STREAM DRIFT. Much variation among taxa: Fraction of benthos drifting: Distance drifted? at any moment? over 24 hr period?

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1 STREAM DRIFT In 1952, the Swedish researcher Karl Müller had 150 m of streambed cleared by caterpillar tractor Eleven days later benthic density in this reachestimated at 4,158,000 organisms! Common drifters: Ephemeroptera (Baetis) Diptera (chironomid midges, blackfies) Plecoptera Trichoptera (not stone-cased) Gammarid crustaceans Mayflies Stoneflie s Blackflies Caddisflies Much variation among taxa: Fraction of benthos drifting: at any moment? 0.01 to 0.5% of benthic density over 24 hr period? up to 100x benthic density! Distance drifted? Nightly up to 75 m Lifetime up to km (?) Amphipods 1

2 Drift nets Cost / Benefit Analysis for that drifts Advantages Find [ growth increased size increased fitness ] Find (dispersal) Avoid predators Baetis will enter drift when touched by a predatory stonefly. Disadvantages? Land in Exposure to predators (esp. fish) [ getting eaten zero fitness! ] Ecological Rule? minimize /g = mortality risk (minimize, i.e., avoid predators when present) g = growth rate (maximize when possible, i.e., predation risk low) If individuals increase fitness then population will be large. 2

3 How do we measure drift? Quantify with drift net 1) Drift density: Number per water filtered Units: #/m 3 Range = per 100m 3 2) Drift rate: Number captured per Units: #/hr Range over 24 hr period: small stream: organisms Missouri River: 64 * 10 6 organisms (200 kg!) Drift FAQs: What are the patterns? Do individuals drift intentionally (active) or accidentally (passive)? What causes drift? How do insect populations persist in drift source areas? Drift Patterns -- 3 types 1) Constant low numbers (background level) 3) pollution, dewatering (can occur any time) 2) ** Diel (24 hr) pattern This is the primary form of drift we are interested in TERRESTRIAL special case "surface drift" -- terrestrial insects falling onto water, ovipositing females Important food resource for fish Drift rate or density Drift rate or density sunset Behavioral Drift: 3 mayflies sunset midnight Time of day Baetis midnight Time of day Constant Catastrophic Behavioral sunrise Ephemerella Paraleptophlebia Total sunrise 3

4 Why are organisms drifting? (1)? Many insects move from sheltered bottoms to erosive tops of stones at night to feed on algae, increasing chance of dislodgment, so nighttime peak in drift numbers. Confirmed with red flashlight technology! Good evidence that commonly drifting species can control their entry into, and exit from, the water column, so not strong case for Determine experimentally in lab? Ho: IF drift is passive THEN distance drifted fits passive fall rate curve. depth 0 10 Distance drifted (m) passive active Which active? Paradox? How can 24-hr drift rate be 100x benthic insect density? Answer: Individuals drift many times per night (maybe up to 75m) so source area for drift is extensive. Implication: Drift might deplete local populations. 4

5 Why are organisms drifting? (2) [Kohler, Fig. 3] Lab experiment: Baetis mayfly * Fed vs. starved (--) * Different amounts of food (--) * Different patchiness (# algal cells/cm 2 ) Measured what fraction of all Baetis in experimental chambers actually drifted at night. NO LOW HIGH How does drift respond to: 1) (-----) Starved drift slightly more 2)? (-----) Strong response: no food > low food > high food 3)? (-----) More drift when high patchiness, but only when not starved ** Which treatment has Highest drift? Lowest drift? Take home message? Baetis will enter drift to find food. Why do organisms drift at night? Distinguish between and cues for behavioral drift (1) Proximate cue How does organisms know it s night? Evidence? Experimentally shortened night [Fig. 10.2] Baetis drift only during controlled nighttime How would you test in real world? Solar eclipse? Full moon? 4 hr night 2 hr 1 hr 5

6 (2) What is the Ultimate cue that has selected for nighttime drift? strong selective pressure to avoid 2 supporting lines of evidence: 1. of drift (many more at night) 2. of drifting insects When should larger individuals drift? Why? [J.D. Allan, 1978]» Larger insects are more easily seen and captured by drift-feeding fish Hypothesis? Ho: Size composition same by day as by night (D:N = 1.0) Ratio of # x -Ho 10.0 x x drifting by Actual x x night vs. day 1.0 x (N:D) Insect size (head width, mm) Larger nymphs show preferential drift at night Experimental evidence of predation avoidance? How would you design? Case #1: Flecker's (1992) Natural Experiment in Venezuela [Fig. 10.6] Ratio of # drifting by 10.0 Ho night vs. day (N:D) low high Predation intensity in stream What does this result show? Ho How do bugs know it s safe to drift in the absence of fish?? 6

7 Is there a cue for fish presence? Case #2): McIntosh, Peckarsky, and others (1999) Controlled experiment with fish odor [Fig. 1, 3, 4] In a fishless stream, looked at the change in drift in response to adding fish odor between large and small Baetis and day vs. night Experimental design: - odor vs. no odor - 5-min drift samples - before/during/after - (with replication) Nighttime (large vs. small) Large at night with odor Daytime (all sizes) All by day with odor Small no change with odor The Drift Paradox? Despite amounts of drift, reaches are not What are some hypotheses? downstream displacement exaggerated drift represents excess production of benthos beyond local carrying capacity upstream movements by adult insects 7

8 The Colonization Cycle Hypothesis (Müller 1954) Population in headwaters due to drift Source areas either by: Upstream crawling occurs for some insects Compensatory upstream flight by adults Evidence? Larvae 99% move downsteam Adults 55% move upstream From Svensson

9 Flight directionality of two mayflies 82-96% of adults fly upstream From Müller 1982 Only need a few females needed to a reach (high ) Thousands of eggs per female! 9

10 Hershey et al. (1993) Experiment [Figs. 1, 4] - fertilized algae in Kuparuk River, AK with stable N isotope ( 15 N) - Use 15 N accumulated in Baetis larvae to track "source" of drifting nymphs and emerging adults. - Almost ½ of Baetis population drifted > 2 km downstream over arctic summer - Found 15 N-labeled adults above drip point - Baetis drifted more from upstream, unfertilized section (low food) -. - fertilized algae in Kuparuk River, AK with stable N isotope ( 15 N) - Use 15 N accumulated in Baetis larvae to track "source" of drifting nymphs and emerging adults. - Almost ½ of Baetis population drifted > 2 km downstream over arctic summer - Found 15 N-labeled adults above drip point - Baetis drifted more from upstream, unfertilized section (low food) -. Mixing model shows that ~1/3 to 1/2 of adult Baetis population flew km upstream from where they emerged 10

11 Macneale et al. (2005) experiment - labeling of detritus in Hubbard Brook, NH Leuctra ferruginea 20% of captured stonefly adults labeled with 15 N Most adults captured along streams 76% of labelled individuals travelled upstream A proposed model of dispersal probabilities for L. ferruginea emerging from a point (arrow), based on mark-recapture data. 11