International Association for Ecology Size-Dependent Drift Responses of Mayflies to Experimental Hydrologic Variation: Active Predator Avoidance or Passive Hydrodynamic Displacement? Author(s): N. LeRoy Poff, R. D. DeCino and J. V. Ward Source: Oecologia, Vol. 88, No. 4 (1991), pp. 577-586 Published by: Springer in cooperation with International Association for Ecology Stable URL: https://www.jstor.org/stable/4219836 Accessed: 28-08-2019 20:23 UTC JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms Springer, International Association for Ecology are collaborating with JSTOR to digitize, preserve and extend access to Oecologia This content downloaded from 24.9.112.12 on Wed, 28 Aug 2019 20:23:01 UTC All use subject to https://about.jstor.org/terms Oecologia (1991) 88:577-586 ~ZZ ? I Oecologia ? Springer-Verlag 1991 Size-dependent drift responses of mayflies to experimental hydrologie variation: active predator avoidance or passive hydrodynamic displacement? N. LeRoy Poff*, R.D. DeCino**, and J.V. Ward Department of Biology, Colorado State University, Ft. Collins, CO 80523, USA Received March 14, 1991 / Accepted in revised form August 7, 1991 Summary. Larger nymphs within aquatic insect taxa creases in drift rates following flow reduction and eleva- have been frequently observed to be transported down- tion, but responses of this large-bodied species were re- stream in the stream drift only at night. Others have stricted to nighttime. Drift responses for these two taxa hypothesized this pattern results primarily from large were largely independent of direction of hydrologie nymphs' behavioural avoidance of entering drift during change, thus indicating a strong behavioural control over daylight, when size-selective, visually-feeding fish preda- drift. By contrast, numbers and sizes of drifting Paralep- tors are most active. This hypothesis assumes that ani- tophlebia heteronea and Ephemerella infrequens depended mals can actively control their entry into the drift, which strongly on direction of flow change. Drift rates for both may not be the case under all flow conditions. We experi- species generally declined after flow reduction and in- mentally induced streamflow increases and decreases in creased after flow elevation. Moreover, after flow eleva- adjacent riffles in a hydrologically-stable stream during tion, larger individuals often drifted diurnally, a finding the daytime to examine whether changes in diel patterns consistent with expectations under a passive hydrody- of drift abundance and size-distribution of mayflies were namic model. These experiments indicate that size- consistent with the hypothesis of active avoidance of dependent mayfly drift reflects not only presumed risk diurnal drift. We assessed the likelihood of active vs. from visual fish predators, but also functional attributes passive mechanisms of diurnal drift entry and transport of species such as morphology, behaviour, and microha- for four taxa that differ with respect to body size, mor- bitat affiliation, which influence aspects of drift entry and pho-behavioural attributes, microhabitat use, andtransport under variable hydrologie conditions. general propensity to drift. In each of three seasons, diurnal and nocturnal drift samples were collected Key in words: Ephemeroptera - Drift - Body size - Stream- three riffles over two diel cycles. Background drift flow pat- manipulation - Pr?dation risk terns were established on the first day (no flow manipula- tion). Six h before sunset on the second day, flow was experimentally increased in one riffle, decreased in the second, and not altered in the third (control). Between- Many stream invertebrates exhibit a marked diel day differences in diurnal and nocturnal drift rate periodicity and in drift (downstream transport), typically size composition were then compared among the treat- with a nocturnal peak in abundance (see reviews in ment and reference riffles. Responses of two taxa Waterswere 1972; Brittain and Eikeland 1988). The observa- consistent with active control over drift entry, transport, tion that larger species and size classes within a species or both. For Baetis spp., drift-prone mayflies typically often drift at night led Allan (1978) to hypothesize that preyed upon by fish, diurnal drift rates immediately selective in- pressure from day-active, visually-cued predato- creased following both flow reduction and flow elevation ry fish have served as the ultimate cause for size-struc- in all seasons, but only small individuals comprised tured the drift patterns. Observations