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COMMUNITY STRUCTURE
AND DYNAMICS
Copyright © 2009 Pearson Education, Inc.
◦ Biological community
– An assemblage of populations living close enough
together for potential interaction
– Described by its species composition
◦ Boundaries of the community vary with
research questions
– What kind of communities exist at Miramar
College?
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◦ Interspecific interactions
– Relationships with other species in the community
– Can have positive (+) or negative (-) effects on
each species
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1. Interspecific competition (usually -/-)
– Two different species compete for the same
limited resource
–
Squirrels and black bears
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Bear and Squirrel by ~magooey on deviantART
◦ Competition occurs when ecological niche of
species overlap
– Ecological niche: Sum of an organism’s use of
biotic and abiotic resources
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2. Mutualism: both populations benefit (+/+)
◦ Example: Reef-building corals
– Photosynthetic dinoflagellates and heterotrophic
cnidarian
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3. Predation: benefits the predator but kills prey
(+/-)

However, can lead to evolutionary adaptation
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4. Herbivory is not usually fatal and leads to
adaptation in plants (+/-)
– Plants must expend energy to replace the loss

Plants have numerous defenses against
herbivores
– Spines and thorns
– Chemical toxins
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5. A parasite lives on or in a host from which it
obtains nourishment (+/-)
– Internal parasites include nematodes and tapeworms
– External parasites include aphids and mosquitos
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
Pathogens are disease-causing parasites (+/-)
– Pathogens can be bacteria, viruses, fungi, or protists
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


Non-native pathogens can have rapid and
dramatic impacts
Non-native pathogens can cause a decline of the
ecosystem
Example: Snakehead fish
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
Trophic structure
– A pattern of feeding relationships consisting of
several different levels

Food chain
– Sequence of food transfer up the trophic levels
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
Producers
– Support all other trophic levels
– Examples??
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
Producers
– Support all other trophic levels
– Examples:
– Phytoplankton and plants

Consumers
– Heterotrophs
–
–
–
–
Primary consumers
Secondary consumers
Tertiary consumers
Quaternary consumers
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


Producers
– Support all other trophic levels
Consumers
– Heterotrophs
– Primary consumers
– Secondary consumers
– Tertiary consumers
– Quaternary consumers
Detritivores and decomposers
– Derive energy from dead matter and wastes
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Plant
A terrestrial food chain
Producers
Phytoplankton
An aquatic food chain
Grasshopper
Plant
A terrestrial food chain
Primary
consumers
Zooplankton
Producers
Phytoplankton
An aquatic food chain
Mouse
Secondary
consumers
Herring
Grasshopper
Primary
consumers
Zooplankton
Producers
Phytoplankton
Plant
A terrestrial food chain
An aquatic food chain
Snake
Tertiary
consumers
Tuna
Mouse
Secondary
consumers
Herring
Grasshopper
Primary
consumers
Zooplankton
Producers
Phytoplankton
Plant
A terrestrial food chain
An aquatic food chain
Trophic level
Hawk
Quaternary
consumers
Killer whale
Snake
Tertiary
consumers
Tuna
Mouse
Secondary
consumers
Herring
Grasshopper
Primary
consumers
Zooplankton
Producers
Phytoplankton
Plant
A terrestrial food chain
An aquatic food chain

Food web
– A network of
interconnecting
food chains
Example:
Mercury
poisoning
Quaternary,
tertiary,
and secondary
consumers
Tertiary
and
secondary
consumers
Secondary
and
primary
consumers
Primary
consumers
Producers
(plants)
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
Species diversity defined by two components
1. Species richness
2. Relative abundance
A
B
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
Keystone species
Keystone
– A species whose
impact on its
community is larger
than its biomass or
abundance indicates
– Occupies a niche that
holds the rest of its
community in place
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Keystone
absent

Disturbances
– Events that damage biological communities
– Storms, fire, floods, droughts, overgrazing, or
human activity
– The types, frequency, and severity of disturbances
vary from community to community
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Invasive species can devastate communities


Invasive species: non-native species spreading
beyond their point of introduction and causing
environmental or economic damage
Example: Snakehead fish
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

Communities change drastically following a
severe disturbance
Ecological succession
– Colonization by a variety of species
– A success of change gradually replaces other species
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
Primary succession
– Begins in a virtually lifeless area with no soil

Secondary succession
– When a disturbance destroyed an existing
community but left the soil intact
Annual
plants
Perennial
plants and
grasses
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Shrubs
Softwood trees
such as pines
Time
Hardwood
trees

Introduction of rabbits in Australia
600 Km
Australia
Key
Frontier of rabbit spread Origin: 1860
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ECOSYSTEM STRUCTURE
AND DYNAMICS
Copyright © 2009 Pearson Education, Inc.

