Communities in Motion

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Communities in Motion

• A biological community is an assemblage of
  populations of various species living close
  enough for potential interaction
• For example, the carrier crab carries a sea
  urchin on its back for protection against
  predators

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Community interactions are classified by whether
they help, harm, or have no effect on the
species involved

• Ecologists call relationships between species in
  a community interspecific interactions
• Examples are competition, predation, herbivory,
  symbiosis (parasitism, mutualism, and
  commensalism)
• Interspecific interactions can affect the
  survival and reproduction of each species, and
  the effects can be summarized as positive (),
  negative (), or no effect (0)

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Competition

• Interspecific competition (/ interaction)
  occurs when species compete for a resource in
  short supply

Competitive Exclusion

• Strong competition can lead to competitive
  exclusion, local elimination of a competing
  species
• The competitive exclusion principle states that
  two species competing for the same limiting
  resources cannot coexist in the same place

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Ecological Niches and Natural Selection

• Ecological niche ? can also be thought of as an
  organisms ecological role (its use of biotic
  and abiotic factors)
• Ecologically similar species can coexist in a
  community if there are one or more significant
  differences in their niches however, TWO SPECIES
  CANNOT OCCUPY THE EXACT SAME NICHE! (One will be
  excluded through competition.)

• Resource partitioning of a niche enables similar
  species to coexist in a community

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Figure 54.2
A. distichus perches on fence posts and
other sunny surfaces.
A. insolitus usually perches on shady branches.
A. ricordii
A. insolitus
A. aliniger
A. christophei
A. distichus
A. cybotes
A. etheridgei
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Predation

• Predation (/ interaction) refers to interaction
  where one species, the predator, kills and eats
  the other, the prey
• Some feeding adaptations of predators are claws,
  teeth, fangs, stingers, and poison

• Prey display various defensive adaptations
• include hiding, fleeing, forming herds or
  schools, self-defense, and alarm calls

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• Animals also have morphological and physiological
  defense adaptations
• Cryptic coloration, or camouflage, makes prey
  difficult to spot

• Animals with effective chemical defense often
  exhibit bright warning coloration, called
  aposematic coloration
• Predators are particularly cautious in dealing
  with prey that display such coloration

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• In some cases, a prey species may gain
  significant protection by mimicking the
  appearance of another species
• In Batesian mimicry, a palatable or harmless
  species mimics an unpalatable or harmful model

• In Müllerian mimicry, two or more unpalatable
  species resemble each other

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Symbiosis

• Symbiosis is a relationship where two or more
  species live in direct and intimate contact with
  one another

- Parasitism

• In parasitism (/ interaction), one organism,
  the parasite, derives nourishment from another
  organism, its host, which is harmed in the
  process
• Parasites that live within the body of their host
  are called endoparasites (worms), or on the
  external surface of a host are ectoparasites
  (ticks)

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Mutualism

• Mutualistic symbiosis, or mutualism (/
  interaction), is an interspecific interaction
  that benefits both species
• A mutualism can be
• Obligate, where one species cannot survive
  without the other
• Facultative, where both species can survive alone

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Figure 54.7
(a) Acacia tree and ants (genus Pseudomyrmex)
(b) Area cleared by ants at the base of an acacia
tree
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Commensalism

• In commensalism (/0 interaction), one species
  benefits and the other is neither harmed nor
  helped
• Commensal interactions are hard to document in
  nature because any close association likely
  affects both species

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Physical laws govern energy flow and chemical
cycling in ecosystems

• An ecosystem consists of all the organisms living
  in a community, as well as the abiotic factors
  with which they interact

• Regardless of an ecosystems size, its dynamics
  involve two main processes energy flow and
  chemical cycling
• Remember Energy flows through an ecosystemwhile
  matter is recycled!

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• Trophic structure is the feeding relationships
  between organisms in a community

Trophic level refers to where a species falls in
the food chain

• Energy and nutrients pass from primary producers
  (autotrophs) to primary consumers (herbivores) to
  secondary consumers (carnivores) to tertiary
  consumers (carnivores that feed on other
  carnivores)

Food chains link trophic levels from producers to
top carnivores
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Energy, Mass, and Trophic Levels
Every food web must contain a PRIMARY PRODUCER!
It is responsible for making all of the organic
materials that will be passed along the food
chain. Primary producers capture all the
ENERGY the ecosystem will have!

• Heterotrophs depend on the biosynthetic output of
  other organisms

Primary production is the amount of light energy
converted to chemical energy by autotrophs during
a given time period ? The extent of
photosynthetic production sets the spending limit
for an ecosystems energy budget
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Figure 55.4
Sun
Key
Chemical cycling Energy flow
Heat
Primary producers
Primaryconsumers
Detritus
Microorganismsand otherdetritivores
Secondary andtertiary consumers
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• Tropical rain forests, estuaries, and coral reefs
  are among the most productive ecosystems per unit
  area
• Marine ecosystems are relatively unproductive per
  unit area, but contribute much to global net
  primary production because of their volume

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Energy transfer between trophic levels is
typically only 10 efficient

• The other 90
• gets used by the organism for cellular process
  growth
• some is lost as heat
• some eliminated as waste

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• In a biomass pyramid, each tier represents the
  dry weight of all organisms in one trophic level
• Most biomass pyramids show a sharp decrease at
  successively higher trophic levels

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Trophic Efficiency and Ecological Pyramids

• Trophic efficiency is the percentage of
  production transferred from one trophic level to
  the next
• It is usually about 10, (with a range of 5 to
  20)

• Approximately 0.1 of chemical energy fixed by
  photosynthesis reaches a tertiary consumer
• Food chains rarely go beyond quaternary
  consumers because there simply isnt enough
  energy left to sustain that level of consumer.

