1. Chapter 54
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. Interspecific interactions
Can have differing effects on the populations involved
Table 53.1

5. Community Interactions: Effects:
Competition -/-
Predation (herbivory) +/-
Symbiosis
Mutualism +/+
Parasitism (disease) +/-
Commensalism +/o

6. *Interspecific (diff. species) competition leads to:
Competitive Exclusion Principle—no two species can share the same niche (complete role), one will be eliminated.
Similar species can coexist if something separates the niche (breeding season, diet, part of tree they live in)
Example:
2 types of barnacles can separate along the beach into separate areas, but if one species removed, the other fills both areas. Thus, a species fundamental niche may be different than its realized niche.

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

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

9. 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

10. Resource Partitioning
Resource partitioning is the differentiation of niches. It 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

11. Again, one can see so many ways species are affecting each other
Again, one can see so many ways species are affecting each other. Resource partitioning is one,others are:
Character Displacement – a variety of traits (ex…beak lengths). When 2 species are sympatric (not separated physically), they develop a greater range of characteristics than if allopatric (ranges do not overlap). *Predation and Herbivory have also led the evolution of many adaptations in both predator (claws, fangs, speed) and prey.

12. 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

13. *Predation and Herbivory have also led the evolution of many adaptations in both predator (claws, fangs, speed) and prey.
Prey adaptations:
Cryptic Coloration (camouflage)
Aposematic Coloration (warning!)
Batesian Mimicry (harmless mimics harmful organism)
Mullerian Mimicry (both unfavorable species resemble each other)
Physical and chemical defenses in plants (spines, poisons) against herbivory

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

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

16. 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

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

18. Symbiosis—2 or more different species living closely together
3 types of symbiosis:

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

20. Parasitism exerts substantial influence on populations
And the structure of communities
Disease can be considered a special type of parasitism involving pathogens such as bacteria, viruses, or protists.

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

22. 3. Commensalism (+/0) In commensalism
One species benefits and the other is not affected
Figure 53.10

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

24. Species Diversity:
Species Diversity (variety of species in community) Includes BOTH:
1. Species Richness (total # of species)
2. Relative Abundance (proportions of each species.

25. 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

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

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

28. 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

29. 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

30. Trophic Structure (food webs and chains):
Each food chain is only a few links long. Why:
*1. Energetic Hypothesis –due to inefficient energy transfer (only 10%) (most supported)
2. Dynamic Stability model- longer chains are less stable. (less accepted model)
Controls:
Bottom Up –abiotic factors at bottom of food chain affect the top. (ex…low light, nutrients, water)
vs.
Top Down -predators influence trickles down
*Food chains are affected by shifting and variations of these two controls.

31. 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

32. Dominant and Keystone Species exert a strong influence on community structure:
Dominant Species– species with highest biomass (either best competitor or avoids predation best)
Keystone Species – have a special ecological role in the community (ex..sea otters eating sea urchins helps kelp growth)
*Keystone species might also be Foundation Species—exert influence on community by physically changing the ecosystem. (ex—beavers)
Facilitators—when a foundation species has positive effects on the survival of many other species. (ex.. Juncus grass prevents salt buildup and oxygen depletion of New England salt marshes).

33. 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

34. 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

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

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

37. 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

38. Concept 54.3: Disturbance influences species diversity and composition
Decades ago, most ecologists favored the traditional view
That communities are in a state of equilibrium

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

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

41. 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

42. The intermediate disturbance hypothesis
Suggests that moderate levels of disturbance can foster higher species diversity than low levels of disturbance
High levels of disturbance reduce species diversity
Low levels of disturbance reduce species diversity be allowing dominant species to exclude others

43. Intermediate Disturbance Hypothesis:
--moderate levels of disturbance are best for species diversity. (ex…Yellowstone fires)
****The Biggest Agent of Disturbance to Ecosystems are HUMANS! .

44. 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

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

46. Ecological Succession (sequence of changes after a disturbance):
Primary—no soil (after volcano erupts)
Secondary—on bare soil (after a fire, plowing)
**we often study succession after a disturbance. Glacier Bay provides many areas to study sec. succ. as glaciers retreat. (global warming again, Rich.)

47. 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

48. 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

49. 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

50. Concept 54.4: Biogeographic factors affect community diversity
Two key factors correlated with a community’s species diversity
geographic location
its size

51. Species Diversity is connected to geographical location and size:
1. Location- Equator to Polar Gradient of Diversity:
Species richness is greatest at the equator and declines towards the poles. Why?
Tropics are older and thus species have been evolving longer.
Climate has more evapotranspiration (sun, water)
2. Size-Species/Area Graph curves show larger areas have more species.
**Island Equilibrium Model—says there are more species on islands that are larger and closer to the mainland than further and smaller ones.

52. 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

53. 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

54. 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

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

56. 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

57. Concept 54.5: Community ecology is useful for studying pathogens and controlling disease
Pathogens are disease-causing microorganisms, viruses, viroids(RNA), prions(proteins).
Zoonotic pathogens are transferred from other animals to humans (malaria, hantavirus) through either direct contact or an intermediate species called a vector.

58. Finally…..
It is important to keep in mind that there are many different community hypotheses and models
most represent extremes
most communities probably lie somewhere in the middle