that more and larger drift. Drift by large individuals was delayed until night-individuals commonly drift nocturnally in streams with time. Epeorus longimanus also exhibited significant drift-feeding in- fishes (e.g., Allan 1978, 1984; Newman and Waters 1984; Skinner 1985; Andersson et al. 1986) but * Present address and address for offprint requests: Department of not necessarily in fishless streams (Flecker 1992), have Zoology, University of Maryland, College Park, MD 20742-4415, provided support for the predation-risk hypothesis. USA A major assumption underlying the notion of diurnal ** Department of Fisheries and Wildlife, Utah State University, predator avoidance is that an individual can actively Logan, UT 84322-5210, USA control its entry into the water column by resisting me- This content downloaded from 24.9.112.12 on Wed, 28 Aug 2019 20:23:01 UTC All use subject to https://about.jstor.org/terms 578 chanical displacement from the substratum by the shear- iour, morphology, and body size. They also differed in ing force of moving water. While behavioural control of general propensity to drift and hence in degree of drift has been attributed to many species (e.g., Elliott presumed risk of pr?dation by drift-feeding fishes. 1971; Ciborowski and Corkum 1980), much debate re- The primary hypothesis tested for each taxon in each mains as to whether animals actively enter the water season is as follows : median diurnal and nocturnal sizes column or are simply passively eroded from the stream- of drifting mayfly nymphs are not altered by diurnal bed (e.g., see Allan et al. 1986; Wilzbach 1990). An decreases or increases in streamflow. We also tested individual's morphology and body size contribute to its whether numbers drifting were altered by manipulated hydrodynamic characteristics, which determine the ex- streamflow. We were primarily interested in comparing pected drag and lift forces encountered when exposed to drift responses under directional flow changes. For ex- shearing flows (Statzner and Holm 1982, 1989). Within ample, an increase in diurnal drift rate and body size a species, larger individuals experience greater erosive after flow elevation, coupled with no change (or a de- forces (Statzner 1988) and are presumably more prone to cline) after flow reduction, would strongly suggest a passive displacement. Thus, as the speed of the current passive drift mechanism. By contrast, no diurnal change increases, so does the likelihood of mechanical dislodge- (or a decline) in size, coupled with diurnal and nocturnal ment, but in some species-specific and size-dependent increases in drift rate, would strongly implicate behav- manner. Also, distance transported (and hence time of ioural control over drift. Field experiments were potential exposure to predators) generally increases with conducted over three seasons, over which time body sizes current, but in a manner dependent on species' morphol- and relative abundances of the four taxa changed. ogy and behaviour (e.g., Elliott 1971; Corkum 1978; Ciborowski and Corkum 1980; Wilzbach et al. 1988) and Materials and methods on individual body size (Corkum and Clifford 1980; Campbell 1985; Allan and Feifarek 1989). The interac- Study site and mayfly taxa tion between hydrologie factors and species' functional attributes may therefore result in drift patterns that re- The study was conducted in the upper Colorado River (2420 m veal the relative importance of active vs. passive controls elevation) in Colorado, USA (^105? 55' W, 40? 07' N) over three seasons (Summer: 27-28 August 1985; Autumn: 15-16 November on drift and that accordingly address the generality of the 1985; Spring: 5-6 June 1986). The site was 7 km below a hypolim- pr?dation risk hypothesis. netic-release storage impoundment constructed in 1954. The Differentiation between active and passive drift is not stream's flow regime was completely regulated by predictable, step- straightforward but should be most pronounced when wise reservoir releases, occurring annually on May 1, August 1, and animals experience a sudden change in current. Rapid, September 1. Thus, streamflow was constant for at least two weeks temporal changes in streamflow are characteristic of prior to each seasonal experiment (see Poff and Ward 1991 for detailed hydrology). Storm-induced spates did not occur at this site. many lotie ecosystems (e.g., see Poff and Ward 1989) and Channel width was 10-12 m, and mean depth of riffles varied from have been widely reported to lead to dramatic changes 25-50 cm, depending on season. The