy of selective exclusion in the U.S
v. 1952 – the Immigration and Nationality Act was passes (aka IRCA)
vi. Currently, the top five countries from which legal U. S. immigrants
migrate are Mexico, the Philippines, Vietnam, the Dominican
Republic, and China