population biology concepts

Copyright © 2009 Benjamin Cummings is an imprint of Pearson

Population Biology Concepts

• Population ecology• Carrying capacity• Reproductive strategies• Survivorship


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.


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.


Population


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.


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.


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


• 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


Survivorship CurvesIndividuals of different ages


Four factors that determine population growth• Population growth or decline is due to:

1. Natality: births within the population 2. Mortality: deaths within the population3. Immigration: arrival of individuals from outside the

population 4. Emigration: departure of individuals from the

population


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


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.


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


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 .


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


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


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


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


Example of low biotic potential

Mature at 10 years

Give birth to a single baby every 8 years


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


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.


K-selected vs. r-selected species

population biology concepts

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