1. Chapter 54
Community Ecology

2. Overview: A Sense of Community
A biological community is an group of populations of various species living close enough for potential interaction

3. Concept 54.1: Community interactions are classified by whether they help, harm, or have no effect on the species involved
Relationships between species in a community are called interspecific interactions
Examples: competition, predation, herbivory, and symbiosis (parasitism, mutualism, and commensalism)
Interspecific interactions affect the survival and reproduction of each species
the effects can be (+), (–), or no effect (0)

4. Competition
Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply
In Alaska, the lynx and the fox must compete with
each other for prey such as the snowshoe hare.

5. Competitive Exclusion
Strong competition can lead to competitive exclusion, local elimination of a competing species
The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place
One species will be more efficient and thus reproduce more rapidly than the other. This will eventually lead to the local elimination of the inferior competitor.

6. Ecological Niches
Ecological niche --The total of a species’ use of biotic and abiotic resources
In other words…a niche is an organism’s ecological role (its “job”)
Similar species can coexist in a community if there are one or more significant differences in their niches

7. Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community
Ex: Seven species of Anolis Lizards live close together in the Dominican Republic. They all eat the same food. Competition, however, is reduced because each lizard species has a preferred perch, thus occupying a distinct niche.

8. A. distichus perches on fence posts and other sunny surfaces.
Fig. 54-2
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

9. As a result of competition, a species’ fundamental niche may differ from its realized niche
Fundamental niche = niche potentially occupied
Realized niche = part of the niche actually occupied

10. Two barnacle species have a stratified distribution on coastal
Fig. 54-3
EXPERIMENT
High tide
Chthamalus
Two barnacle species have
a stratified distribution on coastal
rocks. Is this the result of interspecific
competition between the two?
To find out, researchers removed the
Balanus from the rocks.
Balanus
Chthamalus
realized niche
Balanus
realized niche
Ocean
Low tide
Results were that the Chthamalus
moved into the area formerly
occupied by Balanus. Thus,
interspecific competition makes the
realized niche of Chthamalus much
smaller than its fundamental niche
RESULTS
High tide
Figure 54.3 Can a species’ niche be influenced by interspecific competition?
Chthamalus
fundamental niche
Knowing that Balanus cannot survive high on the
rocks because it dries out during
low tides, how does its realized
niche compare with its fundamental
niche?
Ocean
Low tide

11. Character Displacement
Character displacement is a tendency for characteristics to be more divergent in populations that geographically overlapping rather than populations that are geographically separate
An example is variation in beak size between populations of two species of Galápagos finches
Where these two species live separately, their beak sizes are about the same---indicating they eat the same seeds.
Where they live together (on Santa Maria and San Cristobal) they must compete for food and overtime have evolved differences that allow for different beak sizes thus, eating different seeds.

12. Percentages of individuals in each size class
Fig. 54-4
G. fuliginosa
G. fortis
Beak
depth 60
Los Hermanos 40
G. fuliginosa,
allopatric 20 60
Daphne
Percentages of individuals in each size class 40
G. fortis,
allopatric 20
Figure 54.4 Character displacement: indirect evidence of past competition 60
Santa María, San Cristóbal
Sympatric
populations 40 20 8 10 12 14 16
Beak depth (mm)

13. Predation
Predation (+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey
Some feeding adaptations of predators are claws, teeth, fangs, stingers, and poison

14. Video: Seahorse Camouflage
Prey display various defensive adaptations
Behavioral defenses include hiding, fleeing, forming herds or schools, self-defense, and alarm calls
Animals also have morphological and physiological defense adaptations such as:
Cryptic coloration, or camouflage, makes prey difficult to spot
Canyon Tree Frog
Video: Seahorse Camouflage

15. Aposematic coloration bright warning coloration indicating a chemical defense (poison)
Batesian Mimicry —a palatable or harmless species mimics an unpalatable or harmful species
Poison Dart Frog
Hawkmoth larva
Green parrot snake

16. Müllerian mimicry-- two or more unpalatable species resemble each other. Each species gains an advantage because the more unpalatable prey there are, the more quickly predators adapt to avoid prey with that appearance. (This is also why warning colors seem to be universal: yellow, black, and red)
Cuckoo bee
Yellow jacket

