J. N. Am. Benthol. Soc., 2004, 23(2):297–308 q 2004 by The North American Benthological Society
Conspecific cases as alternative grazing surfaces for larval Glossosoma intermedium (Trichoptera:Glossosomatidae) JENNIFER C. CAVANAUGH1, ROGER J. HARO2,
AND
SHANE N. JONES3
River Studies Center, University of Wisconsin—La Crosse, La Crosse, Wisconsin 54601 USA Abstract. Larval Glossosoma intermedium are dominant benthic grazers that often deplete their food resources (periphyton). We observed G. intermedium grazing periphyton from conspecific cases, a possible result of limited resources on stream cobbles, and we hypothesized that case grazing increases when periphyton resources become scarce. This hypothesis was tested by monitoring frequency of case grazing among G. intermedium in 3 streams in southwest Wisconsin for 1 y, and also in the laboratory using time-lapse video of larvae on tiles with and without periphyton. In situ periphyton biomass was higher on larval cases than on the stream cobbles to which larvae had access, irrespective of density, age, or size structure of G. intermedium. Case grazing was positively related to larval densities at 2 of the 3 streams. In laboratory experiments, larval encounter rates were similar in both tile treatments, although frequency and duration of case grazing increased on tiles lacking periphyton. These results suggest that periphyton on G. intermedium cases can provide an important resource patch for this species, especially when periphyton biomass is low in the ambient environment. Key words: grazers, periphyton, caddisfly cases, foraging behavior, intraspecific interactions, resource structure, Glossosoma intermedium.
Lotic macroinvertebrate grazers often must contend with a low-biomass food resource (epilithic periphyton) that varies both in space and time (Hart and Resh 1980). Spatial and temporal heterogeneity in periphyton have been linked to resource limitation for sessile (Hart and Robinson 1990, Wiley and Warren 1992) and mobile (Hill 1992, Kohler 1992, Kohler and Wiley 1997) grazers, frequently causing growth reductions of grazers (Fuller et al. 1986, Hill 1992, Kohler 1992). However, certain grazers can maintain high densities despite local depletion of periphyton (Wilson et al. 1999). Grazing caddisflies (Trichoptera) often act as strong interactors (sensu Paine 1980) in their benthic communities (Hart and Resh 1980, Lamberti and Resh 1983, McAuliffe 1984, Kohler and Wiley 1997, Kahlert and Baunsgaard 1999). Documentation of strategies used by grazers that allow them to maintain dominance with limited resources is critical for understanding the biotic mechanisms structuring stream benPresent address: US Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, Wisconsin 54603 USA. E-mail:
[email protected] 2 To whom correspondence should be addressed. E-mail:
[email protected] 3 Present address: Department of Zoology, University of Oklahoma, Norman, Oklahoma 73019 USA. E-mail:
[email protected] 1
thic communities (Power et al. 1988). Mouthpart morphology (Arens 1989, Wilson et al. 1999), high feeding efficiency (Kohler 1992), specialized foraging behavior (Kohler and McPeek 1989, Kohler and Wiley 1997), and life-history plasticity (Elzinga 2000) all likely contribute to community dominance by grazers. Lotic grazers often encounter resource depression in periphyton during their lifespan. Periphyton biomass has been used to describe a stream’s carrying capacity for lotic macroinvertebrate grazers (Feminella and Hawkins 1995). Although biomass does not account for periphyton production, it can be used as a measure of instantaneous resource availability for grazers. As grazer density approaches carrying capacity of the occupied resource patch, grazers must locate new patches rapidly while minimizing energy expenditure (Kohler 1984). To graze efficiently, some grazers expand foraging strategies and/or seek new patches with alternative food sources. The energetic costs of finding new food patches may be minimal for highly mobile grazers (e.g., Baetis spp.), but for many caddisflies that deal with the added load of a portable case, such costs may be substantial (Kohler 1992, Becker 2001). As a result, grazers may use joining behaviors, a positive response to stimuli resulting in a brief association between conspecif-
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FIG. 1. Locations of the 3 study streams (Coon, Poplar, and Spring Coulee creeks) in the Coon Creek watershed, Wisconsin, USA. WI 5 Wisconsin, MN 5 Minnesota, IA 5 Iowa.
