The Role of Consumer Diversity in Ecosystem Function

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The Role of Consumer Diversityin Ecosystem
Function
Species Diversity Seminar October 28, 2003
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Outline for Todays Discussion

• Relevance Brief Background
• Some trophic structure models
• The Role of Consumers
• Theory
• Empirical Results (our papers)
• Discussion Questions

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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?

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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

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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

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(No Transcript)
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Fundamental Trophic Structure
CM2
CM1
M
P
CP1
CP2
After Naeem 2002
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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.

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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.

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

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Consumer Diversity in T. Cascades




After Duffy 2002.
Consumer diversity should reduce penetrance of
trophic cascades.
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Todays 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

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Duffys 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)
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Duffys test of the herbivore diversity hypothesis
No grazers
Idotea only
Erichsonella only
All six species
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Are more diverse algal assemblages more
productive?
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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.
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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 were looking at
  trophic relationships?
• (Then again, some consumers with low biomass can
  have
• disproportionate impacts on systems)

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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.
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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
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Questions about Schmitz reading

• In terms of last weeks 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?

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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?