The Role of Consumer Diversity in Ecosystem Function

1. The Role of Consumer Diversityin Ecosystem Function Species Diversity Seminar October 28, 2003

2. Outline for Today’s Discussion • Relevance & Brief Background • Some trophic structure models • The Role of Consumers • Theory • Empirical Results (our papers) • Discussion Questions

3. Why look at trophic interactions? • The recent surge in diversity-ecosystem function studies have largely avoided multi-trophic interactions, and have focused primarily on one trophic level: photosynthetic autotrophs. • What role do trophic interactions play in regulating ecosystem function and community diversity?

4. A few definitions: • Trophic Level: Position within a food chain determined by the number of energy transfer steps to that level. • Trophic Cascade: When consumers or producers have an influence on populations that are two or more trophic levels removed. • Naeem (2002) proposes the following classifications • Producers- photoautotrophs • Decomposers - chemo heterotrophic absorptive, organic-inorganic matter transformers; • Consumers - chemo heterotrophic ingestive, organic-inorganic matter transformers

5. Rich History • Research on trophic interactions is by no means new in ecology: • Hairston, Smith & Slobodkin (1960) - “Green Earth” • Paine (1966) - seastars (Pisaster sp.) as keystone predators in rocky shore communities in WA • Estes & Palmisano (1974) sea otters & kelp • Pimm (1982) - Food Webs • Carpenter (1993) Trophic cascades in lakes

6. wow.nrri.umn.edu/wow/under/ primer/art/pyrsmid.gif (Levine 1992) http://www.imma.org/codvideo/foodwebpic.html

7. MInorganic Inorganic Carbon C - Consumer M - Microbial Decomposer Organic Carbon MOrganic P - Producer Fundamental Trophic Structure CM2 CM1 M P CP1 CP2 After Naeem 2002

8. Principles of Trophic Interactions (Naeem 2002) • Decomposers and producers are locked in an “antagonistic mutualistic” relationship. Environmental changes lead to inseparable responses by both; • Consumers affect rates of movement of materials among different pools; • Consumers can determine the distribution of biomass among trophic groups; • Stability and reliability of systems & populations is affected by trophic structure as well as numbers of species within trophic groups; • Interactions may exist between diversity at one level and diversity at another.

9. What is the role of consumers? Duffy (2002) points out that: • Extinctions tend to be biased towards higher trophic levels; and • Consumers often have a disproportionate impact on natural communities. Therefore, it seems prudent to research how trophic interactions regulate ecosystem function. He reviews a number of theoretical roles consumer diversity might play in regulating community properties.

10. Do these results say it all? (Hector et al. 1999)

11. Consumer Diversity COMPLIMENTARITY After Duffy 2002. - Diversity at the producer level has been shown to increase biomass. - Adding a diverse consumer assemblage to the adjacent trophic level should reduce producer biomass through same mechanisms. (complimentarity & sampling effects) - In some cases, specialized consumers may have lower impacts on producers.

12. Producer diversity After Duffy 2002. Likewise, increased producer diversity may offer enhanced resistance to consumption, as some producers are released from herbivory and can compensate.

13. Community Stability - Insurance • Both consumer and producer diversity should help to regulate community stability. • Through sampling effect (some species resistant to env. perturbations) • Through compensation • Naeem & Li (1997,1998) - Found that diverse algal & bacterial communities showed less variation in aggregate biomass when nutrients and light levels were varied in microcosm experiments. • McGrady-Steed & Morin (1997) - Found that respiration rates were more predictable in diverse aquatic microbial communities.

14. C C Consumer Diversity in T. Cascades + = + = After Duffy 2002. Consumer diversity should reduce penetrance of trophic cascades.

15. Today’s papers • Duffy et al. 2003 Grazer diversity in eelgrass ecosystems • Confirms some theoretical predictions about effects of consumer diversity & identity • Schmitz 2003 • Explores a trophic cascade in terrestrial environments

16. Duffy’s test of the herbivore diversity hypothesis Manipulated micrograzer diversity (0-6 species) Used outdoor mesocosms at VIMS Measured effects on a variety of ecosystem parameters (algal biomass, sediment organic content, benthic diversity)

17. Duffy’s test of the herbivore diversity hypothesis No grazers Idotea only Erichsonella only All six species

18. Are more diverse algal assemblages more productive?

19. Grazer Diversity in Eel Grass System Duffy et al. 2003 • Results: • Increased grazer diversity led to • decreased algal biomass, as predicted. • Total grazer biomass increased with • grazer diversity (similar to grasslands) • Individual grazers specialized on • different algae, and effect seems to • be due to complimentarity. After Duffy et al. 2003.

20. Dr. Emmett Duffy Questions about Duffy et al. reading Q: Given that these grazers seem to specialize on different algal species, does algal diversity have a bottom up influence in the natural system? Q: Does the use of “productivity” as response variable make more sense when we’re looking at trophic relationships? (Then again, some consumers with low biomass can have disproportionate impacts on systems…)

21. Invasibility? Increased grazer diversity resulted in 3 fold increase in tunicate Botryllus schlosseri biomass. Does this finding mean that systems with high grazer diversity are more invasible? From Duffy et al. 2003.

22. Terrestrial Trophic Cascade Schmitz (2003) Mechanism: Predation pressure causes grasshopper to shift grazing pressure from preferred sp (grass) to herb. This cascade alters evenness in plant community. From Schmitz 2003

23. Questions about Schmitz reading In terms of last week’s discussion about arguments for conserving biodiversity - • Does the added complexity of trophic relationships give more weight to the “diversity is valuable” conservation argument?; or • Does the fact that many seemingly complex food webs are dominated by a few strong linkages (Schmitz 2003) leave people worried?

24. Additional Discussion Questions • Do alterations in trophic interactions present as large a conservation threat as habitat loss? • Should future diversity-ecosystem function experiments try to incorporate more research on trophic interactions? • How else might we investigate relationships between trophic levels and diversity?