Community Ecology

1
Chapter 53

• Community Ecology

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

2

• The various animals and plants surrounding this
  watering hole are all members of a savanna
  community in southern Africa

3
Interspecific Interaction

• A communitys interactions
• Competition
• Predation
• Herbivory
• Symbiosis
• Disease
• Populations are linked by interspecific
  interactions that affect the survival and
  reproduction of the species engaged in the
  interaction

4
Interspecific Interaction
5
Competition

• Interspecific competition occurs when species
  compete for a particular resource that is in
  short supply
• Strong competition can lead to competitive
  exclusion

6
The Competitive Exclusion Principle

• The competitive exclusion principle
• States that two species competing for the same
  limiting resources cannot coexist in the same
  place

7
Ecological Niches

• The ecological niche is the total of an
  organisms 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

8

• However, ecologically similar species can coexist
  in a community
• If there are one or more significant difference
  in their niches

9
Resource Partitioning

• Resource partitioning is the differentiation of
  niches
• That enables similar species to coexist in a
  community

10
Character Displacement

• In character displacement
• characteristics are more divergent

11
Predation

• Predation refers to an interaction
• Where one species, the predator, kills and eats
  the other, the prey

12

• Feeding adaptations of predators include
• Claws, teeth, fangs, stingers, and poison
• Animals also display
• A great variety of defensive adaptations

13

• Cryptic coloration, or camouflage
• Makes prey difficult to spot

14

• Aposematic coloration
• Warns predators to stay away from prey

15
Mimicry

• In some cases, one prey species
• May gain significant protection by mimicking the
  appearance of another

16

• In Batesian mimicry
• A palatable or harmless species mimics an
  unpalatable or harmful model

17

• In Müllerian mimicry
• Two or more unpalatable species resemble each
  other

18
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

19
Parasitism

• 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
• The effects of disease on populations and
  communities is similar to that of parasites
• Pathogens, disease-causing agents
• Bacteria
• Viruses
• Protists

21
Mutualism

• Mutualistic symbiosis, or mutualism
• Is an interspecific interaction that benefits
  both species

22
Commensalism

• In commensalism
• One species benefits and the other is not affected

23
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

24

• 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

25

• Two different communities
• Can have the same species richness, but a
  different relative abundance

26
Trophic Structure

• Trophic structure
• Is the feeding relationships between organisms in
  a community
• Is a key factor in community dynamics

27

• Trophic structure
• Is the feeding relationships between organisms in
  a community
• Food chains
• Link the trophic levels from producers to top
  carnivores

28
Food Webs

• A food web

• Is a branching food chain with complex trophic
  interactions

29
Limits on Food Chain Length

• Each food chain in a food web
• Is usually only a few links long
• 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

30

• Most of the available data
• Support the energetic hypothesis

31
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

32
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

33
Keystone Species

• Keystone species
• Are not necessarily abundant in a community
• Exert strong control on a community by their
  ecological roles, or niches

34
Ecosystem Engineers (Foundation Species)

• Some organisms exert their influence
• By causing physical changes in the environment
  that affect community structure
• Beaver dams can transform landscapes on a very
  large scale

35
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

36

• 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

37

• Long-term experiment studies have shown
• That communities can shift periodically from
  bottom-up to top-down

38
What Is Disturbance?

• A disturbance
• Is an event that changes a community
• Removes organisms from a community
• Alters resource availability

39

• Fire
• Is a significant disturbance in most terrestrial
  ecosystems
• Is often a necessity in some communities

40

• The intermediate disturbance hypothesis
• Suggests that moderate levels of disturbance can
  foster higher species diversity than low levels
  of disturbance

41

• The large-scale fire in Yellowstone National Park
  in 1988
• Demonstrated that communities can often respond
  very rapidly to a massive disturbance

Figure 53.22a, b
42
Ecological Succession

• Ecological succession
• Is the sequence of community and ecosystem
  changes after a disturbance

43

• Two key factors correlated with a communitys
  species diversity
• Geographic location
• Size
• Climate
• Is likely the primary cause of the latitudinal
  gradient in biodiversity

44

• The two main climatic factors correlated with
  biodiversity
• Are solar energy input and water availability