ics (sensu Prokopy and Roitberg 2001), which increases resource acquisition and postpones emigration. In this context, grazing caddisflies with portable cases may join conspecifics to briefly increase both the probability of finding food and frequency of feeding, while reducing costs of emigration. Our study examined a joining behavior among larvae of Glossosoma intermedium (Trichoptera:Glossosomatidae), in which larvae graze on the cases of conspecifics. Our objectives were to 1) quantify differences in periphyton abundance on 2 grazed surfaces, stream cobbles and cases of G. intermedium, and 2) determine, using both field observations and laboratory experiments, the environmental condi-
tions under which case grazing occurred. We tested the related hypotheses that case grazing frequency would increase: 1) as G. intermedium density increased, and 2) as periphyton on stream cobbles decreased. Methods Study area We studied three 2nd-order streams (Coon Creek, Poplar Creek, and Spring Coulee Creek) in the Coon Creek watershed of southwestern Wisconsin (Fig. 1). The watershed (196 km2) discharges into the Mississippi River near the city of Stoddard, Wisconsin, and lies within the un-
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glaciated Driftless Area. Land use consisted of row-crop agriculture and grazed pastures, and valley slopes were covered by deciduous forest. The study sites were in groundwater-fed stream sections with stony-bottomed channels. Study animal Larval Glossosoma spp. are benthic grazers that consume periphyton in stony-bottomed, cold-water Holarctic streams (Wiggins 1996). Larvae construct and carry saddle-shaped cases as they graze. Glossosoma intermedium is the only known glossosomatid species inhabiting the Driftless Area of southwestern Wisconsin (Longridge and Hilsenhoff 1973, Deuschle 2001), the area of study. This species is bivoltine, with a winter (November–April) and summer (May– November) cohort (Deuschle 2001) and has 5 instars in which larvae build a new case after each molt. Glossosoma intermedium larvae are highly efficient grazers with hard-scraping and digging mandibles (Shapas and Hilsenhoff 1976), which, combined with high larval densities (1000–10,000 individuals/m2), allows them to deplete periphyton biomass similar to other species of Glossosoma (Kohler 1992). Field observations of Glossosoma density, phenology, and case grazing Larval sampling. The objective of our field study was to quantify seasonal frequency of case grazing within 3 geographically distinct populations of G. intermedium (i.e., sites in Coon, Poplar, and Spring Coulee creeks). We surveyed larval densities monthly for 13 mo (November 1999–November 2000) to evaluate case grazing behavior across all instars and both cohorts. We made monthly observations of the frequency of case grazing at each site by randomly selecting study riffles (1 per site) that allowed for unintrusive observation and recording the number of larvae exhibiting case grazing. We collected G. intermedium larvae used for density, body size, and age estimations with a D-frame dip net (150-mm mesh). We collected larvae from 3 cobbles sampled from single points along 6 random transects at each stream. We collected cobbles from the center of the channel for 3 transects and close to the stream bank (0.3–0.5 m from the wetted perimeter) for the other 3 transects. We separated G. interme-
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dium into 2 groups: larvae or post-larvae (i.e., prepupal, pupal, pharate adult stages), but only larvae were counted and measured. Glossosoma larvae graze almost exclusively on the tops of stones (Kohler 1992); therefore, we determined densities by dividing the number of larvae from each cobble by the exposed surface area of each cobble. We measured the upper surface area of cobbles from a digital image using the OPTIMAS 6.5 image analysis software package (Media Cybernetics, Silver Springs, Maryland). To determine G. intermedium body size and instar, we recorded the planar area of 30 larvae from each cobble with the OPTIMAS program. We then used area measurements to calculate individual dry mass (mg) using the formula: ln larval dry mass (mg) 5 1.61(ln planar body area) 2 3.66 (Deuschle 2001). We used maximum pronotal width to determine instar. Periphyton sampling. We removed 5 or 10 G. intermedium cases from the upper surface of cobbles and then sampled periphyton to estimate algal biomass. Five cases were removed during the first 3 sample dates, and 10 cases during the last 3 sample dates. We removed larvae immediately from their cases and preserved them separately for size and age determination. We then placed empty cases individually in shell vials, covered in Parafilmy and foil, and placed them on ice. After removing G. intermedium and all other macroinvertebrates, we collected a periphyton sample from cobbles with a stiff brush and deionized water and transported samples in opaque bottles to the laboratory. In the laboratory, we placed cases dorsal-side up on a Whatmany Grade 41 paper filter and digitally photographed them. We rinsed periphyton samples onto the filters, which were then frozen. We then determined algal biomass for cobble and larval case periphyton by extracting chlorophyll a from periphyton samples with absolute methanol (Tett et al. 1975). We collected chlorophyll a samples in alternate months to determine the relative difference between periphyton biomass on stream cobbles and G. intermedium cases. To determine the total exposed surface area of each case, we recorded the area of the dome portion of a case using the OPTIMAS program. We used dome measurements to calculate total exposed surface area using the formula: ln total exposed case surface area (mm2) 5 1.50(ln planar case area) 2 0.60 (RJH, unpublished data). We obtained this formula by measuring the dorsal
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view of the dome (planar case area), and then separating individual inorganic particles from the case dome, and then measuring and summing dorsal surfaces of particles to determine total exposed surface area of each case. We used this formula to correct all subsequent measurements of planar case area, and to reduce underestimation of total exposed case surface area. Laboratory experiment We did a laboratory experiment to examine the effect of periphyton resource level on case grazing by G. intermedium. We used time-lapse video to quantify frequency of case grazing among larvae in artificial streams with and without a periphyton source. On 26 May 2000, we incubated a series of unglazed quarry tiles (58 cm2) within a 30-m segment of Spring Coulee Creek for periphyton colonization. We placed 8 metal platforms holding 6 tiles each in situ to elevate tiles ;10 cm above the stream bed to exclude Glossosoma and other crawling grazers (Lamberti and Resh 1983, Kohler 1992). Ranges for depth and current velocity above the tiles were similar to those observed above natural cobble substrates colonized by G. intermedium (Dueschle 2001, JCC, unpublished data). We removed tiles after a 31-d colonization period and transported them back to the laboratory on ice for use in experiments. We constructed a flume (1.0 m L 3 0.15 m W 3 0.15 m D; similar to Vogel and LaBarbara 1978) for use in case grazing experiments. Experimentation involved placing 6 G. intermedium larvae on either a colonized (periphyton present) or uncolonized (periphyton absent) tile in the experimental flume and then quantifying larval activity. Chilled (14–178C), aerated well water (8.5–10.5 mg O2/L) was delivered to the flume. Approximately 20 L of water circulated through the experimental flume at an average velocity of 20 cm/s. We used fresh G. intermedium larvae from Spring Coulee Creek, and only larvae held ,4 d in captivity in experiments. We isolated larvae in individual screened containers within a holding flume the day before a trial to ensure larvae were starved and had no recent (.12 h) conspecific contacts. We conducted trials under indirect fluorescent light, and monitored larval activity for 2 h using a 2-camera time-lapse VHS recording system. We positioned 1 camera di-
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rectly above the tile, and the 2nd camera adjacent to the tile. Recording angle was cycled between the 2 cameras every 30 s using a video switch. We examined 6 small (2nd and 3rd instar) or large (4th and 5th instar) larvae in separate trials, and randomly assigned treatment and trial order (morning or afternoon). We quantified larval activity by recording the number of encounters with other conspecifics, if the encounter resulted in case grazing, and the duration of the encounter and/or case grazing bout. We defined encounters as any contact between a larva and a conspecific case, and case grazing as any physical contact between the mouth of a larva and a conspecific case. We collected a 1-cm2 sample of periphyton from tile surfaces immediately before and after each trial, and we also collected cases of larvae used in experiments. We estimated average algal biomass (as chlorophyll a) on tiles and cases, using the same method as in the field study (see above). Statistical analysis For the field study, we used linear regression to examine the relationship between mean larval density and frequency of case grazing. We used a 1-way ANOVA to determine if chlorophyll a on stream cobbles and larval cases differed among the 3 streams (SPSS, version 10.1, SPSS, Inc., Chicago, Illinois). We also used regression on log-transformed (common log) data to assess the relationship between case area and case chlorophyll a concentration. For the laboratory experiment, we analyzed differences in the frequency of conspecific encounters and the proportion of encounters resulting in case grazing between the treatment combinations (small vs large larvae, periphytoncolonized vs control tiles) using goodness of fit tests (Sokal and Rohlf 1995). We compared the average duration of each case grazing bout between the 2 tile treatments using t-tests with unequal variances (Sokal and Rohlf 1995). Results Field observations of Glossosoma density, phenology, and case grazing Larval analysis. Monthly larval G. intermedium density was similar among the 3 streams,
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FIG. 2. Mean density (no./cm ) of larval Glossosoma intermedium and the % of conspecific case grazing observed at the 3 study streams, Coon Creek (A), Poplar Creek (B), and Spring Coulee Creek (C), November 1999 to November 2000. Regression lines indicate significant relationships at a 5 0.05. NS 5 not significant. 2
with densities being highest in winter (3395 6 1011 to 6616 6 2519/m2, mean 6 SE) and lowest in spring at Spring Coulee Creek (689 6 294/m2) and summer at Coon and Poplar creeks (0 and 65 6 34/m2). The frequency of case grazing generally increased with larval density (Fig. 2). Case grazing frequency was significantly correlated with density in 2 of the 3 streams, Coon Creek (p 5 0.0002, Fig. 2A) and Spring Coulee Creek (p 5 0.0055, Fig. 2C). Trends were similar for the Poplar Creek population, but
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FIG. 3. Mean (6 SD) larval dry mass (mg) plotted against mean (6 SD) pronotal width (mm) for each instar of Glossosoma intermedium from the 3 study streams, Coon Creek (A), Poplar Creek (B), and Spring Coulee Creek (C). In (A) symbols represent instar number I to V.
were not statistically significant (p 5 0.0961, Fig. 2B). Unlike the other 2 streams Poplar Creek experienced a scouring spate in early spring, which disturbed bed materials over the entire stream reach and reduced larval densities to close to 0 (JCC, personal observations). Analysis of G. intermedium pronotal widths showed 5 larval instars (Fig. 3). The first 3 instars exhibited considerable overlap in pronotal size across the 3 populations. In contrast, instars IV and V displayed the greatest pronotal growth, although variation in larval body mass also was high (Fig. 3). Monthly instar distribu-
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TABLE 1. Comparison of algal biomass (as chlorophyll a concentration, mg/cm2) on Glossosoma intermedium larval cases, stream cobbles, and the ratio of the aforementioned means for the 3 study streams.
Stream
Cases (mean 6 SE)
Stream cobble (mean 6 SE)
Case/cobble
Coon Creek Poplar Creek Spring Coulee Creek
2.90 6 0.60 29.73 6 5.12 4.31 6 0.48
0.54 6 0.09 0.38 6 0.04 0.12 6 0.02
5.37 78.24 35.92
tions indicated that G. intermedium was bivoltine, with both cohorts showing high temporal overlap (Cavanaugh 2001). High cohort overlap resulted in the presence of 5 instars in the stream for most of the study. Therefore, we were unable to detect a relationship between modal instar and case grazing (p . 0.05). Periphyton analysis. Chlorophyll a concentration on stream cobbles was at least 1 order of magnitude lower than on G. intermedium cases in all 3 streams throughout the study (p , 0.001, Table 1). There was no significant relationship between mean chlorophyll a concentration and larval density, either for cobbles or for G. intermedium cases (p . 0.05). Chlorophyll a concentration on G. intermedium cases decreased as case size increased (Fig. 4). This relationship was significant within Poplar and Spring Coulee creeks, but not Coon Creek. The frequency of case grazing by G. intermedium was unrelated to chlorophyll a concentration on larval cases, although case grazing may occur more frequently when chlorophyll a concentration on stream cobbles is low (Fig. 5). Laboratory experiment
FIG. 4. Chlorophyll a concentration on larval Glossosoma intermedium cases as a function of case area (mg/cm2) for Coon Creek (A), Poplar Creek (B), and Spring Coulee Creek (C). Regression lines indicate significant relationships at a 5 0.05. NS 5 not significant.
