1. Chapter 53
Community Ecology

2. Overview: What Is a Community? A biological community
Is an assemblage of populations of various species living close enough for potential interaction

3. The various animals and plants surrounding this watering hole
Are all members of a savanna community in southern Africa
Figure 53.1

4. Populations are linked by interspecific interactions
Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease
Populations are linked by interspecific interactions
That affect the survival and reproduction of the species engaged in the interaction

5. Interspecific interactions
Can have differing effects on the populations involved
Table 53.1

6. Interspecific competition
Occurs when species compete for a particular resource that is in short supply
Strong competition can lead to competitive exclusion
The local elimination of one of the two competing species

7. The Competitive Exclusion Principle
States that two species competing for the same limiting resources cannot coexist in the same place

8. Ecological Niches The ecological niche
Is the total of an organism’s use of the biotic and abiotic resources in its environment

9. The niche concept allows restatement of the competitive exclusion principle
Two species cannot coexist in a community if their niches are identical

10. However, ecologically similar species can coexist in a community
If there are one or more significant difference in their niches
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 realized niche of Chthamalus much smaller than its fundamental niche.
RESULTS
CONCLUSION
Ocean
Ecologist Joseph Connell studied two barnacle speciesBalanus 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 is unable 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
Chthamalus fundamental niche
Chthamalus realized niche
Chthamalus
Balanus realized niche
Balanus
Figure 53.2

11. As a result of competition
A species’ fundamental niche may be different from its realized niche

12. Resource Partitioning
Resource partitioning is the differentiation of niches
That enables similar species to coexist in a community
A. insolitus usually perches on 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

13. Character Displacement
In character displacement
There is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species
G. fortis
Beak depth (mm)
G. fuliginosa
Beak depth
Los Hermanos
Daphne
Santa María, San Cristóbal
Sympatric populations
G. fuliginosa, allopatric
G. fortis, allopatric
Percentages of individuals in each size class 40 20 8 10 12 14 16
Figure 53.4

14. Predation refers to an interaction
Where one species, the predator, kills and eats the other, the prey

15. Feeding adaptations of predators include
Claws, teeth, fangs, stingers, and poison
Animals also display
A great variety of defensive adaptations

16. Cryptic coloration, or camouflage
Makes prey difficult to spot
Figure 53.5

17. Aposematic coloration
Warns predators to stay away from prey
Figure 53.6

18. In some cases, one prey species
May gain significant protection by mimicking the appearance of another

19. In Batesian mimicry
A palatable or harmless species mimics an unpalatable or harmful model
(a) Hawkmoth larva
(b) Green parrot snake
Figure 53.7a, b

20. In Müllerian mimicry
Two or more unpalatable species resemble each other
(a) Cuckoo bee
(b) Yellow jacket
Figure 53.8a, b

21. Herbivory, the process in which an herbivore eats parts of a plant
Has led to the evolution of plant mechanical and chemical defenses and consequent adaptations by herbivores

22. In parasitism, one organism, the parasite
Derives its nourishment from another organism, its host, which is harmed in the process

23. Parasitism exerts substantial influence on populations
And the structure of communities

24. The effects of disease on populations and communities
Is similar to that of parasites

25. Pathogens, disease-causing agents
Are typically bacteria, viruses, or protists

26. Mutualistic symbiosis, or mutualism
Is an interspecific interaction that benefits both species
Figure 53.9

27. Commensalism In commensalism
One species benefits and the other is not affected
Figure 53.10

28. Commensal interactions have been difficult to document in nature
Because any close association between species likely affects both species

29. Interspecific Interactions and Adaptation
Evidence for coevolution
Which involves reciprocal genetic change by interacting populations, is scarce

30. However, generalized adaptation of organisms to other organisms in their environment
Is a fundamental feature of life

31. In general, a small number of species in a community
Concept 53.2: Dominant and keystone species exert strong controls on community structure
In general, a small number of species in a community
Exert strong control on that community’s structure

