1. Chapter 54
Community Ecology

2. Overview: A Sense of Community
A biological community is an assemblage of populations of various species living close enough for potential interaction
Figure 54.1 How many interactions between species are occurring in this scene?
Hornworm caterpillar (herbivore) and its preferred food, a tomato plant (producer).
Parasitic wasp – lays eggs inside caterpillar, and the larvae that emerge from the eggs feed on the caterpillar’s tissues.
The larvae then develop into adult wasps inside the white cocoons on their host’s back.
This interaction will eventually kill the caterpillar.

3. Community Interactions
Interactions between and within populations influence patterns of species distribution and abundance:
Competition, parasitism, predation and commensalism can affect population dynamics.
Relationships among interacting populations can be characterized by positive and negative effects.
Many complex symbiotic relationships exist in an ecosystem, and feedback control systems play a role in the functioning of those ecosystems.

4. Concept 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
Examples are competition, predation, herbivory, and symbiosis (parasitism, mutualism, and commensalism)
Interspecific interactions can affect the survival and reproduction of each species, and the effects can be summarized as positive (+), negative (–), or no effect (0)
Interspecific competition (–/– interaction) occurs when species compete for a resource in short supply
This is competition between two different species.

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

6. Ecological Niches
The total of a species’ use of biotic and abiotic resources is called the species’ ecological niche
An ecological niche can also be thought of as an organism’s ecological role
Ecologically similar species can coexist in a community if there are one or more significant differences in their niches

7. 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
Cooperative behavior within or between populations contributes to the survival of the populations:
Resource partitioning is differentiation of ecological niches, enabling similar species to coexist in a community
A. insolitus
A. aliniger
A. christophei
A. distichus
A. cybotes
A. etheridgei

8. 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
Figure 54.3 Can a species’ niche be influenced by interspecific competition?
As a result of competition, a species’ fundamental niche may differ from its realized niche
Chthamalus
fundamental niche
Ocean
Low tide

9. 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
Character displacement is a tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species…this should make sense…why?
An example is variation in beak size between populations of two species of Galápagos finches 60
Santa María, San Cristóbal
Sympatric
populations 40 20 8 10 12 14 16
Beak depth (mm)

10. 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
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
Cryptic coloration, or camouflage, makes prey difficult to spot

11. (c) Batesian mimicry: A harmless species mimics a harmful one.
Fig. 54-5
(a)
Cryptic
coloration
Canyon tree frog
(b)
Aposematic
coloration
Poison dart frog
(c) Batesian mimicry: A harmless species mimics a harmful one.
Hawkmoth
larva
Figure 54.5 Examples of defensive coloration in animals
Cryptic coloration: blend into the environment / camouflage makes prey difficult to spot.
Animals with effective chemical defense often exhibit bright warning coloration, called aposematic coloration. Predators are particularly cautious in dealing with prey that display such coloration
In some cases, a prey species may gain significant protection by mimicking the appearance of another species. In Batesian mimicry, a palatable or harmless species mimics an unpalatable or harmful model.
In Müllerian mimicry, two or more unpalatable species resemble each other.
(d)
Müllerian mimicry: Two unpalatable species
mimic each other.
Cuckoo bee
Green parrot snake
Yellow jacket

12. 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

13. Symbiosis
Symbiosis is a relationship where two or more species live in direct and intimate contact with one another
Parasitism
Mutualism
Commensalism
In parasitism (+/– interaction), one organism, the parasite, derives nourishment from another organism, its host, which is harmed in the process. Parasites that live within the body of their host are called endoparasites; parasites that live on the external surface of a host are ectoparasites
Mutualistic symbiosis, or mutualism (+/+ interaction), is an interspecific interaction that benefits both species. A mutualism can be
Obligate, where one species cannot survive without the other
Facultative, where both species can survive alone
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

14. Fig. 54-UN2
All biological systems from cells and organisms to populations, communities and ecosystems are affected by complex biotic and abiotic interactions involving exchange of matter and free energy.
Cooperative behavior within or between populations contributes to the survival of the populations: Mutualism: lichens, bacteria in digestive tracts, mycorrhizae.

15. Concept 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
Two fundamental features of community structure are species diversity and feeding relationships

16. It has two components: species richness and relative abundance
Species Diversity
Species diversity of a community is the variety of organisms that make up the community
It has two components: species richness and relative abundance
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
The stability of populations, communities and ecosystems is affected by interactions with biotic and abiotic factors. The structure of a community is measured in terms of species richness/diversity.

