Ecology - Chapter 10 (Population Growth)

Population Growth

The difference between births and deaths

J-shaped population growth curves

• From unlimited population growth

• Geometric growth

• Exponential growth

The _________ the population growth rate, the _________ a population grows

Greater population growth rate, faster a population grows

Geometric Growth

• Population breeds seasonally (often once a year)

Example of geometric growth

Wolves reintroduced in Montana and Idaho

Exponential Growth

• Species reproduce almost continuously

• Generations overlap

Species that grow exponetially

• Bacteria

• Internal parasites

• Humans

• Invasive species

2 examples of breeds that fit the J-shaped curve in a geometric pattern

• Tule elk

• Wolves

Who introduced rabbits in Australia?

Thomas Austin

At what time is exponential growth exhibited?

• When an organism is first introduced into a new environment

  • Exhibited for a short period of time

What is the best way to fight to control invasive species? Who came up with this?

• Dan Simberloff

• Discovering them *early* and eradicating or containing them before they spread  

What leads to S-Shaped population growth?

Limited resources

What causes resources to become limited for many species

Growing popluations

Logistic growth

A mathematical model used to describe how a population grows under conditions of limited resources

Where does logistic growth occur?

• Where there are limited resources

• Creates an upper boundary for population size

Carrying Capacity (K)

The upper boundary for the population size

For what type of breeders can logistic growth occur?

Both continuous and periodic

What shape does logistic growth yield?

An S-shaped population growth curve

Density-Dependent Factor

A factor whose influence varies with the density of the population

• Increases mortality rate

What do density dependent factors affect?

• A higher proportion of individuals when population densities are higher

• A lower proportion of individuals when population densities are lower

Why may birth rates decrease as populations increase?

• Resources become more limited

• Density-dependent competition for those resources increases

Density-Dependent Mortality

• Occurs as population densities increase

• Competition for resources increases

• Reduces offspring production or survival

Density-independent factors

• Keeps mortality rate unchanged (flat line)

• Influence not affected by changes in population size or density

• Physical factors

Inverse density-dependent factors

Decreases mortality rate

Examples of density-independent factors

• Weather

• Drought

• Freezes

• Floods

• Fire

Example of a scenario with a density-independent factor

• Hard freezes

• Kills a considerable % of animals and plants no matter how large

• Most individuals are susceptible 

Introduced diseases to which native organisms are not adapted can act in a ___________ (+ example)

Density independent manner

• May kill virtually all their hosts regardless of their density

• Ex. Chestnut blight

Life history strategies

Sets of physiological and behavioral features that incorporate:

• reproductive traits

• survivorship

• length-of-life characteristics

• Preferred habitat type

• Competitive ability

What is the importance of life history strategies?

Have important implications for how populations grow and the reproductive success of populations and species

Semelparity

When offspring are produced in a single reproductive event

• May live for many months or years before reproducing once and dying

• Common in insects and other invertebrates 

3 examples of semelparous species

• Salmon 

• Surgeonfish

• Century plants

Iteroparity

Repeated reproduction at intervals throughout the life cycle

• Common in vertebrates and perennial plants (trees)

• Number of reproductive events/offspring varies

Examples of continuously iteroparous species

• Chimpanzees

Seasonal iteroparity

• Have distinct breeding seasons

• Leads to distinct groups of individuals who are all born at the same time

Examples of species that go through seasonal iteroparity

• Birds

• Mammals

• Temperate forest trees

Why do some species reproduce in a semelparous mode?

If the environment is stable then selection favors a single act of reproduction

• Organism can devote all its energy to making offspring

• Does not have to maintain its own body

Why do some species reproduce in a iteroparous mode?

If survival of juveniles is very poor and unpredictable

• Repeated reproduction + long reproductive life increases the chance that juveniles will survive in at least some years

K-selected species

Populations adapted to exist at or near the carrying capacity, K, of the environment

• Density dependent

r-selected species

High rate of per capita population growth, r, but poor competitive ability

• Density independent

Example of K-selected species

Acorn + Oak Tree

• Tree has large size

• Slow growth

• Long life span

• Has fewer, larger seeds

• Poor seed dispersal

Example of r-selected species

Dandelion

• Plant has small size

• Rapid growth

• Short life span

• Many small seeds

• Good seed dispersal

Characteristics of r- and K-selected species

Which selected species is at risk of extinction in a human-dominated world? Why?

• K-selected species

  • Tend to be bigger = need more habitat

  • Have fewer offspring, cannot recover as fast from disturbances

  • Breed at larger age, generation time is long

What K-selected species runk the risk of extinction?

• Giant sequoia

• Terrestrial mammals (rhinoceros, elephants, grizzly bears)

• Marine malls (blue whales and sperm whales)

Are humans r- or K-selected?

• Depends on area of the world

• Industrial nations tend to be more K-selected

• Areas like Africa and Asia have highest offspring rates