32. The species diversity of a community
Is the variety of different kinds of organisms that make up the community
Has two components

33. Species richness Relative abundance
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

34. Two different communities
Can have the same species richness, but a different relative abundance
Community 1
A: 25% B: 25% C: 25% D: 25%
Community 2
A: 80% B: 5% C: 5% D: 10% D C B A
Figure 53.11

35. A community with an even species abundance
Is more diverse than one in which one or two species are abundant and the remainder rare

36. Trophic Structure Trophic structure
Is the feeding relationships between organisms in a community
Is a key factor in community dynamics

37. A terrestrial food chain
Food chains
Quaternary consumers
Tertiary consumers
Secondary consumers
Primary consumers
Primary producers
Carnivore
Herbivore
Plant
Zooplankton
Phytoplankton
A terrestrial food chain
A marine food chain
Figure 53.12
Link the trophic levels from producers to top carnivores

38. Smaller toothed whales
Food Webs
A food web
Humans
Baleen whales
Crab-eater seals
Birds
Fishes
Squids
Leopard
seals
Elephant
Smaller toothed whales
Sperm whales
Carnivorous
plankton
Euphausids
(krill)
Copepods
Phyto- plankton
Figure 53.13
Is a branching food chain with complex trophic interactions

39. Food webs can be simplified
By isolating a portion of a community that interacts very little with the rest of the community
Sea nettle
Fish larvae
Zooplankton
Fish eggs
Juvenile striped bass
Figure 53.14

40. Limits on Food Chain Length
Each food chain in a food web
Is usually only a few links long
There are two hypotheses
That attempt to explain food chain length

41. The energetic hypothesis suggests that the length of a food chain
Is limited by the inefficiency of energy transfer along the chain

42. The dynamic stability hypothesis
Proposes that long food chains are less stable than short ones

43. Most of the available data
Support the energetic hypothesis
High (control)
Medium
Low
Productivity
No. of species
No. of trophic links
Number of species
Number of trophic links 1 2 3 4 5 6
Figure 53.15

44. Species with a Large Impact
Certain species have an especially large impact on the structure of entire communities
Either because they are highly abundant or because they play a pivotal role in community dynamics

45. Dominant Species Dominant species
Are those species in a community that are most abundant or have the highest biomass
Exert powerful control over the occurrence and distribution of other species

46. One hypothesis suggests that dominant species
Are most competitive in exploiting limited resources
Another hypothesis for dominant species success
Is that they are most successful at avoiding predators

47. Keystone Species Keystone species
Are not necessarily abundant in a community
Exert strong control on a community by their ecological roles, or niches

48. Number of species present
Field studies of sea stars
Exhibit their role as a keystone species in intertidal communities
(a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.
With Pisaster (control)
Without Pisaster (experimental)
Number of species present 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.
Figure 53.16a,b

49. Otter number (% max. count)
Observation of sea otter populations and their predation
Figure 53.17
Food chain before killer whale involve- ment in chain
(a) Sea otter abundance
(b) Sea urchin biomass
(c) Total kelp density
Number per 0.25 m2
1972
1985
1989
1993
1997 2 4 6 8 10
100
200
300
400
Grams per 0.25 m2
Otter number (% max. count) 40 20 60 80
Year
Food chain after killer whales started preying on otters
Shows the effect the otters have on ocean communities

50. Ecosystem “Engineers” (Foundation Species)
Some organisms exert their influence
By causing physical changes in the environment that affect community structure

51. Beaver dams Can transform landscapes on a very large scale
Figure 53.18

52. Some foundation species act as facilitators
That have positive effects on the survival and reproduction of some of the other species in the community
Salt marsh with Juncus (foreground)
With Juncus
Without Juncus
Number of plant species 2 4 6 8
Conditions
Figure 53.19

