Chapter 6

The Human Population and Its Impact

Case StudyNew England Forests Come Full Circle

1. Describe the temperate forest as seen by the Pilgrims.

2. What happened to the temperate forest between 1620 and 1800s? Explain why this happened.

3. What happened to the temperate forest between 1850 and 1950. Explain why this happened.

4. What caused the appearance of the leaf beetle?

5. What did the “outbreak” of the leaf beetle cause to happen?

6. Explain the rise and fall of the goldenrod population.

7. Explain the rise and fall of the other plant species.

8. What was the first tree species to dominate and why?

9. What led to the rise of the broadleaf deciduous trees?

10. What does this process demonstrate?

Levels of Complexity

Population Ecology

• Definition:

• Knowing the factors that affect a species’ population size helps to implement measures to improve its survival and reproduction.

Population Characteristics

1. Population size

2. Population density

Population Characteristics

3. Population Distribution

– Occupation of a population with respect to one another.

– Random– Uniform– Clumped

Population Characteristics

4. Population Sex Ratio– Ratio of males to females.– Number of offspring is primarily:

5. Population Age Structure– How many individuals are in particular age

categories.– Large portion of older individuals …– Large number of young individuals …

Factors that Influence Population Size

• Density-Dependent factors– Factors that

influence the individual’s probability of survival and reproduction that depend on:

• Gause’s experiments

Limiting Resources

• Definition

– Limiting resource decreases …– Limiting resources for autotrophs …– Limiting resources for heterotrophs …

• Carrying capacity - K– Definition

Density-Independent Factors

• Factors that influence the individual’s probability to survive that have nothing to do with the size of the population.

• Examples:

Growth Models• To explain and predict changes in population size.• Exponential Growth Model

– Predict the population size at any moment of time.

– Growth rate - r• Definition:

– Intrinsic growth rate• Definition:

Exponential Growth Model

• Nt = N0ert

• Nt = population’s future size

• N0 = number of individuals currently in population

• e is the base of the natural logarithms

• r = intrinsic growth rate• t = time passed

Do The Math• Consider a population of rabbits that has an initial

population size of 10 individuals.

– N0 = 10

• Let’s assume that the intrinsic rate of growth is 0.5 – 50%, which means that each rabbit produces and net increase of 0.5 rabbits each year. – r = 0.5

• With this information, we can predict the size of the rabbit population a year from now.

• Nt = N0ert

• What about in 10 years?

Logistic Growth Model• Populations do not

experience exponential growth indefinitely, therefore:

• Initial growth is exponential but a slowing of growth as the population reaches the carrying capacity.

• S-shaped curve

Variations on the Logistic Growth Model

• Overshoot

• Die-off

Predation as a Factor in Limiting Population Growth.

Reproductive Strategies

• K-selected species– Species with a low growth rate.– Population increases slowly until they reach the carrying

capacity.– Number of organisms is determined by the carrying

capacity.– Fluctuations in population are small.

• Characteristics:

r- Selected Species

• High intrinsic growth rate• Do not typically remain near their carrying

capacity.– Rapid population growth– Overshoots– Die-offs

• Characteristics:

Survivorship Curves• Patterns of survival over time

– Type I– Type III– Type II

Metapopulations

• A group of spatially distinct populations that are connected by occasional movement of individuals between them.

• Corridors:

Species Interactions

• Factors that determine distributions of species:

• Community ecology– Study of interactions of

populations within a community.

• Competition– Definition:

• Competitive Exclusion Principle

• Resource Partitioning

Types of Resource Partitioning

• Temporal

• Spatial

• Morphological

Predation

• Use of one species as a resource by another species.

• True predators

• Herbivores

• Parasites– Pathogens– Parasitoids

Prey Defenses

• To avoid being eaten or harmed by a predator.

Symbiotic Relationships• Relationship of two species that live in close

association with each other.

• Mutualism– Both interacting species benefit increasing both

species chances of survival or reproduction.

Commensalism• One species benefits and the other is not

harmed or helped.

Cattle egret and cows and sheep.

Keystone Species• A species that plays a role in its

community that is far more important than its relative abundance might suggest.

Predator-mediated Competition

Types of Keystone Species

• Finding food for the community

• Mutualistic interactions

• Ecosystem engineers

Ecological Succession

• Predictable replacement of one group of species by another group of species over time.

• Terrestrial communities

– Primary succession

– Secondary succession

Primary Succession

• Growth of species on land that has never had growth before – no soil.

Primary Succession• The type of community that eventually develops is

determined by the temperature and rainfall of the region.– U.S.

• Forest in the East• Grasslands in the Midwest• Shrubland in the Southwest

• In some areas the number of species increases as succession proceeds.

• In some areas there are fewer late successional species than early ones.

Secondary Succession

• Growth of species in areas that have been disturbed but have not lost their soil.

• Pioneer Species

Aquatic Succession

Factors Influencing Species Richness

• The number and types of species present is determined by:– Colonization of the area by new species– Speciation in the area– Loses in the area by extinction

• Factors that affect these processes:– Latitude– Time– Size of the habitat– Distance of the habitat from the source of the

colonizing species

Theory of Island Biogeography• Theory that demonstrates the dual importance

of habitat size and distance in determining species richness.

• Habitat size

– Larger habitats usually contain more species.

• Because:

• Distance from source of colonizing species

– Distance matters because: