1. Ecology: Lecture 17
Community structure

2. Vertical stratification in lakes

3. River Continuum concept (pictures from Stroud Water Research Center)
Headwater Mid-sized stream Large river

5. Biological Structure Species dominance Numbers Impact
Dominant groups in stream sample?
Impact
Impact of introduced lampreys on fish
Keystone predators such as Pisaster ochraceus (type of sea star)

6. Biological Structure Species diversity
Number of taxa (families or species for ex.)
First approximation
Shannon diversity index considers 2 factors:
Number of taxa
Evenness (or “relative abundance”)
For a given number of families, the Shannon index is higher when there are approximately equal numbers of individuals in each group (more even distribution of individuals)

7. Rank abundance of species Method of examining evenness
Pi values are on the y-axis
Groups placed in order of pi values

8. Effects of population interactions on community structure
Competition and community structure
Focus: Zebra mussels
Invasive
Freshwater filter-feeder
Attach by byssal threads
Encrust any submerged hard surface
Envirozine (Canada)

9. Zebra mussels: major effects
Outcompetes native clams for space (how?)
 zooplankton (efficient filter-feeding)   other filter-feeders or fish that feed on zooplankton
 water clarity   growth of benthic algae
Why so successful?
Good “fit” with new environment
No native parasites or predators

10. Local example: Himalayan blackberry
Note robust stem with heavy thorns
Grows tall and dense
Outcompetes native shrubs
Blocks light for shade-intolerant tree seedlings
Net result:  in community diversity of both plants and animals (Why animals too?)
Also limits animal movement

11. Two hypotheses for level of community diversity
Bottom-up hypothesis
Top-down hypothesis

12. Bottom-up hypothesis
Defined: The abundance and diversity of primary producers (the “bottom”) affects the diversity at higher levels of the food web.
Stream-related hypothesis: Diversity in different parts of a river is a consequence of the diversity and abundance of primary producers.
Diversity: High diversity in the mid-reaches from multiple sources  more feeding niches.
Review: Sources of primary production along river continuum
Abundance: Higher total primary production  increase in food resources  reduction of competition among taxa (i.e. reduces the chance of competitive exclusion)

13. Top down hypothesis
Defined: Levels of diversity are controlled by predators
Classic example: Pisaster ochraceus, the ochre star (native inhabitant), is a keystone predator in the community (Bob Payne)

14. Keystone predator: Pisaster ochraceus
Pisaster maintains (promotes) diversity
Mussels would otherwise dominate mid-intertidal

15. Pisaster ochraceus predation and diversity (2 mechanisms)

16. Keystone predator (invasive) Non-native rainbow trout
Keystone predator decreases diversity in this case
Understand the mechanism!
Step-by-step…