Chapter 53Chapter 53
Community EcologyCommunity Ecology
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What Is a Biological Community?
• a grouping of populations of various species living close enough for potential interaction
• How big is a community?
– As big as you define it
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25000 species of plants1600 birds369 mammals350 reptiles400 amphibians800 fish in Amazon450 in Pacific100,000s of inverts
413 birds17 reptiles12 amphibians56 fish
What’s in a Community?
2,301,000 km2
256,370 km2
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How do communities interact? Who eats who?
• interactions include
– competition
– predation
– herbivory
– symbiosis
– disease
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Inter-species interactions
Table 53.1
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Competition
• Species compete for resource in short supply
• Competitive exclusion principle
– Competing species cannot coexist in the same niche
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Ecological Niches
• The ecological niche
– Use of abiotic and biotic resources
– Competitors cannot coexist unless niches change
When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area.
The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche.
RESULTS
CONCLUSION
Ocean
Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland.
EXPERIMENT
In nature, Balanus fails to survive high on the rocks because it isunable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata.
Low tide
High tide
Chthamalusfundamental niche
Chthamalusrealized niche
Low tide
High tideChthamalus
Balanusrealized niche
Balanus
Ocean
Figure 53.2
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Competition
• Fundamental niche (what you want)
• Realized niche (what you can get)
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A. insolitususually percheson shady branches.
A. distichus perches on fence posts and
other sunny surfaces.
A. distichus
A. ricordii
A. insolitus
A. christophei
A. cybotes
A. etheridgei
A. alinigar
Figure 53.3
Resource Partitioning
• Resource partitioning allows similar species to coexist in a community
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Predation
• Predator kills and eats prey (+/-)
• Includes herbivory and parasitism
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Predation
• Feeding adaptations
– Claws, teeth, fangs, stingers, poison
• Defensive adaptations
– Camouflage, mimicking
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Plant defenses
Tobacco plant - nicotine
Cinnamon plant
Thorns
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Animal Defenses
Cryptic coloration
Chemical defenses
Fleeing
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Aposematic coloration
• Warns predators - stay away!
Figure 53.6
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Batesian mimicry
• palatable or harmless species mimics an unpalatable or harmful model
(a) Hawkmoth larva
(b) Green parrot snake
Figure 53.7a, b
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Parasitism (+/-)
Ectoparasite: a parasite that feeds on the external surface of a host
Endoparasite: a parasite that lives within the host
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Parasitism
• Derive nourishment from another organism,
– Host is harmed in the process
– Complex life cycle
– May effect predation
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Behavior changes
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Symbiosis
Mutualism: interspecific interaction benefits BOTH species
Commensalism: interspecific interaction benefits ONE species, neutral to the other
Symbiosis can include parasitism, mutualism, and commensalism
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25000 species of plants1600 birds369 mammals350 reptiles400 amphibians800 fish in Amazon450 in Pacific100,000s of inverts
413 birds17 reptiles12 amphibians56 fish
2,301,000 km2
256,370 km2
SPECIES DIVERSITY
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Diversity has 2 components:
• Species richness
– Is the total number of different species in the community
• Relative abundance
– Is the proportion each species represents of the total individuals in the community
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2 communities
• Same richness – different abundance
– More diversity where abundance is similar
Community 1A: 25% B: 25% C: 25% D: 25%
Community 2A: 80% B: 5% C: 5% D: 10%
D
C
BA
Figure 53.11
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Trophic Structure
• Feeding relationships between organisms in a community
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Food chains
• Link the trophic levels from producers to top carnivores
Quaternary consumers
Tertiary consumers
Secondary consumers
Primary consumers
Primary producers
Carnivore
Carnivore
Carnivore
Herbivore
Plant
Carnivore
Carnivore
Carnivore
Zooplankton
Phytoplankton
A terrestrial food chain A marine food chainFigure 53.12
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Food Webs
• Branching food chain, complex trophic interactions
Humans
Baleen whales
Crab-eater seals
Birds Fishes Squids
Leopardseals
Elephant seals
Smaller toothed whales
Sperm whales
Carnivorous plankton
Euphausids (krill)
Copepods
Phyto-plankton
Figure 53.13
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Limits on Food Chain Length
• Each usually only a few links long
• Energetic hypothesis suggests length limited by inefficiency of energy transfer
– 100kg – 10 kg – 1 kg
• Dynamic stability hypothesis - long food chains less stable than short ones
– Shocks travel up the food chain
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Dominant and keystone species
• In general, a small number of species in a community exert strong control on community’s structure
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Dominant Species
• Most abundant or have the highest biomass
– Exert control over occurrence and distribution of other species
– Exploit limited resources better or..
– Successfully avoid predators
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Keystone species
• Not necessarily abundant
• Control by niche or role
Figure 53.16a,b
(a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.
With Pisaster (control)
Without Pisaster (experimental)
Num
ber
of s
peci
es
pres
ent
0
5
10
15
20
1963 ´64 ´65 ´66 ´67 ´68 ´69 ´70 ´71 ´72 ´73
(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity.
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Brazil nut tree
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No longer a keystone
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Community organization
• The bottom-up model
– influence from lower to higher trophic levels
– presence or absence of abiotic nutrients
– think “primary producers”
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Community organization
• Top-down model:
– Control comes from the trophic level above
– Predators control herbivores
– Herbivores control primary producers
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Disturbance
• Influences species diversity and composition
• Equilibrium model – stable unless disturbed by humans
• Non-equilibrium model – constant change due to disturbances
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What Is Disturbance?
• A disturbance
– Is an event that changes a community
– Removes organisms from a community
– Alters resource availability
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Community Stability
Communities are constantly changing, they are in nonequilibrium – many are in some state of recovery from disturbance
Drought
Fire Clearcut
Flood
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Fire
• Is a significant disturbance in most terrestrial ecosystems
– Is often a necessity in some communities
(a) Before a controlled burn.A prairie that has not burned forseveral years has a high propor-tion of detritus (dead grass).
(b) During the burn. The detritus serves as fuel for fires.
(c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.
Figure 53.21a–c
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Yellowstone fires, 1988
• Communities can often respond very rapidly to a massive disturbance
Figure 53.22a, b
(a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.
(b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.
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Yellowstone wolves
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Dying aspen grove
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Streamside elk grazing
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Hey, what’s that noise?
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Yellowstone Ecosystem
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Lunch grew back!
• Beaver dams
– Can transform landscapes on a very large scale
Figure 53.18
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Succession
• Moraines in Glacier Bay, Alaska
– Follows a predictable pattern of change in vegetation and soil characteristics
(b) Dryas stage
(c) Spruce stage
(d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content.
Soi
l nitr
ogen
(g/
m2)
Successional stagePioneer Dryas Alder Spruce
0
10
20
30
40
50
60
(a) Pioneer stage, with fireweed dominant
- Some species improve
environment for followers
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Ecological Succession
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