ECOSCIENCETHE ECOLOGY OF

POPULATIONS( Population group properties: density,

natality, mortality, age distribution, biotic potential, growth and regulation fluctuation)

Reported by:FELICES, ANGELICA

2010-10910BS ARCH 5-2

As the number of people grows and the amounts of goods and services provided per person

increases, the associated demands on resources,

technology, social organization, and environmental processes

become more intense and more complicated, and the

interactions among these factors become increasingly

consequential.

POPULATION DYNAMICS

• The study of changes in population size-population dynamics—is of enormous practical importance to humankind.

• Population Dynamics is the branch of life sciences that studies short-term and long-term changes in the size and age composition of populations, and the biological and environmental processes influencing those changes.

• The analysis basically amounts to keeping track of inputs and outputs in a population.

NATALITY & IMMIGRATION

• The inputs are natality and immigration. (Natality (birth rate) is used instead of births because individuals that hatch from eggs or grow from seeds are not normally said to have been “born.”)• The outputs are mortality

(deaths) and emigration.

AGE COMPOSITION• In populations with overlapping

generations (such as human population), the age composition--that is, the proportion of individuals in various age classes—has a substantial effect on the future course of population growth.• In populations where adults of one

generation invariably die before their offspring mature (like many insects), the generations do not overlap, and the analysis are greatly simplified.

Demographers vs. Ecologists

• Demographers concern themselves primarily with factors that influence natality (birth rate). They have viewed populations they study as having varying inputs and fixed outputs. • Ecologists tend to focus on mortality.

They have viewed populations as having fixed inputs and varying outputs.

Their point of view…

Density• Biological Population Densities - Population density is

population divided by total land area or water volume, as appropriate.

Low densities may cause an extinction vortex and lead to further reduced fertility. This is called the Allee effect after the scientist who identified it. Examples of the causes in low population densities include:

Increased problems with locating sexual matesIncreased inbreeding 

• Human Population Density -For humans, population density is the number of people per unit of area usually per square kilometre or mile (which may include or exclude cultivated or potentially productive area). Commonly this may be calculated for a country, city, another territory, or the entire world.

total area population / land area in square miles (or square kilometers)

Symbols Used in Population Dynamics

Instantaneous Rate of Increase

• A logical beginning to consider the problem of measuring the rate of change in a population– that is, the rate of increase or decrease—at a given moment. An average rate is easy to calculate. It is reasonable to expect that the rate of change in the size of a population (let the size be N) in the course of time (t) will itself defend on the population size. Whatever the average individual’s contribution to population growth, it must be multiplied by the population size in order to determine the amount of change in population as a whole.

Exponential Growth

“The most important fact in demography is

that we all get one year older every year”

Population Age Structure

 The reason for this is a population's age structure: the proportion of the population (or of each sex) at each age level.Population Geographers typically construct  a population age structure diagram by plotting the percentages of numbers of males and females in the total population in each of three age categories:•Pre reproductive (ages 0-14)•Reproductive (ages 15-44)•Post reproductive (ages 45 and up) Generalized population age structure diagrams for countries with:•rapid (1.5-3%) growth rates/slow (0.3-1.4%) growth rates•zero (0-0.2%) growth rates/negative growth rates

Definition of Age

• Age is a critical variable in understanding population change of all kinds

• Exact age refers to time elapsed since birth• Age in completed years refers to the greatest integer less than exact age• Age at last birthday means the same• ‘Age’ is often used without further specification, with the appropriate meaning inferred from context

Age Distribution of a Population

• Exists at any point in time• Changes with time except in a ‘stationary population’• May refer to a list of the ages of all persons in the population• But usually refers to numbers of persons in age groups