Transcript The Role of Consumers in Community Diversity

The Role of Consumer Diversity
in Ecosystem Function
Species Diversity Seminar
October 28, 2003
Outline for Today’s Discussion
• Relevance & Brief Background
• Some trophic structure models
• The Role of Consumers
– Theory
– Empirical Results (our papers)
• Discussion Questions
Why look at trophic interactions?
• The recent surge in diversity-ecosystem
function studies have largely avoided multitrophic interactions, and have focused
primarily on one trophic level:
photosynthetic autotrophs.
• What role do trophic interactions play in
regulating ecosystem function and
community diversity?
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, organicinorganic matter transformers
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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Fundamental Trophic Structure
C - Consumer
M - Microbial Decomposer
P - Producer
CM2 CM1 M
Organic
Carbon
Inorganic
Carbon
P
MInorganic
CP1 CP2
MOrganic
After Naeem 2002
Principles of Trophic Interactions
(Naeem 2002)
1. Decomposers and producers are locked in an
“antagonistic mutualistic” relationship.
Environmental changes lead to inseparable responses
by both;
2. Consumers affect rates of movement of materials
among different pools;
3. Consumers can determine the distribution of biomass
among trophic groups;
4. Stability and reliability of systems & populations is
affected by trophic structure as well as numbers of
species within trophic groups;
5. Interactions may exist between diversity at one level
and diversity at another.
What is the role of consumers?
Duffy (2002) points out that:
1.
Extinctions tend to be biased towards higher trophic
levels; and
2.
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.
Do these results say it all?
(Hector et al. 1999)
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.
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.
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.
Consumer Diversity in T. Cascades
+
C
=
+
C
=
After Duffy 2002.
Consumer diversity should reduce penetrance of trophic cascades.
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
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)
Duffy’s test of the herbivore diversity hypothesis
No grazers
Idotea only
Erichsonella only
All six species
Are more diverse algal assemblages more productive?
% change in wet mass
250
200
150
100
Poly
50
150
0
red
mix
100
50
0
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mi
xtu
re
mo
no
rg.
av
g.
Alga
Sa
a
Ulv
ter
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En
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Po
Gr
ac
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-50
Figure 3. Algal net production in the absence of
herbivores, for single species (n = 5), a mixture
of all 5 species (mix; n = 5) and the average of
all species (average monoculture; n = 25). Inset
show s data forPolysiphonia harveyi, in the red
versus mixed treatments. Data from pilot study,
described in Box 1. Bars means + 1 SE.
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.
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…)
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.
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
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?
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?