Long-term movements of self-marked caddisfly larvae (Trichoptera: Sericostomatidae) in a California coastal mountain stream

Transcription

1 Freshwater Biology (1999) 42, 525±536 Long-term movements of self-marked caddisfly larvae (Trichoptera: Sericostomatidae) in a California coastal mountain stream J.K. JACKSON,* ERIC P. MCELRAVYy AND VINCENT H. RESHy Department of Entomological Sciences, University of California, Berkeley, California, U.S.A. SUMMARY 1. Over larvae of the case-building caddisfly Gumaga nigricula were self-marked as they incorporated glitter into small portions of their cases while reared in streamside troughs. These marked individuals were released into stream pools and their movements monitored in the dry season, when base flow was low and no spates occurred, and in the wet season when base flow was high and several spates occurred. 2. Of the 9,000± larvae released in each of two stream pools in the dry season, 4± 20% (i.e. 377±1817 marked individuals) were observed on three sampling dates (4, 11 and 24 d after release). Most larvae (87±93%) remained within 4 m up- or downstream of the release line after 24 d. No larvae were found outside of the release pools, even after 37 d. 3. Of the > larvae released in one stream pool near the beginning of the wet season, 408 larvae were recaptured 130±167 d later, a period that included 30 days of high flow associated with six spates. Estimated survivorship over this period was 0.7±6.2%; there was no relationship between survival and larval size at release. Most (75%) recaptured larvae were found in the pool where they were originally released. The remaining larvae were found downstream of the release pool. Larvae had generally dispersed only a short distance downstream of the release pool (median = 18 m, maximum = 222 m). In addition, four marked pupae were later found 436 m downstream of the release pool. 4. These results illustrate the sedentary nature of larval G. nigricula as well as the important role that high flow events play in larval mortality and dispersal. These casebuilding larvae move very little during low flow periods, even when food resources appear limiting. In contrast, the frequency and distance of larval dispersal are much greater during periods with high flow. 5. Our observations for G. nigricula support previously published inferences that larval dispersal within a stream can be limited for some aquatic insects. However, our observations also suggest that, even for a relatively sedentary species like G. nigricula, larval dispersal during periods with high flow may contribute significantly to gene flow within a stream reach. Keywords: aquatic insect, dispersal, survivorship, spate, seasonal Introduction *Correspondence and present address: John K. Jackson, Stroud Water Research Center, 970 Spencer Road, Avondale, PA 19311, U.S.A. jkjackson@stroudcenter.org ypresent address: Department of Environmental Science, Policy & Management, University of California, Berkeley, CA 94720, U.S.A. Movement from one location to another is integral to the ecology of larval stream insects. Movement may reflect an active or passive response to environmental conditions. For example, an individual insect may move a short distance (e.g. < 1±10 m) upstream, ã 1999 Blackwell Science Ltd. 525

2 526 J.K. Jackson et al. downstream or laterally in response to flow, substratum, food availability, or interactions with a predator or a competitor (e.g. Resh & Rosenberg, 1984; Allan, 1995). Concurrently, longer-distance movement (e.g. 100±1000 s of metres) may occur as a result of passive transport (e.g. catastrophic drift during spates; Anderson & Lehmkuhl, 1968) or by active dispersal [e.g. migrations by mayflies, shrimp, or snails (Neave, 1930; Covich, 1988; Schneider & Lyons, 1993)]. Together, these short- and long-distance movements help define biological units such as neighbourhoods, demes and genetic populations and subpopulations for stream insects, and also the degree of connectivity between these units (Richardson, Baverstock & Adams, 1986). Diel and seasonal patterns of larval movements have been studied extensively, as have the stimuli that contribute to these movements (Waters, 1972; MuÈ ller, 1974; Wiley & Kohler, 1984; Brittain & Eikeland, 1988; Allan, 1995). Much less is known about the distance travelled or the fate of individuals that disperse (Palmer, Allan & Butman, 1996; Downes & Keough, 1998). Progress in these areas has been very slow because it is difficult to mark the large number of insects needed so that an adequate sample can be recaptured after dispersal and losses from mortality. As a result of the limited empirical data available, the discussion of stream insect dispersal remains speculative in most studies of disturbance and recolonization, biological interactions, population structure and dynamics, and habitat heterogeneity. The purpose of this study was to examine dispersal of the larvae of a case-building caddisfly, the sericostomatid Gumaga nigricula (McLachlan), using tens of thousands of larvae that were self-marked as they incorporated glitter into small portions of their cases. We assessed movements of marked individuals over a 37-day period when stream discharge was low and no spates occurred. We also assessed movements and survivorship of marked individuals over a 4±5-month period when baseflow was high and several spates occurred. Methods Field site and study organism This study was conducted in a third-order reach of Big Sulphur Creek, a coastal mountain stream in northern California U.S.A. The Mediterranean climate of this region results in a dry season (May±September) when base flow discharge is low (e.g. 0.02±0.1 m 3 s ±1 ) and spates are rare, and a wet season (November± March) when base flow discharge is high (e.g. 0.5± 1m 3 s ±1 ) and spates (i.e. peak discharge > 3.35 m 3 s ±1 or mean daily discharge > 2.03 m 3 s ±1 ) are frequent (McElravy, Lamberti & Resh, 1989; Feminella & Resh, 1990; Fig. 1). Jackson & Resh (1998) concluded from genetic and morphological analyses that Gumaga (Sericostomatidae) in Big Sulphur Creek was a distinct group that Fig. 1 Mean daily discharge for Big Sulphur Creek at Geysers Resort (U.S. Geological Survey ) between 1 September 1986 and 1 April 1988.

3 closely resembled published descriptions of Gumaga nigricula (McLachlan). We believe this name is appropriate at this time, although examination of specimens from many additional sites is needed to clarify taxonomic relationships within the genus Gumaga. G. nigricula is one of the most common aquatic insects in Big Sulphur Creek, with peak densities often exceeding larvae m ±2 in summer (Wood, 1988; McElravy et al., 1989; Resh et al., 1997). G. nigricula is primarily univoltine in Big Sulphur Creek, with larval development requiring» 11 months (e.g. June-April). Unlike most caddisflies, G. nigricula has an indeterminate number of instars (maximum observed = 15). Larval growth is rapid during summer and early autumn, slows during December and January, and is rapid again in late winter and spring. Larvae are omnivorous, feeding on algae, leaves, seeds, fine detritus and animal tissue. As they feed and grow, larvae use fine sand and silk to construct long, thin cases that taper gradually toward the posterior end. The cases are much longer than the larvae, and older portions of cases are eventually trimmed off. Final larval size is» 12± 14 mm long (5±6 mg dry mass) while cases can be 20± 25 mm long. Preliminary marking tests In 1985, mid-sized larvae were provided with a variety of potential case-building materials in separate laboratory troughs. These test materials included small glitter ( mm aluminium flakes in purple, yellow or green), medium glitter ( mm aluminium flakes in blue), stainless steel shavings, red clay tile ground to a fine powder, and fine white aquarium sand. The filamentous green alga, Cladophora sp., was added to each trough for larval food. Larvae actively incorporated all of the materials into their cases, and a clear mark was visible within a few weeks. When the test materials were removed and natural case-building materials (i.e. fine sand from Big Sulphur Creek) added to the troughs, the larvae immediately begin adding sand to the case so that the mark was surrounded on both sides by sand case. We chose to work with purple and yellow glitter ( mm) in our field experiments because cases marked with these materials were easily seen in Big Sulphur Creek (Fig. 2). Movement of larval