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2. Communities in Motion
A biological community is an assemblage of populations of various species living close enough for potential interaction
Some interactions are beneficial to both of the species involved
For example, the bluestreak cleaner wrasse swims inside the mouth of a moray eel and eats tiny parasites inside its mouth
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3. Figure 54.1 Which species benefits from this interaction?

4. Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved
Ecologists call relationships between species in a community interspecific interactions
Examples are competition, predation, herbivory, parasitism, mutualism, and commensalism
Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)
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5. Competition
Competition (–/– interaction) occurs when species compete for a resource that limits survival and reproduction
Resources must be in short supply for competition to occur
Strong competition can lead to competitive exclusion, local elimination of a competing species
Russian ecologist G.F. Gause concluded that two species competing for the same limiting resources cannot coexist permanently in the same place
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6. Ecological Niches and Natural Selection
An ecological niche is the sum of an organism’s use of biotic and abiotic resources; it can be thought of as an organism’s ecological role
Ecologically similar species can coexist in a community if there are one or more significant differences in their niches
Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community
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7. A. insolitus usually perches on shady branches.
A. distichus perches
on fence posts and
other sunny surfaces.
A. insolitus usually perches
on shady branches.
A. ricordii
Figure 54.2 Resource partitioning among Dominican Republic lizards
A. insolitus
A. aliniger
A. christophei
A. distichus
A. cybotes
A. etheridgei

8. Both species are normally nocturnal (active during the night)
The common spiny mouse and the golden spiny mouse show temporal partitioning of their niches
Both species are normally nocturnal (active during the night)
Where they coexist, the golden spiny mouse becomes diurnal (active during the day)
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9. Character Displacement
Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species
An example is variation in beak size between populations of two species of Galápagos finches
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10. Floreana, San Cristóbal Sympatric populations 40
G. fuliginosa
G. fortis
Beak
depth
Percentages of individuals in each size class 60
Los Hermanos 40
G. fuliginosa,
allopatric 20 60
Daphne 40
G. fortis,
allopatric 20
Figure 54.4 Character displacement: indirect evidence of past competition 60
Floreana, San Cristóbal
Sympatric
populations 40 20 8 10 12 14 16
Beak depth (mm)

11. Exploitation
Exploitation refers to any +/– interaction in which one species benefits by feeding on the other species
Exploitative interactions include predation, herbivory, and parasitism
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12. Predation
Predation (+/– interaction) refers to an interaction in which one species, the predator, kills and eats the other, the prey
Predators have adaptations that enable them to find, identify, catch, and subdue their prey
For example, many predators have claws, fangs, or poison
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13. Predation Prey display various adaptations to avoid being eaten
Behavioral defenses include hiding, fleeing, and forming herds or schools
Animals also have morphological and physiological defense adaptations
For example, mechanical and chemical defenses protect species such as porcupines and skunks
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14. Cryptic coloration: camouflage
Mechanical defense
Chemical defense
Porcupine
Skunk
Cryptic coloration: camouflage
Aposematic coloration: warning coloration
Poison
dart frog
Canyon
tree frog
Batesian mimicry: A harmless species mimics a harmful one.
(Müllerian mimicry: Two unpalatable species mimic each other.
Figure 54.5 Examples of defensive adaptations in animals
Venomous green
parrot snake
Yellow jacket
Nonvenomous
hawkmoth larva
Cuckoo bee

15. Cryptic coloration, or camouflage, makes prey difficult to spot
Animals with effective chemical defenses often exhibit bright warning coloration, called aposematic coloration
Predators are particularly cautious in dealing with prey that display such coloration
Cryptic coloration, or camouflage, makes prey difficult to spot
In some cases, a prey species may gain significant protection by mimicking the appearance of another species
In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model
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16. In Müllerian mimicry, two or more unpalatable species resemble each other
Mimicry has also evolved in many predators to enable them to approach prey
For example, the mimic octopus can take on the appearance and movement of more than a dozen marine animals
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17. Mimicking a sea snake Mimicking a flounder Mimicking a stingray
Figure 54.6 The mimic octopus
Mimicking a stingray

18. Herbivory
Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga
Large mammals are the most familiar herbivores, but most herbivores are invertebrates
Herbivores have many specialized adaptations
For example, many herbivores have specialized teeth or digestive systems for processing vegetation
Plants may produce toxic or distasteful chemicals or mechanical defenses, such as spines or thorns
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19. Figure 54.7 An herbivorous marine mammal

