gen bio - lecture 23: population ecology

biological hierarchy

atoms, molecule, cell, tissue, organ, body system, organism, population, community, ecosystem, biosphere

population + role of individuals within populations

group of potentially interbreeding organisms IN NATURE of the SAME SPECIES

• individuals within populations…

  • rely on and compete for the same resources

  • are influenced by similar environmental factors

ecology

study of interactions between living organisms and physical environment

population ecology

considers # of individuals of a species in an area as well as population dynamics

population dynamics

study of change in populations

• how and why population size changes over time

• processes common to all populations

• different features of populations

population size (N)

# of individuals in a population

• scale matters!

population density

# of individuals per unit area/volume @ given time

• scale matters!

changes in population size

all populations have the potential to increase (birth, immigration) or decrease (death, emigration) in size

• change = (births + immigration) - (death + emigration)

population growth rate EQUATION

ΔN/Δt = B - D

ΔN = change in population size

Δt = time interval

B = birth rate

D = death rate

per capita definition + birth/mortality rate equations

“by head” / per individual

b = per capita birth rate (B=bN)

m = per capita mortality rate (D=mN)

rate of change (r) - definition, equation, and parameters

• change in population over time = births - deaths

• rate of change = birth rate - mortality rate (r = b - m)

ΔN/Δt = rN

parameters:

• r < 0 → population declining

• r = 0 → zero population growth, @ equilibrium

• r > 0 → population increasing

instantaneous growth rate

can be expressed as dN/dt = r_{instantaneous}N

intrinsic rate of increase + conditions/who has high vs low r_max

r_max: maximum rate at which population could increase under ideal conditions

• conditions: abundant resources + low density

• highest r_max: microorganisms

• lowest r_max: large organisms

exponential growth model (definition, equation, + limitations)

• definition: population increase under idealized conditions

  • rate of increase @ max = r_max > 0 and is constant

• equation: dN/dt = r_maxN (J shaped curve)

• limitations:

  • environmental resistance (aka limits set by environment)

    • run out of food, resources, living space, build up toxic waste

logistic growth mode (definition, equation/parameters, + limitations)

• definition: fits few real populations but better than exponential model bc takes into account ENVIRONMENTAL RESISTANCE

  • carrying capacity (k): maximum population size that the environment can support

    • population grows more slowly near carrying capacity

• equation: dN/dt = r_instN (K-N)/K (S shaped curve)

• parameters:

  • if N is not close to K then (K-N)/K = 1 → high growth rate

  • if N is close to K then (K-N)/K approaches 0 → low growth rate

  • when N = K then (K-N)/K = 0 → no growth

• limitations:

  • curve sin nature are usually not perfectly logistic bc…

    • populations tend to fluctuate around K

    • environment changes

    • K value may change

an organism’s life history is defined as…

the trade offs between survival and reproductive success

3 main variables of life history traits

1. age @ first reproduction

2. how often organism reproduces

3. how many offspring produced per reproductive episode

semelparity (definition + where it’s favored)

• definition: reproduce once and die (big-bang reproduction)

  • many insects, plants, fish

• favored where:

  • offspring survival rate is low

  • live in highly variable/unpredictable environment

iteroparity (definition + where it’s favored)

• definition: repeated reproduction

  • many vertebrates

• favored where:

  • environment is stable

  • adults more likely to survive and reproduce again

  • high competition

r selected species characteristics

• maximize reproduction rate

• small body size

• early maturity

• many babies @ same time

• little to no parental care

• short life span

• found in variable/unpredictable environments

k selected species

• low reproductive rate

• few offspring

• high parental investment/care

• slow development

• large body size