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Population Biology
Biotic Potential: highest rate of reproduction under ideal conditions.
-populations very rarely reach their biotic potential, because conditionsare not perfect all of the time.
Population size: the number of organisms in a population (N)
Under perfect conditions, a populationwill continue to increase indefinitely.
The human population shows this pattern (so far) . Why?
-unlimited resources, such as food andwater, unlimited space, all waste is removed so that it does not build up, etc.
Unlimited Population
0100200300400500600
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Time
Pop
ulat
ion
Num
bers
This is an example of an EXPONENTIAL growth curve.
Carrying Capacity: the number of individuals that a given environmentcan support.
Ex: Number of lions in the African plains, or the number of zebrasat carrying capacity, birth rate is equal to the death rate.
Normal Population Growth
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1 2 3 4 5 6 7 8 9
Time
Popu
latio
n N
umbe
rs Carrying Capacity
This is an example of a LOGISTIC growth curve.
Environmental Resistance: the sum of limiting factors facing a population.
They all add up to the Environmental Resistance, which holds numbers down
Ex: water requirements, space, need for food, competition, lack ofmates.
Limiting Factors:-control population size and growth-can change depending onenvironmental conditions.
Population Density: the number of organisms per unit area, or volume.
Ex: rain after a long drought
Limiting Factors can be Density Dependent (related to the density of the population), or Density Independent (does not matter what the density of the population is)
Ex: flood/storm (DI) vs levels of disease(DD).
Density Dependent Factors1)Predation: size of prey population is held down by predators, size of predator population is dependent on size of prey population.
Population Relationships
0
10
20
30
1 2 3 4 5 6 7
Time in Years
Nu
mb
er
of
Ind
ivid
ua
ls
Foxes
Rabbits
3) Food/water resources: the higherthe density of organisms, the faster the food and water supply will be used up.
4) Space for nesting: High density populations will have a high level of competition for the best areas to raiseyoung.
2) Disease: Disease spreads more rapidly through a dense population, populations can be reduced by disease
Interspecific and Intraspecific Competition:
Interspecific: competition between different populations.
Intraspecific: within a population.
Ex: lions vs. hyenas
Ex: lions vs. lionsBoth types are Density Dependent
Dispersion and Competition
Dispersion is the pattern of distribution theindividuals within a population take.
Random: Individuals are placed by chance, or by self determination.
Ex: trees in a forest, a coral reef.
Even Distribution: Individuals are evenly spaced, located at regular intervals. Usuallyoccurs because of intraspecific competition.
Ex: Competition between pine trees for sunlight
Clumped Distribution: Individuals are bunched together in clusters, for protection,reproduction, or because of narrow habitattolerances.Ex: schools of fish, flocks of birds, clumps ofone type of plant within a forest.
Population Growth Patterns1)Rapidly growing populations: Many species, such as insects, plants, and fungi are found inrapidly changing environments.
Such species are called r-strategists
r-strategists populations grow exponentially when environmental conditions allow them toreproduce.
When conditions worsen, the population size drops quickly.
R-strategists use the strategy of creating many offspring, but spending little energy and time oneach individual.
Many offspring, little input
EX: Dandelions: Lots of seeds, blown by the wind,no care at all.
Blue Crabs: Eggs are released by the thousands,but no energy is spent on the young.
1 2 3 4 5
Years
Pop
ulat
ion
R-Strategist Population Growth
2) Slowly Growing Populations
Organisms that grow slowly often have small population sizes.
These organisms are called k-strategists, because their population is usually below the carryingcapacity (K).
k-strategists have fewer young, but spend more time and energy on each individual.
K-strategists grow slowly, have a longer life span,and are less susceptible to environmental change.
Many endangered k-strategists are in troublebecause they are being hunted in numbers thatcannot be supported.
Examples: Sharks Gorillas Tigers Lions
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Years
Pop
ulat
ion
K-strategist Population Growth
Summary of R vs. K strategists
R:
Mature rapidlyShort-lived
Have many offspring
Invest little energy in young
Boom or Bust population
K:
Mature Slowly
Long-lived
Few offspring at a time
Care for their young
More stable population