1. Community Ecology
Chapters 54
PART 2

2. Community Ecology (Ch 54)
Concept 4: Community Ecology – Analyzing the interactions and relationships within and between species and the effects of environmental factors on species diversity and composition.
Community Ecology (Ch 54)
The difference between a fundamental niche and a realized niche
The role of competitive exclusion in interspecific competition
The symbiotic relationships of parasitism, mutualism, and commensalism
The impact of keystone species on community structure
The difference between primary and secondary succession

3. Community Structure
How do the following principles relate to each other?
What are some examples?
Trophic Structure
Food Chain
Food Web
Species Diversity
Dominant Species
Invasive Species
Keystone Species
Foundation Species

4. Trophic Structure
The feeding relationships between organisms in a community
Key factor in community dynamics!
Illustrated by:
food chains - link trophic levels from producers to top carnivores
food webs - branching food chain with complex trophic interactions

5. Trophic St.
Food chain!

6. Humans
Smaller
toothed
whales
Baleen
Sperm
Elephant
seals
Leopard
Crab-eater
Birds
Fishes
Squids
Carnivorous
plankton
Copepods
Euphausids
(krill)
Phyto-
Trophic St.
Food web!
Species may play a role at more than one trophic level

7. Trophic Structure
Food webs can be simplified by isolating a portion of the community that interacts very little with the rest of the community
Sea nettle
Fish larvae
Juvenile striped bass
Fish eggs
Zooplankton

8. Trophic Structure How is the food chain limited?
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 suggests that length is limited by inefficient energy transfer
The dynamic stability hypothesis proposes that long food chains are less stable than short ones

9. Try This
Which of the following organisms and trophic levels is mismatched?
A) algae – producer
B) phytoplankton – primary consumer
C) fungi – decomposer
D) carnivorous fish larvae – secondary consumer
E) eagle – tertiary or quaternary consumer
1) B – phytoplankton is a primary producer

10. Try This
Which of the following organisms and trophic levels is mismatched?
A) algae – producer
B) phytoplankton – primary consumer
C) fungi – decomposer
D) carnivorous fish larvae – secondary consumer
E) eagle – tertiary or quaternary consumer
1) B – phytoplankton is a primary producer

11. Species Impact
Certain species have a very large impact on community structure
Dominant species are those that are most abundant or have the highest biomass (total mass of all individuals in a population).
Ex) sugar maple
Exert powerful control over the occurrence and distribution of other species
Two hypotheses:
1) Most competitive in exploiting resources
2) Most successful at avoiding predators
Invasive species, typically introduced to a new environment by humans, often lack predators or disease.
Ex) Atlantic salmon in pacific waters

12. Species Impact
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
Examples to follow…
Foundation species (ecosystem “engineers”) cause physical changes in the environment that affect community structure
Ex) beaver dams can transform landscapes on a very large scale
Some foundation species act as facilitators that have positive effects on survival and reproduction of some other species in the community

13. Focus: Keystone Species
Field studies of sea stars exhibit their role as a keystone species in intertidal communities
With Pisaster (control)
Without Pisaster (experimental)
Number of species
present
Year 20 15 10 5
1963
’64
’65
’66
’67
’68
’69
’70
’71
’72
’73
RESULTS

14. Focus: Keystone Species
Observation of sea otter populations and their predation shows how otters affect ocean communities

15. Focus: Keystone Species
Video clip:
Bees as keystone species (8 min)

16. Species Diversity The variety of organisms that make up the community
Two components:
1) Species richness is the total number of different species in the community
2) Relative abundance is the proportion each species represents of the total individuals in the community

17. Ecological Succession
The change in the composition of species over time
Traditional view:
One community with certain species is gradually and predictably replaced by another community of different species
Species diversity and total biomass increase
Climax community is attained at final stage… persists until destroyed by catastrophe (ex: fire)

18. Ecological Succession
Pioneer species: the plants and animals first to colonize a newly exposed habitat
Typically opportunistic (r-selected), and can tolerate harsh conditions (ie: intense sunlight, arid climate, nutrient-deficient soil)
Ex) plants whose roots support nitrogen-fixing bacteria (mutualism)
Gradually, K-selected species replace r-selected species

19. Ecological Succession
Two kinds: Primary and Secondary Seccession
Primary succession: substrate never had life before!
Ex) Rock or lava
1) lichens
2) bacteria, protists, mosses, and fungi
3) insects and other arthropods
4) nitrogen fixing bacteria
5) grasses, weeds, herbs (r)
6) perennial shrubs and trees (K)

21. Ecological Succession
Two kinds: Primary and Secondary Seccession
Secondary succession: substrate where community completely or partially destroyed by a disturbance (flood, deforestation..)
Ex) lakes and ponds
1) body of water
2) marsh land
3) meadow
4) climax community of native vegetation (ie: part of the forest)
If a community returns to same structure prior to the disturbance, that community is resilient (one type of ecological stability)
Some small scale disturbances may enhance environmental patchiness and help maintain species diversity. Human prevention of these small scale disturbances such as small fires, may lead to large scale disturbances, such as large, destructive fires.

24. Try This
You survey two plots of trees. Plot 1 has six species of trees and 95% of all trees belong to just one species. Plot 2 has five different species of trees, each representing ~20%. How would you describe plot 2 compared to plot 1 in terms of tree species diversity, richness, and abundance?
During succession, inhibition by early species may have what consequence?
Plot 2 has a lower richness (lower number of species), exhibits relatively equal abundance of species, and therefore has a higher species diversity
Slow down the successful colonization of other species... Will take longer to reach climax community

25. Try This! (Answer)
Plot 2 has a lower richness (lower number of species), but exhibits relatively equal abundance of species, and therefore has a higher species diversity
Slow down the successful colonization of other species... Will take longer to reach climax community

26. Community Ecology (Ch 54)
Concept 4: Community Ecology – Analyzing the interactions and relationships within and between species and the effects of environmental factors on species diversity and composition.
Community Ecology (Ch 54)
The difference between a fundamental niche and a realized niche
The role of competitive exclusion in interspecific competition
The symbiotic relationships of parasitism, mutualism, and commensalism
The impact of keystone species on community structure
The difference between primary and secondary succession