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Predation. One species benefits, the other incurs a cost; also herbivory & parasitism. Important in adaptive evolution: predator and prey adaptations. Orcinus orca calls. Phoca vitulina response (most common prey). Predator (usually silent).

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R w baird

Predation. One species benefits, the other incurs a cost; also herbivory & parasitism. Important in adaptive evolution: predator and prey adaptations.

Orcinus orca calls

Phoca vitulina response (most common prey)

Predator (usually silent)

Changes in number of seals visible at the surface

Distance of nearest seal to playback source

Familiar non-predator (frequently vocal)

Unfamiliar non-predator (frequently vocal)

Nature 2002

R. W. Baird

pages 1178-1179

Plant defenses against herbivory.

Also important in shaping communities.

K. Starks

R w baird

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.




H. bihai

H. caribaea


H. bihai

H. bihai

H. caribaea


St. Lucia

H. bihai

H. caribaea

H. caribaea



Science 2003

Also important in adaptive evolution

pages 1180-1181

R w baird

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 couldn’t they both co-exist assuming they were equally fit?

R w baird

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


R w baird

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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page 1182

Fig. 53.11

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Proc. Natl. Acad. Sci. 18 Feb 2003

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Productivity in holes on tree trunks

Fig. 53.13

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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For example, sugar maple and impact on shading and soil and thus forest community.

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

R w baird

Keystone species. thus forest community. 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 thus forest community.

pages 1185-1186

western Alaska

Science 1998

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Bottom-up model of regulation of community structure predators

Mar. Ecol.

Prog. Ser. 2005

pages 1185-1186

R w baird

Oikos 2004 predators

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Disturbance- predatorsData 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 predators




Despite the importance of natural disturbances, humans are the greatest agents of disturbance.



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Ecological succession- predators 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.

R w baird

Secondary succession- predators Occurs where an existing community has been cleared by some disturbance that leaves the soil intact.


One year later