Characteristics of Populations

A population is a group of organisms from the same species that live in the same geographical area.

There are three important characteristics of all populations:1.Geographical area. (Location)2.Density (# of individuals per unit of area).3.Growth rate.

Population Growth Some populations may stay the same size for

years. Some may increase or decrease in size over time. Three factors affect the size of a population:1.Birth rate2.Death rate3.Migration (Immigration and emigration).

If BR + I < DR + E = DECREASING If BR + I > DR + E = INCREASING

Exponential Growth

What if a population had “ideal” conditions

(no predators, no disease, plenty of food and space).

Ex.) Bacterium “Doubling time” = 10 min

Exponential Growth

10 min 1 70 min 64 130 min 4096

20 min 2 80 min 128 140 min 8192

30 min 4 90 min 256 150 min 16,384

40 min 8 100 min 512 160 min 32,768

50 min 16 110 min 1024 170 min 65,536

60 min 32 120 min 2048 180 min 131,072

Can bacteria continue to grow like this?

Exponential Growth

J- shaped curve Population grows

slowly at first and then rapidly to infinitely high number.

Does NOT happen in natural populations for very long. Time

# of Organisms

Exponential Growth

• Exponential growth occurs under “ideal conditions”.

Growth is “un-restricted.” Individuals will reproduce at “constant rate.” In case of bacteria, they constantly double

every 10 minutes. If a population continues to reproduce at this

rate, it will reach its “biotic potential.” Biotic potential- highest reproduction rate

possible for a species

Logistic Growth Occurs when population’s growth slows or

stops following a period of exponential growth.

What could cause this to happen? There are several factors that “limit” a

populations growth. These factors can be abioitc or biotic. The environment can only support a certain

amount of individuals. This is known as the “carrying capacity.”

Logistic Growth

Logistic Growth Summarized

S- shaped curve Lag phase – growth is stable but

somewhat slow, population is getting established

Exponential phase- population well established and growing quickly

Equilibrium phase- population levels out as carrying capacity reached and remains relatively same size

Limiting Factors

Factors that cause a populations numbers to level out or decrease are known as “limiting factors.”

Two types of limiting factors:1.Density – Dependent Factors2.Density – Independent Factors

Density – Dependent Limiting Factors

Factors only become limiting when the population density reaches a certain level.

Population size is the issue. These factors impact large, dense populations

the most.1.Competition2.Predation3.Parasitism and Disease

Competition

When populations get larger, resources are used up more quickly. Ex: food, water, space, and other essentials to survival.

Interspecific Between two different species.

Intraspecific Between members of the same species.

Competitive Exclusion Principle

Two different species competing for the same resource, one will be better adapted than the other and will eventually win that resource.

Density – Independent Factors

Affect all populations same way regardless of population size (density).1.Natural disasters

(storms, hurricanes, etc.).

2.Human activity (deforestation, pollution, etc.).