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Ecology
Populations, Communities,
Ecosystems
Population Growth
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Population: all indiv. of the same
species that live together in the same
place at the same time
Demography: statistical study of
populations
3 Key features of population:
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Population size
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Population density
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Tend to grow in size, b/c indiv. Have
more than 1 offspring
Very small populations likely to
become extinct
Number of indiv in a given area
Widely spaced are less likely to
reproduce
Dispersion
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Clumped (most common)
Even
Random
Population Models
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Models: hypothetical pop. w/key
characteristics of pop being studied
Growth – more indiv. are born than
die
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Number of indiv added to pop as it
grows (ΔN)
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r (rate of growth) = birth rate – death
rate
Exponential growth curve (ΔN vs.
time)
Carrying capacity (K) – the pop
size that an environment can sustain
Logistic model – accounts for
declining resources as pop grows
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As N approaches K the population
ceases to grow (birth rate = death
rate)
Limiting Factors
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Density dependent
Density independent
Strategies of Pop Growth
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r-strategists
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K-strategists
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Exponential growth than pop size crashes
Reproduce early in life, quick gestation
Have many offspring at one time
Offspring mature rapidly with little
parental care
Live in changing environments
Ex.
Small pop size and slow growth
Reproduce later in life, few offspring
Offspring mature slowly and receive
parental care
Live in stable environments
Ex.
Human population
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Humans expanded carrying capacity of
environment
Nearly 94 million people are added to the
population each year
How populations evolve
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In order to understand how populations change in
response to evolutionary forces, you need to
understand how they evolve in the absence of
these forces
Hardy-Weinberg Principle
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Early 1900’s scientists wondered if dominant alleles would
replace recessive alleles
Mathematician GH hardy and physician Wilhelm Weinberg
independently demonstrated that dominant alleles DO
NOT replace recessive alleles, frequency of alleles
remains constant from one generation to the next
unless evolutionary forces act upon them
Hardy-Weinberg Principle
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Populations do not change unless evolutionary
forces act upon them
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Allele frequency: proportion of the group with a
specific allele
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p: dominant allele
q: recessive allele
Easily predict the freq of each genotype in a large,
randomly mating populations
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Calculate freq of recessive allele
Calculate the freq of dominant allele
Determine the freq of heterozygotes
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P2+2pq+q2 = 1
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(AA)2 + 2(Aa) + (aa)2 = 1
5 Forces that cause Populations
to Evolve
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Mutation: ultimate source of
genetic variation that makes
evolution possible
Migration (Gene Flow):
movement of alleles into or out
of a pop
Nonrandom mating
(Inbreeding): increases the
proportion of homozygotes,
deceases heterozygotes
Genetic Drift: in small isolated
populations, allele frequencies
are drastically affected by
natural disasters
Natural Selection: directly
changes freq of alleles
Patterns of Natural Selection
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Only characteristics that are
expressed can be selected for or
against
Directional Selection – eliminates
one extreme from a range of
phenotypes and favors the other
extreme
Stabilizing Selection – eliminates
extremes at both ends of a range
of phenotypes
Disruptive Selection – eliminates
the intermediate phenotype…aids
in speciation
Ecosystems
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Ecology: study of the interactions of
living organisms with one an other
and their physical environment
Habitat: the place where a particular
pop of species live
Community: many different species
living together in a habitat
Ecosystem: community and physical
aspects of habitat
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Biotic – living organisms
Abiotic – physical aspects (non-living)
Biodiversity – variety of organism,
genetic differences and the
communities and ecosystems in which
they occur
No distinct boundaries of an
ecosystem
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Changes in an ecosystem
Physical Environment
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Succession – progression
of species replacement
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Changes sets of colonization
Volcano – creates an island
Glacier recedes – new soil
Eventually, organisms
colonize new habitat
10 succession: occurs on land
where nothing has grown
before (pioneer species)
20 succession: occurs in a n
area with previous growth
(forest clearing, abandoned
fields)
No 2 successions are alike!