In the artificial streams, larvae were engaged in case grazing behavior 79% of the time in treatments where tiles lacked periphyton, compared to only 41% when periphyton abundance on tiles was high (i.e., colonized tiles, Fig. 6). Frequency of larval encounters between periphyton treatments was not significantly different. Thus, we assumed the likelihood of a case-grazing event was independent of a larval encounter. The difference in overall frequency of case grazing between periphyton treatments was not significant when observations were separated by larval size, but was significant when trials for small and large larvae were pooled (p , 0.05, Fig. 6). In addition, duration of case grazing events also was significantly higher on tiles
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FIG. 5. Mean chlorophyll a concentration (mg/cm2) on stream cobbles plotted against frequency of conspecific case grazing by larval Glossosoma intermedium.
lacking periphyton than on periphyton-colonized tiles when size classes were pooled (p , 0.05, Fig. 7). Thus, larvae spent more time on conspecific cases when tile periphyton was absent. Chlorophyll a concentration was higher on larval cases than on tiles in both periphyton treatments (p , 0.001, Table 2). Discussion Periphyton–grazer interactions The 3 G. intermedium populations we examined reached annual peak densities of up to 6616/m2, even during periods of extremely low food resource availability. Chlorophyll a analysis showed higher algal biomass on larval cases than on stream cobbles grazed by G. intermedium. Although encounter rates between conspecifics and opportunities for case grazing increased with larval density, our laboratory experiments also suggested that frequency and duration of conspecific case grazing may increase as food resources on natural substrates decline. Taken together, these results indicate periphyton on G. intermedium cases may provide a supplemental food source for larvae, and that case grazing is a facultative feeding behavior for this caddisfly. Other studies have observed periphyton, specifically diatoms, growing on aquatic insect cas-
es or retreats. Substantial algal growth on the inner ventral strap of Agapetus fucipes (Glossosomatidae) cases also was documented by Cox and Wagner (1989). Bergey (1999) examined the cases of Gumaga nigricula (Sericostomatidae), and concluded that case architecture traps diatoms and other microalgae and also provides a refugium for these organisms from abrasion caused by larval burrowing. Unlike G. nigricula, G. intermedium does not burrow, so it is unlikely that diatoms colonizing G. intermedium cases are affected by abrasion. Poff and Ward (1988) found that interstitial spaces in Glossosoma verdona cases were large enough to harbor early instar Baetis mayflies. We observed a similar phenomenon in G. intermedium cases (JCC, unpublished data), which are structurally the same as G. verdona, and we speculate that Baetis also may use periphyton on G. intermedium cases as a supplemental food source. Chlorophyll a levels on G. intermedium cases were within the range of those found on other caddisfly cases (Bergey and Resh 1994a, b), although these studies did not compare periphyton on cases to that on stream cobbles. Glossosomatid cases have relatively large interstitial spaces between adjacent mineral particles that provide refugia for diatoms and other periphyton components. Pringle (1985) and Hershey et al. (1988) reported that the tube-shaped
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FIG. 6. Total number of encounters and the % frequency of conspecific case grazing per encounter for small and large Glossosoma intermedium larvae on artificial substrate tiles with (1) and without (–) periphyton. The last 2 bar groups combine both larval sizes. Letters above shaded bars indicate significant (p , 0.05) differences in frequency of case grazing between periphyton treatments.
FIG. 7. Duration of conspecific case grazing events (mean 6 SE) for Glossosoma intermedium larvae. Treatment combinations included small and large larvae on tiles with (1) and without (–) periphyton. The last 2 bar groups combine both larval sizes. Letters above the bars indicate significant (p , 0.05) differences between periphyton treatments.
