Community ecology examines the structure and dynamics of species interactions within biological communities. Key factors influencing community structure include species richness and abundance, interactions like predation and competition, and environmental conditions. Ecological succession describes how communities change over time following a disturbance like fire, with pioneer species colonizing first and later species competing until a stable climax community develops.

1. Community ecology
Outline:
• Community structure: attributes
• Factors influencing the structure of communities
• Community dynamics

3. Community attributes
• Numbers of species
• Relative abundance of species
• Nature of species interactions (food webs)
• Physical structure

4. Community structure
• Species richness (# of species within
community)
10

5. Community structure
• Relative abundance (% each species
contributes to the total number of
individuals)

6. Simpson’s diversity index
• D = 0 - 1
– 0: high diversity
– 1: low diversity
• Stand one (Table 16.1): D=0.13
• Stand two (Table 16.2): D=0.36

7. Functional groups
• Feeding level
• Exploitation of common resources (guilds)
• Photosynthetic pathway
• Shade tolerance
• Life history

8. Physical structure

9. Zonation

10. Association
• Relatively consistent species composition
• Uniform general appearance
• Distribution that is characteristic of a
particular habitat

11. Factors controlling community structure

12. Fundamental niche

13. Species interactions

14. • Interspecific interactions

15. 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

16. 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

17. Parasitism
• In parasitism, one organism, the parasite
– Derives its nourishment from another organism,
its host, which is harmed in the process

18. 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

19. Mutualism
• Mutualistic symbiosis, or mutualism
– Is an interspecific interaction that benefits both
species

20. Commensalism
One species benefits and the other is not affected
• Commensal interactions have been difficult
to document in nature
– Because any close association between
species likely affects both species

21. competition

22. Species Diversity
• The species diversity of a community
– Is the variety of different kinds of organisms that
make up the community
– Has two components

23. • 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. 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

25. Keystone species
A keystone species is a species which has a disproportionately
large effect on its natural environment relative to its abundance

26. Keystone Species
– Are not necessarily abundant in a community
– Exert strong control on a community by their
ecological roles, or niches

27. Ecosystem “Engineers”
(Foundation Species)
• Some organisms exert their influence
– By causing physical changes in the environment
that affect community structure
• Example:
Beaver dams: Can transform landscapes on a very large scale

28. Top-down vs. bottom-up control
• The bottom-up effect means that a lower trophic level in the biological
network affects the community structure of higher trophic levels by means
of resource restriction.
• The top-down effect refers to a higher trophic level influences the
community structure of a lower trophic level through predation

29. Bottom-Up 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

30. Top-Down Controls
• 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

31. Stress tolerance and competition

32. Environmental heterogeneity

33. Environmental quality

34. Community stability
Types of stability:
1. Resilient community: returns to former
state after disturbance
2. Resistant community: changes little in
response to disturbance

35. What Is Disturbance?
• A disturbance
– Is an event that changes a community
– Removes organisms from a community
– Alters resource availability

36. • 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.

37. Human Disturbance
• Humans
– Are the most widespread agents of disturbance
• Human disturbance to communities
– Usually reduces species diversity
• Humans also prevent some naturally
occurring disturbances
– Which can be important to community structure

38. Community dynamics:
Succession
Ecological succession is the process of change in
the species structure of an ecological community over time.

39. Ecological Succession
• Ecological succession
– Is the sequence of community and ecosystem
changes after a disturbance

40. • Primary succession
– Occurs where no soil exists when succession
begins
• Secondary succession
– Begins in an area where soil remains after a
disturbance

41. Pioneer species
Primary succession
• Primary succession is the beginning step of ecological
succession after an extreme disturbance, which usually occurs in an
environment devoid of vegetation and other organisms. These
environments are typically lacking in soil, as disturbances like lava
flow or retreating glaciers shred the environment of nutrients.
Secondary succession
Secondary succession is the secondary ecological succession of a
plant's life. As opposed to the first, primary succession, secondary
succession is a process started by an event (e.g. forest
fire, harvesting, hurricane, etc.) that reduces an already
established ecosystem (e.g. a forest or a wheat field) to a smaller
population of species, and as such secondary succession occurs on
preexisting soil.
Pioneer species are hardy species that are the first
to colonize barren environments or previously biodiverse steady-
state ecosystems that have been disrupted, such as by wildfire.

42. 1. Nudation: Succession begins with the development of a bare site,
called Nudation (disturbance).
2. Migration: refers to arrival of propagules
3. Ecesis: involves establishment and initial growth of vegetation.
4. Competition: as vegetation becomes well established, grows, and
spreads, various species begin to compete for space, light and
nutrients.
5. Reaction: during this phase autogenic changes such as the buildup
of humus affect the habitat, and one plant community replaces
another.
6. Stabilization: a supposedly stable climax community forms.
Mechanisms of succession

43. Bark
beetle/wood-
boring beetle
Fungi/bacteria
Predatory
insects
Moss and
lichen
Invertebrates/
mice/
salamanders
Succession of heterotrophs

45. Changes over geologic time