1. Introduction to community ecology: lecture topics
How do we describe communities?
How do we characterize, compare, and ordinate (order, organize) communities?
What indices can we use to quantify species diversity of a community, and what are their strengths and weaknesses?
What, exactly, is a community?
Is it something real, easily delineated and highly structured biologically?
Or is it simply a collection of independent species, an arbitrary assemblage?
How can we use this information about communities?

2. Communities are assemblages of species--here fairly distinctive (California coast) (from Rickleffs 2001)
Chaparral
Grassland
Oak woodland

3. Let’s look at bird species in 3 habitats: questions is how to compare habitats?
(Data from tin-mined areas in Indonesia; from Stiling text, Table 16.1)

4. Ecologists have devised a variety of indices to species diversity of sites
One can use abundance of individuals, or other indices of “importance” (such as biomass, energy consumed)
Species richness = no. species (simplest index!)
Dominance indices reflect concentration of importance by a few species, and thus emphasize the most abundant (or important) species
Simpson’s index: Ds = Si=1 to s{ni*(ni-1)/(N*(N-1))}
ni = number of individuals in species i, N = total number of individuals in sample, s = no. species
Diversity indices like Shannon’s emphasize rare spp.
Shannon’s index: Hs = -Si=1 to s{pi*loge(pi)}
pi = what proportion of sample is species i, i.e., ni/N

5. Other ways to quantify species diversity...
Evenness or equity of distribution = Actual diversity/maximum possible diversity
Using Shannon’s index, Evenness = Hs/Hmax
Hmax = -loge(1/S), where S = number of species in sample (this formula is derived easily from Hs index)
(One can also calculate Hmax simply by apportioning the species evenly--dividing total individuals by total number of species--and then calculating Hs)
Evenness expresses the extent to which resources (or energy, etc.) are divided equitably among species in the community

6. Now we can ask, for the actual communities given in this extended example, what are values of these various indices for species diversity?

7. What do diversity indices tell us about the bird communities in the example?
First, the most species-rich habitats, by far, are the unmined sites (native vegetation)
What a surprise?
This pattern is partly function of the number of individuals (diversity is often related to productivity!)
Second, the mined site shows the greatest dominance by a few species (presumably those species tolerant of the disturbance!)
The greatest “species diversity” (Shannon index) is in the unmined site
This diversity is not due to evenness
Therefore it must be due to the richness component
Note: Rank order of communities differs by index!

8. We can also ask what are the similarity indices among the three habitats containing Indonesian bird species?
Here, we use the Jaccard Coefficient (Cj), in a matrix of between-habitat comparisons
Cj = a/(a+b+c), where a = no. species shared between two samples, b = no. species in sample 1 but not 2, and c = no. species in sample 2 but not 1
This analysis shows mined (unrestored) site and 2nd-yr. Restored sites are most similar, unmined site most distinctive

9. One application of similarity (or distance) indices such as we’ve calculated is to “ordinate” the sites (arrange graphically):
Unmined (native) site
“Distance” = 1- similarity = 0.68
0.61
Mined (unrestored) site
0.30
2nd-yr. Restored site
Such an analysis suggests here that the 2nd-yr. restored site has moved in the direction of the unmined site (a step in right direction), but not very far after only two years.

10. Applicability of some of these species diversity indices?
Conservation biologists, for example, tend to be interested more in species richness, rather than species diversity components
One can weight the species in an assemblage when calculating overall community metrics
Weight by rarity of species
Weight by taxonomic (genetic) distinctiveness of species

11. Yet another way that ecologists have looked at communities is in terms of Species-abundance curves
Method: Plot species abundance on logarithmic y-axis, versus rank on x-axis
Best known types of species-abundance curve:
Lognormal model--arises in diverse communities with many ecological processes influencing abundance
Broken stick model--arises when species are fairly equitable in abundance & don’t dominate each other, e.g., due to simultaneous arrival in community & equal competitive abilities
Geometric series model--corresponds with competitive preemption of resources by first arriving species, or stressful effects of pollutants such that only a few species are well adapted

