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

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Time

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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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Time

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n N

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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

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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.

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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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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