# Demecology – the Ecology of Populations

The main idea: Note how mathematical models are used to examine variation in growth of a population.

Lecture outline:

1. Statistic and dynamic characteristics of population.

2. Growth curves patterns: J-shaped curve and S-shaped curve 3. Population regulation: Density-dependent and density-independent factors. 4. Human population patterns:

- Population numbers.

- Demographic transition and structure

- Population urbanization

1. Statistic and dynamic characteristics of population

Population – is a group of individuals of the same species that live in the same area. Population is an elementary and evolutionary unit of biosphere. Statistic characteristics:

• Population Number - number of individuals in a given population. • Population Density – the number of individuals in a population in given area. Population density = 100 fish / 1.000m3 of water = 1fish/ 10 m3 of water or 10 plants/ 1m2 Dynamic characteristics:

• Population growth – an increase in a population at a given time. • Population growth rate – the change in the number of individuals in a population over time

2. Growth Curves Patterns:

Growth Curves – a graph showing the number of individuals in a population over time. • The J-shaped curve – a growth curve that shows population growth which occurs indefinitely at a constant reproductive rate.

The J-shaped curve presents two phases of population growth: Lag-phase: little or no increase occurs in a population.

Exponential phase: an increase occurs in a population so rapidly that the number of individuals doubles in a specific time interval and keeps doubling in increasingly shorter periods of time. The J-shaped curve is also called as Exponential curve.

Biotic potential – the maximum rate at which a population could increase under ideal conditions – an environment with unlimited resources. The biotic potential is rarely achieved by population in natural environment. • The S-shaped curve – a growth curve that shows the period of relative stability in a population that occurs because of environmental resistance (after its lag and exponential phases). Carrying capacity – the largest population that a particular environment can support sustainably (long-term), assuming there are no changes in that environment. Environmental resistance – unfavorable environmental conditions that prevent organisms from reproducing indefinitely at their biotic potential. Factors that tend to reduce population growth rates: limits of food, light, space, oxygen etc. Notice that the J-shaped curve is the lower part of the S-shaped curve!

|[pic] |[pic] | | | | |J-shaped curve (Exponential curve) |S-shaped curve (Sigmoidal or Logistic curve) shows that | |Note that a population grows slowly during the early lag |population growth levels off after the exponential phase. The | |phase and then very rapidly during the exponential phase. |top of this curve indicates the carrying capacity of an | | |ecosystem. |

Survivorship rate – a number of individuals survived over specific period of time. Types of bell curves of survivorship rate: strait diagonal, convex, concave. Population dynamics – periodical or not periodical changes in number, sex and age of a population affected by abiotic and biotic factors (stable, changeable, explosive).

3. Population Regulation:

• Density-dependent factors – are factors that affect populations (reproduction and mortality rates) in different ways depending on population density (availability of food, space,...

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