12. Hypothetical lognormal (left), broken stick (right) species-abundance curves
Note: “normal” distribution, log axis

13. Geometric series model of species abundance curve

14. Some examples of species-abundance curves (from Rickleffs 2001) ...
Forest birds
Vascular plants in Eastern (U.S.) deciduous forest
Vascular plants in alpine fir forest

15. What is community ecology (based on what we’ve seen so far this lecture)?
New level of organization (from populations)
Define community = a group of species populations (e.g., plants, animals, fungi) in a given place; also, an ecological unit or assemblage of species at a particular spatial scale
Emergent properties of communities lead to new set of questions (that make no sense at population level of organization)
How many species are there in a community, and why?
What are relative abundances of species, and why?
How are communities structured (e.g., ordinated)?
How interdependent are the species in their distributions?

16. What is the nature of communities?
One extreme view: Community as “superorganism”
Concept most closely associated with Clements (20th Century), Forbes (1883)
Local communities hypothesized to be closed, discrete units, separated by sharp boundaries (of coincident species), and structured by strongly interdependent ecological (and co-evolutionary) relationships among component species
Gaia Hypothesis is related concept—idea of entire Earth system as a superorganism, with homeostatic emergent properties (e.g., ability to regulate atmospheric composition of gases), and different species have particular functional roles
One impetus for this Clementsian view—to allow classification of local communities based on dominant spp.

17. Are communities “real”, highly structured by strongly interdependent species interactions?

18. Or are communities simply assemblages of species that happen to co-occur in some locales?

19. Are communities closed or open?

20. What is the nature of communities?
Second, extreme view: open communities
Community viewed as loose assemblage (continuum) of individualistic (independent) species, each responding to slightly different ecological factors
Concept most closely associated with Gleason, who challenged Clementsian view
If Gleasonian view correct, then few natural boundaries exist between communities, and precise classification of communities at best arbitrary; each species best viewed as distributed independently of others with respect to competition, predation, & other factors that affect distribution
Ecotones are natural, relatively abrupt, changes in community caused by physical environment, and are not support for Clementsian view

21. Most ecologists today accept Gleasonian view, based on data such as these tree data of R.H. Whittaker (from Rickleffs 2001)

22. Geographic ranges, e.g. these independent distributions of Kentucky tree species, also support Gleasonian view (Rickleffs 2001)

23. Plant communities (Gt. Smoky Mts
Plant communities (Gt. Smoky Mts.) again support Gleasonian view: independent distributions, & each species occurs widely outside the forest type bearing its name (communities arbitrary) (from Rickleffs 2001)

24. Ecotones are important, and do create relatively abrupt boundaries (from Rickleffs 2001)...
Plant distributions in southwestern Oregon; see also next slide showing soil data for same sites

25. …But these ecotonal boundaries generally result from physical environment, not from tight species interactions (from Rickleffs 2001)

26. This debate about the tightness of community structuring continued into latter part of 20th Century
E.g., debate about the prevalence of competition structuring communities of birds, and other organisms
Cody, MacArthur, & others believed that competitive interactions are constant, strong, the norm among species
Wiens, Simberloff, & others argued that competition not particularly strong
European community ecologists still emphasize plant community classifications (termed biocoenoses)
Recent evidence on invasive species suggests that more structuring may be going on in some communities than previously thought
The example given involves Centaurea grass allelochemicals, and coevolved plants (discussed early in semester)

27. Conclusions:
Communities have emergent properties such as diversity of species, and distributions of species abundances
Ecologists have devised diverse metrics to quantify community characteristics and relationships, and to characterize community types
These metrics can be useful for conservation efforts, which emphasize particular species, rare & threatened species
Most communities are probably not tightly structured, not “real” entities, but rather local assemblages of independently distributed species occupying particular area

28. Acknowledgements: Some illustrations for this lecture from R. E
Acknowledgements: Some illustrations for this lecture from R.E. Ricklefs The Economy of Nature, 5th Edition. W.H. Freeman and Company, New York.