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, inter- and 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?