Gumaga nigricula 527 Field Release 1 ± Movement during dry season with low flow For Field Release 1, two plastic-lined artificial pools (1 m 2 wooden frames,» 0.25 m deep) were constructed in a dry stream-side channel adjacent to Big Sulphur Creek. Stream water was gravity fed continuously to the artificial pools. Purple glitter was added to one artificial pool for case-building material while yellow glitter was added to the other artificial pool. Large amounts of Cladophora sp. were collected along the edges of the stream, washed thoroughly to remove fine sand, and then added to each artificial pool as larval food. On 8 September 1986, several thousand mid-sized larvae were hand collected and placed in the artificial pools. Case marking was stopped on 29 September 1986 following a very small spate (0.18 m 3 s ±1 on 26 September 1986) that partially flooded the dry channel. Most larval cases (92% for purple glitter and 94% for yellow glitter) had been successfully marked by then. Larvae marked with purple glitter (» individuals) were released along a transect across the middle of a 20-m long stream pool (Release Pool 1). Larvae marked with yellow glitter (» 9000 individuals) were released in a similar manner in a second stream pool (Release Pool 2) that was 16 m long and > 100 m downstream of Release Pool 1. Substratum in the release pools consisted primarily of sand, gravel and small cobbles. Some leaves, algal mats and fine detritus and a few large rocks were also present. The distribution of larvae within each release pool was surveyed by dividing the length of each pool into 1 m sections up- and downstream of the release line, and then counting all marked larvae observed in each section. Buckets with clear-plastic bottoms were used to view larvae on the stream bed. Pools were surveyed after 4, 11 and 24 d (i.e. 3, 10 and 23 October 1986, respectively). We also looked for larvae immediately up- and downstream of the release pools after 37 d (5 November 1986) to determine if there was evidence of emigration from the release pools. Field Release 2 ± Survival and movement during wet season with periods of high flow Because we anticipated high mortality and greater dispersal as a result of spates during the wet season, a larger number of larvae was marked for the second field release to increase the probability of recapturing

4 528 J.K. Jackson et al. Fig. 2 Gumaga nigricula larvae with either yellow or purple glitter incorporated into the anterior portions of their cases. marked larvae. We set up twelve plastic trays (0.5 m 2,» 0.15 m deep) alongside Big Sulphur Creek, and provided them with a continuous supply of stream water and with large quantities of Cladophora sp. as food. On 31 August 1987,» mid-sized larvae were distributed among the twelve trays and given yellow glitter as case-building material. These larvae were transferred to Release Pool 2 on 2 October Because it was possible that a significant portion of the smaller yellow larvae might lose their marks (i.e. larvae would remove older portions of their case containing the mark when it is outgrown) before the end of the wet season, an additional mid-sized larvae were distributed among the twelve trays on 2 October 1987 and given purple glitter as case-building material. These larvae were transferred to Release Pool 2 on 2 November An examination of Release Pool 2 was carried out when the purple larvae were released to determine if there was emigration of yellow larvae from the release pool after 31 d of low flow (i.e. for comparison with results from Field Release 1). The fate of marked larvae was assessed at the end of the wet season and before adult emergence. We searched for marked larvae on 11, 12 and 17 March 1988, 161±167 d after the release of yellow larvae and 130±136 d after the release of purple larvae. A total of 95 m of stream (i.e. 10 m upstream of Release Pool 2, 16 m within Release Pool 2, and 69 m downstream of Release Pool 2) was divided into 1 m sections and examined with the aid of buckets with clear-plastic bottoms. Because no marked larvae were initially observed in riffles further downstream, search efforts then focused on the next three pools downstream (i.e. 132±140 m, 145±172 m and 206±226 m downstream of Release Pool 2). Following pupation and emergence in April and May, pupal clumps in several pools 250± 600 m downstream of Release Pool 2 were examined for marked pupal cases. Case diameters at the anterior end of the glitter mark were measured for larvae released in October and November and for larvae recaptured in March. A x&syp;2 test that compared size (as case diameter) distributions of released and