Energy Flow
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Energy flows thru Earth’s
ecosystems in one
direction
Producers, consumers,
decomposers
Heterotrophs and
autotrophs
Herbivore, carnivore,
omnivore, detrivore
Food chain & food web
Only 10% of energy moves
to the next trophic level
Biogeochemical cycles: Water
cycle
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Biogeochemical cycles:
substance enters living
organisms, then returns to the
non-living environment
Water Cycle – driven by the sun
Nonliving (aquatic ecosystems)
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Water vapor in atm. Condenses
and precipitates to earths
surface
Some water seeps into soil and
becomes ground water retained
by earth
Remaining water collects in
lakes, rivers, and oceans where
it is heated by the sun and
evaporates into the atm
Living (terrestrial ecosystems)
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Water is taken up by plant roots
Transpiration – sun heats earth
and creates wind currents which
draw moisture out of leaves
Carbon Cycle
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0.035% CO2 in air and dissolved
in water
CO2 used by plants, algae,
bacteria to build organic
molecules (photosynthesis)
CO2 returns to atm by 3 ways
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Cellular respiration
Combustion/ burning
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Carbon in wood
Fossil fuels – remains of organisms
buried in sediments for thousandsmillions of years (coal, oil, natural
gas)
Erosion
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Marine organisms use dissolved CO2
to build shells (CaCO3)
Shells covered with sediment – form
limestone, then erode when exposed
Nitrogen Cycle
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Atm 78% N2
Organisms are unable to use N2
Nitrogen Fixation: break the N2
triple bond and combines w/H to
form ammonia (NH3)
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Assimilation: absorption and
incorporation of nitrogen into plant
and animal compounds (proteins,
nuc. acids)
Ammonification: production of
ammonia by bacteria during the
decay of nitrogen containing
organic matter
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Occurs only in the absence of O2
Nitrogen fixing bacteria live in soil
and nodules underground
Urea
Dead organisms
Nitrification: production of nitrate
(NO3-)by bacteria from ammonia
Denitrification: bacteria converts
nitrates into nitrogen gas
Phosphorus Cycle
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PO4-3 found in rock and soil
Dissolves in water and is absorbed by
plants
Plants use PO4-3 to build ATP and DNA
Animals eat these plants and reuse
phosphorus
When plants and animals die, bacteria in
the soil convert phosphorus in organic
molecules back to PO4-3
Phosphorus can move to other
ecosystems
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Trapped in sediments form rock
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If rock is exposed, weathering
releases phosphorus
Phosphorus level in freshwater is low
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Prevents growth of photosynthetic
algae
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If added by humans
(fertilizers/detergents), so much
algae forms that fish and other
invertebrates suffocate
Communities: How organisms
interact
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Coevolution: back-and-forth
evolutionary adjustments
between interacting members of
an ecosystem
Coevolving in opposition
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Predation: one organism feeds on
another
Parasitism: one organism feeds
on and usually lives on or in host
(usually don’t kill host)
Defense
Overcoming defense
Competition: 2 spp use the same
resource (s) in short supply
Coevolving in Cooperation
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Symbiosis: 2 or more
spp living together in
a close, long term
association
Mutualism: both spp
benefit
Commensialism: one
spp benefits, the other
is neither helped nor
harmed
Niche
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Niche: the functional role a particular
spp performs in an ecosystem…
multidimensional
Fundamental niche: the total niche
an organism is potentially able to
occupy within an ecosystem, the
entire range of conditions it can
tolerate
Realized niche: the part of the niche
that the spp actually occupies
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Ex. Cape May Warbler & character
displacement
Principle of competitive exclusion:
if 2 spp are competing, the spp that
uses the resource more efficiently will
eventually eliminate the other locally –
no 2 spp can occupy the same niche
Predation can lesson competition
among prey – promote biodiversity
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Ex. Sea stars eat bivalves
The Physical Environment
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Climate depends on…
The amount of E from the sun that
reaches diff parts of earth and seasonal
variation in that E
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Equator – sun’s rays are
perpendicular
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Poles – rays spread out over grater
s.a. (less E)
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Earth’s daily rotation – prevents
build up of extreme temps
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23.50 tilt – seasons further from the
equator
Global pattern of atmospheric and
oceanic circulation created by unequal
global distribution of solar energy
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Latitude
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Distance from ocean
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Elevation
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Position relative to mountain
ranges
Major Biological Communities
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Marine
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Fresh Water
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Strongly connected w/ terrestrial
habitat
Wetlands
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Shallow water
Open Sea
Deep Sea
Lots plant growth
Biomes: terrestrial communities
w/similar climate
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Tropical Rainforest
Savanna
Desert
Temperate Grassland
Temperate Deciduous Forest
Temperate Evergreen Forest
Taiga
Tundra
Biome Project
You are to select a biome and research its
biotic and abiotic characteristics. List and
describe types of organisms you would
expect to find there. Where in the world
would you find this biome. And then select
one of the following:
 Create a 3-D shoebox model of the biome
and create a new organism that is well
adapted (in at least 4 ways) to its biome
 Write a paper comparing and contrasting 2
biomes
Human Impact on the
Environment
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Acid Rain
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Coal burning power plants release
smoke high in sulfur
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Sulfur combines w/water vapor 
sulfuric acid
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Rain & snow precipitate H2S back
to earth’s surface
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Northeast US has pH 3.8
Destruction of Ozone
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1985 – discovered low conc of
ozone over Anatartica, now hole
over the Artic too
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More UV radiation to reach Earth’s
surface, increase in skin cancer,
cataracts, and retina cancer
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Chlorofluorocarbons (CFC’s) are
major cause of ozone depletion
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CFC’s found in refrigerator coolant,
air conditioners, aerosol propellant,
foaming agents
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CFS’s are banned in US
Global Warming
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Excess CO2 released in atmosphere
from burning fossil fuels
CO2 bonds absorb solar energy
trapping heat, methane, and nitrous
oxide
Greenhouse Effect: the warming of
atmosphere as a result of
greenhouse gases
Studies indicate direct correlation
between amt of CO2 in atmosphere
and temp increase over the last 130
years
Problem: rising sea level (already
rose >5cm), if polar ice caps melt,
sea levels would rise 150m flooding
entire Atlantic Coast of N. America