Population Ecology

1. Species populations  in an ecosystem are dynamic.  Their size is controlled by four (4) parameters. The rates of birth and immigration increase the population size while the rates of death and emigration decrease the population size.  When the rates of increase and decrease are equal, the population size is constant and we say the population is in a steady state.

Unconstrained population growth is exponential (see graph below). Assuming organisms reproduce at a fixed rate, population size doubles at each reproductive event.  For example, fast growing bacteria divide every 20 min.  In a twenty four hour period (72 reproductive events), one bacteria gives rise to 4,722,366,482,869,645,213,696 individuals.  The graph below illustrates this exponential growth.

2. Within an ecosystem, growth does not continue unchecked. Rather it reaches a steady state size. which in turn reflects the limitations imposed on the population by factors within the ecosystem. The graph below shows population growth as it reaches steady state. Initial growth is slow. After ten to eleven generations the growth rate accelerates, becoming exponential. This is the period of fastest growth rate.  As the population approaches its steady state, the growth rate slows.

The steady state size of the population defines  the ecosystem's carrying capacity for that species, i.e. the number of individual organisms of a species that the ecosystem can support.

3. Ecologists determine the carrying capacity of an ecosystem by identifying the limiting factors that impact population size.   Limiting factors fall into one of two categories: density independent factors and density dependent factors. 

Density independent factors  are factors that impact  a population's size regardless of the size of the population. These factors include earthquakes, fires, floods and seasonal changes that trigger migrations to and from an ecosystem.

Density dependent factors are factors that are a function of population size such as disease, food availability, mate availability, etc.

4. The predator prey relationship is an example of a density dependent factor. Population curves for predator prey are often sinusoidal.  As the population of prey increases, predators have more food, are healthier and can give birth to and sustain  more offspring.  As the predator population increases, the prey population decreases.  Without enough food, the predator population also declines.  Carrying capacity here is a range based on predator and prey sinusoidal population curves.

5. The lynx and hare predator prey population case study is widely taught.  Here is an interesting video that showcases both animals in their native habitat.

Play the predator - prey game by clicking on the image below: 

6. The following graph shows the impact of disease on population size. More specifically this shows the impact of the plague on the human population in the middle ages and the follow on recovery and growth.

7. This video of populations from Bozeman Science does an excellent job of explaining the material related to population dynamics.  You are not expected to do the mathematical calculations but you should be familiar with the graphs and what they portray. 

Below you will find the links to the Population Dynamics Unit Organizer and the slide presentation 

                                 Population Unit Organizer.doc                  Population Dynamics.ppt  Population Dynamics (1).pdf