Download Biotic Potential

Biotic Potential
Limits to Growth
Biotic Potential
• Population vs. Individuals
– Individuals are born; they grow, and they die.
– Populations have a birth rate, a growth rate and a death rate.
– Populations can evolve and the show a dispersion pattern.
• Recruitment
– survival of the young through the early growth stages of a
species.
• Plants generally reproduce through seeds and spores.
• Animals reproduce by laying eggs or bearing live young.
Biotic Potential
• The biotic potential is the maximum rate at
which members of a species can reproduce
given unlimited resources and ideal
environmental conditions.
– The number of offspring that a species may
produce under ideal conditions.
• This kind of growth starts slowly and then
increases rapidly producing an exponential or
J-shaped curve.
Exponential Growth
J-curve
• Bacterial growth shows this
type of growth.
• With plenty of food and the
right temperature, bacteria
can divide into two about
every 20 minutes.
• After 24 hours, you would see
about
1,000,000,000,000,000,000,00
0,000 (1024), or one million
billion billion bacteria!
Exponential
Growth
Exponential or
logarithmic growth
leads to a much
faster population
growth.
Human Population
Growth Over Time
Limits to Growth
• If bacteria are transferred onto a growth medium,
the population increases exponentially initially
but then stops increasing.
• The reason for this is that the environment can
no longer support the vast number of new cells
being produced.
• A food shortage may result.
• Bacteria also produce waste products which may
be toxic to themselves.
Limits to Growth
•A decrease in births and
an increase in deaths lead
to a population decline.
•The J-curve has become
an S-shaped curve
representing sigmoidal or
logistic growth.
•Changes in the organism’s
environment have
prevented the population
from realizing its biotic
potential.
Population Growth
• If migration is ignored (to make a simpler
model) and population change is monitored
based on births and deaths only:
• Rate of Change in Population Size =
(Births – Deaths) x Population number
– If births > deaths => population increases
– If deaths > births => population decreases
– If births >>> deaths population increases
exponentially or logarithmically
Causes of Population Fluctuations
• The number of individuals in a population changes
according to fluctuations in the birth and death rate.
• Birth Rate =
number of births =
# adults in population
Natality
• Death Rate =
Number of Deaths =
# adults in population
Mortality
Population Changes
Carrying Capacity
• The maximum population size a particular environment can
support.
• The number of individuals of a species that can survive, in an
area over time.
• For most populations, four factors interact to set the carrying
capacity:
–
–
–
–
Availability of raw materials – food, water.
Availability of energy
Accumulation of waste products and their disposal.
Interactions among organisms – competition, predation, parasitism.
• All of these forces acting together to limit population size is
known as environmental resistance.
Carrying Capacity
•Certain factors have
a primary role in
limiting the size of a
population.
•These are called
limiting factors.
•Some are easy to
identify whilst
others are less
obvious.
Limiting Factors
• Grass plants need both nitrogen and magnesium in
their soil for chlorophyll production.
– These are limiting factors as without them the grass
population will not increase.
– Adding fertilizer containing the two minerals makes the
grass grow and reproduce increasing the population.
• The carrying capacity has increased because a limiting
factor was removed.
– The carrying capacity has not been removed because other
factors such as water, light, etc. will be a limiting factor.
Environmental Resistance
• Environmental resistance is
– all the limiting factors that tend to reduce population growth
rates and set the maximum allowable population size or
carrying capacity of an ecosystem.
• Accumulation of waste products is often a limiting factor
for animals especially when they exist in a small space like
goldfish in a bowl. If there is no way to dispose of waste
products they can become toxic and kill the organism.
• This decline in population size is known as the death or
decline phase.
• A rapid decrease in population size is known as a crash or
bust.
Environmental Resistance
• Environmental resistance
lowers the biotic potential of a
population converting the Jcurve to an S-curve.
• In this diagram, a fish
population could be subjected
to the environmental
resistance factors of:
–
–
–
–
–
Decreased oxygen supply
Low food supply
Disease
Predators
Limited space
Changes in Population Size
• Both biotic and abiotic factors affect population size.
• Some populations fluctuate each year but have a
fairly obvious pattern whereas other populations can
fluctuate wildly from year to year not producing an
obvious trend.
– Boom and bust
– Colonizers
– Random cycles
Population Changes
• The number of algae in a lake may increase
rapidly, producing a choking algal bloom.
– This bloom may produce toxins which enter the food
chain or can actually make the algae itself die off in an
effect called “boom and bust”.
• When new species are introduced, the
population can fluctuate for many years until it
settles down.
• Sometimes a population fluctuates based on
climate or another abiotic factor.
Boom and Bust
• In 1911, 25 reindeer were
introduced onto an island
off Alaska. No predator
existed on the island.
• The herd grew
exponentially until it
reached 2000 in 1938.
• Overgrazing resulted and
the reindeer population
declined significantly.
• By 1950 only 8 reindeer
survived.
Colonizing Species
• Sheep were introduced into Tasmania by Europeans. The
population showed exponential increase initially but once the
environmental resistance kicked in, it leveled off to show an Scurve.
Wild Fluctuations in Population
• Mule Deer in Texas
• Fluctuations in mule
deer populations are
normal.
• Possible factors:
–
–
–
–
Drought
Habitat degradation
Low reproduction
Water due to shift in
agricultural practices
– Predators