17. Herbivory
Herbivory (+/– interaction) refers to an interaction in which an herbivore eats parts of a plant or alga
It has led to evolution of plant mechanical and chemical defenses and adaptations by herbivores

18. Symbiosis
Symbiosis is a relationship where two or more species live in direct and intimate contact with one another
Symbiotic relationships can be harmful, helpful or neutral

19. Parasitism
In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process
Endoparasites live within the body of their host (tapeworm); Ectoparasites live on the external surface of a host (leech)
Parasites may account for 1/3 of all the species on Earth
Many parasites have a complex life cycle involving a number of hosts (blood fluke—infects snails and humans)

20. Video: Clownfish and Anemone
Mutualism
Mutualism (+/+ interaction), is an interspecific interaction that benefits both species
A mutualism can be
Obligate, where one species cannot survive without the other (termites and digestive microbes)
Facultative, where both species can survive alone (acacia trees and ants)
Ants get food in the form of nectar
Trees get protection as ants keep the
area at the base of the tree free from
fungus, herbivores, debris, etc.
Video: Clownfish and Anemone

21. Commensalism
In commensalism (+/0 interaction), one species benefits and the other is apparently unaffected
Commensal interactions are hard to document in nature because any close association likely affects both species
Cattle egrets and water buffalo

22. Concept 54.2: Dominant and keystone species exert strong controls on community structure
Two fundamental features of community structure are species diversity and feeding relationships

23. Species Diversity
Species diversity of a community is the variety of organisms that make up the community
It has two 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

24. Which forest is more diverse?
Fig. 54-9
Which forest is more diverse?
Both of these forest communities have the same species richness because
they each have four species of trees. A B C D
Community 1
Community 2
Figure 54.9 Which forest is more diverse?
A: 25% B: 25% C: 25% D: 25%
A: 80% B: 5% C: 5% D: 10%
Their relative abundance, however is very different. Most people would
therefore describe Community 1 as being more diverse.

25. Video: Shark Eating a Seal
Trophic Structure
Trophic structure is the feeding relationships between organisms in a community
It is a key factor in community dynamics
Video: Shark Eating a Seal

26. Food chains link trophic levels from producers to top carnivores
Fig
Quaternary
consumers
Food chains link trophic levels from producers to top carnivores
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Figure Examples of terrestrial and marine food chains
Primary
consumers
Herbivore
Zooplankton
Primary
producers
Plant
Phytoplankton
A terrestrial food chain
A marine food chain

27. A food web is a branching food chain with complex trophic interactions
Fig
Humans
A food web is a branching food chain with complex trophic interactions
Smaller
toothed
whales
Baleen
whales
Sperm
whales
Crab-eater
seals
Leopard
seals
Elephant
seals
Birds
Fishes
Squids
Figure An antarctic marine food web
Carnivorous
plankton
Euphausids
(krill)
Copepods
Phyto-
plankton

28. Species may play a role at more than one trophic level This is true for many omnivores.
Sea nettle
Juvenile striped bass
Fish eggs
Zooplankton

29. Limits on Food Chain Length
Each food chain in a food web is usually only a few links long (about 5)
Two hypotheses attempt to explain food chain length: the energetic hypothesis and the dynamic stability hypothesis

30. The energetic hypothesis suggests that length is limited by inefficient energy transfer
Only about 10% of the energy stored in one level of a food chain is passed on to the next
The dynamic stability hypothesis proposes that long food chains are less stable than short ones
The longer the food chain, the more slowly top predators can recover from environmental shocks.
Most data support the energetic hypothesis

31. Species with a Large Impact
Certain species have a very large impact on community structure
Such species are highly abundant or play a pivotal role in community dynamics

32. Dominant Species
Dominant species are those that are most abundant or have the highest biomass
Biomass is the total mass of all individuals in a population
Dominant species exert powerful control over the occurrence and distribution of other species
Ex: Maple trees in Northeastern American Forests impact abiotic factors such as shading and soil.

