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Population Growth Curves
and Mechanisms
Species and Population
• Organism
• Species: genetically similar organisms that
reproduce
• Population: all members of a species in an
area
Characteristics of
populations:
Same species living in
same area
Age structure:
–Prereproductive
–Reproductive
–Postreproductive
•What limits population
growth?
Biotic Potential
vs.
Environmental
Resistance
Biotic Potential
an organism’s maximum reproductive
rate, provided:
• unlimited resources are available
(food, water, shelter, etc.)
• freedom from inhibiting factors (no
predators or disease, etc.)
Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Table 6.2
•Biotic potential –
capacity for growth
•Intrinsic rate of increase (r)
•Environmental
Resistance –
Biotic and Abiotic
factors limiting
population size
•Recruitment - ability to
survive environmental
resistance and become
part of the breeding
population
•Reproductive
strategies
•High biotic potential
and low recruitment
•Low biotic potential
and high recruitment
r - strategists
K – strategists
• Transparency
R strategists:
• Many small offspring
• Little /no parental care
• Early reproductive age
• Most offspring die
• Small adults
• High growth rate (r)
• Population fluctuates wildly above and below
carrying capacity (K)
• Generalist niche
• Low ability to compete
• Early successional species
K strategists:
• Fewer, larger offspring
• High parental care
• Later reproductive age
• Larger adults
• Lower population growth rate (r)
• Population size fairly stable and close to carrying
capacity (K)
• Specialist niche
• High ability to compete
• Late successional species
•Growth Curve - Result
of biotic potential and
environmental
resistance.
•Exponential growth (J-curve) before
environmental
resistance factors kick
in to reduce
population size
• Population explosion
•Logistic growth (S - curve) Balance
between
environmental
resistance and biotic
potential
•Carrying Capacity The maximum
population a habitat
can support without
being degraded over
the long term.
Population Size
Carrying Capacity
-maximum # individuals
of a given species that the
environment can support
Time
Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.4
Carrying Capacity
J-curve
-maximum # individuals
of a given species that the
environment can support
Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.4
•Critical number –
minimum population
required to provide
protection and support
for members of
population
Calculating Fertility Rates and
Doubling Times
(CBR - CDR)/10 = Rate of Increase in percentage
To get r (intrinsic growth rate), divide by 100
70/ Rate of Increase = Doubling Time
•A population had a
growth rate of 1.7 in
2000. In what year
would the population be
double its current
population?
•70 / 1.7 = 41 years
•2000 + 41 = 2041
Calculating Fertility Rates and
Doubling Times: Practice
Country
CBR
CDR
Kenya
46
7
Mexico
29
6
USA
17
9
Denmark 12
12
Rate of
Increase
Doubling
Time
•Threatened – species
whose populations are
declining rapidly
•Endangered – population is
near the critical number
Mechanisms of
population
equilibrium:
•Predator – prey
dynamics
Snowshoe hare
Cunningham, Cunningham and Saigo, “Environmental Science, 8th ed.” McGraw Hill, Fig. 6.9
Predator-prey Balance:Wolves and Moose
Isle Royale
Isle Royale
• As Wolves decreased Moose increased.
• As Wolves increased the Moose decreased.
• Wolves kept the Moose population stabilized until
canine parvovirus probably killed off large #’s of Wolves.
• Moose prospered until overgrazing of Ash and Aspen
caused mass death.
Plant Herbivore
dynamics:
–Herbivores are the
natural enemies of plants
–Overgrazing
Mechanisms of Population
Equilibrium: Plant-Herbivore
Introduced Species: Rabbits in
Australia
• Introduced into Australia from England in 1859
• No natural enemies – rabbit population exploded
• Overabundant herbivore population devastated
natural vegetation (see Fig. 4-11 in text).
• Using disease as control measure – why will this
procedure fail in the long term?
•Eliminating predators
upsets basic plant –
herbivore
relationships.
•Competition
–Interspecific
–Intraspecific
• Density dependent factors
affecting population growth:
–Increasing effect as population
increases
–Examples
•Competition
•Disease
•Parasites
•Stress due to crowding
• Density independent factors
controlling population:
–affect all populations, regardless of
density
–Most are abiotic
•Forest fires
•Floods
•