1. Herbivory (text, Ch. 9, pp. 197-205)
Definition: consumption of primary producers by heterotrophs
Primary Producers:
Benthic algae (Periphyton)
Consumers = Herbivores:
Insects: 6 orders, 38 families
Majority caddisflies and mayflies
Snails
Vertebrates: some fish and larval amphibians
Use term “GRAZERS” for consumers that rely on algae

2. Algal-based Food Web
What regulates energy flow from algae to consumers?
1) Algal Characteristics
2) Grazer Characteristics
3) Environmental Characteristics

3. Algal-Grazer interactions
Algal characteristics:
A) growth form [Fig. 4.1]
filaments vs. low-lying diatoms
B) palatability / nutritional value (C:N ratio)
generally, diatoms > unicellular green algae >> filamentous greens/bluegreens

4. Leucotrichia territories
Herbivore characteristics:
A) size and mobility
larger organisms more generalists, higher mobility to track resources, broader effects
e.g., fish, crayfish
smaller organisms more specialized
select types (not spp.) of algae
e.g., Leucotrichia is a sessile, territorial caddisfly that "weeds” inedible bluegreen algae (Microcoleus) from its “lawns.”
Leucotrichia territories
Microcoleus

5. B) mouthpart morphology (depth of feeding in algal mat)
SNAILS
rasping (radula)
CADDISFLIES
scraping (sclerotized mandibles)
MAYFLIES
gathering (soft mouthparts)
brushing (surface of mat)
biting (“browsing”) (mid-depth into mat)
C) population abundance
Snail radula
rasping & scraping
Heptagenia - scraping & gathering
Baetis - gathering & shredding

6. 3) Environmental Factors: Affect both algae and grazers:
Current Velocity (Shear stress)
type of algae and amount
grazer mobility / movement
MOVIE
Disturbance

7. Density of diatom cells
No. grazers
Density of diatom cells
QUESTION: Does Grazing regulate stream algae?
1) Early evidence – correlative [Fig. 5, Douglas diatoms decline with increasing # caddisfly grazers]
Lots of correlative evidence
2) 1980s -- experimental evidence showing grazer reduction of algal biomass
[Fig. 8.4b]

8. Amount periphyton removed
Feminella & Hawkins (1995)
Reviewed 89 experimental studies
Strong finding: Increase in grazer density  periphyton reduction
Amount periphyton removed
[Fig. 3, F&H] patterns?
As periphyton biomass increases (X-axis), so does amount of periphyton removed!
[Fig. 4, F&H] patterns?
Fish, Caddis, Crayfish, Frogs, Snails, Mayflies -- all reduce periphyton

9. Mechanisms of Grazer Impact on Algae:
1) Consume cells
reduces biomass
2) Physically dislodge dead cells, etc.
enhances circulation through mat (nutrients)
3) Open canopy to light
reduce self-shading
4) Regenerate nutrients
increase production

10. Types of periphytic responses to grazers:
1) Biomass
generally declines with increasing grazer density
Units of measurement: chl-a, AFDM (Ash-free dry mass)
Grazers differ in effect!
2 literature reviews here:
[Fig. 1, Steinman]
Snails > Caddisflies > Mayflies
Caddisflies > Mayflies > Snails

11. Types of periphytic responses to grazers:
2) Growth form and structure
Overstory more vulnerable
Grazer type: snail > caddisfly > mayfly (Fig.2 Steinman)
Overstory
Understory
3) Algal species number
Snails and some caddisflies reduce algal species richness by removing all but the most resistant (small) diatoms

12. 4) Primary production (new biomass / time)
Ratio of grazer to algal biomass in streams up to 20:1
How can so much biomass be supported?
Low-biomass diatoms have high production rate and can thus support more herbivore biomass than filamentous algae.

13. Total areal production
A proposed "model"? [Fig. 9.4 in text]
Total areal production
Photosynthetic rate
Biomass
Biomass decreases
Mechanism?
Grazers remove algae!
Photosynthetic rate per unit biomass increases
Mechanism?
Conversion to more productive diatoms
Total areal production highest at Intermediate Grazing
Mechanism?
reduced self-shading, mix of algal species ?

14. How do grazers track algal patchiness?
1) Patchiness within a habitat (e.g., on cobble surface or among cobbles within a reach)
Individual response:
Area restricted search – stay in good algal patch (Fig and 9.1)
Drift to new habitat when food depleted (drift lecture)
2) Patchiness at larger scales (e.g., between reaches or between streams)
Population response: population size larger where whole-stream algal production is high, so grazers can still suppress algae

15. How do biotic and abiotic factors interact to influence herbivory?
Ongoing discussion/debate among stream ecologists about biotic vs. abiotic controls.
Much evidence for grazer control [Figs. 3,4 F&H]
70-80% of all experimental studies in streams show grazer control of algae according to Feminella & Hawkins (1995), implying strong biotic control, but …

16. Question: Is this experimental evidence representative?
1) limited taxa that have strong effects (snails, caddis)
2) Often at greater than ambient density
3) in artificial channels
4) limited range of natural conditions (flow)
Physical factors also important
1) physical harshness (current velocity)
Does grazer control depend on current?
2) disturbance (Talk about in upcoming lecture!)
Can keep algal populations low (and grazers too!)

