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Population
Growth Cycles
and Stresses
Chapter 5
Section 2
Population Growth
No Population Can Grow Indefinitely:
J-Curves and S-Curves
Biotic potential – capacity for
population growth under ideal
conditions
Intrinsic rate of increase (r) –
rate the population of a
species would grow if it had
unlimited resources
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
This shows exponential growth leading to overshoot and
population dieback of a species
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
S or K-Curve Fluctuations
r-Curve Fluctuations
Bottom-up Regulation: Population is limited by available resources
(boom and bust cycle)
Top-down Regulation
Predation keeps both populations within the limits
of available resources
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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