53. Bottom-Up and Top-Down Controls
The bottom-up model of community organization
Proposes a unidirectional influence from lower to higher trophic levels
In this case, the presence or absence of abiotic nutrients
Determines community structure, including the abundance of primary producers

54. The top-down model of community organization
Proposes that control comes from the trophic level above
In this case, predators control herbivores
Which in turn control primary producers

55. Percentage of herbaceous plant cover
Long-term experiment studies have shown
That communities can shift periodically from bottom-up to top-down
Figure 53.20
100
200
300
400
Rainfall (mm) 25 50 75
Percentage of herbaceous plant cover

56. But through biomanipulation
Pollution
Can affect community dynamics
But through biomanipulation
Polluted communities can be restored
Fish
Zooplankton
Algae
Abundant
Rare
Polluted State
Restored State

57. Concept 53.3: Disturbance influences species diversity and composition
Decades ago, most ecologists favored the traditional view
That communities are in a state of equilibrium

58. However, a recent emphasis on change has led to a nonequilibrium model
Which describes communities as constantly changing after being buffeted by disturbances

59. What Is Disturbance? A disturbance
Is an event that changes a community
Removes organisms from a community
Alters resource availability

60. 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 for several 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

61. The intermediate disturbance hypothesis
Suggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbance

62. The large-scale fire in Yellowstone National Park in 1988
Demonstrated that communities can often respond very rapidly to a massive disturbance
(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.
Figure 53.22a, b

63. Human Disturbance
Humans
Are the most widespread agents of disturbance

64. Human disturbance to communities
Usually reduces species diversity
Humans also prevent some naturally occurring disturbances
Which can be important to community structure

65. Ecological Succession
Is the sequence of community and ecosystem changes after a disturbance

66. Primary succession Secondary succession
Occurs where no soil exists when succession begins
Secondary succession
Begins in an area where soil remains after a disturbance

67. Early-arriving species
May facilitate the appearance of later species by making the environment more favorable
May inhibit establishment of later species
May tolerate later species but have no impact on their establishment

68. Retreating glaciers
Provide a valuable field-research opportunity on succession
McBride glacier retreating 5 10
Miles
Glacier Bay
Pleasant Is.
Johns Hopkins Gl.
Reid Gl.
Grand Pacific Gl.
Canada
Alaska
1940
1912
1899
1879
1949
1935
1760
1780
1830
1860
1913
1911
1892
1900
1907
1948
1931
1941
Casement Gl.
McBride Gl.
Plateau Gl.
Muir Gl.
Riggs Gl.
Figure 53.23

69. Succession on the moraines in Glacier Bay, Alaska
Follows a predictable pattern of change in vegetation and soil characteristics
Figure 53.24a–d
(b) Dryas stage
(c) Spruce stage
(d) Nitrogen fixation by Dryas and alder increases the soil nitrogen content.
Soil nitrogen (g/m2)
Successional stage
Pioneer
Dryas
Alder
Spruce 10 20 30 40 50 60
(a) Pioneer stage, with fireweed dominant

70. Concept 53.4: Biogeographic factors affect community diversity
Two key factors correlated with a community’s species diversity
Are its geographic location and its size

71. Equatorial-Polar Gradients
The two key factors in equatorial-polar gradients of species richness
Are probably evolutionary history and climate

72. Species richness generally declines along an equatorial-polar gradient
And is especially great in the tropics
The greater age of tropical environments
May account for the greater species richness

73. Climate
Is likely the primary cause of the latitudinal gradient in biodiversity

74. Vertebrate species richness (log scale)
The two main climatic factors correlated with biodiversity
Are solar energy input and water availability
(b) Vertebrates
500
1,000
1,500
2,000
Potential evapotranspiration (mm/yr) 10 50
100
200
Vertebrate species richness (log scale) 1
300
700
900
1,100
180
160
140
120 80 60 40 20
Tree species richness
(a) Trees
Actual evapotranspiration (mm/yr)
Figure 53.25a, b

