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Community Structure
• Community – An assemblage of all the populations interacting in an area
• A community has attributes – Number of species– Relative abundance of species– Nature of species interactions– Physical structure
Community Structure
• Dominant species are typically the dominant competitors under the prevailing environmental conditions– Other factors may determine dominance within
communities
Species Dominance
Species Diversity
• Species diversity– Variety, Abundance of Species in a Particular Place
• Species richness:– The number of different species in a given area
• Species evenness:– Comparative number of individuals
Variations in Species Richness and Species Evenness
Fig. 4-12, p. 93
Higher richness and evenness
lower richness and evenness
Species Diversity:
• Diversity varies with geographical location
• The most species-rich communities– Tropical rain forests– Coral reefs– Ocean bottom zone– Large tropical lakes
Global Map of Plant Biodiversity
Supplement 8, Fig. 6, p. S36
Subsets of species can be defined by:• Taxonomic affinity (e.g., all bird
species in a community)
• Guild—group of species that use the same resources
• Functional group—species that function in similar ways
What Are Communities?
• Food webs organize species based on trophic or energetic interactions.
• Trophic levels: • Primary producers—autotrophs
• Primary consumers—herbivores
• Secondary consumers—carnivores
• Tertiary consumers—carnivores
Concept 16.3 Interactions of Multiple Species
Trophic facilitation:
A consumer is indirectly facilitated by a positive interaction between its prey and another species.
Example: Interactions between salt marsh plants affect aphids.
Figure 16.13 Indirect Effects in Interaction Webs
Concept 16.3 Interactions of Multiple Species
Indirect effects can arise from horizontal interactions.
Buss and Jackson (1979) hypothesized that competitive networks allow the coexistence of competitors and thus maintain species richness.
Interactions of Multiple Species
• In a community, multiple species interactions generate a multitude of connections.
• Direct interactions: Between two species (e.g., competition, predation, facilitation).
• Indirect interactions: The relationship between two species is mediated by a third (or more) species.
Food Webs and Interaction Webs (Part 1)
Food Webs and Interaction Webs
Coevolution produces non-random species assemblages
Species are not assembled randomly in nature.
Physical stress
(Sousa 1979 Ecology)
0
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Low Mid High
Disturbance level
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ies
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ess
Nov-75May-76Oct-76May-77
(Lubchenco 1978 Am Nat)
Herbivory
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Herbivore Density (snails/m2)
# s
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ies
Nitrogen availability
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log
# s
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log ammonium loading(micromol/L/h)
(Bracken & Nielsen 2004 Ecology)
2 species 17 species
~ 59 seaweed species 20 – 26 common species
30 cm
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The changing nature of community structure in response to:– Changing environmental conditions– Interactions among species
Community structure is dynamic– Shifting pattern of species’ dominance and diversity
through time
Community Dynamics
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Community Dynamics
Factors affecting changes in community structure– Intermediate Disturbance Hypothesis– Keystone Species– Trophic Cascades– Dominant species
Foundation species Ecosystem engineers
– Exotic (introduced species)– Community Instability (Alternate stable states)– Ecological Succession
Primary Secondary
Intermediate Disturbance Hypothesis
Areas with moderate levels of disturbance have greater species diversity
Physical stress
(Sousa 1979 Ecology)
0
1
2
3
4
5
Low Mid High
Disturbance level
Spec
ies
richn
ess
Nov-75May-76Oct-76May-77
(Lubchenco 1978 Am Nat)
Herbivory
0
5
10
15
0 100 200 300
Herbivore Density (snails/m2)
# s
eaw
eed
spec
ies
Nitrogen availability
0.25
0.50
0.75
1.00
0.0 0.2 0.4 0.6 0.8 1.0
log
# s
eaw
eed
spec
ies
log ammonium loading(micromol/L/h)
(Bracken & Nielsen 2004 Ecology)
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Keystone Species
A species that has a disproportionately large effect on community structure
Removal of a keystone species can cause drastic changes in a community– can decrease diversity
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sea star community 15 species coexist
- 1 sea star is removed8 remain in community
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Keystone speciesexert an importantregulating effecton other speciesin a community.
Fig. 53.14
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Populationsare limited by
competition for resources
Interactions of Multiple Species
Interaction strength: Magnitude of the effect of one species on the abundance of other species.
How Much Does Predation by Sea Stars Matter? It Depends
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If Keystone Species are removed, community structure is greatly affected.
Interactions of Multiple Species
Trophic cascade:
Indirect interaction caused by a consumer affecting other members in a food web.
Trophic Cascade
Interactions of Multiple Species
• In kelp forests, sea otters feed on sea urchins, which feed on the kelp.
