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Population
Growth Cycles
and Stresses
Chapter 35
Section 2
Population Growth
No Population Can Grow Indefinitely:
J-Curves and S-Curves
Biotic potential – capacity for
population growth under ideal
conditions
o Larger organisms tend to
have low potential
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
o
o
o
o
Reproduce early in life
Have short generation times
Can reproduce many times
Have many offspring each time they
reproduce
Population Stresses
Any population growing
exponentially goes through a “J”
shaped growth, but most of the
time environmental influences
create an “S” shaped pattern in
growth
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
Carrying Capacity and J shaped growth
This shows exponential growth leading to overshoot and
population dieback of a species
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
o Many small offspring
o Little to no parental care or protection
o Reproductive opportunists
• K-selected species, competitors – reproduce
later in life and have a small number of
offspring with fairly long life spans
o Few large offspring
o High parental care
r- and K- selected Organisms
r
K
Unstable environment,
density independent
Stable environment, density
dependent interactions
small size of organism
large size of organism
energy used to make each
individual is low
energy used to make each
individual is high
many offspring are produced
few offspring are produced
early maturity
late maturity, often after a
prolonged period of parental
care
short life expectancy
long life expectancy
each individual reproduces
only once
individuals can reproduce more
than once in their lifetime
most of the individuals die
within a short time
but a few live much
longer
most individuals live to near
the maximum life span
Courtesy of www.bio.indiana.edu
Positions of r- and K-Selected Species on the SShaped Population Growth Curve
Types of Population
Change
• Stable – population fluctuates slightly
above and below its carrying capacity
o Characteristic of undisturbed rain forests
o Late loss curve
• Irruptive – short-lived rapidly reproducing
species
o Linked to seasonal changes in weather or
nutrient availability
o Algal Blooms
o Early loss curves
S-Curve Fluctuations
Types of Population
Change
• Cyclic fluctuations, boom-and-bust
cycles
o Top-down population regulation
• Controlled by predation
o Bottom-up population regulation
• Controlled by scarcity of one or more resources
• Irregular – changes in population size with
no recurring pattern
o chaos
r-Curve Fluctuations
Top-down Regulation
Survivorship Curves
These curves demonstrate
o Type I: Late Loss
Populations (K –
strategists)
o Type II: Constant Loss
Populations (K –
strategists)
o Type III: Early Loss
Populations (usually rstrategists)
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