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Food Webs

• A food web is a branching food chain with complex
  trophic interactions
• Species may play a role at more than one trophic
  level

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Be able to identify the primary producers,
primary consumers (herbivores), etc
How would a decomposer fit into a food web?
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• Detritivores, or decomposers, are consumers that
  derive their energy from detritus, nonliving
  organic matter
• Prokaryotes and fungi are important detritivores
• Decomposition connects all trophic levels

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Species with a Large Impact

• Certain species have a very large impact on
  community structure
• Such species are highly abundant (Dominant
  species) or play a pivotal role in community
  dynamics (Keystone species)

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Dominant Species

• Dominant species are those that are most abundant
  or have the highest biomass
• Dominant species exert powerful control over the
  occurrence and distribution of other species
• For example, sugar maples have a major impact on
  shading and soil nutrient availability in eastern
  North America this affects the distribution of
  other plant species

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Keystone Species

• Keystone species exert strong control on a
  community by their ecological roles, or niches
• In contrast to dominant species, they are not
  necessarily abundant in a community
• Keystone predators usually increase diversity in
  a community by holding down prey populations.

For example It could be a small predator that
keeps down a population of herbivores and
prevents them from wiping out a plant species.
They're effective especially when the other
animal has no other predators.
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Figure 54.17
EXPERIMENT
RESULTS
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With Pisaster (control)
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Number of species present
10
Without Pisaster (experimental)
5
0
73
72
71
70
69
68
67
66
65
64
1963
Year
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Ecological Succession

• Ecological succession is the sequence of
  community and ecosystem changes after a
  disturbance
• Primary succession occurs where no soil exists
  when succession begins. Retreating glaciers
  provide a valuable field-research opportunity for
  observing primary succession.
• Secondary succession begins in an area where soil
  remains after a disturbance

Early arrival species may facilitate appearance
of later species by making the environment
favorable
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The first stage of succession is called the
pioneer stage.

• Succession is the result of changes induced by
  the vegetation itself
• A CLIMAX COMMUNITY is a mature, stable community
  that is the final stage of ecological succession

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Biological and geochemical processes cycle
nutrients and water in ecosystems

• Life depends on recycling chemical elements
• Nutrient circuits in ecosystems involve biotic
  and abiotic components and are often called
  biogeochemical cycles

• A model of nutrient cycling includes main
  reservoirs of elements and processes that
  transfer elements between reservoirs
• All elements cycle between organic and inorganic
  reservoirs

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Biogeochemical Cycles

• Gaseous carbon, oxygen, sulfur, and nitrogen
  occur in the atmosphere and cycle globally
• Less mobile elements include phosphorus,
  potassium, and calcium

In terrestrial ecosystems, nitrogen is the most
common limiting nutrient.
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• Water is essential to all organisms
• Liquid water is the primary physical phase in
  which water is used
• The oceans contain 97 of the biospheres water
  2 is in glaciers and polar ice caps, and 1 is
  in lakes, rivers, and groundwater
• Water moves by the processes of evaporation,
  transpiration, condensation, precipitation, and
  movement through surface and groundwater

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Figure 55.14a
The Water Cycle
Movement overland by wind
Precipitationover land
Evaporationfrom ocean
Precipitationover ocean
Evapotranspira-tion from land
Percolationthroughsoil
Runoff andgroundwater
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• The Carbon Cycle
• Carbon-based organic molecules are essential to
  all organisms
• Photosynthetic organisms convert CO2 to organic
  molecules that are used by heterotrophs
• Carbon reservoirs include fossil fuels, soils and
  sediments, plant and animal biomass, the
  atmosphere, and sedimentary rocks

• CO2 is taken up and released through
  photosynthesis and respiration additionally,
  volcanoes and the burning of fossil fuels
  contribute CO2 to the atmosphere

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CO2 inatmosphere
Photosynthesis
Photo-synthesis
Cellularrespiration
Burningof fossilfuels andwood
Phyto-plankton
Consumers
Consumers
Decomposition
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• The Nitrogen Cycle
• Nitrogen is a component of amino acids, proteins,
  and nucleic acids
• The main reservoir of nitrogen is the atmosphere
  (N2), though this nitrogen must be converted to
  other usable forms for the plant ? this is done
  through nitrogen fixation by bacteria

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Terrestrialcycling
N2
Denitri-fication
Assimilation
Decom-position
NO3
Uptakeof aminoacids
Fixationin root nodules
Ammonification
Nitrification
NH3
NO2
NH4
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Decomposition and Nutrient Cycling Rates

• Decomposers (detritivores) play a key role in the
  general pattern of chemical cycling
• Rates at which nutrients cycle in different
  ecosystems vary greatly, mostly as a result of
  differing rates of decomposition
• The rate of decomposition is controlled by
  temperature, moisture, and nutrient availability