• Sea otters have a positive indirect effect on kelp.
• Kelp, in turn, can positively affect abundance of other seaweeds, which serve as habitat and food for marine invertebrates and fishes.
Interactions of Multiple Species
• Dominant species or foundation species: Have large effects on other species.
Concept 16.3 Interactions of Multiple Species
• Some dominant species are ecosystem engineers—they create, modify, or maintain physical habitat.
• Example: Trees, grasses, coral
Trees Are Dominant Species and Ecosystem Engineers
Beavers Are Keystone Species and Ecosystem Engineers
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Species Introductions
Exotic species– A species moves out of its home range and takes up
residence in a new place– No natural enemies or controls– Can outcompete native species
Geographic dispersal slow or rapid movement
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1925 – 30 million acres covered (500 ton/acre)
Prickly Pear Cactus in Australia 1916 – introduced to Australia
1926 – Cactus moth (Cactoblastis) introduced
1934 – only 1/10 of the cactus remained
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Kudzu in Georgia
Imported for erosion control No natural herbivores, pathogens, or
competitors Grows over landscapes and cannot be dug up or
burned out May turn out to have some
commercial use
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Aquatic Invasives: Unintended Stow-aways
Ballast water has brought many invaders to Bay Area
Great Lakes affected along with ALL other shipping ports world-wide
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Community Instability
Disturbances can cause a community to change in ways that persist even if the change is reversed. In some
cases these are considered “alternate stable states”
Interactions of Multiple Species
• Competitive networks: Interactions have a negative effect on every other species.
• Competitive hierarchy: one species dominates the interaction.
Competitive Networks versus Competitive Hierarchies
Connections in Nature: Stopping Invasions Requires Commitment
• In 2000, Caulerpa was discovered near San Diego, California.
• A team of scientists and managers was immediately assembled to design an eradication plan.
• It eventually took six years and $7 million to eradicate the alga.
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Caulerpa taxifolia
•Native to Indian Ocean, used as ornamental in aquaria
•Specific strain was found to thrive in cold aquarium environments
•Selective breeding under exposure to chemicals and ultra-violet light made the strain even hardier
•Escaped from Oceanographic Museum in Monaco (France)
•Smothering parts of Mediterranean Sea
•Continues to spread
• The introduction of Caulerpa to the Mediterranean dramatically changed native species interactions, and thus community structure and function.
• Seagrass meadows support a multitude of species and were overgrown by Caulerpa.
Spread of Caulerpa in the Mediterranean Sea
• The seagrass Posidonia and Caulerpahave different growth cycles: Posidonialoses blades in the summer when Caulerpa is most productive.
• This allows Caulerpa to overtop Posidonia and dominate.
A Mediterranean Seagrass Meadow
• Caulerpa acts as an ecosystem engineer, accumulating sediments around its roots, which changes the invertebrate community.
• There is also a decrease in numbers and sizes of fish after Caulerpainvades, suggesting the habitat is no longer suitable.
Invading Seaweed
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Community structure varies in time and space– Across the landscape (zonation)– In one position as time passes
Succession is the gradual change in community structure through time
Community Structure Changes through Time
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Types of Succession
Primary succession: new environments – Begins with “foundation species”– Ends with “climax community”
Secondary succession: communities destroyed or displaced– May not have “foundation species”– Also ends with “climax community”
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Climax Community
Stable array of species that does not change over time
In a particular habitat, succession produces the same climax community
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Volcanic Island Succession
Foundation Species Climax Community
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Ecological Succession
Over time, one array of species is replaced by another
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Primary succession begins on sites that have never supported a community– Rock outcrops and cliffs– Sand dunes– Lakes or ponds to meadows
Primary Succession
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Glacial Succession
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Sand Dune Succession
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Primary succession –begins in a new or lifeless area where soil has not yet formed.
Lake Succession
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Secondary succession –occurs where an existing community has been cleared, but the soil is left intact.
Relatively rapid succession
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Most communities are disturbedby events like fire, weather, or human activities
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Marine communities are subject to disturbance by tropical storms.
Fig. 53.17Any major changes in the living community has significant affects on local environmental conditions.
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Secondary Succession: Disturbed Habitats
Abandoned Fields
Along Roadsides
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Disease Damage – Sudden Oak Death
Storm Damage
Fire Damage
Cyclic Replacement
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Chaparral –fire adapted plant community
Cyclic Replacement:Fire Climax
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Patterns of succession are not random Early successional species or pioneer
species are usually characterized by high growth rates, smaller size, high degree of dispersal, and high rates of per capita population growth
Late successional species have lower rates of dispersal and colonization, slower per capita growth rate, and they are larger and longer-lived
Community Structure Changes through Time
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