1. Chapter 54
Community Ecology

2. A biological community is an assemblage of populations of various species living close enough for potential interaction

3. Can affect the survival and reproduction of each species
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
Competition
Predation
Herbivory
Symbiosis (parasitism, mutualism, and commensalism)
Can affect the survival and reproduction of each species

4. Competition
(–/– interaction) occurs when species compete for a resource in short supply
Example?

5. Competitive exclusion principle
**The competitive exclusion principle states that two species competing for the same limiting resources cannot coexist in the same place**

6. Niche
Ecological niche- The total of a species’ use of biotic and abiotic resources
an organism’s ecological role
Ecologically similar species can ONLY coexist in a community if there are one or more significant differences in their niches
Why??????????????????????????

7. Resource partitioning
differentiation of ecological niches
enabling similar species to coexist in a community
Example?
Species can change niches when there is overlap with another species
Fundamental vs. realized 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
A. insolitus
A. aliniger
A. christophei
A. distichus
A. cybotes
A. etheridgei

9. EXPERIMENT High tide Chthamalus Chthamalus Balanus realized niche
Fig. 54-3
EXPERIMENT
High tide
Chthamalus
Chthamalus
realized niche
Balanus
Balanus
realized niche
Ocean
Low tide
RESULTS
High tide
Chthamalus
fundamental niche
Ocean
Low tide

10. Character Displacement
The tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species
In other words:
two species are MORE different from each other when they live in the same area than when they live apart from each other.

11. Predation
(+/– interaction) refers to interaction where one species, the predator, kills and eats the other, the prey
Predator adaptations include____
Prey adaptations include______

12. Cryptic coloration- camouflage, makes prey difficult to spot
Aposematic coloration- Bright warning coloration that animals with effective chemical defense often exhibit as a warning

13. Mimicry
Batesian mimicry- a harmless species mimics an unpalatable or harmful model
Müllerian mimicry, two or more unpalatable species resemble each other

14. (c) Batesian mimicry: A harmless species mimics a harmful one.
Fig. 54-5c
(c) Batesian mimicry: A harmless species mimics a harmful one.
Hawkmoth
larva
Green parrot snake

15. Müllerian mimicry: Two unpalatable species mimic each other.
Fig. 54-5d
(d)
Müllerian mimicry: Two unpalatable species
mimic each other.
Cuckoo bee
Yellow jacket

16. 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

17. Fig. 54-6

18. Symbiosis
Symbiosis is a relationship where two or more species live in direct and intimate contact with one another

19. 3
parasitism (+/– interaction) one organism derives nourishment from another organism, which is harmed in the process
the parasite
the host
What do you think is the difference btwn an endoparasite and an ectoparasite?

20. Why are many/most parasites NOT extremely deadly?

21. Mutualism
(+/+ interaction), is an interspecific interaction that benefits both species
Obligate, where one species cannot survive without the other
Facultative, where both species can survive alone

22. (a) Acacia tree and ants (genus Pseudomyrmex)
Fig. 54-7
(a) Acacia tree and ants (genus Pseudomyrmex)
(b) Area cleared by ants at the base of an acacia tree

23. 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

24. Fig. 54-8

25. 54.2: Dominant and keystone species exert strong controls on community structure
In general, a few species in a community exert strong control on that community’s structure

26. Species Diverstiy
Species diversity of a community is the variety of organisms that make up the community
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

27. A B C D Community 1 Community 2 A: 25% B: 25% C: 25% D: 25%
Fig. 54-9 A B C D
Community 1
Community 2
A: 25% B: 25% C: 25% D: 25%
A: 80% B: 5% C: 5% D: 10%

28. H = –[(pA ln pA) + (pB ln pB) + (pC ln pC) + …]
Two communities can have the same species richness but a different relative abundance
Diversity can be compared using a diversity index
Shannon diversity index (H):
H = –[(pA ln pA) + (pB ln pB) + (pC ln pC) + …]

29. Trophic Structure
Trophic structure is the feeding relationships between organisms in a community
Food chains link trophic levels from producers to top carnivores
REMEMBER WHICH WAY THE ARROWS GO!

30. A terrestrial food chain A marine food chain
Fig
Quaternary
consumers
Carnivore
Carnivore
Tertiary
consumers
Carnivore
Carnivore
Secondary
consumers
Carnivore
Carnivore
Primary
consumers
Herbivore
Zooplankton
Primary
producers
Plant
Phytoplankton
A terrestrial food chain
A marine food chain

31. Food Webs
A food web is a branching food chain with complex trophic interactions
Species may play a role at more than one trophic level
How is a food web diff from a food chain?