5 recaptured larvae was used to examine the relationship between larval size and survival. Larvae that were not found were assumed to be alive, but shallowly buried in the stream bed, or dead as a result of being crushed, transported downstream, or stranded on the streambank by high flow events. There is little evidence that Gumaga larvae enter the hyporheric zone in Big Sulphur Creek (McElravy & Resh, unpublished). Results Field Release 1 ± Movement during dry season with low flow Stream discharge during the dry season study was relatively low and stable (0.05±0.17 m 3 s ±1 ; Fig. 1). Numerous marked larvae were observed on Day 4 (377 purple and 954 yellow larvae), Day 11 (404 purple and 1317 yellow larvae), and Day 24 (729 purple and 1817 yellow larvae). The number of marked larvae observed nearly doubled between Day 4 and Day 24. Based on the estimated number of larvae released, we were able to find 4±7% of purple larvae in Release Pool 1 and 11±20% of yellow larvae in Release Pool 2. Larval dispersal away from the release line was gradual in both pools over the 24-day period (Fig. 3). Maximum dispersal after the first 4 d was 6±7 m upstream and downstream in Release Pool 1, and 4± 5 m upstream and 6±7 m downstream in Release Pool 2. Maximum net-displacement after 24 d was 5±6 m upstream and 10±11 m downstream in Release Pool 1 and 5±6 m upstream and 6±7 m downstream in Release Pool 2. Although marked larvae were found throughout each pool after 24 d, most (87±93%) were still within 4 m up- or downstream of the release line, and none were found outside the release pools. Searching immediately up- and downstream of the release pools in early November found no evidence that larvae had left the release pools after 37 d of low, relatively stable flow (Fig. 1). Field Release 2 ± Survival and movement during wet season with periods of high flow The second field release began with low, relatively stable flow (0.02±0.16 m 3 s ±1 ) during October and Movement of larval Gumaga nigricula 529 November 1987 (Fig. 1). This was followed by increased discharge in December 1987 and January 1988, with a total of 30 d of high flow (i.e. mean daily discharge > 2.03 m 3 s ±1 ) associated with six small to moderate spates (peak mean daily discharge was 4.1± 21.8 m 3 s ±1 for the six spates). Base flow declined gradually (from 1.0 to 0.2 m 3 s ±1 ) between February and April 1988 when no spates occurred (Fig. 1). As in the first study (see observations above), no yellow larvae were found outside of Release Pool 2 after one month of low, relatively stable flow conditions. However, most of these larvae were not found in Release Pool 2 after the wet season (i.e. 4±5 months later). Of the > larvae released near the beginning of the wet season, only 408 larvae and pupae (106 yellow and 302 purple) were recaptured 130±167 d later in the 252 m reach of stream that was examined. This represents 0.1% of the yellow larvae and 0.6% of the purple larvae released the previous autumn. Because wet season spates in Big Sulphur Creek scour and rearrange the stream bed sediments, we believe most larvae that were not recaptured were no longer alive because they were crushed by moving sediments, washed downstream, or deposited on the streambank by the spates. Very few empty (marked or unmarked) cases were found in the stream in March. However, some marked larvae were presumably alive but not visible because they had burrowed shallowly into the stream bed as in Field Release 1. If we assume that only 10±20% of the marked larvae present in the study reach were visible (i.e. 80±90% were hidden from view, as in Field Release 1 and Bergey & Resh, 1994), then the estimated survival rate increases to 0.7±1.5% for the yellow larvae and 3.1±6.2% for the purple larvae. Survival may also have been somewhat underestimated if larvae that were small when marked removed the older portions of their cases that included the mark when it was outgrown. This loss of marks was expected a priori for the yellow larvae and motivated the release of purple larvae in November. The removal of marks by small yellow larvae was evident in the field in November and December, as well as in the analysis comparing sizes of released and recaptured larvae. Relative abundance of small individuals (i.e. case diameters 0.9±1.7 mm) was greater for yellow larvae released in October than for yellow larvae recaptured in March (x 2 = 31.2, 12

