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Exponential Growth
• Populations can grow
exponentially if not
kept in check.
~ for instance:
one bacteria will
become 2, then 4 then
8,then 64, then
512,then at the end of
one day, there will be
4.72 x1021
Exponential Growth
• only happens when populations have
every offspring survive to reproduce.
• As resources become less available,
offspring don't live as well. This stops
population growth.
• Carrying capacity
– max # org. an ecosystem can support
Exponential Growth
• Human population has grown
exponentially since the 1700's.
– due to medicine and farming technology:
• people live longer lives
• vaccines lower death rates
• more food supports more people
• We have not reached carrying capacity yet, but
some evidence shows we may reach 9 bill. by
• Ecosystems are influenced by a
combination of biotic (living organisms)
and abiotic (physical) factors.
Limiting Factors for populations
• Density-dependent
– Competition for food - more org. = less
– Predation - predator-prey relationships
keep each other in balance
– Parasitism and Disease - keeps populations
down by killing host
Limiting Factors for populations
• Density-indepenant
– Drought / Climate extremes – extreme heat
or cold, lack of or too much water...all can kill
– Human disturbances - changing the
ecosystem ~ deforestation, waste dumping,
killing off predators
• DDF - only become limiting when the pop.
density reaches a certain level
• DIF - affect all pops. similarly regardless of
pop. size
• Through a combination of all of these
factors, populations can vary from being in
balance, to being way out of balance.
Interaction in Communities
• There are many different ways species
can interact with each other within their
• Competition
– species have similar needs and compete for
• Predation
– one org. eats another
– back and forth adjustment between the 2
results in coevolution
– ex: the faster the prey gets, the faster the
predator gets and vice versa
• Symbiosis - relationship where 2 species live
closely together.
3 types:
a.) Mutualism - both species benefit
ex: flowers and insects
b.) Commensalism - one benefits, the other is
neither helped or harmed
ex: barnacles on a whale
c.) Parasitism - one org. lives on another and
the host is harmed
ex: tapeworms in mammals
Shaping Communities
• Habitat
– area in which an org. lives
• Niche - all conditions in which an org. lives and
how it uses those conditions
type of food eaten
place in the food web
temp. in which they survive
reproduction style
– Fundamental niche - where a species could live
– Realized niche - where a species actually lives
• No two species will share the same niche
in the same habitat!
• Competitive exclusion principle
– no 2 species can occupy the same niche in
the same habitat at the same time
– this would result in the elimination of one of
the species
• keystone species
– these are critical to the ecosystem because
they affect the survival and number of many
other species in their community
– Ex. Sea otters
• Ex: sea otters:
– sea urchins eat kelp - sea otters eat sea
– when sea otters are over hunted, too much
kelp gets eaten by urchins - all other species
that use the kelp to live in lose their home.
– sea otters are critical to the survival of the
• The more species in a community, the
• It will recover from disasters.
• The predators help insure that no one
species takes over.
• More biodiversity = better resiliency