1. Chapter 17 Lecture Outline See PowerPoint Image Slides
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2. Population Characteristics
A population is a group of organisms of the same species located in the same place at the same time.
A species is the largest possible population.
Usually the term population is used to describe portions of a species.
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3. Gene Flow The movement of genes
From one generation to another by reproduction
From one population to another by migration or dispersal
Both types of gene flow can occur at the same time.
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4. Gene Frequency
A measure of how often a specific gene shows up in the gametes of a population
Two populations of the same species can have very different gene frequencies.
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5. Age Distribution
The number of organisms in each age category in the population
Organisms can be grouped by their reproductive status.
Prereproductive juveniles
Reproductive adults
Postreproductive adults
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6. Age Distribution
The age categories of a population often do not contain the same proportion of individuals.
The age structure of some species will change dramatically over the course of a season.
Birds are all reproductive adults as they nest in the spring.
In summer there are a high percentage of prereproductive juveniles.
Annual plants are all prereproductive at start of growing season; They become reproductive in middle of growing season; and postreproductive at end of growing season.
Age structure impacts how fast the population will grow.
Large numbers of prereproductive or reproductive individuals lead to a rapid growth in the population.
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7. Age Distribution in the Human Population
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8. Age Distribution in Selected Populations
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9. Sex Ratio The number of males compared to the number of females
Animals that have strong pair-bonding usually have a 1:1 ratio of males to females.
Animals without strong pair-bonding often have more females than males.
Harems
Game animals in which the males are killed more frequently
Some insects (bees, wasps, ants) have far more females than males.
Sex ratios do not apply to hermaphroditic species.
Most plants
Some animals—earthworms
Sex ratios do not apply to organisms that change their sex or those that reproduce asexually
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10. Population Distribution
Describes the way individuals within a population are arranged with respect to one another
Three basic arrangements
Even
Random
Clumped
Even distribution
Random distribution
Clumped distribution
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11. Population Density The number of organisms of a species per unit area
Population density varies with
Reproductive success
Resources available
As population density increases competition becomes more intense.
Population pressure describes how the intensity of competition for resources affects populations.
Dispersal relieves the pressure and can lead to new populations.
If dispersal doesn’t happen, individuals will begin to die.
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12. Reproductive Capacity
The theoretical number of offspring that could be produced by a population
Also called biotic potential
Usually much larger than the number of offspring needed to maintain the population
Much larger than the number of offspring that actually survive (reproductive rate)
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13. Reproductive Capacity
Two strategies for maintaining the species
Have a lot of offspring, but do not invest a lot of energy in them after birth
The probability of survival to reproductive age is slim.
Have fewer offspring, but invest a lot of energy in them after birth
The probability of survival to reproductive age is quite good.
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14. The Population Growth Curve
The growth of a population is dependent on
The rate at which organisms enter the population
Natality = the number of new individuals added by birth/1000 individuals
Immigration
The rate at which organisms leave the population
Mortality = the number of individuals leaving by death/1000 individuals
Migration from the population
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15. The Population Growth Curve
Population growth curves graph the change in population size over time.
Four phases can be seen in the curve.
Lag phase
Exponential growth phase
Deceleration phase
Stable equilibrium phase
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16. A Typical Population Growth Curve
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17. Lag Phase
A period of time immediately following the establishment of the population
Population is small.
Growth is happening slowly.
Both natality and mortality are low
Occurs slowly because reproduction takes time
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18. Exponential Growth Phase
The period of time when the population is growing rapidly.
Natality is much greater than mortality.
As the number of individuals increases, there are more individuals available to reproduce.
During this phase, populations often increase by the same percent each year.
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19. Deceleration Phase Occurs when population growth rate begins to slow
Population continues to grow but at a slower rate.
Eventually birthrate and deathrate become equal.
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20. Stable Equilibrium Phase
The period of time when the population stops growing and maintains a stable size
The availability of resources eventually will limit population growth.
When a population becomes large enough, resources will be in short supply.
Mortality will increase to the point where it equals natality.
Large organisms that live a long time tend to reach stable equilibrium.
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21. Alternate Population Growth Strategies
K-strategists are large organisms that a live long time and reach a population size that can be sustained.
Typical of large mammals, birds, etc.
r-strategists are small organisms with short life spans and tend to have fluctuating population that do not reach a stable equilibrium phase.
Population growth is characterized by a rapid growth followed by a “crash”, followed by rapid growth.
Typical of insects, some small mammals, bacteria, protozoa, etc.
