1. Food webs and trophic cascades …going from species interactions to community structure
How do species interactions organize groups of organisms into communities?
How do trophic interactions influence species abundance?
What influence does species composition have on ecosystem productivity ?

2. Coexisting groups of organisms are called communities
Communities are structured according to feeding relationships (energy flow). These relationships describe the food web and are determined by species trophic levels
Green food webs: consist of producer organisms (almost always plants, herbivores, carnivores (1-2 levels). Terrestrial or aquatic
Brown food webs: based on detritus (food for bacteria and fungi) which support consumer organisms (mites, ants etc…)

3. (4) 3 2 1 What determines how many trophic levels exist in a food web?
Energy present per sq m per yr in the bodies of plants, heribvores and predators.
Most terrestrial systems have 3 levels not 4… 2 1

4. Hairston, Smith, Slobodkin (HSS) in 1960 proposed a hypothesis for ‘Why the world is green’
If predators (carnivores) are an important force regulating prey (herbivore) communities.
Then how does the effect of predation influence the trophic level below the herbivores?
If predator abundance can regulate herbivore abundance, then predators may exert a top-down effect on the abundance of producer organisms via a trophic cascade

5. Top-down control of food webs
Trophic level
No predators, must be food limited 3
Carnivores
(competition)
Herbivores
Lots of food?? Must be predation
Limited? 2
Producers
Resource limited (light, water
nutrients). (competition) 1
HSS argue that plants are abundant because herbivores cannot eat them all.
Can you come up with an alternative hypothesis??

6. Alternative to HSS:Bottom-up control of food webs
Most plant material cannot be eaten by herbivores. Resource availability to plants determines productivity and thus herbivore and carnivore biomass

7. Bottom-up versus top-down
What would happen if you add another level?

8. Why the lake is blue hypothesis?

9. Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish) (big fish)
Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity
Whole lake manipulations (Carpenter)

10. Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish) (big fish)
Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity
Whole lake manipulations (Carpenter)

11. Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish) (big fish)
Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity
Whole lake manipulations (Carpenter)

12. Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish) (big fish)
Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity
Whole lake manipulations (Carpenter)

13. Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish) (big fish)
Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity
Whole lake manipulations (Carpenter)

14. Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish) (big fish)
Top down? Lakes with similar levels of limiting nutrients have 1000-fold variation in productivity
Whole lake manipulations (Carpenter)

15. What about bottom-up limitation?
Bottom-up effects evident when relative sizes of trophic levels are positively correlated
Each point is a lake. Lots of variation…
Chlorophyll (µg per L)

16. What happens when fish (3rd level) are added?
Reduce zooplankton…
And increase Chlorophyll
Bottom up and top down are both important

17. Under what conditions might you expect only two trophic levels?
Outbreaks of moths can completely defoliate their hosts (oak and aspen trees). Sustained defoliation over 4 yr will kill the host.
Gypsy moth (Lymantria dispar) - introduced from Europe to USA during the Civil war to start a silk industry. Moth lacks natural enemies in introduced range. Although the female moth cannot fly, eggs and larvae are transported on firewood, furniture, and SUVs... Slow spread across US.

18. How does food web structure influence species diversity?
Classic studies by Robert Paine (Univ. Washington) in inter-tidal zone
Qu: What role do predators have in controlling the diversity of lower trophic levels?
Pisaster (sea star) is a keystone predator in NW tidal communities. Removal of Pisaster leads to dominance by Mytilus (mussel)

19. Both Pisaster and Heliaster are keystone predators

20. Why would Mytilus dominate when Pisaster
is absent?

21. Keystone predators also important in terrestrial ecosystems
Look familiar? What is the yellow flowering plant that dominates the plot on the right?

22. Carson - sprayed old field plots with insecticide for 8 years
control
Here solidago is analagous to Mytilus (mussel) and the insect herbivores fulfill the role of Pisaster (the seastar)

23. Spraying of fields reduces species richness
Spraying prevents outbreaks of a chrysomelid beetle that defoliates goldenrod
Solidago altissima, its preferred food plant and species dominant.
Species richness
This shows the Carson experiment results
Sprayed treatment

24. Conservation significance of trophic cascades and top-down effects?
More than just charismatic!!

25. Killer whale feeding relationships

26. How would you measure functionality?
Food webs indicate species roles but can also emphasize energy flow or functional diversity
Acmaea - limpit = fiilter feeder
Strongylocentrotus sea urcin
Katharina = nocturnal browsing mollusc (chiton)
Tonicella = chiton
How would you measure functionality?

27. How does food web structure vary?
Dotted line connect to other prey species that are not shown
Looping lines show ominivory
Omnivory tends to de-stabilize food webs.
Web complexity determined by number of linkages (a) has 7 and 7 spp, (b) has 12 and 8 spp
mudflat
Baccharis plant
What makes (b) more complex
What makes (a) more stable?

28. How does community diversity influence foodweb complexity and stability?
Food webs vary greatly in complexity. Arctic/boreal food webs often v. simple (few nodes, few links). But are stable (been around a long time).
Might argue that stability is a species trait not an emergent trait of the food web?
Food web complexity (linkage) however does remain rather constant, while trophic structure tends to increase with species diversity
Example of pitcher plant communties:

29. If stability is a property of the number of interactions that each species has with other species , then the number of interactions (here links per species) seems to be rather independent of food web diversity. Instead as diversity increases we see an increase in the structural complexity of the web - more feedlng guilds, more trophic levesl and often more sub-webs.

30. Summary
Food webs depict the feeding relationships among species in a community. Food webs can be characterized by diversity (nodes) and complexity (linkage).
Functional webs emphasize linkage effects on population growth. Energy flow webs emphasize energy flux through the link
Manipulations of food webs show the importance of top down effects which cascade through webs and can ultimately affect ecosystem productivity.
Resource supply (determining energy flux) also affects trophic complexity via bottom up effects.