Algal-based Food Web

Transcription

1 Herbivory (text, Ch. 9, pp ) Definition: Primary Producers: Benthic algae ( ) Consumers = Herbivores: Insects: 6 orders, 38 families Majority caddisflies and mayflies Snails Vertebrates: some fish and larval amphibians Use term for consumers that rely on algae Algal-based Food Web What regulates energy flow from algae to consumers? 1) Characteristics 2) Characteristics 3) Characteristics 1

2 Algal-Grazer interactions Algal characteristics: A) [Fig. 4.1] filaments vs. low-lying diatoms B) / nutritional value (C:N ratio) generally, > unicellular green algae >> greens/bluegreens Grazer characteristics: A) and - larger organisms more, higher to track resources, broader effects - e.g., fish, crayfish - smaller organisms more specialized - select types (not spp.) of algae (e.g., diatoms vs filamentous algae - e.g., Leucotrichia is a sessile, territorial caddisfly that "weeds low quality bluegreen algae (Microcoleus) from its lawns. Leucotrichia territories Microcoleus 2

3 B) morphology (depth of feeding in algal mat) SNAILS rasping (radula)» Removes algal overstory CADDISFLIES scraping (sclerotized mandibles)» Can remove MAYFLIES (mid-depth into mat) gathering (soft mouthparts) brushing (surface of mat) biting ( browsing ) Heptagenia - scraping & gathering Snail radula rasping & scraping Baetis - gathering & shredding C) abundance 3) Environmental Factors: Affect algae and grazers: Current Velocity (Shear stress) type of algae and accumulation grazer Disturbance When the streambed moves, algae is and grazers often suffer, which reduces 3

4 Algal responses to grazers: 1) Reduced algal biomass Measured as chlorophyll-a ( ) or as Ash Free Dry Mass ( ) Directly consume cells Physically dislodge cells while foraging that are not consumed. Grazer species differ in effect! 2) Growth form and structure reducee % of algal species and % of understory species mayfly snail caddis Overstory Understory snail 3) Algal species number Snails, fish, tadpoles and caddisflies (but not ) reduce algal species richness by removing all but the most resistant (small) diatoms mayfly caddis Fish/ tadpoles 4) Increased productivity of surviving cells Remove overstory algae and reduce shading of mid- to understory algae Enhance water flow through mat and nutrient delivery Regenerate nutrients through consumption and defecation Ratio of grazer to algal biomass in streams up to 20:1 How can so much biomass be supported?. 4

5 QUESTION: Does Grazing regulate stream algae? 1) Early evidence correlative [Fig. 5, Douglas diatoms with caddisfly density] Lots of evidence 2) 1980s -- evidence showing grazer reduction of algal biomass [Fig. 8.4b] What do these 2 graphs show? Density of diatom cells No. grazers Clay tiles in stream to grow algae in presence vs. absence of invertebrate grazers Example effect of a mayfly grazer (Ameletus validus) on algae Streambed ambient Caged mayflies at different densities What do we see in the experimental results in 9.3a? (1) Compare the streambed ambient to the 1X density of caged mayflies. (2) What happens if 0 grazers? (3) What is general effect of increasing grazer density on algal biomass? photos_invertebrates_mayflies.html Take home message: What do we see in the experimental results in 9.3b? (1) What are the X and Y axes in this figure? (2) What is general effect of increasing grazer biomass on algal productivity? (new biomass produced over time of experiment) -How do you explain this???!! 5

6 How does Figure 9.5 support the Biggs model of periphyton biomass model (Fig. 6.2)? What s going on here? Amount consumed Why is the N+P treatment different? Snails are able to consume algae in N+P treatment. (Just like in figure to right, which shows how, in general, grazers are able to consume Amount periphyton removed How do they keep up? How do grazers track algal patchiness? 1) Patchiness within a habitat (e.g., on cobble surface or among cobbles within a reach) response: Area restricted search stay in good algal (Fig and 9.1) Drift to new habitat when food depleted (drift lecture) 2) 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 6

7 Which is more important in regulating algal biomass: grazers or current velocity? (Opsahl, Wellnitz, and Poff, 2003, Hydrobiologia) Current Grazers Algae Stream survey The streambed pattern What are some hypotheses? Algal AFDM (g) Experiment: grow algae on tiles Current velocity (cm/s) Slow Fast Which treatment has more algae? Algae AFDM (mg cm -2 ) Grazers present! (like above fig.) Controls Electric shock Current Velocity (cm s -1 ) 7

8 Using electric shock to exclude grazers Electrified wire Results Controls (grazers +) Electric shock (- grazers) 1. 2 Slow Fast Algae AFDM (mg cm -2 ) Current Velocity (cm s -1 ) Pattern Reversal: Without electricity, With electricity, mediates herbivory and algal biomass on streambed! 8

9 Grazer density with electricity All grazers Mayflies Caddisflies Chironomids Effect ê ê ç è é P-value Predominant algae on tiles Cyanophytes Chlorophytes Diatoms Controls Electricity 99% 78% 0.5% 7% 0.5% 15% à Grazer species have different sensitivities à Why does relative abundance of algal types change? 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) Among reaches Individual search, Population recruitment Ancistrus (catfish), Baetis Power (1983) Fuller et al. (1986) Among rocks Among patches on rock Individual search behavior Individual search behavior Baetis, Campostoma (fish) Baetis, Dicosmoecus (caddis) Richards & Minshall (1988), Power & Matthews (1983) Kohler (1984), Hart (1981) 9