6 530 J.K. Jackson et al. Fig. 3 Relative abundance of marked larvae at selected distances upstream and downstream of the release line in Release Pools 1 and 2 during the dry season (Field Release 1). Larval distributions recorded 4 d (total recovered = 377 purple and 954 yellow larvae), 11 d (404 purple and 1317 yellow larvae), and 24 d (729 purple and 1817 yellow larvae) after release. Arrow indicates line where larvae were released. d.f., P = 0.002; Fig. 4). Small individuals were not as abundant among the purple larvae released in November, and there was no significant difference between the size distributions of larvae released in November and recaptured in March (x 2 = 16.6, 12 d.f., P = 0.17; Fig. 4). Based on the data for purple larvae, it does not appear that there was a relationship between larval size and survivorship during the wet season. Most (75%, 76 yellow and 231 purple individuals) of the marked larvae that were recaptured in March were found in Release Pool 2, where they were originally placed. A total of 101 marked larvae (30 yellow and 71 purple) was found in the 226-m reach downstream of Release Pool 2. No marked larvae were found upstream of Release Pool 2. Most larvae that had dispersed downstream had moved only a short distance (median, 18 m; maximum, 222 m downstream of Release Pool 2; Fig. 5). Four marked

7 Discussion Movement of larval Gumaga nigricula 531 Fig. 4 Relative abundance of sizes (as case diameter) for yellow and purple larvae released before (total measured = 290 and 188 larvae, respectively) and recaptured after (total measured = 101 and 282 larvae, respectively) the wet season (Field Release 2). pupae were later found 436 m downstream of Release Pool 2. Individuals found 222 m downstream of Release Pool 2 had successfully moved (presumably during spates) through five riffles and four pools while individuals found 436 m downstream had successfully moved through nine riffles and eight pools. Individual aquatic insects have been marked using a variety of methods such as applying paint or stain (Elliott, 1971a,b; Neves, 1979; Erman, 1981; Spence, 1981; Stout, 1982), attaching labels (Hart & Resh, 1980; Aiken & Roughley, 1985; Freilich, 1986, 1989), spraying fluorescent powders (Brusven, 1970), labeling with radioactive or stable isotopes (Bishop & Bishop, 1968; Hershey et al., 1993), and even attaching radio transmitters (Hayashi & Nakane, 1989). Late instar larvae of caddisflies and stoneflies, and adult beetles and Hemiptera have been most commonly marked because they are large and have a durable integument or case for marking. Thus, it is less likely that the mark or handling will affect their subsequent activities. The marking technique used in our study was similar to the one described by Erman (1986), who found that the limnephilid caddisfly Chyranda centralis (Banks) would incorporate pieces of plastic flagging markers into their cases. Our technique offers several advantages for studies of cased caddisflies such as G. nigricula. First, a highly visible mark can be placed simultaneously on thousands of individuals. This is essential if low recapture rates are expected because of high mortality, extensive dispersal, poor visibility, or burrowing. Second, small (as well as large) individuals can be marked. This facilitates the inclusion of early instars, which are often ignored in ecological and behavioural studies. Third, the mark can last for a few to several months, depending on individual and Fig. 5 Abundance of yellow and purple larvae (total recaptured = 30 yellow and 71 purple larvae, respectively) downstream of Release Pool 2 after the wet season (Field Release 2).