Ecosystem
– All the organisms in a community as well as the
abiotic environment

Components of ecosystems
– Energy flow
– Passage of energy through the ecosystem
– Chemical cycling
– Transfer of materials within the ecosystem
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
A terrarium has the components of an ecosystem
Chemical cycling
Energy
flow
Light
energy
Chemical energy
Heat
energy
Chemical
elements
Bacteria
and fungi
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
Primary production
– The amount of solar energy converted to chemical
energy
– Carried out by ________________
– Produces biomass
– Amount of living organic material in an ecosystem
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
Primary production of different ecosystems
Open ocean
Estuary
Algal beds and coral reefs
Desert and semidesert scrub
Tundra
Temperate grassland
Cultivated land
Boreal forest (taiga)
Savanna
Temperate deciduous forest
Tropical rain forest
0
500 1,000 1,500 2,000 2,500
Average net primary productivity (g/m2/yr)
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
A pyramid of production
– Illustrates the cumulative loss of energy transfer in a
food chain
Tertiary
consumers
10 kcal
Secondary
consumers
100 kcal
Primary
consumers
Producers
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1,000 kcal
10,000 kcal
1,000,000 kcal of sunlight
Trophic level
Secondary
consumers
Human
meat-eaters
Human
Primary
consumers vegetarians
Producers
Corn
Cattle
Corn

Biogeochemical
cycles
– Cycle chemicals
between
organisms (biotic)
and the Earth
(abiotic)

Decomposers play
a central role in
biogeochemical
cycles
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Consumers
3
2
Producers
Decomposers
1
Nutrients
available
to producers
Abiotic
reservoir
Geologic processes
4

Biotic: Carbon is basis of organic molecules

Abiotic: Carbon is found ___________

The return of CO2 to the atmosphere by
respiration closely balances its removal by
photosynthesis
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CO2 in atmosphere
5 Burning
3
Cellular respiration
Photosynthesis
1
Higher-level
consumers
Wood
and fossil
fuels
Primary
consumers
Wastes; death
Decomposers
(soil microbes)
Plants, algae,
cyanobacteria
4
Detritus
2
Plant litter;
death


Biotic: organisms require phosphorus for
___________.
Abiotic: No atmospheric component; abiotic
resevoir found in rocks, soil
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6
Uplifting
of rock
3
Weathering
of rock
Runoff
Phosphates
in rock
Animals
Plants
1
Assimilation
2
Phosphates
in solution
Detritus
Phosphates
in soil
(inorganic)
5
Rock
Precipitated
(solid) phosphates
Decomposition Decomposers
in soil
4

Biotic: Nitrogen is essential for ________.

Abiotic: Nitrogen found in air and soil

Nitrogen fixation converts N2 to nitrogen used by
plants
– Carried out by some bacteria and cyanobacteria
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Nitrogen fixation
Leguminous root nodules
TEM of rhizobia in root nodule cell
Rhizobium in symbiosis
Azotobacter
Klebsiella
some Clostridium
Nitrogen (N2) in atmosphere
8
Animal
Plant
6
Assimilation
by plants
Organic
compounds
Nitrogen
fixation
Death; wastes
5
Denitrifiers
Organic
compounds
3
Nitrates
in soil
(NO3–)
Nitrogen-fixing
bacteria in
root nodules
Detritus
Free-living
nitrogen-fixing
bacteria and
cyanobacteria
Decomposers
4
1
Nitrifying
bacteria
7
Decomposition
Ammonium (NH4+)
in soil
Nitrogen fixation
2

Chemical cycling in an ecosystem depends on
– The web of feeding
– Relationships between plants, animals, and
detritivores
– Geologic processes

Altering an environment can cause severe losses
in chemical cycling
– Erosion
– Acid rain
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Producer
Herbivore
(primary
consumer)
Energy flow
Chemical cycling
Decomposers
Carnivore
(secondary
consumer)