32. Fig
Humans
Smaller
toothed
whales
Baleen
whales
Sperm
whales
Crab-eater
seals
Leopard
seals
Elephant
seals
Birds
Fishes
Squids
Carnivorous
plankton
Euphausids
(krill)
Copepods
Phyto-
plankton

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

34. The energetic hypothesis suggests that length is limited by inefficient energy transfer
The dynamic stability hypothesis proposes that long food chains are less stable than short ones
Most data support the energetic hypothesis

35. 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

36. Dominant species
Species that are the most abundant or have the highest biomass
Maybe dom b/c most exploitive, competitive
Maybe b/c they avoid predation
Biomass is the total mass of all individuals in a population

37. Invasive species (what do you think that means?)
typically introduced to a new environment by humans, often lack predators or disease

38. Keystone species exert strong control on a community by their ecological roles, or niches
What’s “keystone” mean to you?
In contrast to dominant species, they are not necessarily abundant in a community
Often, we figure out “keystone” species b/c when they are removed… whoa changes!

39. Number of species present
Fig b
RESULTS 20 15
With Pisaster (control)
Number of species
present 10
Without Pisaster (experimental) 5
1963
’64
’65
’66
’67
’68
’69
’70
’71
’72
’73
Year

40. Otter number (% max. count) Grams per 0.25 m2 Number per 0.25 m2
Fig
100 80
Otter number
(% max. count) 60 40 20
(a) Sea otter abundance
400
300
Grams per
0.25 m2
200
100
(b) Sea urchin biomass 10 8
Number per
0.25 m2 6 4 2
1972
1985
1989
1993
1997
Year
(c) Total kelp density
Food chain

41. Foundation Species (Ecosystem “Engineers”)
Cause physical changes in the environment that affect community structure
beaver dams can transform landscapes on v. large scale

42. Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community
*small distinction btwn “facilitators” and “keystone”

43. Number of plant species
Fig b 8 6
Number of plant species 4 2
With Juncus
Without Juncus
(b)

44. How are these communities controlled?
Bottom-up model
Top-down model

45. Bottom up model
Unidirectional influence from lower to higher trophic levels
Presence or absence of mineral nutrients determines community structure, including abundance of primary producers

46. Top-down model Control comes from the trophic level above
also called the trophic cascade model
Predators control herbivores, which in turn control primary producers

47. Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control
Next slide: biomanipulation is used to fix the effects of pollution

48. Polluted State Restored State Fish Abundant Rare Zooplankton Rare
Fig. 54-UN1
Polluted State
Restored State
Fish
Abundant
Rare
Zooplankton
Rare
Abundant
Algae
Abundant
Rare

49. 54.3: Disturbance influences species diversity and composition
Decades ago, most ecologists favored the view that communities are in a state of equilibrium
Recent evidence of change has led to a nonequilibrium model, which describes communities as constantly changing after being buffeted by disturbances

50. Characterizing Disturbance
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

51. The intermediate disturbance hypothesis suggests that moderate levels of disturbance can foster greater diversity than either high or low levels of disturbance
High levels of disturbance exclude many slow-growing species
Low levels of disturbance allow dominant species to exclude less competitive species

52. Example (FYI)
The large-scale fire in Yellowstone National Park in 1988 demonstrated that communities can often respond very rapidly to a massive disturbance

53. Fig
(a) Soon after fire
(b) One year after fire

54. 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
Secondary succession begins in an area where soil remains after a disturbance

55. How does succession work?
Early arrivals may facilitate appearance of later species by making the environment favorable
Early may inhibit establishment of later species
Early may tolerate later species but have no impact on their establishment

56. Example following a retreating glacier
Why glacier??

57. 1941 1907 Dryas stage Pioneer stage, with fireweed dominant 1860
Fig
1941
1907 2
Dryas stage 1
Pioneer stage, with
fireweed dominant 5 10 15
1860
Kilometers
Glacier
Bay
Alaska
1760 4
Spruce stage 3
Alder stage

58. 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

59. Before….
Fig a

60. …after

61. 54.4: Biogeographic factors affect community biodiversity
Latitude and area are two key factors that affect a community’s species diversity
Species richness generally declines along an equatorial-polar gradient and is especially great in the tropics
(perhaps b/c OLDER species are found in those areas… meaning what??)