1. A biological community is an association of interacting populations
Community Structure
A biological community is an association of interacting populations

2. Community Diversity & Community Structure
Community diversity relates the number of different species in a community
A community’s structure is based on the interactions between all of its species
Individualistic
holistic

3. Community Structure
Consumer-resource interactions are a significant component of community structure
Feeding relationships are often used to describe community structure

4. Community Structure
Consumer-resource (feeding) interactions are critical to community structure, because they represent the travel of energy through the ecosystem

5. Community Structure – Trophic Levels
Primary producers - autotrophs (plants and other ‘self-feeders’)
Primary consumers – herbivores
Secondary consumers – carnivores that eat herbivores
Tertiary consumers – carnivores that eat secondary consumers…

6. Community Structure – Food Web
Food web = a summary of all the feeding relationships within a community

8. Effects of species richness on food web structure
Is there a relationship between the species richness of a community and the complexity of its feeding interactions?

9. Complexity of a food web can be characterized based on:
number of feeding interactions
number of trophic levels

10. Effect of species richness on food web complexity
The number of feeding interactions that each species has with other species is independent of the overall diversity of the community a b

11. Effect of species richness on food web complexity
However, the number of trophic levels does tend to increase with species richness, so higher community diversity is usually associated with increased food web complexity

12. Effect of species richness on food web complexity

13. Effect of food web structure on species diversity
feeding relationships (and other species interactions) can affect species diversity within a community
for example: when a predator controls the population of an otherwise dominant competitor, it may allow other less competitive species to persist

14. Effect of food web structure on species diversity
The number of species at one trophic level may depend on the presence of consumer species at a higher trophic level

15. There are different ways to visualize community structure
Robert Paine – noted that some feeding relationships are more important than others in terms of shaping community diversity
“strong interactions”
defining criterion not necessarily the quantity of energy flow, but the degree of influence on population sizes and community structure

16. There are different ways to visualize community structure

17. Effect of food web structure on species diversity: Keystone species
Keystone species are those species whose interactions (usually feeding) have a disproportionately large influence on the structure of their community
influence is disproportionate relative to the keystone species’ actual biomass
distinct from dominant species (whose influence is due to high biomass, e.g., redwoods)

21. Community structure - alternative stable states
Similar to population sizes in predator-prey cycles, community structure can exist at different stable equilibria
e.g., kelp forest / urchin barren

22. Community structure & trophic cascades
Trophic levels are influenced from above by predation and from below by production
Top-down control (predator-mediated)
Bottom-up control (producer-mediated)
Trophic Cascade – the activity of one trophic level has far-reaching effects on the biomass of distant trophic levels

23. Community structure & trophic cascades

24. Community structure & trophic cascades

25. Community structure & trophic cascades

27. Community structure & trophic cascades
Trophic cascades can have far-reaching and unpredictable effects

28. Introduced Norway rats indirectly alter the intertidal community in the Aleutian Islands through direct predation on birds that forage in the intertidal
Introduced Norway rats indirectly alter the intertidal community in the Aleutian Islands through direct predation on birds that forage in the intertidal. Dotted arrows indicate indirect effects, whereas solid arrows indicate direct effects. Rats keep Glaucous-winged gull and Black Oystercatcher numbers low, which releases intertidal invertebrates such as barnacles and herbivorous snails and limpets from foraging pressure. Greater numbers of grazing invertebrates leads to a significant decrease in algal cover, which allows more settling space for sessile invertebrates. The marine rocky intertidal is altered from an algae- to an invertebrate-dominated system.
Kurle C. M. et.al. PNAS 2008;105:
©2008 by National Academy of Sciences

29. Introduced species & community structure
Introduced species tend to reduce the complexity of community structure by simplifying trophic structure / eliminating trophic levels

31. Energy flow constrains community structure
Trophic dynamics: as energy is transferred from one trophic level to the next, some of it is lost due to:
limited assimilation
respiration
heat production

32. Energy flow constrains community structure - Demo
paper clips = energy units
must have at least 2 energy units to survive
front row = primary producers (3 clips)
second row = primary consumers (forage 3 clips from primary consumers, pay 2 clips for respiration)
third row = secondary consumers (get 3 clips worth of energy from prey, pay 2 clips for respiration)

33. Energy flow constrains community structure
Energy transfer between trophic levels is inefficient enough to make numerous trophic levels an impossibility
The most productive ecosystems usually max out at six trophic levels

35. Energy Pyramids