BIOLOGY 157:
LIFE SCIENCE: AN
ENVIRONMENTAL APPROACH
(Populations)
POPULATIONS
What is a population?
all the individuals of a certain species in a
particular area
adjacent populations of the same species usually
have some degree of interaction (immigration,
emigration, gene flow, resource exchange, etc.)
thus forming a METAPOPULATION
EMERGENT PROPERTIES
• Each level of organization has certain properties.
• As we go from one level to the next (e.g. from
atoms to molecules or individuals to populations)
we see that the higher level has many of the
properties of the lower level(s) that make it up.
• HOWEVER, we also see properties or attributes
‘emerging’ in the whole which were not evident in
the parts that make it up.
• In other words, the whole is more than the sum of
its parts.
Some Important Population
Factors / Attributes
•
•
•
•
•
•
•
•
•
Natality (crude, age or sex-specific) (0 or +)
Mortality (crude, age or sex-specific) (0 or +)
Growth Rate (+ or 0 or -)
Carrying Capacity (K)
Density
– Density Dependent Factors
– Density Independent Factors
Age Distribution
Dispersion
Survivability
Competition (intra species)
POPULATION GROWTH
The rate at which a population grows depends on:
• Natality
• Mortality
• Immigration
• Emigration
• Of course, each of the above factors is itself
affected by other factors.
FACTORS AFFECTING POPULATION SIZE
TYPES OF GROWTH (I)
• Linear Growth
– a quantity increases by a constant amount
per unit of time; additive
– produces a straight line when graphed
• Exponential Growth
– a quantity increases by a fixed percentage of
the whole per unit of time; same
phenomenon as compound interest
– produces a curve when graphed
TYPES OF GROWTH (II)
POPULATION GROWTH PATTERNS
• Exponential (J-shaped curve)
largely density independent
• Logistic (s-shaped or sigmoid)
more density dependent
factors such as territoriality, aggression, interand intraspecific competition, predation and
disease are of major importance here
POPULATION GROWTH
CURVES: J vs. S (I)
POPULATION GROWTH
CURVES: J vs. S (II)
POPULATION GROWTH
CURVES: J vs. S (III)
• Populations which follow a J-shaped curve usually
lack control by density dependent factors.
• Populations which follow an S-shaped curve have
one or more DENSITY DEPENDENT factors
controlling their growth (e.g., territoriality,
aggression, inter- or intraspecific competition,
predation, disease).
• All populations can be affected by DENSITY
INDEPENDENT factors such as catastrophic
weather, earthquakes, volcanic activity, etc.
HUMAN POPULATION GROWTH
• What type of Growth pattern? J or S?
• Why?
HUMAN POPULATION
GROWTH FACTORS
• EXPONENTIAL ?????
• Until rather recently in our evolutionary
history human numbers were held in check
by famine, disease, war, lack of technology,
etc. (= Environmental Resistance)
• Recently these factors have been greatly
minimized (= less environmental resistance).
• Population is growing rapidly due to good
death control and poor birth control.
THE PLAGUE AND HUMAN
POPULATION GROWTH
The impact of recurrent plague on the population of Europe. After.
W. Langer, “The Black Death”, Scientific American 210:14-19,
Feb. 1964.
THE NEW PLAGUE (HIV/AIDS) ?
• Will AIDS do what the Bubonic Plague did?
• Are there any differences as regards the
spread of these diseases?
• Is AIDS having a major effect?
• READ: AIDS cutting Africans’ life
expectancy to 30 years… (© 2000 Nando Media, AP Online)
INFANT MORTALITY REDUCTION (I)
INFANT MORTALITY REDUCTION (II)
THE DEMOGRAPHIC SHIFT (I)
(= the Demographic Transition)
• According to this hypothesis, as countries
industrialize they experience, in order:
1) a decrease in death rates
2) a resulting growth in population because birth
rates stay the same or even increase slightly
3) a decrease in birth rates
4) a slowing of population increase and eventually
a stabilizing (or even a DECREASE) of the
population size
Why do death rates, and eventually birth rates,
decrease?
DEMOGRAPHIC SHIFT (II)
DEMOGRAPHIC TRANSITION (III)
DEMOGRAPHIC TRANSITION (IV)
SOME ADDITIONAL
POPULATION GROWTH ITEMS
• Replacement Level Fertility
• Zero Population Growth
• Doubling Time For A Population
The “Rule” of 70
70 / % growth rate = doubling time
e.g.: 70 / 1.6% = 43.75 years
70 / 5% = 14 years
70 / 2% = 35 years
U.S. FERTILITY RATES
AGE DISTRIBUTION IN A
POPULATION
• The proportion of various age groups in a
population can have a profound effect on
growth of a population.
• Three “Ecological” Ages
Pre-reproductive
Reproductive
Post Reproductive
PRE-REPRODUCTIVE AGE
• Humans (and some other organisms as well)
have two types of pre-reproductive ages.
Absolute (= biological)
12 to 15 years ?????
Dropping a bit over the last 50 years ?????
Socially Acceptable
varies with the culture
POPULATION AGE STRUCTURE (I)
POPULATION AGE STRUCTURE (II)
GENERALIZED
SURVIVORSHIP CURVES
SURVIVORSHIP
• Many organisms do not fit neatly into one
type or another.
• Humans, in developed countries, have close
to a Type I
• Humans, in less developed countries,
somewhat Type III
• Oysters and salmon show a Type III curve.
• Many annual plants and some lizards show
Type II.
SOME THOUGHT QUESTIONS
• As regards reproductive strategy, are humans
r-selected or K-selected? Explain.
(please note: #11 is reversed in Table 3.3 on
page 62)
• (see table 4.1 on page 75)
From 800 BC to 200 BC (600 years) the
human population doubled (from 100 to 200
million). It then took 1,400 years for the
population to double again (200 to 400
million). Any ideas as to why this increase in
doubling time when doubling time was
shorter before that and after that?