17. Q1: Role of current velocity in mediating stream herbivory?
(Poff, Wellnitz, and Monroe Oecologia)
Hypothesis: abilities of grazers to control algal biomass change in response to near-bed current velocities
RATIONALE: Heterogeneity in near-bed velocities is dominant feature of natural streams, but it has been largely ignored in herbivory experiments.
Implications:
• “Functional redundancy” - Do all grazers “do” the same thing under all conditions? If not, heterogeneity in near-bed current can offer different “niches” for grazer species.
Table 1. Characteristics of the three grazer species used in this study. Illustrations of the grazers are not to scale, but are presented to show the distinct morphology of each species. The “Biomass” column shows the mean (and range of) size of the grazers. “Mode of Movement” indicates each grazer’s relative speed, and primary means, of movement across the streambed. “Mode of Feeding” indicates the mouth or body parts known (for Glossosoma and Baetis; see Arens, 1989) or observed (for Drunella) to remove periphyton from substrates. “Current Range” shows the mean (and range of) velocity and the number of observations made for each grazer in the upper Colorado River.
Grazers
Biomass (mg indiv. –1)
Mode of Movement
Mode of Feeding
Current Range (cm s–1)
Glossosoma
Baetis
Drunella
1.02 ( )
1.00 ( )
1.90 ( )
slow crawler, non-drifter
fast crawler and swimmer, frequent drifter
slow crawler,
infrequent drifter
scraping (mandibles)
shearing (mandibles + maxilla)
scraping + shearing
(maxilla + mandibles leg movements)
26 (2-67), n=44
29 (2-74), n=20
26 (4-64), n=17
Upper Colorado River:
3 dominant grazers that vary
in body morphology,
mobility and mode of
feeding. Are they
ecologically “redundant”?

18. The Experiment
Algae grown in troughs for 30 days on clean tiles prior to experiment
3 grazer species x 3 velocity treatments (5, 15, 30 cm/s), replicated 4 times
Total biomass of grazers same in each trial, equal to streambed ambient
3 and 7 day experiments to determine efficacy of algal removal

19. Fraction AFDM Removed Current Velocity a b c b c c Conclusions:
0.00
0.25
0.50
0.75
1.00
Slow
Medium
Fast
Glossosoma
Fraction AFDM Removed
Current Velocity a b c b c c
Drunella
Baetis
[Poff, Wellnitz, Monroe (2003), Oecologia]
Conclusions:
• Grazing performance for some species changes with current. (Glossosoma, Baetis)
• Functional redundancy among these 3 grazers depends on local flow. (species have equal performance at high flow but not low)

20. Q2. Do grazers regulate algal production and composition in the stream as a function of current?
(Opsahl, Wellnitz, and Poff, 2003, Hydrobiologia)
Algae
Current
Grazers

21. Grazers present! (like above fig.)
Stream survey
The streambed pattern…
Current velocity (cm/s)
Algal AFDM (g)
0.00
0.06
0.12
0.18
0.24 10 20 30 40 50 60 70 80
“Fast”
“Slow”
Controls
Electric shock
Experiment: grow algae on tiles . 2 4 6 8 1 5 3
Algae AFDM (mg cm-2)
Current Velocity
(cm s-1)
Grazers present! (like above fig.)
Which treatment has more algae?

22. Using electric shock
to exclude grazers
Electrified wire

23. Which treatment has more algae?
Results . 2 4 6 8 1 5 3
Algae AFDM (mg cm-2)
Current Velocity
(cm s-1)
without grazers
“Slow”
Which treatment has more algae?
with grazers
“Fast”
Controls
Electric shock
Pattern Reversal:
• Without electricity, grazers have access to algae in slow flow, and more algae in fast flow habitats.
• With electricity, grazers excluded, and faster-growing algae in slow accumulates more biomass.
• Current mediates herbivory and algal biomass on streambed!

24. Predominant algae on tiles
Grazer density with electricity
Effect P-value
0.0001
0.001
0.72
0.05
All grazers
Mayflies
Caddisflies
Chironomids
Predominant algae on tiles
Controls Electricity
99% %
0.5% %
0.5% %
Cyanophytes
Chlorophytes
Diatoms    
 Why does relative abundance of algal types change?
 Grazer species have different sensitivities

25. END

26. Scales of Heterogeneity in Algal Patchiness
How do grazers track patchy periphyton?
Scales of Heterogeneity in Algal Patchiness
Biggs’ model

27. Herbivore Tracking of Periphyton Heterogeneity Across Scales
Scale of algal
Patchiness
Herbivore tracking mechanism(s)
Example taxa
Reference in text
Among streams
Population recruitment
Baetis
Wallace & Gurtz (1986)
Individual search, Population recruitment
Ancistrus (catfish),
Baetis
Power (1983)
Fuller et al. (1986)
Among reaches
Individual search behavior
Baetis,
Campostoma (fish)
Richards & Minshall (1988), Power & Matthews (1983)
Among rocks
Among patches on rock
Individual search behavior
Kohler (1984), Hart (1981)
Baetis,
Dicosmoecus (caddis)