Exploring Trophic Cascades in Aquatic and Terrestrial Ecosystems: A Comparative Analysis

3. Trophic position Biomass Terrestrial systems have regular pyramids

4. Trophic position Biomass Aquatic systems have inverted pyramids

5. Bottom-up vs. top-down control • Bottom-up: The flow of energy and materials from producers to consumers • Top-down: The impact of consumers on resource populations and communities • Does the strength of top-down control vary between ecosystems?

6. Does the strength of trophic cascades vary among ecosystems? • Data from 102 experiments that • manipulated primary predators • measured herbivore, plant community biomass • were done in six different ecosystems • Plant and herbivore log ratios- ln(NP+/NP-)

7. Terrestrial herbaceous plant communities

8. Marine benthos mostly kelp beds, urchin herbivores

9. Lake plankton and benthos

10. Stream benthos

11. Marine plankton coastal and estuarine

12. lake lake marine marine stream plankton benthos plankton benthos benthos terrestrial 0 -1 -2 -3 -4 -5 Predators reduce herbivore abundance n=33 n=18 n=22 n=9 Predator effect (log NP+/NP-) n=8 R2=0.35 P<0.0001 n=12

13. 3 n=8 2 Predator effect (log NP+/NP-) n=12 n=22 n=33 1 n=9 n=18 0 lake lake marine marine stream terrestrial plankton benthos plankton benthos Predators increase plant biomass Effect is bigger in water than on land R2=0.29 P<0.0001

14. Bigger effects on herbivores than plants Shurin et al. Ecology Letters5:785

15. What controls variation within and among systems? Borer et al. Ecology (in press) 1. Predator size

16. What controls variation within and among systems? 3. Experimental duration (plant generations)

17. Concordant vs. discordant factors

18. Conclusions from meta-analysis • Stronger trophic cascades in aquatic systems • Lots of variation among aquatic systems • Predator and herbivore types (vertebrate vs. invertebrate, endotherm vs. ectotherm) were important

19. How are wet and dry systems different? • Micro-algae are smaller than herbivores • plants consumed entirely • higher mass-specific production • Plants are more nutritious in water • C:N:P of plants and herbivores more similar • Food webs are structurally different

21. How does body size affect trophic interactions?Shurin and Seabloom, J. Animal Ecology, in review • Small plants have high growth rates • Big animals have low metabolic rates • Strong top-down effects with big herbivores eating small plants

22. How are wet and dry systems different? • Micro-algae are smaller than herbivores • plants consumed entirely • higher mass-specific production • Plants are more nutritious in water • C:N:P of plants and herbivores more similar • Food webs are structurally different

23. Aquatic plants are more nutritious from Cebrian (1999) Am. Nat. 154: 449-468

24. How does plant quality affectWith Spencer Hall, Roger Nisbet and Seb Diehl • Top-down control? • impact of herbivore on plant • Bottom-up control? • impact of nutrients on plant and herbivore, pyramid of numbers

25. Modeling approach Plant N C Herbivore N C Predator N C

26. How are wet and dry systems different? • Micro-algae are smaller than herbivores • plants consumed entirely • higher mass-specific production • Plants are more nutritious in water • C:N:P of plants and herbivores more similar • Food webs are structurally different

27. Two views of nature: food chains vs. food webs Carpenter and Kitchell 1993

28. Two views of nature: food chains vs. food webs Carpenter and Kitchell 1993 Martinez 1997 vs.

29. Two views of nature: food chains vs. food webs

30. Are aquatic food webs “simpler”? • Are trophic cascades all wet? • “My assertion is not necessarily that top-down forces are unimportant (on land). Rather, consumption is so differentiated in speciose systems that its overall effects are buffered.” Don Strong 1992 Ecology 747-754

31. 3 n=8 Predator effect (log NP+/NP-) 2 n=12 n=22 n=33 n=9 1 n=18 0 lake lake marine marine stream terrestrial plankton benthos plankton benthos Stronger plant response in freshwater planktonthan marine Are lake zooplankton more herbivorous (cladocerans vs. copepods)?

32. How can we quantify structural differences among food webs? with Martin Hemberg,Brian Starzomsky and Ross Thompson • Use topological webs to calculate trophic position for all species • Ask whether discrete “trophic levels” are apparent • Does the degree of “lumpiness” vary by system?

33. Kye Burn New Zealand

34. Benguela Shelf, Caribbean

35. Broadstone Stream Ythan Estuary Skipwith Pond Grassland Maine Streams (2) US NE Shelf Chesapeake Bay Duffin Creek Coweeta Streams (2) Coachella Valley Broom food web Little Rock Lake Dung arthropod El Verde rainforest Carribean reef Benguela St Martin Island St Marks Lerderberg River Taieri Streams (18) Company Bay

36. How to calculate trophic position? TP = 2.5 TP = 2 TP = 1 TP = 0

37. Are Trophic Position distributions lumpy and/or bumpy? • Calculate TP for every species in a web • Calculate mean and variance for TP in intervals of 0.5-1.5, 1.5-2.5, 2.5-3.5, etc. • With perfect trophic levels, mean=integer, variance=0

38. Trophic position histograms

39. Is observed variance less than expected by chance? • Randomize the matrix 100 times *subject to a few constraints • Calculate the mean and variance for each TP interval (0.5-1.5, 1.5-2.5, etc) • Is the observed variance < in random webs?

40. Is the observed variance < in random webs?Yes!

41. Are aquatic webs lumpier than terrestrial (is omnivory more common on land)?Stay tuned

42. Conclusions • Aquatic ecosystems show stronger trophic cascades than terrestrial • Effect attenuates from herbivores to plants • Predator and herbivore metabolism predict effect • Why?

43. Potential explanations • Body size differences, allometry of growth and metabolism • Stoichiometry, plants and animals have more similar composition in water • Degree of omnivory • Real food webs have real trophic levels • Are there aquatic-terrestrial differences?