1. Community Ecology Community concepts Niche concepts
Emergent properties of communities
Species richness, evenness, diversity
Species interactions
Consequences of competition
Factors that influence biodiversity
Succession

2. Community Definition All populations within a defined geographic area
Most people only study a small subset of the community they study, often focusing on a small number of obviously interacting species

3. Community concepts
Clements viewed species in a community as interdependent superorganism with distinct ecotones
Gleason viewed species as responding idiosyncratically to environmental gradients
Most research supports Gleason’s individualistic model

4. Niche concepts An organism’s role in the environment
E.g., The “woodpecker niche”
G. Evelyn Hutchinson’s
N-dimensional hypervolume
Many one-dimensional niche axes combined

5. A single niche axis
e.g., pH, temperature, or soil moisture

6. Number of seeds consumed
Figure 55.2a
Another axis might include food eaten.
Number of seeds consumed
Figure 55.2a Niche Overlap Leads to Competition.
Seed size 6

7. Multiple axes can then be combined…
In two dimensions…

9. …or three … or more!

10. Emergent Properties of Communities
Species interactions, e.g., Competition, predation
Species richness - how many species are present?
Evenness - relative abundances of commonest to rarest species
Evenness and richness often combined into an index of species diversity

11. Species Evenness and Diversity
Top: diversity is high because evenness is high
Bottom: diversity is lower because one species dominates

12. Indices of Biodiversity
Simpson’s Index: D = 1/∑pi2
pi= proportion of sample composed of ith species
Result is on an arithmetic scale (Dmax=S)
Shannon’s Index H´ = ∑piln(pi)
Result is on a logarithmic scale [Hmax= ln(S)]
Take eH to get back to units of number of spp
Evenness: ED = D/Dmax , EH´ = H´/Hmax
A measure of how equally abundant species are

13. Interpretation fuzzy – something like “what you might encounter in a small sample”

14. Species diversity (Shannon index):
Problem with interpretation: there can be many ways to arrive at similar index value
Community 1
Community 2
Community 3 A B C
Species D
Figure Species Diversity Can Be Quantified. E F
Species richness:
More species 6
Fewer species
Evenness:
with lower
evenness 1
with greater
evenness
Species diversity (Shannon index):
=lower index
1.794
= higher index 14

15. Relative abundances of species
Sample any community and you will learn that most species are rare.
And a few abundant species account for most individuals in sample

16. Most abundant Species Rank Least abundant
Rank-abundance curve
Few common
Many rare
Most abundant Species Rank Least abundant

17. Species Interactions
Competition -- Each species harms (has a negative effect on the per capita vital rates of) the other (-/-)
Predation -- One species benefits, while the other is harmed (+/-)
Mutualism -- Both species benefit (+/+)
Commensalism -- One species benefits, other is unaffected (+/0)
Can modify exponential or logistic equation to model these interactions

18. “Red Queens”
Important difference between adaptation to the abiotic and the biotic environment
Abiotic environment does not respond to an organism’s adaptation to it (e.g., polar bear and cold)
Organisms, however, can adapt to changes in other organisms they interact with (e.g., cheetah and gazelle, host and pathogen, males and females)

19. Competition: What happens when two species niches overlap?
Resource use (e.g., seed size eaten)

20. Competitive Exclusion
Species reared alone tend to follow logistic growth pattern
But when reared together, one displaces the other through interspecific competition for shared limiting resources.

21. (a) Observation: asymmetric competition
Figure 55.3
(a) Observation: asymmetric competition
Paramecium aurelia
Number of individuals
Paramecium caudatum
Time (days)
(b) Explanation: competitive exclusion due to complete niche overlap
Figure 55.3 Competitive Exclusion in Paramecium.
Species 1: Strong competitor
Number of individuals using resource
Species 2: Weak competitor, driven to extinction
Niche (range of resources used) 21

22. But what if niches overlap less?
Resource use (e.g., seed size eaten)

23. What if niches overlap less?
Joe Connell (Tepper’s undergrad ecology prof!)
Observed young Chthamalus barnacles (an arthropod – remember?) settle throughout intertidal
But adults only found in upper intertidal

