Population Growth and Species Strategies

Population Growth

• Population: a group of individuals of the same species living in the same area.
• Populations change over time due to factors such as specialist and generalist species.

Specialist Species

• Specialist species are adapted to specific habitats and have a narrow diet.
• They may be advantaged in constant habitats.
• Many specialist species are becoming threatened, endangered, and extinct.

Generalist Species

• Generalist species can live in many environments and have a varied diet.
• They are better adapted to changing conditions.
• They are less likely to be threatened.

Life Strategies (r versus K)

• r-strategists are small, easily dispersed, reproduce quickly, and have short lifespans.

• They thrive in unstable environments, reproducing rapidly.

• Most r-strategists are generalists.

• K-strategists are larger, mature slowly, and have long lifespans.

• They thrive in stable environments, with relatively few offspring.

• Most K-strategists are specialists.

• Populations of K-strategists are mostly stable.

Measuring Populations

• Ecologists measure populations by counting individuals at various times or areas.
• This is similar to a census.

Population Growth

• Population growth is the change in the number of individuals in a population over a time frame, often yearly and is known as the growth rate (gr).

Processes Changing Population Size

• Births (b)

• Immigration (I)

• Deaths (d)

• Emigration (e)

• Change in population = [b + I] - [d + e]

• Growth rates can be expressed as a percentage of the original population.

Percent Change in Population

• The change in population can be expressed as a percentage.
• The formula uses the number of initial individuals and the change over time (final-initial)/initial. This is then multiplied by 100 to show the percentage change.

Population Density

• The population density is the number of individuals in a definite area or volume (A or V), which is shown as D= N/A or N/V where N is the number of individuals, A is the area and V is the Volume.
• When it is not possible to count every individual, several sample areas are used to estimate the actual population size.

Population Distribution Patterns

• Population distribution patterns are based on resource availability.
• When resources are scarce (water or food), populations tend to clump around them.
• When resources needed for growth are abundant, populations tend to spread out randomly.

Fluctuations in Population Growth

• Population growth may increase or decrease slowly or rapidly, and follow a particular pattern.
• When organisms are placed in a favorable habitat their population will increase slowly at first. This is known as the lag phase.
• In the exponential or rapid growth phase reproduction of individuals increases rapidly.

Biotic Potential, r

• Biotic potential is the maximum reproductive rate for a population in ideal conditions.
• The higher the biotic potential, the faster the growth rate.
• This is the intrinsic rate of growth that is possible given unlimited resources and ideal living conditions.

Growth is Never Exponential Indefinitely

• Competition for resources and other limiting factors will slow the rate of growth.
• Most populations show logistic (S-shape) growth.
• The initial J-stage is followed by levelling off of the curve over time.
• The levelling off is the habitat's carrying capacity (K), which is the maximum population size.

Density Dependent Factors

• Density-dependent factors are factors whose effect increases when the population size increases.
• The factors include starvation (competition for food), parasitism, and disease transmission.

Boom Then Bust

• For r-strategists, once the population density reaches its peak, the individuals begin to die rapidly.

• This causes a sudden crash in the population density (bust).

Factors Limiting Carrying Capacity

• Factors limiting a habitat's carrying capacity can be density-dependent or density-independent.
• Density-independent factors affect populations regardless of population size.

Per Capita Rates

• To compare populations of different sizes, the per capita rate is used.
• Per capita is the rate of change per person.
• To calculate the per capita rate you divide the rate by the total number of people in the population.

Human Carrying Capacity

• Experts estimate humans have not yet reached carrying capacity.
• The carrying capacity is the maximum number of individuals the area can support.

Human Population Growth

• The current global population growth rate is about 1.05%
• The rate of global population increase is determined by daily births and daily deaths.

Demographic Transition

• The demographic transition is the movement of a country from high to low birth and death rates.
• It typically happens when a country develops economically.

Age Structure Diagrams

• Population growth rates can be interpreted from the shape of the age structure diagram.
• An age structure diagram shows the relative number of individuals in each age group (males and females) of the population.

Infant Mortality Rates

• Death rates of children are higher in less developed countries (LEDCs) because mothers do not have easy access to good healthcare and nutrition.

Less Developed Countries (LDCs)

• Less developed countries (LDCs) are the fastest-growing countries.

Cultural Challenges to Declining Birth Rates

• In order for a transition to a higher-developed country (MEDC) from a low-developed country (LEDC), the country must reduce the birth rate.

• The reduction in birth rates is often achieved by offering family planning methods, such as contraceptives. Additionally, educational opportunities for women, encouraging later marriages, and encouraging postponement of childbearing can be effective.

Total Fertility Rates (TFR) and Replacement Fertility

• TFR is the number of children a woman will have in her lifetime.

• The earlier a woman starts having children, the TFR will be higher.

• Education of women lowers the TFR since it postpones childbearing.

• Replacement fertility is the number of children needed for a couple to replace themselves and keep the population stable.

• Replacement fertility is 2.1 due to infant mortality.

China's One Child Policy

• China implemented a one-child policy in 1979 in an attempt to reduce its population.
• This policy led to a reduction in the total fertility rate (TFR) in China.

Urbanization

• Urbanization is the redistribution of people from rural to urban areas as countries industrialize.