Date post: | 20-Jun-2015 |
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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.).