33. How does a species become the dominant species?
One hypothesis suggests that dominant species are most competitive in exploiting resources
Another hypothesis is that they are most successful at avoiding predators
Invasive species lend support to the second hypothesis. They are typically introduced to a new environment by humans and often lack predators or disease, therefore attaining a high biomass.

34. The impact of a dominant species on a community can be seen when it is accidentally lost
Ex: American Chestnut: formally the dominant tree of Northeastern American forests. All were killed by a fungus between 1910 and Few mammals and birds were harmed by the loss, but 7 species of moths and butterflies went extinct.

35. Keystone Species
Keystone species exert strong control on a community by their ecological roles, or niches
In contrast to dominant species, they are not necessarily abundant in a community
Example: Pacific Sea Otter
Observation of sea otter populations and their predation shows how otters affect ocean communities

36. Otter number (% max. count) Grams per 0.25 m2 Number per 0.25 m2
Fig
100 80
Otter number
(% max. count) 60 40 20
When sea otters
are abundant, kelp
forests thrive
As the otters are
hunted by orcas,
their numbers
decline. Sea
urchin populations
rise, causing the
loss of kelp forests.
(a) Sea otter abundance
400
300
Grams per
0.25 m2
200
100
(b) Sea urchin biomass
Figure Sea otters as keystone predators in the North Pacific 10 8
Number per
0.25 m2 6 4 2
1972
1985
1989
1993
1997
Year
(c) Total kelp density
Food chain

37. Foundation Species (Ecosystem “Engineers”)
Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structure
For example, beaver dams can transform landscapes on a very large scale

38. Bottom-Up and Top-Down Controls
The bottom-up model of community organization proposes a unidirectional influence from lower to higher trophic levels
N V H P
In this case, presence or absence of mineral nutrients (N) determines the amount of vegetation (V) which controls the number of herbivores (H) and the number of predators (P)

39. The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above
N V H P
In this case, predators control herbivores, which in turn control the amount of vegetation and the uptake of nutrients
The effects of any manipulation thus move down the trophic structure as alternating +/- effects

40. Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control

41. Concept 54.3: Disturbance influences species diversity and composition
The nonequilibrium model of a community structure describe describes communities as constantly changing following disturbances
A disturbance is an event that changes a community, removes organisms from it, and alters resource availability
Fire is a significant disturbance in most terrestrial ecosystems
It is often a necessity in some communities

42. The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance. They do this by opening up habitats for occupation by less competitive species
High levels exclude many slow-growing species
Low levels allow dominant species to exclude less competitive species

43. The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance
The dominant species in Yellowstone is the Lodgepole pine. It releases seeds only after exposure to extreme heat. The fire is actually healthy for a forest like this.

44. (a) Soon after fire (b) One year after fire Fig. 54-21
Figure Recovery following a large-scale disturbance
(a) Soon after fire
(b) One year after fire

45. Ecological Succession
Ecological succession is the sequence of community and ecosystem changes after a disturbance
Primary succession occurs where no soil exists when succession begins. May take hundreds or thousands of years
Secondary succession begins in an area where soil remains after a disturbance

46. Early-arriving species and later-arriving species may be linked in one of three processes:
Early arrivals may facilitate appearance of later species by making the environment favorable
They may inhibit establishment of later species
They may tolerate later species but have no impact on their establishment

47. Retreating glaciers provide a valuable field-research opportunity for observing succession
Succession on the moraines in Glacier Bay, Alaska, follows a predictable pattern of change in vegetation and soil characteristics

48. Current Iceberg Front
Glacier Bay

49. 1941 1907 Pioneer stage, with fireweed dominant 1860 Glacier Bay
Fig
1941
1907 1
Pioneer stage, with
fireweed dominant 5 10 15
1860
Kilometers
Glacier
Bay
Figure Glacial retreat and primary succession at Glacier Bay, Alaska
Alaska
1760

50. Dryas stage—after 30 years Pioneer stage, with fireweed dominant 1860
Fig
1941
1907 2
Dryas stage—after 30 years 1
Pioneer stage, with
fireweed dominant 5 10 15
1860
Kilometers
Glacier
Bay
Figure Glacial retreat and primary succession at Glacier Bay, Alaska
Alaska
1760

