K. Starks

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Predation. One species benefits, the other
incurs a cost also herbivory parasitism.
Important in adaptive evolution predator and
prey adaptations.
pages 1178-1179
Plant defenses against herbivory.
Also important in shaping communities.
K. Starks
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Mutualism. Both species benefit from the
interaction. Some are obligatory both species
cannot persist without the other others are
facultative the association is nonessential.
Overlooked as a mechanism in community structure
it may be more significant than either
competition or predation.
Also important in adaptive evolution
pages 1180-1181
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SOME questions from February 17th
1- When determining human population growth, do
the only consider natural death or all causes?
2- In the barnacle example, are you saying that
Balanus is better suited to the environment and
that if desiccation was not a problem at the
higher water level, they would force out the
Chthamalus?
3- Assuming two species occupy the same
ecological niche, why couldnt they both co-exist
assuming they were equally fit?
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Species Interactions
Competition, Predation, Mutualism, Commensalism
Coevolution reciprocal evolutionary change in
interacting species, ending in some degree of
specialization.
Trophic Structure Dominant and Keystone
Species Community Control
COMMUNITY STRUCTURE
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TROPHIC STRUCTURE Feeding relationships between
organisms. Describe species interactions. Food
chain- Transfer of food energy all the way until
decomposers. Trophic levels- Links in the food
chain. Usually four or five. Food web-
Branching and interconnected food chains.
Fig. 53.10
page 1181
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ANTARCTIC FOOD WEB
Fig. 53.11
page 1182
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FOOD WEB TUESDAY LAKE, MI
Fish
Zooplankton
Phytoplankton
Proc. Natl. Acad. Sci. 18 Feb 2003
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What limits the length of a food chain?
Energetic hypothesis. Inefficiency of energy
transfer along the chain limits the number of
levels.
Dynamic stability hypothesis. Long food chains
are less stable than short food chains.
page 1183
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Species that control community structure
Dominant species. Have the highest abundance or
highest biomass (sum weight of all individuals).
Exert control over the distribution and abundance
of other species.
pages 1183-1184
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Keystone species. Exert strong control on
community structure by their ecological roles
(niches). They are not particularly abundant.
For example, Pisaster ochraceous (ochre sea star)
and Mytilus californianus (mussel).
Fig. 53.14
Keystone in wave-exposed sites, not so in
wave-protected sites due to low food supply and
sand burial.
pages 1184-1185
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Top-down model of regulation of community
structure
western Alaska
pages 1185-1186
Science 1998
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Examples of changes in community structure
regulated by predators
Bioscience 2005
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Bottom-up model of regulation of community
structure
Mar. Ecol. Prog. Ser. 2005
pages 1185-1186
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Oikos 2004
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Disturbance- Data suggest that the norm for most
communities IS disturbance and nonequilibrium
(constantly changing), rather than stability and
equilibrium (species composition relatively
constant). Disturbances are events that damage
communities, remove species and alter resource
availability.
Fig. 53.17
pages 1186-1188
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Mount St. Helens
1980
1978
ÙSFS
Despite the importance of natural disturbances,
humans are the greatest agents of disturbance.
1999
1981
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Ecological succession- Transitions in species
composition over ecological time. Primary
succession- Begins in a virtually lifeless area
where soil has yet not formed (new volcanic
island or rubble left by retreating glaciers).
Glacier Bay, Alaska
Spruce butting in 80-200 yrs.
Alder, cottonwoods 30-80 yrs.
Spruce and hemlock 200-300 yrs.
Secondary succession- Occurs where an existing
community has been cleared by some disturbance
that leaves the soil intact.
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Secondary succession- Occurs where an existing
community has been cleared by some disturbance
that leaves the soil intact.
Yellowstone
One year later