copyright © 2009 benjamin cummings is an imprint of pearson population biology concepts population...
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Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Population Biology Concepts
• Population ecology
• Carrying capacity
• Reproductive strategies
• Survivorship
Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Objective 1
• Outline the characteristics of populations that help predict population growth.
- Populations are characterized by population size, population density, population distribution, sex ratio, and age structure.
- Birth and death rates, as well as immigration and emigration, determine how a population will grow or decline.
Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Objective 2
• Assess logistic growth, carrying capacity, limiting factors, and other fundamental concepts in population ecology.- Populations unrestrained by limiting factors will undergo
exponential growth until they meet environmental resistance.
- Logistic growth describes the effects of density dependence; growth slows as population size increases, and population size levels off at a carrying capacity.
- Carrying capacity is the maximum size a population can attain in a given environment
- K-selection and r-selection describe theoretical extremes in how organisms can allocate growth and reproduction.
Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Population
Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Population characteristics
• Population size: the number of individual organisms present at a given time
- Numbers can increase, decrease, cycle, or remain the same
In 100 years, passenger pigeons — billions of birds — were driven to extinction.
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Population characteristics
• Population density: the number of individuals within a population per unit area- Generally, larger organisms have lower population
densities because they need more resources.- High densities make it easier to find mates, but
increase competition and vulnerability to predation.- Low densities make it harder to find mates, but
individuals enjoy plentiful resources and space.- Reduced resources can lead to overcrowding, disease,
predators, parasites, and extinction.
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Population characteristics• Population distribution
(dispersion): spatial arrangement of organisms within an area - Random — haphazardly
located individuals, with no pattern
- Uniform — individuals are evenly spaced due to territoriality or competition
- Clumped — arranged according to availability of resources- Most common in nature
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• Sex ratio: proportion of males to females
- In monogamous species, a 50/50 sex ratio maximizes population growth.
• Age structure (age distribution): the relative numbers of organisms of each age within a population
- Age structure diagrams (pyramids): show the age structure of populations
Population characteristics
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Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Survivorship CurvesIndividuals of different ages
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Four factors that determine population growth• Population growth or decline is due to:
1. Natality: births within the population
2. Mortality: deaths within the population
3. Immigration: arrival of individuals from outside the population
4. Emigration: departure of individuals from the population
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Calculating Population Growth
• Population Growth rate = (crude birth rate + immigration rate) - (crude death rate + emigration rate)
takes into account effects of migration
Natural rate of population growth= (crude birth rate) – (crude death rate)
number of births and deaths per 1000 individuals/year
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Exponential population growth
• Exponential growth: a population increases by a fixed percent
- A fixed percent of a large number produces a large increase.
- Graphed as a J-shaped curve
• Exponential growth cannot be sustained indefinitely.
- It occurs in nature with a small population and ideal conditions.
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Limiting factors restrain growth
• Exponential growth rarely lasts for long.
• Limiting factors: physical, chemical, and biological characteristics that restrain population growth
- Water, space, food, predators, and disease
• Environmental resistance: all limiting factors taken together that stop exponential growth
- Stabilizes the population size
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Carrying capacity• Carrying capacity: the
maximum population size of a species that its environment can sustain
- An S-shaped logistic growth curve
- Initial exponential increase is slowed and stopped due to limiting factors..
• Carrying capacities changeHumans have raised their carrying capacity by decreasing the carrying capacities for other .
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Perfect logistic curves aren’t often found
(a) Ideal(b) and (c) Fluctuate around carrying capacity(d) Grow rapidly, use resources too quickly, crash suddenly
Copyright © 2009 Benjamin Cummings is an imprint of Pearson
Population density affects limiting factors
• A population’s density can increase or decrease the impact of certain factors.
• Density-dependent factors: limiting factors whose influence is affected by population density
- Increased risk of predation and competition for mates occurs with increased density.
- The logistic growth curve represents the effects of density dependence.
• Density-independent factors: limiting factors whose influence is not affected by population density
- Events such as floods, fires, and landslides
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Biotic potential and reproductive strategies vary
• Population regulation is not only due to environmental limiting factors, but to attributes of the organism itself.
• Biotic potential: an organism’s ability to produce offspring
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Cabezon “Scorpion Fish
Example of high biotic potential
Mature at 3-5 years
Release 50,000-100,000 eggs/year
Fertilized eggs hatch in 12-16 days
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Example of low biotic potential
Mature at 10 years
Give birth to a single baby every 8 years
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K-selected species: animals with long gestation periods and few offspring (“quality, not quantity”)
Have a low biotic potentialStabilize at or near carrying capacityMust compete for resources…so offspring must be of high quality and be good competitors
Natural selection favors individuals that invest in offspring
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r-selected species: animals which reproduce quickly (“quantity, not quality”)
Have a high biotic potential
Devote their energy and resources to producing as many offspring as possible in a relatively short time.
Their offspring do not require parental care after birth- survival left to chance.
Many fish, plants, frogs, and insects.
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K-selected vs. r-selected species
Copyright © 2009 Benjamin Cummings is an imprint of Pearson