Why Population Ecology?

• Scientific goal- understanding the factors that influence the size of populations

  - general principles

  - specific cases

• Practical goal- management of populations

  - increase population size

  Ex: endangered species

  - decrease population size

  Ex: pests

  - maintain population size

  Ex: fisheries management 

  - maintain & maximize sustained yield

Life takes place in populations

• Population

  • group of individuals of same species in same area at same time

  • rely on same resources, interact, and interbreed

Factors that affect Population Size

• Abiotic factors

  - sunlight & temperature

  - precipitation / water

  - soil / nutrients

  Biotic factors

  - other living organisms

  - prey (food)

  - competitors

  - predators, parasites,
  disease

  Intrinsic factors

  - adaptations

Characterizing a Population

• Describing a population

  - population range

  - pattern of Dispersion

  Density of population

  - #individuals per unit 

  area

Population Range

• Geographical limitations

  - abiotic & biotic factors

  Ex: temperature, rainfall, food, predators, etc.

  - habitat

Population Dispersion

• Clumped

• Uniform

• Random

• Provides insight into the environmental associations & social interactions of individuals in population

Population Size

• Changes to population size can occur by:

  -Births, Deaths, Immigration, and Emigration

Population Growth Rates

• Factors affecting population growth rate

  sex ratio

  - how many females vs. males

  generation time

  - at what age females reproduce

  age structure

  - #females at reproductive age

Demography

• Study of a populations vital statistics and how they

  change over time

  - Life tables, Age Structure Diagrams and Survivorship Graphs

Survivorship curves

• Graphic representation of life table

• The relatively straight lines of the plots indicate relatively constant rates of death; however, males have a lower survival rate overall than females.

• What do these graphs tell about survival &
  strategy of a species?

  • I. High death rate in post-reproductive years

  • II. Constant mortality rate throughout life span

  • III. Very high early mortality but the few survivors then live long (stay reproductive)

Age structure

• Relative number of individuals of each age

• What do these data imply about population growth in these countries?

Trade-offs: survival vs. reproduction

• The cost of reproduction

      To increase reproduction may decrease survival

  - age at first reproduction 

  - investment per offspring

  - number of reproductive cycles per lifetime

  - parents not equally invested

  • offspring mutations

• Life History determined by costs and benefits of all adaptations.

• Natural selection favors a life history that maximizes lifetime reproductive success    

Reproductive strategies

• K-selected

  - late reproduction

  - few offspring

  - invest a lot in raising offspring

  Ex: primates

  Ex: coconut

  r-selected

  - early reproduction

  - many offspring

  - little parental care

  Ex: insects

  Ex: many plants

Trade offs

• Number & size of offspring vs. Survival of offspring or parent

Population Growth Rate Models

• Exponential growth

  - Rapid growth

  - No constraints

• Logistic growth

  - Environmental constraints

  - Limited growth

Population Growth Math

• Change in population = Births – Deaths

• Per capita birth rate = b

• Per capita death rate = d

• # of individuals = N

• Rate of population growth (r) = b – d

• Survivorship = % surviving

Ex: If there are 50 deer in a population, 13 die and 27 are born the next month. What is the population size the following month?

             Answer: 27-13 = 14, so new population is 64

Ex: What are the birth and death rates for the deer?   #Births/N = b

Answer: 27/50 = 0.54

Death rate (d) = 13/50 = 0.26

Ex: What is the rate of growth for the deer?   r = 0.54 -0.26 = 0.28

Exponential Growth  (ideal conditions)

• No environmental barriers and Growth is at maximum rate

  dN/dt = rmaxN

  N = # individuals

  Rmax = growth r

Exponential Growth

• Characteristic of populations without limiting factors 

  - introduced to a new environment or rebounding 

     from a catastrophe

Logistic Growth Equation

• K = carrying capacity of population

  
  

  Ex: If a population has a carrying capacity of 900 and the rmax is 1, what is the population growth when the population is 435?    

  Answer = 1 x 435 (900-435)/900 = 224 

  What if the population is at 850?   

  What if it is at 1010?

  Explain the results of each problem.

Carrying capacity

• Maximum population size that the environment can support with no degradation of the habitat

  *varies with changes in resources

Changes in Carrying Capacity

• Population cycles Ex: predator – prey interactions

Regulation of population size

• Limiting factors

  density dependent

  • competition: food, mates, nesting sites

  • predators, parasites, pathogens

  density independent

  • abiotic factors

    • sunlight (energy)

    • temperature

    • rainfall

Introduced species

• Non-native species (INVASIVE)

  - transplanted populations grow exponentially in new area

  - out-compete native species 

  - reduce diversity 

  Examples:

  - African honeybee

  - gypsy moth