r and K selected species
No Population Can Grow Indefinitely:
J-Curves and S-Curves
• Biotic potential –
capacity for
population growth
under ideal
conditions
– Larger organisms
tend to have low
potential
Population Growth
• Exponential growth – population that
increases at a fixed rate
– J-Curve
• Logistic growth – rapid exponential
population growth followed by a steady
decrease in population growth
– S-Curve
Population Growth
J-Curves
• Intrinsic rate of
increase (r) – rate
the population of a
species would grow
if it had unlimited
resources
J-Curves
• Individuals in populations
with high r
– Reproduce early in life
– Have short generation
times
– Can reproduce many times
– Have many offspring each
time they reproduce
S-Curves
• Environmental resistance –
combination of all factors that act to limit
the growth of a population
• Carrying capacity (K) – maximum
population of a given species that a
habitat can sustain indefinitely without
being degraded
S-Curves
Phases of Logistic Growth Curve
1. Lag Phase – little initial growth.
2. Rapid Growth Phase
3. Stable Phase – stabilizing factors limit
growth
Species Reproductive Patterns
• r-Selected species, opportunists – species with
a capacity for a high rate of population increase
– Many small offspring
– Little to no parental care or protection
– Reproductive opportunists
• K-selected species, competitors – reproduce
later in life and have a small number of offspring
with fairly long life spans
– Few large offspring
– High parental care
Positions of r- and K-Selected Species on the SShaped Population Growth Curve
Transitioning between J and S
curves…
• Carry capacity isn’t fixed
– Varies depending on climate and season
– Unpredictable changes can be devastating to the
species AND the habitat
• Reproductive time lag – period needed
for the birth rate to fall and the death rate to
rise in response to resource
overconsumption
– May lead to overshoot
– Dieback (crash)
r-Curve Fluctuations
Types of Population Change
• Stable – population fluctuates slightly
above and below its carrying capacity
– Characteristic of undisturbed rain forests
– Late loss curve
• Irruptive – short-lived rapidly reproducing
species
– Linked to seasonal changes in weather or
nutrient availability
– Algal Blooms
– Early loss curves
S-Curve Fluctuations
Types of Population Change
• Cyclic fluctuations, boom-and-bust
cycles
– Top-down population regulation
• Controlled by predation
– Bottom-up population regulation
• Controlled by scarcity of one or more resources
• Irregular – changes in population size with
no recurring pattern
– chaos
Top-down Regulation
Your Turn!
• Make a K selected  r selected
continuum on your desk
• Organize your cards ON YOUR OWN
• Discuss with partner
Survival Strategies
• Which organisms were difficult to
classify? Why?
Survival Strategies
• K selected
• r selected
•
•
•
•
•
•
Red-tailed hawk
Coyote
Western rattlesnake
Roadrunner
Kangaroo Mouse
Whiptail lizard
Your Turn!
• Live for Today Life Table
• Review:
– What does late loss look like?
– What does early loss look like?
– What does constant-loss look like?
• Remember to include a key (molting
species)