17. 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
Figure 54.9 Which forest is more diverse?
Two communities can have the same species richness but a different relative abundance
A: 25% B: 25% C: 25% D: 25%
A: 80% B: 5% C: 5% D: 10%

18. Trophic Structure
Trophic structure is the feeding relationships between organisms in a community
It is a key factor in community dynamics
Food chains link trophic levels from producers to top carnivores
A food web is a branching food chain with complex trophic interactions

19. A terrestrial food chain A marine food chain
Fig
Quaternary
consumers
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

20. Fig
Humans
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

21. Sea nettle Juvenile striped bass Fish larvae Fish eggs Zooplankton
Fig
Sea nettle
Juvenile striped bass
Figure Partial food web for the Chesapeake Bay estuary on the U.S. Atlantic coast
Species may play a role at more than one trophic level
Food webs can be simplified by isolating a portion of a community that interacts very little with the rest of the community
Fish larvae
Fish eggs
Zooplankton

22. 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 and the dynamic stability hypothesis
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

23. Number of trophic links
Fig 5 4 3
Number of trophic links 2 1
High (control):
natural rate of
litter fall
Medium: 1/10
natural rate
Low: 1/100
natural rate
Figure Test of the energetic hypothesis for the restriction of food chain length.
Researchers manipulated the productivity of tree-hole communities by providing leaf litter input at three levels.
Reducing energy input reduced food chain length, a result consistent with the energetic hypothesis.
Productivity

24. 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
Field studies of sea stars exhibit their role as a keystone species in intertidal communities

25. 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
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, typically introduced to a new environment by humans, often lack predators or disease

26. Keystone Species
A population of organisms has properties that are different from those of the individuals that make up the population.
The cooperation and competition between individuals contributes to these different properties.
Species-specific and environmental catastrophes affect species distribution and abundance, and sometimes these can be severe.
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

27. EXPERIMENT RESULTS Fig. 54-15
Figure Is Pisaster ochraceus a keystone predator?
Conclusions: acts as a keystone species, exerting an influence on the community that is not reflected in its abundance. 20 15
With Pisaster (control)
Number of species
present 10 5
Without Pisaster (experimental)
1963
’64
’65
’66
’67
’68
’69
’70
’71
’72
’73
Year

28. 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
Figure Sea otters as keystone predators in the North Pacific
Observation of sea otter populations and their predation shows how otters affect ocean communities
Sea otters feed on sea urchins, and sea urchins feed mainly on kelp. In areas where sea otters are abundant, sea urchins are rare and kelp forests are well developed. Where sea otters are rare, sea urchins are common and kelp is almost absent.
Over the last 20 years, orcas have been preying on otters as the whales’ usual prey has declined. As a result, sea otter populations have declined and the loss of this keystone species has allowed sea urchin populations to increase, resulting in a loss of kelp forests. 10 8
Number per
0.25 m2 6 4 2
1972
1985
1989
1993
1997
Year
(c) Total kelp density
Food chain

29. 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

30. Fig
Figure Beavers as ecosystem “engineers”

31. Number of plant species
Fig 8 6
Number of plant species 4 2
Figure Facilitation by black rush (Juncus gerardi) in New England salt marshes
Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community
(a)
Salt marsh with Juncus
(foreground)
With Juncus
Without Juncus
(b)

32. 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, presence or absence of mineral nutrients determines community structure, including abundance of primary producers
The top-down model, also called the trophic cascade model, proposes that control comes from the trophic level above
In this case, predators control herbivores, which in turn control primary producers

33. (number of individuals
Fig
RESULTS
Control plots
300
Warmed plots
(number of individuals
Nematode density
per kg soil)
200
100
Figure Are soil nematode communities in Antarctica controlled by bottom-up or top-down factors?
Predator: E. antarcticus
Prey: S. lindsayae
Conclusion: the prey species’ increase in density as the predator density declined suggests that this soil nematode community is controlled by top-down factors.
Long-term experimental studies have shown that communities vary in their relative degree of bottom-up to top-down control
E. antarcticus
S. lindsayae

34. Polluted State Restored State Fish Abundant Rare Zooplankton Rare
Fig. 54-UN1
Polluted State
Restored State
Fish
Abundant
Rare
Zooplankton
Rare
Abundant
Pollution can affect community dynamics
Biomanipulation can help restore polluted communities
Algae
Abundant
Rare

35. Concept 54.3: Disturbance influences species diversity and composition.
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

36. Log intensity of disturbance
Fig 35 30 25
Number of taxa 20 15
Figure Testing the intermediate disturbance hypothesis
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
Conclusions: number of invertebrate taxa peaks when the intensity of flooding was at intermediate levels. 10
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
Log intensity of disturbance

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

38. 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

39. Primary Succession

40. Secondary Succession

41. Ecological Succession
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

42. 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

43. Fig
Figure Disturbance of the ocean floor by trawling

44. Island Equilibrium Model
Species richness on islands depends on island size, distance from the mainland, immigration, and extinction
The equilibrium model of island biogeography maintains that species richness on an ecological island levels off at a dynamic equilibrium point

45. 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

46. Fig
Figure White-band disease visible on a coral
Pathogens can have dramatic effects on communities
For example, coral reef communities are being decimated by white-band disease
Human activities are transporting pathogens around the world at unprecedented rates
Community ecology is needed to help study and combat them

47. 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