8 532 J.K. Jackson et al. seasonal variation in growth rates. For example, small individuals often add to their cases faster (and therefore remove the mark sooner) than large individuals while low temperature in winter can slow growth of small and large individuals. Finally, case diameter at the mark can be used as a measure of individual larval size, and can be used to address size related issues (e.g. survivorship or individual growth rates). Field Release 1 ± Movement during low flow We were able to find» 4±7% of the purple larvae and 11±20% of the yellow larvae from the first field release. This suggests that many larvae left the release pools during this low flow period; however, the absence of marked larvae outside of the pools does not support this conclusion. The low recovery rates appear to reflect the burrowing activities of G. nigricula as well as the presence of rocks, leaves and fine detritus on the surface of the stream bed that obscured our view of marked larvae. Bergey & Resh (1994) found most larvae of G. nigricula in Big Sulphur Creek burrowed shallowly (< 4 cm) into the stream bed during the day, leaving only a fraction (average = 26%, with the majority of daytime observations between 10 and 20%) visible on the surface of the stream bed. Elliott (1969) observed similar behaviour for the caddisfly Sericostoma personatum Spence (Sericostomatidae). It is not clear why recovery rates were higher for yellow larvae in Release Pool 2 vs. purple larvae in Release Pool 1, but it may reflect different quantities of detritus covering the stream bed and obscuring our view of larvae. Recovery rates in both pools appeared to increase between Day 4 and Day 24 as quantities of leaves, algae and fine detritus on the stream bed decreased and larvae were more easily seen. Most larvae were relatively sedentary during the period of low flow in Field Releases 1 and 2, and there was no evidence that larvae left the release pools. In fact, movement rates were so low that marked larvae had not dispersed evenly throughout either pool after 24 d, and few had reached either the upstream or downstream ends of the release pools (Fig. 3). This failure to disperse does not reflect an inability to move on the part of the larvae of G. nigricula: larvae are easily capable of moving 1±2 m per day (e.g. 6±7 m in 4 d in Release Pools 1 and 2). Neither does the failure to disperse reflect the presence of formidable barriers within the pools or at the ends of pools. We had a priori expected greater dispersal at this time of year because grazing insects (e.g. the caddisflies G. nigricula and Helicopsyche borealis (Hagen)) are abundant (McElravy et al., 1989; Feminella & Resh, 1990) and algal food resources appear scarce in Big Sulphur Creek near the end of the dry season. While food limitation can significantly reduce larval growth of these species (Lamberti, Feminella & Resh, 1987; Feminella & Resh, 1990, 1991), the conditions that occurred in 1986 and 1987 apparently did not induce extensive dispersal of G. nigricula. Limited larval movement has also been found in other marking studies conducted in streams during periods of low, stable flow. For example, Hart & Resh (1980) observed no movement out of pools by larvae of the cased caddisfly Dicosmoecus gilvipes (Hagen) (Limnephilidae). Elliott (1969) found that larvae of the cased caddisfly S. personatum moved very little during low flow, and often returned to the same location each day. Freilich (1991) observed that larvae of the stonefly Pteronarcys californica (Newport) (Pteronarcyidae) generally remained within a few metres of the point where they were released, with no movement out of a 100-m riffle over a 3-month period (maximum distance, 40 m upstream and 44 m downstream). In contrast, Neves (1979) found that the cased caddisfly Pycnopsyche guttifer (Walker) (Limnephilidae) moved actively out of pools during periods of low flow. Drift studies have suggested that some individuals move significant distances over time while others travel only short distances (e.g. McLay, 1970; Elliott, 1971a; Allan & Feifarek, 1989; Lancaster, Hildrew & Gjerlov, 1996; Winterbottom, Orton & Hildrew, 1997). Hershey et al. (1993) estimated that one third to one half of a larval Baetis population drifted at least 2100 m downstream over a summer. Thus, it appears that the insect assemblage in a single section of stream can include individuals that tend to be mobile and other individuals that tend to be sedentary. The caddisfly G. nigricula appears to belong to the latter group during periods of low flow. Field Release 2 ± Survival and movement during wet season with periods of high flow High discharge events (i.e. spates with peak discharges > 3.35 m 3 s ±1 ) that scour and rearrange sedi-

