1. Community Structure & Function
A community has emergent properties beyond the population level
Number of species
Relative abundance of each species
Coadaptation of species within community
Associated abiotic conditions

2. Community Structure and Diversity
Species richness (S) = the number of species in the community
Species even-ness (or relative abundance) = the proportion that individuals of each species contribute to total individuals of all species

3. Two measures of community diversity
Simpson’s index
D = 1/((pi2))
pi = relative abundance of each species = ni / N
D = 1 if only 1 species is present (no diversity)
Dmax = S if all species have equal numbers
(perfect even-ness)

4. Two measures of community diversity
Shannon (or Shannon-Weiner) index
H = -(pi)(lnpi)
pi = relative abundance of each species = ni / N
H = 0 if only 1 species is present (no diversity)
Hmax = ln(S) if all species have equal numbers
(perfect even-ness)

5. Species Rank - Abundance Curves incorporate both
richness and even-ness
Figure 17.1

6. Species Accumulation Curves also reveal richness and even-ness
primary forest
secondary forest
plantation forest
Individual-based species accumulation curves for primary (unbroken line with shaded 95% confidence intervals) and secondary forests (dashed line) and Eucalyptus plantations (dotted-dash line). (Letters a–e) Five response types, grouping taxa according to our analytical criteria (see Materials and Methods) that showed the following: significant differences between samples from all habitat types (letter a), no clear significant difference between samples from secondary forest and Eucalyptus (letter b), no clear significant difference between samples from primary and secondary forest (letter c), no clear difference between any habitat (letter d), and primary forest appearing as less species-rich than other forest habitats (letter e).
Barlow J et al. PNAS 2007;104:

7. Not all species within a community are equivalent
KEYSTONE
DOMINANT
Species Ecological
Effect
RARE
UMBRELLA
INDICATOR
Species abundance (or biomass)

11. Sea otter

12. Kelp

13. Sea urchin

16. OP OA OT
Otter presence affects kelp density (especially understory kelps)

17. Otter presence increases
kelp density…
by reducing urchin density.

18. OTTER RECOLONIZATION
Typically leads to urchin declines
- kelp recovery in some cases
- but not in others (hysteresis)

19. Recruitment (of kelp) vs Offtake (of kelp by urchins)

20. Sea otter – urchin – kelp: trophic cascade

21. Two Contrasting Views of Communities
Clements: ORGANlSMAL concept of community structure: a community is a set of species that are co-adapted to biotic interactions with one another
Gleason: INDIVIDUAL concept of community structure: community is a set of species, but each species responds to environmental gradients independently

22. Clements
Figure 17.12
Gleason

23. Ecotones ARE easily recognizable breaks in community structure,
but often arise due to adaptations to abiotic factors,
rather than biotic interactions among species
Some support for each of the two views

24. Figure 17.3

25. Figure 17.4