75. The species-area curve quantifies the idea that
Area Effects
The species-area curve quantifies the idea that
All other factors being equal, the larger the geographic area of a community, the greater the number of species

76. Number of species (log scale)
A species-area curve of North American breeding birds
Supports this idea
Area (acres) 1 10
100
103
104
105
106
107
108
109
1010
Number of species (log scale)
1,000
Figure 53.26

77. Island Equilibrium Model
Species richness on islands
Depends on island size, distance from the mainland, immigration, and extinction

78. Number of species on island
The equilibrium model of island biogeography maintains that
Species richness on an ecological island levels off at some dynamic equilibrium point
Number of species on island
(a) Immigration and extinction rates. The equilibrium number of species on an island represents a balance between the immigration of new species and the extinction of species already there.
(b) Effect of island size. Large islands may ultimately have a larger equilibrium num- ber of species than small islands because immigration rates tend to be higher and extinction rates lower on large islands.
(c) Effect of distance from mainland. Near islands tend to have larger equilibrium numbers of species than far islands because immigration rates to near islands are higher and extinction rates lower.
Equilibrium number
Small island
Large island
Far island
Near island
Immigration
Extinction
(small island)
(large island)
(far island)
(near island)
Rate of immigration or extinction
Figure 53.27a–c

79. Area of island (mi2) (log scale)
Studies of species richness on the Galápagos Islands
Support the prediction that species richness increases with island size
The results of the study showed that plant species richness increased with island size, supporting the species-area theory.
FIELD STUDY
RESULTS
Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island.
CONCLUSION
200
100 50 25 10
Area of island (mi2) (log scale)
Number of plant species (log scale)
0.1 1
1,000 5
400
Figure 53.28

80. Two different views on community structure
Concept 53.5: Contrasting views of community structure are the subject of continuing debate
Two different views on community structure
Emerged among ecologists in the 1920s and 1930s

81. Integrated and Individualistic Hypotheses
The integrated hypothesis of community structure
Describes a community as an assemblage of closely linked species, locked into association by mandatory biotic interactions

82. The individualistic hypothesis of community structure
Proposes that communities are loosely organized associations of independently distributed species with the same abiotic requirements

83. Population densities of individual species
The integrated hypothesis
Predicts that the presence or absence of particular species depends on the presence or absence of other species
Population densities of individual species
Environmental gradient (such as temperature or moisture)
(a) Integrated hypothesis. Communities are discrete groupings of particular species that are closely interdependent and nearly always occur together.
Figure 53.29a

84. Population densities of individual species
The individualistic hypothesis
Predicts that each species is distributed according to its tolerance ranges for abiotic factors
Population densities of individual species
Environmental gradient (such as temperature or moisture)
(b) Individualistic hypothesis. Species are independently distributed along gradients and a community is simply the assemblage of species that occupy the same area because of similar abiotic needs.
Figure 53.29b

85. Number of plants per hectare
In most actual cases the composition of communities
Seems to change continuously, with each species more or less independently distributed
600
Number of plants per hectare
400
200
Wet
Moisture gradient
Dry
(c) Trees in the Santa Catalina Mountains. The distribution of tree species at one elevation in the Santa Catalina Mountains of Arizona supports the individualistic hypothesis. Each tree species has an independent distribution along the gradient, apparently conforming to its tolerance for moisture, and the species that live together at any point along the gradient have similar physical requirements. Because the vegetation changes continuously along the gradient, it is impossible to delimit sharp boundaries for the communities.
Figure 53.29c

86. Rivet and Redundancy Models
The rivet model of communities
Suggests that all species in a community are linked together in a tight web of interactions
Also states that the loss of even a single species has strong repercussions for the community

87. The redundancy model of communities
Proposes that if a species is lost from a community, other species will fill the gap

88. It is important to keep in mind that community hypotheses and models
Represent extremes, and that most communities probably lie somewhere in the middle