1. STREAM ‘DRIFT’

2. Common drifters: –Ephemeroptera (Baetis) –Diptera (chironomid midges, blackfies) –Plecoptera –Trichoptera (not stone- cased) –Gammarid crustaceans Much variation among taxa: –When and how often drift (daily, seasonal) –Distance drifted? Nightly … up to 75 m Lifetime … up to 10-15 km (?)

3. How do we measure drift? Quantify with drift net 1)Drift density: Number per volume water filtered Range = 10 2 - 10 3 per 100m 3 2) Drift rate: Number captured per time Range over 24 hr period: –small stream: 10 4 - 10 5 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?

4. Drift rate or density Time of day sunrisemidnightsunset Drift Patterns -- 3 types AQUATIC 1) Constant –Constant low numbers (background level) 2) Behavioral ** –Diel (24 hr) pattern 3) Catastrophic –pollution, dewatering TERRESTRIAL special case "surface drift" -- terrestrial insects, ovipositing females –NOT really same sense as above –Important food resource for fish Total Baetis Ephemerella Paraleptophlebia Drift rate or density Time of day sunrisemidnightsunset Catastrophic Constant Behavioral Behavioral Drift: 3 mayflies

5. Behavioral drift - what causes it? –1) Accidental dislodgment? Many insects move from sheltered bottoms to erosive tops of stones at night to feed on algae, increasing chance of dislodgment –Confirmed with red flashlight technology!

6. depth Distance drifted (m) 010 Behavioral drift - what causes it? –2) Active entry? Do invertebrates control drift distance? Determine experimentally in lab? Hypothesis for passive drift? Ho: IF drift is passive THEN distance drifted fits passive ‘fall rate’ curve. passive active Paradox? How can 24-hr drift rate be 100x benthic insect density? Answer: Implication: Individuals drift many times per night (maybe up to 75m) so source area for drift is extensive. Drift might deplete local populations. Which active?

7. Cost / Benefit Analysis for individual that drifts –Advantages Find food – [  growth  increased size  increased fitness ] Find habitat (dispersal) Avoid tactile invertebrate predators –Baetis will enter drift when touched by a predatory stonefly. –Disadvantages? Land in poor habitat Exposure to visual, drift-feeding 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.

8. Evidence for costs and benefits? –Avoid drift-feeding predators (go at night) –Search for food [Kohler, Fig. 3] Lab experiment: Baetis mayfly * Fed vs. starved (--) * Different amounts of food (--) * Different patchiness Measured what fraction of all Baetis in chambers actually drifted. How does drift respond to: 1) starvation level (-----) –Starved drift slightly more 2) access to food? (-----) –Strong response: –no food > low food > high food 3) algal patchiness? (-----) –More drift when high patchiness, but only when not starved ** Which treatment has Highest drift? Lowest drift? NO LOW HIGH Take home message? Baetis will enter drift to find food.

9. Distinguish between proximate and ultimate cues for behavioral drift (1) Proximate cue –What does organism sense How does individual know it is “safe” to drift?  Light ! Evidence? –Experimentally shortened night [Fig. 10.2] Baetis drift only during controlled nighttime –How would you test in “real world”? Solar eclipse ? Full moon ? 2 hr 1 hr 4 hr “night”

10. (2) Ultimate cue –The evolutionary force behind drift, i.e., what populations are adapted to. –Why do organisms drift at night? –  Avoid drift-feeding predators –2 supporting lines of evidence: Time of day Size of drifting insects –When should large drift? Why? [J.D. Allan] –Hypothesis? Ho: Size composition same by day as by night (D:N = 1.0) Insect size (head width, mm) 0.41.2 Ratio of # drifting by night vs. day (N:D) 0.8 0.5 1.0 10.0 - Ho x x x x x x Actual More drift at night, and they are larger

11. Experimental evidence of predation avoidance? –How would you design? –Case #1: Flecker's (1992) Natural Experiment in Venezuela [Fig. 10.6] Predation intensity in stream low high Ratio of # drifting by night vs. day (N:D) 0.5 1.0 10.0 Ho How do bugs ‘know’ it’s safe to drift in the absence of fish?? What does this result show? Ho

12. Experimental design: - odor vs. no odor - 5-min drift samples - before/during/after - (with replication) Is there a cue for fish presence? –Case #2): McIntosh, Peckarsky, and others (1999) Controlled experiment with fish odor[Fig. 1, 3, 4] –Looked at the change in drift in response to adding fish odor between large and small Baetis and day vs. night Nighttime (large vs. small) Daytime (all sizes) Large  at night with odor Small no change with odor All  by day with odor Why do large nymphs increase in drift with odor only at night, while all nymphs decline with odor during the daytime?

13. Testing active vs. passive drift by experimentally manipulating shear stress … Experimentally increase / decrease discharge during the daytime in upper Colorado River (lots of trout). Measure drift rates in daytime vs. nighttime. Baetis Epeorus Ephemerella Paraleptophlebia 4 species of mayflies Baetis and Epeorus: small individuals increase drift during day larger individuals waited until nighttime Ephemerella and Paraleptophlebia: larger individuals drifted during day in high flow riffle.

14. Causes of Behavioral Drift? –find Food and Habitat –avoid Invert Predators –Parasitism Some evidence for gammarid amphipods # in benthos # in drift K –Excess production hypothesis (EPH) Idea that local populations exceed carrying capacity (K) of environment  competition  resource depletion  dispersal Support? –What would this look like? –Is there evidence that drift rate increases with benthic density in the field? –Not really !!! Ho EPH

15. –Colonization cycle (Müller) Population depletion in headwaters due to drift Recolonize either by: –Upstream crawling occurs for some insects, but not “enough” –Compensatory upstream flight by adults Evidence? –Anectodal –Experimental How are headwaters not depleted?

16. Hershey et al. (1993) Experiment [Figs. 1, 4] -fertilized Kuparuk River, AK -Baetis drifted more from upstream, unfertilized section (low food) -Almost half total population drifted > 2 km downstream over summer -Used stable isotope ( 15 N) to track "source" of drifting nymphs and emerging adults. Found 15 N-labeled adults above drip point.  “Mixing model” show sthat ~1/3 to 1/2 of adult Baetis population flew 1.6-1.9 km upstream from where they emerged