4 Important characteristics of Populations

• 1. GEOGRAPHICAL

DISTRIBUTION– Area that is inhabited

by a population

• Varies in size from a few cm3 (bacteria) to millions of square kilometers

2. Density of Area

• # of individuals per unit area

• Ex: desert cactus = low density

• Cactus brush = high density

Partner activity: Density & Growth Rate

• Density formula = # of indiv/area

• 1. What is the density of 150 bullfrogs in

a pond that covers an area of 3 sq.

km?

50 bullfrogs/sq. km

Growth Rate

• Pf - Pi = Growth Rate **can be a neg.

∆Time number

1. In 1940, 4,800 sea otters were counted in a field study off the coast of Monterey, Calif. The field study continued until 1945 when scientists counted only 2,200 sea otters. What was the growth rate of sea otters?

2. -520 sea otters/year

More Problems

• In the Fall of 2000, there were 1857 students at RAHS. This fall in 2008 the student count was 2,164. Growth rate?

• 38 students/year

3. Growth Rate (factors that affect populations

• 1. Number of births (natality)

• 2. Number of deaths

(mortality)

4. # of members that enter or leave the population

• Immigration = movement of individuals into an area

• Emigration = movement of individuals out of an area

Limiting Factors (affecting Populations Growth)

• Competition for• water• shelter• food• Predation• Parasitism & disease• Drought & climate

change• Human disturbances

More Later

Graphs that represent Growth Rates

Exponential Growth (J-curve)

LAG PHASE

EXPONENTIAL PHASE

Exponential Growth

• Occurs when a population reproduces at a constant rate.

• At 1st, growth is slow

• Under ideal conditions with unlimited resources, pop. will grow exponentially

Partner Activity• Example: bacteria

– Within 20 min. bacteria will divide & reproduce asexually

– If an area starts with 2 bacteria cells on it, how many bacteria will have grown in 3 hours?

ANSWER = 1024

Logistic Growth (S-curve)

Logistic Growth• As a resource becomes less available, populations

slows or stops

LAG PHASE

EXPONENTIAL PHASE

STABILIZATION PHASE

CARRYING CAPACITY

Predator-Prey Graph

Predator-prey Graph

• The best known mechanism of pop.control

• Pattern can repeat indefinitely

Extinction Graph

Extinction

• Usually occurs:– After a catastrophic

disaster • Asteroid impacts(?)• Fire• flood

Terms to Know• Carrying capacity - the largest # of indiv. That an

environment can support.

EXPONENTIAL PHASE

STABILIZATION PHASE

• Stabilization phase – population has meet its carrying capacity and population growth levels off

• Exponential growth – a population that reproduces at a constant rate