9 ments in Big Sulphur Creek occur approximately six times during an average wet season, and as many as 10±18 times during above average wet years (McElravy et al., 1989). Previous benthic studies (1977±88) found that wet season spates scoured and rearranged stream bed sediments extensively in Big Sulphur Creek, and this resulted in dramatic declines in the densities of both mobile and sedentary aquatic insects (including G. nigricula) (Wood, 1988; McElravy et al., 1989; Feminella & Resh, 1990; Resh et al., 1997). Of the > larvae we marked and released in October±November 1987, we estimate that only 0.7± 6.2% survived the wet season (this assumes that only 10±20% of the marked larvae present in the study reach were visible, as in Field Release 1 and Bergey & Resh, 1994). While survivorship may be underestimated because of small larvae losing their marks or increased burrowing following the wet season, the estimated mortality rates of 93.8±99.3% that we observed for G. nigricula during the 1987±88 wet season are similar to the mortality rates of 89.0±98.2% estimated for G. nigricula based on benthic samples collected in Big Sulphur Creek during the 1981±82 and 1982±83 wet seasons (Wood, 1988; Resh et al., 1997). A significant proportion of this mortality appears to occur early in the wet season; for example, density of G. nigricula declined by 71±90% as a result of the first few spates of the 1981±82 and 1982±83 wet seasons (Wood, 1988; Resh et al., 1997). While mortality during the wet season is very high for G. nigricula, it represents only 10.5±14.7% of total (egg to adult) mortality (which is > 98%; Wood, 1988; Resh et al., 1997). Most of the larvae recaptured at the end of the wet season were found in Release Pool 2 where they were originally placed. There was no evidence of upstream movement out of the release pool and the larvae found downstream of Release Pool 2 had generally moved only into the first riffle or pool immediately downstream. We found no location within 600 m of Release Pool 2 that served as a refugium for the thousands of larvae that left (actively or passively) Release Pool 2. In addition, we found no relationship between larval survival and size at release. Our recovery of most marked larvae in Release Pool 2 and in the first riffle and pool immediately downstream of Release Pool 2, and our inability to find thousands of larvae, suggests that the probability of survival was greatest for G. nigricula that remained in or near the release pool. Movement of larval Gumaga nigricula 533 The larvae we found downstream of Release Pool 2 provide an estimate of larval dispersal distances for G. nigricula during the wet season. Dispersal distance over extended periods that included variable flow conditions have been measured for very few aquatic insects. Like G. nigricula, greatest dispersal for the cased caddisflies S. personatum and P. guttifer, and the tropical hemipterans Limnocoris insularis Champion and Cryphocricos latus Usinger (Naucoridae) occurs during high flow events (Elliott, 1969; Stout, 1978, 1982; Neves, 1979). Elliott (1969) observed little upstream or downstream movement of S. personatum except when some individuals were swept downstream by spates; the maximum distance travelled was 15 m, with many other individuals never being found. Neves (1979) found most P. guttifer dispersed downstream of release pools (average, 400±700 m; maximum, 1495 m in 20±25 d), with greatest dispersal distances being associated with spates. The propensity for Pycnopsyche to drift during high and low flow (Mackay & Kalff, 1973; Neves, 1979) presumably contributed to the greater dispersal distance for P. guttifer relative to G. nigricula. Others have used indirect observations and extrapolations (e.g. from recolonization or modeling studies) to suggest that some taxa drift long distances downstream similar to P. guttifer (e.g. Leudtke & Brusven, 1976; Hemsworth & Brooker, 1979; Walton, 1980; Vinikour, 1981; Hershey et al., 1993). Stout (1978, 1982) found that 76% of C. latus were recaptured at the release site after 1 month, while most L. insularis were captured within 35 m of the release site. However, several L. insularis were recaptured 100 m upstream of the release site, with greatest dispersal (up to 27±35 m within 17 h) occurring when extensive backflooding by the nearby river reduced stream velocities. Hancock (1996) observed that most individuals of the freshwater shrimp Paratya australiensis Kemp (Atyidae) moved little over 1 years, and that most movement was upstream (maximum, 123±232 m upstream). Erman (1986) found direction and distance of movement of the cased caddisfly C. centralis in spring brooks depended on environmental conditions (e.g. temperature, oxygen, intermittent flow) and developmental stage. The maximum distance moved by C. centralis was 57 m; longer distances, such as were observed for G. nigricula, were not possible for C. centralis because of the small size of the spring brook habitat. In summary, the larvae of G. nigricula were rela-