51. Alder stage—after another few decades
Fig
1941
1907 2
Dryas stage—after 30 yrs. 1
Pioneer stage, with
fireweed dominant 5 10 15
1860
Kilometers
Glacier
Bay
Figure Glacial retreat and primary succession at Glacier Bay, Alaska
Alaska
1760 3
Alder stage—after another few
decades

52. Spruce stage—after two centuries Alder stage—after another few decades
Fig
1941
1907 2
Dryas stage—after 30 yrs. 1
Pioneer stage, with
fireweed dominant 5 10 15
1860
Kilometers
Glacier
Bay
Figure Glacial retreat and primary succession at Glacier Bay, Alaska
Alaska
1760 4
Spruce stage—after two centuries 3
Alder stage—after another few
decades

53. Succession is the result of changes induced by the vegetation itself
On the glacial moraines, vegetation lowers the soil pH and increases soil nitrogen content
As this occurs, more and more plant species can develop over time

54. 60 50 40 Soil nitrogen (g/m2) 30 20 10 Pioneer Dryas Alder Spruce
Fig 60 50 40
Soil nitrogen (g/m2) 30 20
Figure Changes in soil nitrogen content during succession at Glacier Bay 10
Pioneer
Dryas
Alder
Spruce
Successional stage

55. Human Disturbance
Humans have the greatest impact on biological communities worldwide
Human disturbance to communities usually reduces species diversity
Humans also prevent some naturally occurring disturbances, which can be important to community structure
These photos show disturbances on the
ocean floor before (top) and after (bottom)
deep-sea trawlers have passed.

56. Concept 54.4: Biogeographic factors affect community biodiversity
Latitude and area are two key factors that affect a community’s species diversity

57. Latitudinal Gradients
Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics
Two key factors in equatorial-polar gradients of species richness are probably evolutionary history and climate
The greater age of tropical environments may account for the greater species richness
Climate is likely the primary cause of the latitudinal gradient in biodiversity
Two main climatic factors correlated with biodiversity are solar energy and water availability
They can be considered together by measuring a community’s rate of evapotranspiration
Evapotranspiration is evaporation of water from soil plus transpiration of water from plants

58. Area Effects
The species-area curve quantifies the idea that, all other factors being equal, a larger geographic area has more species
A species-area curve of North American breeding birds supports this idea

59. Fig
1,000
100
Number of species 10
Figure Species-area curve for North American breeding birds 1
0.1 1 10
100
103
104
105
106
107
108
109
1010
Area (hectares)

60. Concept 54.5: Community ecology is useful for understanding pathogen life cycles and controlling human disease
Ecological communities are universally affected by pathogens, which include disease-causing microorganisms, viruses, viroids, and prions
Pathogens can alter community structure quickly and extensively

61. Pathogens and Community Structure
Pathogens can have dramatic effects on communities
For example, coral reef communities are being decimated by white-band disease
As the staghorn coral dies, algal species take its place, and the fish community is dominated by herbivores feeding on the algae.

62. Human activities are transporting pathogens around the world at unprecedented rates
Community ecology is needed to help study and combat them

63. Community Ecology and Zoonotic Diseases
Zoonotic pathogens have been transferred from other animals to humans
The transfer of pathogens can be direct or through an intermediate species called a vector
Many of today’s emerging human diseases are zoonotic

64. 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

65. Fig. 54-UN2

66. You should now be able to:
Distinguish between the following sets of terms: competition, predation, herbivory, symbiosis; fundamental and realized niche; cryptic and aposematic coloration; Batesian mimicry and Müllerian mimicry; parasitism, mutualism, and commensalism; endoparasites and ectoparasites; species richness and relative abundance; food chain and food web; primary and secondary succession

67. Define an ecological niche and explain the competitive exclusion principle in terms of the niche concept
Explain how dominant and keystone species exert strong control on community structure
Distinguish between bottom-up and top-down community organization
Describe and explain the intermediate disturbance hypothesis

68. Explain why species richness declines along an equatorial-polar gradient
Define zoonotic pathogens and explain, with an example, how they may be controlled