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22. Population Growth Curve for Short-Lived Organisms
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23. Limits to Population Size
Limiting factors are environmental factors that limit population size.
All the limiting factors on a population are collectively known as environmental resistance.
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24. Extrinsic and Intrinsic Limiting Factors
Extrinsic limiting factors come from outside the population.
Predators, availability of nutrients, natural disasters
Intrinsic limiting factors come from within the population.
Crowded conditions leading to decreased reproductive success
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25. Density-Dependent Limiting Factors
Density-dependent limiting factors become more effective as population size increases
Predators are more likely to capture individuals in a large population.
Disease epidemics are more common in large populations.
Competition for resources increases as population size increases.
Intrinsic factors such as decreased reproductive rates occur as population size increases.
Populations of large, long-lived organisms are usually controlled by these factors (K-strategists).
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26. Density-Independent Limiting Factors
Density-independent limiting factors are not related to population size.
Usually accidental or occasional extrinsic factors
Weather patterns
Natural disasters
Drought/excessive rainfall
Populations of small, short-lived organisms are often controlled by these factors (r-strategists).
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27. Categories of Limiting Factors
Factors that limit population growth
Availability of raw materials
Soil nutrients, water, etc., for plants
Nesting sites, water, food, etc., for animals
Availability of energy
Availability of light to plants
Availability of food to animals
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28. Categories of Limiting Factors
Factors that limit population growth (continued)
Accumulation of waste products
Usually not relevant for plant species
Animal species generate waste products.
Waste can be toxic.
As waste builds up, reproduction slows, death rates increase
Accounts for the death phase in bacterial populations
Follows stable equilibrium phase
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29. Bacterial Population Growth Curve
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30. Categories of Limiting Factors
Factors that limit population growth (continued)
Interactions with other organisms
Population growth is limited by predation, parasitism, and competition.
Mutualistic interactions allow populations to grow at a faster rate than they would have alone.
Interactions between two or more species of organisms can lead to population cycles.
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31. Population Cycles
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32. Carrying Capacity
The carrying capacity is the maximum, sustainable population size.
Determined by a set of limiting factors
The carrying capacity for a species can change with factors such as ecological succession, disease, and natural disasters
The size of the organism and the size of its habitat can determine carrying capacity.
Large individuals require more space and resources than small individuals.
Example: fish in an aquarium; many small fish or a few large fish.
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33. Carrying Capacity
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34. The Effect of Biomass on Carrying Capacity
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35. Limiting Factors to Human Population Growth
Human populations follow the same pattern of growth as other organisms.
Human populations are limited by the same kinds of factors as other organisms.
The human population growth curve
Shows a long lag phase and a rapid exponential phase
Technology continues to increase carrying capacity.
Agriculture, sanitation, disease control, etc.
Nevertheless, limiting factors eventually will cause human population growth to level off.
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36. Human Population Growth Curve
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37. Limiting Factors to Human Population Growth
Available Raw Materials
Include
Drinking and irrigation water
Minerals
Food
Etc.
Many portions of the global human population do not have enough food.
However, the world can produce enough food to feed everyone.
Political and economic barriers prevent equal distribution of food.
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38. Limiting Factors to Human Population Growth
Availability of Energy
Ultimately, humans depend on sunlight for energy.
Solar energy can be stored in fossil fuels.
However, limited reserves of petroleum, natural gas, and coal exist.
It is hard to predict how long these will last.
Rate of use of the reserves increases as population size increases.
It took millions of years to build up these reserves, but we are using them at a rate that will deplete them in a matter of hundreds of years.
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39. Limiting Factors to Human Population Growth
Accumulation of Wastes
Human biological wastes can be recycled by decomposers.
Technological wastes cannot be decomposed by organisms.
Excess technological wastes cause pollution.
Most of this waste is toxic and is damaging the environment.
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40. Limiting Factors to Human Population Growth
Interactions with Other Organisms
We are hosts for a variety of parasites.
We are predators on a number of species.
We have mutualistic relationships with many organisms.
We compete with other species for resources.
We convert natural ecosystems to agriculture.
Because we win, we frequently cause extinctions.
We compete with other humans for resources.
High infant mortality in poor countries is evidence of competition for resources.
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41. Control of Human Population Is a Social Problem
Humans have the information to control the human population and improve health and quality of life.
However, we tend to consider what is best for us individually, not the world as a whole.
Continued population growth will present more social, political, and economic problems as well as biological problems.
Will likely result in increased famine and starvation
Improving the education of women may be one social strategy for limiting population growth.
Women with higher levels of education have fewer children.
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