24. Figure
Why is the distribution of adult Chthamalus restricted to the upper intertidal zone?
Adult Chthamalus are outcompeted in the lower intertidal zone.
Adult Chthamalus do not thrive in the physical conditions of the lower intertidal zone.
Chthamalus in upper intertidal zone
1. Transplant rocks containing young Chthamalus to lower intertidal zone.
Mean tide level
Semibalanus in lower intertidal zone
2. Let Semibalanus colonize the rocks.
Figure Experimental Evidence of Competition.
3. Remove Semibalanus from half of each rock. Monitor survival of Chthamalus on both sides.
Chthamalus 
Chthamalus
Semibalanus 24

25. Percent survival of Chthamalus Competitor present Competitor absent
Figure
Chthamalus will survive better in the absence of Semibalanus.
Chthamalus survival will be low and the same in the presence or absence of Semibalanus.
Percent survival
of Chthamalus
Competitor present
Competitor absent
Figure Experimental Evidence of Competition.
Chthamalus do not occur in the lower intertidal zone because they are outcompeted by Semibalanus.
Fundamental Niche: Niche a species COULD use, based on abiotic environment
Realized Niche: Niche a species DOES use in the presence of other species 25

26. Number of individuals using resource Niche (range of resources used)
Figure 55.4
Species 1 (strong competitor)
Species 2 (weak competitor)
Fundamental niche
Number of individuals using resource
Realized niche
Figure 55.4 Fundamental Niches Are Broader than Realized Niches.
Niche (range of resources used) 26

27. Alternatively, species may coexist, but evolve to differ in resource use
Character displacement – a shift in phenotype of two species when they coexist (compared to regions of allopatry)
Competition results in divergent selection

28. Evidence for Character Displacement (Peter & Rosemary Grant)
Galapagos Finches
Islands where  sympatric
Islands where allopatric {

29. Resource partitioning: the ghost of competition past?
Robert MacArthur − Congeneric warblers forage in different parts of trees

30. Gradients in species richness
Given problem with diversity indices, most unequivocal measure of diversity is richness
Why do we find many species in some places and few species in others?
Still not well understood, but some general patterns and mechanisms

31. Latitude In general, more species in the tropics
Is this because environment is more benign?
Primary causal factor must be abiotic

33. Equilibrium Theory of Island Biogeography
Robert MacArthur and E. O. Wilson
Number of species on islands reflects a balance between colonization (immigration) and extinction

34. Both colonization and extinction vary with
Island size
Distance from source of colonists

35. The Distance Effect
Richness tends to be low on islands and other isolated regions
As distance from the mainland increases, richness decreases

36. Species-area curves
S=cAz
Or:
log S = log c+z log A

37. Habitat diversity
This example begs the question (why is there more complex vegetation?)
But similar pattern with abiotic (topographic or climatic) diversity

38. Intermediate Disturbance Hypothesis (Joe Connell-again!)
Moderate intensity or frequency of disturbance results in maximum richness

39. Intermediate disturbance mechanism related to succesion
Succession = change in species composition at a site over time
Members of late successional stages eventually displace members of earlier stages until “climax” is achieved
Primary succession – on bare rock
Secondary succession – on existing soil

40. Succession and intermediate disturbance
So when disturbance is moderate in frequency or intensity, either:
Creates mosaic of different successional stages (different ages since disturbance) Or
Maintains an intermediate successional stage, in which pioneers not all gone, mid-successional species established, and climax species arriving

41. Prescribed burns: disturbance as a management tool
Tallgrass prairie, savanna, some coniferous forests persist because of fire
Fires from lightning common in dry seasons, but humans have intentionally burned for millennia
European settlers historically prevented fire
Fires release nutrients, remove shade, stimulate release and germination of seeds
Question: how are animals impacted by fire?

42. Community Ecology Community concepts Niche concepts
Emergent properties of communities
Species interactions
Consequences of competition
Factors that influence biodiversity
Succession