10 534 J.K. Jackson et al. tively sedentary, leaving release pools only during the wet season when stream bed sediments were scoured and rearranged by spates. Most larvae apparently died during these high flow events; however, the presence of marked larvae up to 436 m downstream of the release pool shows that at least some larvae are able to survive being swept downstream by spates and are therefore very mobile. Downstream dispersal during the wet season appears to occur annually for G. nigricula because the flow conditions presumably associated with this dispersal occur several times in Big Sulphur Creek during most wet seasons (McElravy et al., 1989; Feminella & Resh, 1990; Fig. 1). Recent genetic studies of stream macroinvertebrates in Australia found evidence that (1) local subpopulations (i.e. individuals within a riffle or pool) consist of offspring from relatively few matings and (2) instream movement by larvae (or aquatic adults) is limited (Hughes et al., 1995, 1996, 1998; Schmidt, Hughes & Bunn, 1995; Bunn & Hughes, 1997). This appears to have resulted in genetic differentiation among local subpopulations of these macroinvertebrates. Our observations for larval G. nigricula are also evidence of limited in-stream movement for a cased caddisfly (especially in the absence of spates), and this clearly affects local distribution and abundance patterns. However, G. nigricula also appears to differ from the species included in the Australian studies. First, G. nigricula larvae in a single pool (and presumably riffle) are the product of thousands of separate matings rather than just a small number of matings. This is evident because there are 84±234 egg masses m ±2 at the beginning of a life cycle in Big Sulphur Creek (Wood, 1988), and individual females produce only one or two (rarely three) egg masses (Resh et al., 1997). Second, dispersal associated with high flow mixes offspring among locations. As a result,» 25% of the individuals in a riffle or a pool at the end of the wet season presumably come from upstream locations. The distance travelled may range from a few metres to several hundred metres. Thus, although adults of G. nigricula are vagile (Jackson & Resh, 1989) and adult flight is presumably the primary mechanism for dispersal within and between catchments, our results suggest that the distance and frequency of larval dispersal (i.e. up to several hundred metres, annually) may be sufficient to contribute significantly to gene flow within a stream reach. Acknowledgments We thank J.W. Feminella for assistance in the field, and W. Smith and L.J. Dondanville (Union Oil Company of California) for providing access to the study site. This study was supported by the University of California Water Resources Center as part of Water Resources Center Project UCAL-WRC-W-646 to V.H. Resh, and by funds from the Stroud Foundation, the Pennswood no. 2 Research Endowment, and the Francis Boyer Research Endowment. References Aiken R.B. & Roughley R.E. (1985) An effective trapping and marking method for aquatic beetles. Proceedings of the Academy of Natural Sciences of Philadelphia, 137, 5±7. Allan J.D. (1995) Stream Ecology. Chapman & Hall, London. Allan J.D. & Feifarek B.P. (1989) Distances travelled by drifting mayfly nymphs: factors influencing return to the substrate. Journal of the North American Benthological Society, 8, 322±330. Anderson N.H. & Lehmkuhl D.M. (1968) Catastrophic drift of insects in a woodland stream. Ecology, 49, 198± 206. Bergey E.A. & Resh V.H. (1994) Effects of burrowing by a stream caddisfly on case-associated algae. Journal of the North American Benthological Society, 13, 379±390. Bishop J.E. & Bishop J.B. (1968) A technique for selectively marking aquatic insects with 32 P. Limnology and Oceanography, 13, 722±724. Brittain J.E. & Eikeland T.J. (1988) Invertebrate drift ± a review. Hydrobiologia, 166, 77±93. Brusven M.A. (1970) Fluorescent pigments as marking agents for aquatic insects. Northwest Science, 44, 44±49. Bunn S.E. & Hughes J.M. (1997) Dispersal and recruitment in streams: evidence from genetic studies. Journal of the North American Benthological Society, 16, 338±346. Covich A.P. (1988) Geographical and historical comparisons of neotropical streams: biotic diversity and detrital processing in highly variable habitats. Journal of the North American Benthological Society, 7, 361±386. Downes B.J. & Keough M.J. (1998) Scaling of colonization processes in streams: parallels and lessons from marine hard substrata. Australian Journal of Ecology, 23, 8±26. Elliott J.M. (1969) Life history and biology of Sericostoma personatum Spence (Trichoptera). Oikos, 20, 110±118. Elliott J.M. (1971a) The distances travelled by drifting invertebrates in a Lake District stream. Oecologia, 6, 350±379. Elliott J.M. (1971b) Upstream movements of benthic

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