20. Parasitism
In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process
Parasites that live within the body of their host are called endoparasites
Parasites that live on the external surface of a host are ectoparasites
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21. Parasitism
Many parasites have a complex life cycle involving multiple hosts
Some parasites change the behavior of the host in a way that increases the likelihood that the parasite will be transmitted to the next host
Parasites can significantly affect the survival, reproduction, and density of their host population, directly or indirectly
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22. Positive Interactions
Ecological communities are heavily influenced by positive interactions, where at least one species benefits and neither is harmed
Mutualism (+/+) and commensalism (+/0) are positive interactions
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23. Mutualism
Mutualism (+/+ interaction) is a common interspecific interaction that benefits both species
In a mutualism, both species incur costs, but the benefits to each partner exceed the costs
In some mutualisms, each species depends on the other for their survival and reproduction; in others, both species can survive alone
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24. Hollow thorns that house stinging ants of the genus Pseudomyrmex
Figure 54.8 Mutualism between acacia trees and ants
Hollow thorns that house stinging ants of the genus Pseudomyrmex
Area cleared by ants around an acacia tree

25. Commensalism
Commensalism (+/0 interaction) is another common interaction in which one species benefits and the other is neither harmed nor helped
For example, shade-tolerant wildflowers depend on the shade provided by forest trees, but the trees are not affected by the wildflowers
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26. Commensalism
Some interactions that are typically commensal may at times become mutualistic
For example, cattle egrets typically benefit from the insects flushed out of the grass by bison and have no effect on the bison
At times, they may remove and eat ectoparasites from the bison’s skin, making the interaction mutualistic
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27. Figure 54.9 Commensalism between cattle egrets and African buffalo

28. Commensalism
Positive interactions can have significant influence on the structure of ecological communities
For example, the black rush affects community diversity by making the soil more hospitable for other plant species
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29. With Juncus Without Juncus
Number of plant species 8 6 4 2
Figure Facilitation by black rush (Juncus gerardii) in New England salt marshes
Salt marsh with Juncus
With
Juncus
Without
Juncus

30. Concept 54.2: Diversity and trophic structure characterize biological communities
The species diversity of a community is the variety of organisms that make up the community
It has two components: species richness and relative abundance
Species richness is the number of different species in the community
Relative abundance is the proportion each species represents of all individuals in the community
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31. H = – (pA ln pA + pB ln pB + pC ln pC + …)
Shannon Diversity Index
Diversity can be compared using a diversity index
Shannon diversity index (H)
H = – (pA ln pA + pB ln pB + pC ln pC + …)
where A, B, C are the species, p is the relative abundance of each species, and ln is the natural logarithm
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32. Shannon Diversity
We can calculate the Shannon diversity index for the two communities discussed in the text
For community 1, p = 0.25 for each species, so
H = – 4(0.25 ln 0.25) = 1.39
For community 2,
H = –[0.8 ln (0.05 ln 0.05) ln 0.1] = 0.71
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33. A B C D Community 1 A: 25% B: 25% C: 25% D: 25% Community 2
Figure Which forest is more diverse?
Community 2
A: 80% B: 5% C: 5% D: 10%

34. Shannon Diversity Communities with higher diversity are
more productive; they produce more biomass (the total mass of all organisms)
more stable in their productivity
better able to withstand and recover from environmental stresses
more resistant to invasive species, organisms that become established outside their native range
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35. Trophic Structure
Trophic structure is the feeding relationships between organisms in a community
It is a key factor affecting community structure and dynamics
Food chains link trophic levels from producers to top carnivores
The position an organism occupies in a food chain is called its trophic level
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36. A terrestrial food chain A marine food chain
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Carnivore
Secondary
consumers
Carnivore
Primary
consumers
Herbivore
Figure Examples of terrestrial and marine food chains
Zooplankton
Plant
Primary
producers
Phytoplankton
A terrestrial food chain
A marine food chain

37. Food Webs
A food web is a group of food chains linked together forming complex trophic interactions
A species may play a role at more than one trophic level in a food web
Food webs can be simplified by
grouping species with similar trophic relationships into broad functional groups
isolating a portion of a community that interacts very little with the rest of the community
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38. Sperm
whales
Smaller
toothed
whales
Baleen
whales
Elephant
seals
Crabeater
seals
Leopard
seals
Birds
Fishes
Squids
Figure An Antarctic marine food web
Carniv-
orous
plankton
Copepods
Krill
Phyto-
plankton

39. Sea nettle Juvenile striped bass Fish larvae Zoo- plankton Fish eggs
Figure Partial food web for Chesapeake Bay estuary
Zoo-
plankton
Fish eggs

