1. Review questions
A few members of a population have reached a favourable habitat with few predators and unlimited resources, but their population growth rate is slower than that of the parent population. What is a possible explanation for this situation?
The genetic makeup of these founders may be less favourable than that of the parent population.
The parent population may still be in the exponential part of its growth curve and not yet limited by density-dependent factors.
The Allee effect may be operating; there are not enough population members present for successful reproduction.
a, b, and c may apply.
This scenario would not happen.

2. Review questions
As N approaches K for a certain population, which of the following is predicted by the logistic equation?
The growth rate will not change.
The growth rate will approach zero.
The population will show an Allee effect.
The population will increase exponentially.
The carrying capacity of the environment will increase.

3. Review questions an abandoned field in Saskatchewan
In which of the following habitats would you expect to find the largest number of K-selected individuals?
an abandoned field in Saskatchewan
the sand dunes south of Lake Michigan
the rain forests of Brazil
south Florida after a hurricane
a newly emergent volcanic island

4. Review questions the high percentage of young people
Which of the following variables is (are) important in contributing to the rapid growth of human populations?
the high percentage of young people
the average age to first give birth
carrying capacity of the environment
only A and B
A, B, and C

5. Chapter 53
Community Ecology

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

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

8. Concept 53.1: A community’s interactions include competition, predation, herbivory, symbiosis, and disease
Populations are linked by interspecific interactions (symbiotic, parasitic, competition, disease.)
that affect the survival and reproduction of the species engaged in the interaction

9. Interspecific interactions
can have differing effects on the populations involved
Table 53.1

10. Competition Interspecific competition
occurs when species compete for a particular resource that is in short supply
Strong competition can lead to competitive exclusion Such as, weeds growing in a garden, compete for nutrients and sunlight.
the local elimination of one of the two competing species

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

12. Ecological Niches The ecological niche “job” or “profession”
is the total of an organism’s use of the biotic and abiotic resources in its environment
The niche concept allows restatement of the competitive exclusion principle
two species cannot coexist in a community if their niches are identical

13. Competitive exclusion
However, ecologically similar species can coexist in a community
if there are one or more significant difference in their niches
As a result of competition
a species’ fundamental niche may be different from its realized niche
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

14. 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

15. Character Displacement
In character displacement
there is a tendency for characteristics to be more divergent in sympatric (geographically overlapping) populations of two species than in allopatric (distinct) populations of the same two species
Sympatric means geographic overlapping.
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

16. Predation Predation refers to an interaction
where one species, the predator, kills and eats the other, the prey
Feeding adaptations of predators include
claws, teeth, fangs, stingers, and poison
Animals also display
a great variety of defensive adaptations

17. Cryptic colouration, or camouflage
makes prey difficult to spot
Figure 53.5

18. Aposematic colouration
warns predators to stay away from prey
Figure 53.6

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
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. Parasitism In parasitism, one organism, the parasite
derives its nourishment from another organism, its host, which is harmed in the process
Parasitism exerts substantial influence on populations
and the structure of communities
Parasitoidism, eggs being laid inside and organism and the larvae slowly eat them.

23. Disease The effects of disease on populations and communities
is similar to that of parasites
Pathogens, disease-causing agents
are typically bacteria, viruses, or protists

24. Mutualism Mutualistic symbiosis, or mutualism
is an interspecific interaction that benefits both species
Figure 53.9

25. Commensalism In commensalism
one species benefits and the other is not affected
Figure 53.10

26. Interspecific Interactions and Adaptation
Evidence for co-evolution
which involves reciprocal genetic change by interacting populations, is scarce
However, generalized adaptation of organisms to other organisms in their environment
is a fundamental feature of life
The Australian Chiloglottis orchid attracts male thynnine wasps by mimicking the appearance, the feel and the smell of the female wasp. The males go to the flowers to try and mate, abandoning the females.

27. 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

28. Species Diversity The species diversity of a community
is the variety of different kinds of organisms that make up the community
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

29. Two different communities
can have the same species richness, but a different relative abundance
A community with an even species abundance
is more diverse than one in which one or two species are abundant and the remainder rare
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

30. Trophic Structure Trophic structure
is the feeding relationships between organisms in a community
is a key factor in community dynamics

31. 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

32. 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

33. 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

34. Limits on Food Chain Length
Each food chain in a food web
is usually only a few links long (energy loss if too big).
There are two hypotheses
that attempt to explain food chain length

35. The energetic hypothesis suggests that the length of a food chain
is limited by the inefficiency of energy transfer along the chain
The dynamic stability hypothesis
proposes that long food chains are less stable than short ones

36. 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

37. 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

38. 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

39. 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

40. Keystone Species Keystone species
are not necessarily abundant in a community
exert strong control on a community by their ecological roles, or niches

41. 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

42. 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

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

44. Beaver dams can transform landscapes on a very large scale
Figure 53.18

45. 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

46. 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

47. 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

48. 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

49. 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

50. Concept 53.3: Disturbance influences species diversity and composition
Decades ago, most ecologists favoured the traditional view
that communities are in a state of equilibrium
However, a recent emphasis on change has led to a nonequilibrium model
which describes communities as constantly changing after being buffeted by disturbances

51. What Is Disturbance? A disturbance
is an event that changes a community
removes organisms from a community
alters resource availability

52. 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

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

54. 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

55. Human Disturbance
Humans
are the most widespread agents of disturbance

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

57. 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

58. Early-arriving species
may facilitate the appearance of later species by making the environment more favourable
may inhibit establishment of later species
may tolerate later species but have no impact on their establishment

59. 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

60. 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

61. 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

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

64. Climate
is likely the primary cause of the latitudinal gradient in biodiversity

65. 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

66. 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

67. 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

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

69. 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

70. 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