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

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

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

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

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

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

Density of diatom cells No. grazers Density of diatom cells QUESTION: Does Grazing regulate stream algae? 1) Early evidence – correlative [Fig. 5, Douglas 1958 - diatoms decline with increasing # caddisfly grazers] Lots of correlative evidence 2) 1980s -- experimental evidence showing grazer reduction of algal biomass [Fig. 8.4b]

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

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

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

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

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.

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 ?

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. 5.14 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

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 …

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!)

Q1: Role of current velocity in mediating stream herbivory? (Poff, Wellnitz, and Monroe. 2003. 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 (0.57-1.29) 1.00 (0.61-1.21) 1.90 (1.81-1.92) 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”?

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

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)

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

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?

Using electric shock to exclude grazers Electrified wire

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!

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% 78% 0.5% 7% 0.5% 15% Cyanophytes Chlorophytes Diatoms      Why does relative abundance of algal types change?  Grazer species have different sensitivities

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

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)