1. Lotic Communities What is a community? A) The Dictionary B) The Ideal
A term applied to any grouping of populations of organisms found living together in a particular environment; essentially, the biotic component of an ecosystem. The organisms interact (by competition, predation, mutualism, etc.) and give the community a structure.
B) The Ideal
All species that interact over a spatial scale that includes the lifetime range of the widest ranging species and the temporal scale of the longest-lived species
C) The Fanciful
“Co-occurring species about which it’s interesting to think, talk and experiment.” (Robert MacArthur)
D) The Pragmatic: “Assemblage”
A group of interacting species defined by the investigator at a particular spatial and temporal scale.
(e.g., the assmeblage of macroinvertebrates in a riffle)

2. Community “structure”
The organization of a biological assemblage based on the numbers of individuals within different taxonomic groups.
We expect similar community structure where environmental conditions are similar.
Some measures of structure:
Number of species
Relative abundances of species (common to rare)
Functional roles of species [Fig. 6.14]
Important underlying processes:
• Abiotic environmental conditions (esp. disturbance)
• Biotic interactions (competition, predation)

3. A goal of community ecology:
To understand and “predict” community structure
Are communities repeatable in space?
Do they show same structure under similar environments?
Or are they “randomly” vary in response to chance events and dispersal
Are communities persistent through time?
Our perception of a community will change with scale. That scale may be…
Taxonomic
e.g., algae, inverts or fish
Spatial
e.g., cobble, riffle, reach, etc. -- within stream or among streams
Temporal
e.g., snapshot, seasonal, annual, etc.

4. Important aspects of community structure
(1) Taxonomic perspective: Diversity
Taxonomic Diversity incorporates 3 aspects of organization:
(1) Richness (# species)
(2) Abundance of species
(3) Evenness of species abundance
Converse of evenness is dominance
A formal measure of diversity
e.g., the Shannon-Wiener Index:
i = 1 S
H' = - S [(ni/N) * loge (ni/N)]
S = # species
ni = abundance of species “i”
N = total abundance

5. 20 species with 5 each, H’ = 3.00 H' = - S [(ni/N) * loge (ni/N)]
What gives highest diversity?
---> High Richness and High Evenness
Low diversity and High dominance characterize polluted or naturally stressful habitats.

6. Some rules about community diversity & richness
1) Most species are rare [Fig. 11.3]
Remaining 20% of individuals comprised of 133 species (which are thus rare).
15 species comprise 80% of all individuals
Germany’s Breitenbach has >1000 described species (Table 10.1)
Most abundant species, 12% of all individuals

7. 2) Species richness increases with …
sampling intensity
Number individuals sampled (Fig. 10.4a)
Number of samples (Fig. 10.4b shows 4 different streams)
Different streams have different maximum species richness. Why?
area sampled
Species-area relationship (Figs. 11.2, 11.5)
Mussel richness increases with river size.
But note variation among rivers of similar size (refer to Fig. 11.1b)
Figure 11.1A should be plotted with axes reversed!!!
Why differences among streams in number of species? Biogeography. Also, stream SIZE.
Fish richness increases with basin area.
Area ~ size, complexity

8. 3) Evolutionary History matters! Biogeogrpahy
Some regions much more speciose (Fig. 10.2)
Local communities are potentially more diverse in regionally diverse areas
“Keystone”/Dominant species often have limited ranges
Region Native fish species
Tropical S.A. 3000
Europe 250
North America 700
- So. Appalach. 250
- Alaska 180
- California < 50

9. Connecting Global to Local Richness
(Tonn’s “filtering” concept)

10. Both historical and contemporary factors influence distributions of lotic species
Zoogeography,
Regional conditions
Local conditions,
Temperature
Habitat complexity
Biotic interactions

11. Local community composition can change over time because species move.
Species can use habitat at
multiple scales
For example, fish move to meet needs at different life stages or to find refugia (Fig. 2).
Fausch et al., BioScience 52:483 (2002)

12. So, what factors regulate Community Structure?
Via dispersal, species spread throughout stream networks and among drainages
Many barriers to dispersal
Natural - waterfalls (fish), steep drainage divides (insects)
Human - e.g., dams
Assuming arrival, flourish where conditions are good
Two dominant processes that influence “suitability” and thus regulate …
biotic interactions
abiotic factors and chance

13. Field study showing interplay of spatial and temporal and biotic and abiotic factors: David Hart (1992, Oecologia)
Augusta Creek, MI
Observation: Filamentous alga, Cladophora, important habitat structure, and it occurs along velocity gradient.
- Figure 2

14. Types of invert communities varies with Cladophora cover
- Table 1 shows positive/negative associations
Crayfish forage on Cladophora, but not > 50 cm/s, because … ?

15. Caging experiments show crayfish controls Cladophora biomass (Fig. 3)
Why might there be less algae in “enclosure” vs. “open” treatment?
Other grazers (like Leucotrichia) can also control Cladophora (Fig. 4) ... but how?
Priority Effect: They must arrive on bare substrates first and get established before Cladophora attains a “size refuge”. (Weed the Cladophora?)
• What creates bare patches?
DISTURBANCE!
• Who gets there first?
CHANCE!

16. Initial Colonization of space by:
Current Velocity
< 50 cm/s,
crayfish present
> 50 cm/s,
crayfish absent
Sparse Cladophora
on streambed
Initial Colonization of space by:
Cladophora
Insect Grazers
Sparse Cladophora
on streambed
Dense Cladophora
on streambed
What maintains heterogeneity of habitat? (timing of disturbance, sediment size, channel features)