NIS - BIOLOGY

Lecture 16Population Dynamics

Ozgur Unal

1

Population Characteristics

2

Population: the members of a single species that share the same geographic location at teh same time.How can we describe populations?What are some observations you can make about populations of insects over the course of a year?Populations of species are described by density, spatial distribution and growth rate.These characteristics are used to classify all populations of organisms including bacteria, animals and plants.

Population DensityPopulation density is the number of organisms per unit area.In order to find the average population density, simply divide the total number of organisms by total area.Figure 4.1 and 4.2!

3

Spatial DistributionPopulation density might not be uniform everywhere.Dispersion is the pattern of spacing of a population within an area.Mainly three types of dispersion: uniform, clumped and random.

4

Spatial DistributionFigure 4.2Which type of population distribution allows you to predict more accurately how many individuals reside in a given area?

5

What advantage do smaller fish gain by forming clumped groups?

Population Ranges

6

No population (not even the human population) occupies all habitats in the biosphere.Some species have very limited population range or distribution.Examples: Iiwi, peregrine falcon etc.

What are the reasons why some species are not able to expand their population range?

Population Limiting Factors

7

All species have limiting factors, which keep them from continuing to increase indefinitely.Example: Food supply..If the food supply increases, a larger population might result. If it decreases a smaller population might result.

Population limiting factors can be grouped into two: density-independent factors and density-dependent factors.

Population Limiting Factors

8

Density-independent factors: Any factor in an environment that does not depend on the number of members in a population per unit area is a density-independent factor.These factors are ususally abiotic and include natural phenomena such as weather events.Example: Ponderosa pine trees

Density-Independent Factors

9

Example: Humpback chub fish in Colorado River.

Density-Dependent Factors

10

Density-dependent factors: Any factor in an environment that depends on the number of members in a population per unit area is a density-dependent factor.These factors are often biotic factors such as predation, disease, parasite and competition.Predation:

Density-Dependent Factors

11

Disease: This is a density-dependent factor.When population density is high, diseases are transmitted easily from one individual to another because contact between individuals is more frequent.

Competition:Competition between organisms increases when density increases.Competition can occur within a species or between two different species that use the same resource.

Density-Dependent Factors

12

Parasites:The presence of parasites is a density-dependent factor that can negatively affect population growth at higher densities.

NIS - BIOLOGY

Lecture 17Population Growth

Ozgur Unal

13

Population Growth RateCheck this out!http://www.molecularstation.com/science-videos/video/85/bacteria-growth/

How is the growth rate of these bacteria?Do they grow fast or slow by time?How would you plot the number of bacteria vs time?

14

Population Growth RatePopulation growth rate (PGR) explains how fast a given population grows.What factors do you think affect the size of a population?

15

The natality of a population is the birthrate, or the number of individuals born in a given time period.The mortality of a population is the number of deaths that occur in a given time period.Emigration is the term to describe the number of individuals moving away from a population.Immigration is the number of individuals moving into a population.

Population Growth ModelsThere are two mathematical models used to describe population growth: Exponential growth model and logistic growth model.

Exponential Growth Model:Remember the bacteria growth. The growth rate was small at first, but then it increases rapidly.The initial growth rate is small and called lag phase.Exponential growth occurs after the lag phase.Similarly, a mice population in an area with large food supply grows exponentially.

16

Exponential Growth ModelThe graph of exponential growth is J-shaped.The rate of growth is proportional to the size of the population.All populations grow exponentially until some limiting factor slows the population’s growth.

17

Logistic Growth ModelLogistic growth occurs when the population’s growth slows or stops after exponential growth, at the population’s carrying capacity.Many populations grow like in this model, rather than exponential growth.S-shaped curve is typical for logistic growth.A population stops increasing when natality is less than mortality, or when emigration exceeds immigration.

18

Logistic Growth ModelCarrying capacity is the maximum number of individuals in a species that an environment can support for the long term.Carrying capacity is limited by the energy, water oxygen, and nutrients.

19

Reproductive PatternsThe logistic model shows the number of individuals increasing until the carrying capacity is reached.However, there are additional factors that must be considered for real populations.Number of births per reproduction cycle, the age that reproduction begins, life span of the organism..

20

r-strategists: The rate strategy is an adaptation for living in an environment where fluctuation in biotic and abiotic factors occur (such as availability of food or changing temperature).Example: Fruit fly or locustsr-strategists usually have short life span and produce many offsprings.

Reproductive PatternsPopulations of r-strategists usually are controlled by density-independent factors and they usually do not maintain a population near the carrying capacity.

21

k-strategists: The carrying capacity strategy is an adaptation for living in environments that do not fluctuate much.k-strategists are generally larger organisms that have a long life span and produce few offspring.Their population reaches equilibrium at the carrying capacity.Populations of k-strategists usually are controlled by dnesity-dependent factors.