40. Limits on Food Chain
Each food chain in a food web is usually only a few links long
The energetic hypothesis suggests that length is limited by inefficient energy transfer
Only about 10% of the energy stored in organic matter at each trophic level is converted to organic matter at the next trophic level
For example, a producer level consisting of 100 kg of plant material can support about 10 kg of herbivore biomass and 1 kg of carnivore biomass
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41. Limits on Food Chain
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42. Limits on Food Chain
Dominant species are those that are most abundant or have the highest biomass
One hypothesis suggests that dominant species are most competitive in exploiting resources
Another hypothesis is that they are most successful at avoiding predators or disease
Invasive species, typically introduced to a new environment by humans, may become dominant because they lack natural predators or parasites
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43. Limits on Food Chain
Keystone species exert strong control on a community by their ecological roles, or niches
In contrast to dominant species, they are not usually abundant in a community
Field studies of sea stars illustrate their role as a keystone species in intertidal communities
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44. Number of species present
Experiment
Results
Number of species
present 20 15
With Pisaster (control)
Figure Inquiry: Is Pisaster ochraceus a keystone species? 10
Without Pisaster (experimental) 5
1963
’64
’65
’66
’67
’68
’69
’70
’71
’72
’73
Year

45. Ecosystem engineers (or “foundation species”) cause physical changes in the environment that affect community structure
For example, beaver dams can transform landscapes on a very large scale
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46. A disturbance is an event that changes a community, removes organisms from it, and alters resource availability
The nonequilibrium model describes communities as constantly changing after disturbance
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47. Characterizing Disturbance
The types of disturbances and their frequency and severity vary among communities
Storms and fire are significant sources of disturbance in many ecosystems
The large-scale fire in Yellowstone National Park in demonstrated that communities can often respond very rapidly to a massive disturbance
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48. Soon after fire One year after fire
Figure Recovery following a large-scale disturbance
One year after fire

49. Ecological Succession
Ecological succession is the sequence of changes in community composition following a disturbance
Primary succession occurs where no soil exists when succession begins
Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics
Succession is the result of changes induced by the vegetation itself
On the glacial moraines, pioneer plant species facilitate later arrivals by increasing soil nitrogen content
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50. Ecological Succession
1941
1907
1860 1
Pioneer stage 2
Dryas stage
Glacier
Bay
Alaska
Figure 54.23_5 Glacial retreat and primary succession at Glacier Bay, Alaska (step 5)
1760 5 10 15
Kilometers 4
Spruce stage 3
Alder stage

51. Ecological Succession
Secondary succession begins in an area where soil remains after a disturbance
For example, abandoned agricultural land may return to its original state through secondary succession
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52. Human Disturbance
Humans have the greatest impact on biological communities worldwide
Both terrestrial and marine ecosystems are subject to human disturbance
Human disturbance to communities usually reduces species diversity
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53. Before trawling After trawling
Figure Disturbance of the ocean floor by trawling
After
trawling

54. Concept 54.4: Biogeographic factors affect community diversity
Latitude and area are two key biogeographic factors that affect the species diversity of biological communities
Species richness is especially great in the tropics and generally declines in a gradient toward the poles
Two key factors affecting latitudinal gradients of species richness are evolutionary history and climate
Tropical environments may have greater species richness because there has been more time for speciation to occur
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55. Two main climatic factors correlated with biodiversity in terrestrial communities are sunlight and precipitation
They can be considered together by measuring a community’s rate of evapotranspiration, the evaporation of water from soil plus transpiration of water from plants
Potential evapotranspiration is the measure of potential water loss, assuming water is available
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56. Vertebrate species richness
200
Vertebrate species richness
(log scale)
100 50
Figure Energy, water, and species richness 10
500
1,000
1,500
2,000
Potential evapotranspiration (mm/yr)

57. Concept 54.5: Pathogens alter community structure locally and globally
Community structure is universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prions
Pathogens can be particularly virulent in a new habitat
Human activities are transporting pathogens around the world at unprecedented rates
For example, H1N1, the virus that causes “swine flu,” spread around the world from Veracruz, Mexico, causing more than 18,000 deaths within two years
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58. Community Ecology and Zoonotic Diseases
Zoonotic pathogens are transferred to humans from other animals
The transfer of pathogens can be direct or through an intermediate species called a vector
Many of today’s emerging human diseases are zoonotic
Avian flu is a highly contagious virus of birds
Ecologists are studying the potential spread of the virus from Asia to North America through migrating birds
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59. Figure 54.UN06 Summary of key concepts: interspecific interactions