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TABLE 2. Mean (6 SE) of algal biomass (as chlorophyll a concentration, mg/cm2) on artificial substrate (tiles) and Glossosoma intermedium larval cases. Treatment combinations included small and large larvae on tiles colonized with (1) and without (2) periphyton. (1) Periphyton
n
(2) Periphyton
n
Tiles Pre-trial Post-trial
0.34 6 0.18 0.30 6 0.18 0.38 6 0.19
16 8 8
0.08 6 0.03 0.09 6 0.03 0.07 6 0.03
16 8 8
Larval cases Small Large
3.53 6 1.82 1.03 6 0.45
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4.21 6 2.08 1.24 6 0.55
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retreats of chironomid larvae possessed higher standing crops of periphyton than the surrounding benthic substrates, thereby providing a correspondingly greater surface area for microalgal colonization than that of natural substrates. In a field experiment, Bergey and Resh (1994b) observed conspecific grazing on mineral cases of G. nigricula, and hypothesized that larvae consume algae on their own cases while protecting algae from other mobile grazers. In a related study (Cox and Wagner 1989), starved A. fucipes larvae inhabiting periphyton-rich cases, survived longer than those inhabiting lowperiphyton cases. If this pattern is true for G. intermedium, then the benefits of refugia to microalgae provided by cases of G. intermedium may be offset by grazing. The inverse relationship between chlorophyll a concentration and case area requires further investigation. Recent studies (Kahlert and Baunsgaard 1999, Hillebrand and Kahlert 2001) suggested a physiological coupling between periphyton and grazers through release of grazer excretory products (nutrients). Nutrient delivery to periphyton on larval cases may be greater than that for substrate periphyton because nutrients may be delivered both from the water column and from the larvae within its case. Younger, smaller organisms have increased metabolism and, therefore, presumably increased nutrient output as excretory products (Wen and Peters 1994). However, Bergey and Resh (1994b) reported that increased release of feces did not increase chlorophyll a concentration on G. nigricula cases to the same degree as direct injection of N and P. Gumaga nigricula construct cases that are less permeable than those built by G. intermedium, so it is possible that excretory nutrients may diffuse from cases of G. intermedium more
effectively. Such increased nutrient delivery in and around case crevices could cause increased periphyton biomass on G. intermedium cases relative to that on stream cobbles, although this hypothesis needs to be tested. Glossosoma foraging behavior Periphyton-covered larval cases, while resource-rich, may be suboptimal foraging patches. Grazing efficiency of G. intermedium may differ on cobbles and cases, being lower on cases than cobbles, and may explain why cases are used less frequently than cobbles by grazing larvae. The relatively rough case may provide a greater surface area than cobbles for periphyton, but also may be more difficult to graze than smoother surfaces such as cobbles. Comparisons of grazing behavior between smooth and rough substrates suggest that substrate texture affects algal–grazer dynamics. For example, Geller (1989) found that littorine gastropods were less effective at grazing periphyton on barnacle-covered tiles than on smooth tiles. Similar difficulties could diminish the energy gain of case grazing and, thus, may explain why larvae do not graze case surfaces more than cobbles. When benthic periphyton resources decline, case grazing may increase the benefit of aggregation and, under these conditions, case grazing by G. intermedium could be classified as joining behavior (Prokopy and Roitberg 2001). Population dispersion increases in other lotic grazers when benthic resources decline. Helicopsyche borealis shifted from an aggregated to a random distribution after grazing depleted periphyton to background levels (Lamberti and Resh 1983). Leucotrichia pictipes actively defended territories containing periphyton resources, which led to
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uniform distributions (Hart 1983). For G. intermedium, larval aggregation actually may facilitate greater per capita resource intake when periphyton on stream cobbles becomes limiting. More studies are needed to evaluate the costs and benefits to both the case grazer (i.e., the focal individual) and the case bearer. Encountering conspecifics bearing resourcerich cases also could increase area-restricted search behavior, further reinforcing aggregation (Kohler 1984). In laboratory experiments, Giller and Malmqvist (1998) reported that A. fuscipes emigrated from stones without periphyton and spent more time on periphyton-covered stones. Hart and Resh (1980) observed similar behaviors for Dicosmoecus gilvipes from field observations. In coldwater streams of Michigan, Glossosoma nigrior displayed optimal patch-use foraging behavior to achieve competitive dominance within the benthic community (Kohler 1992). Like G. nigrior, it is likely that G. intermedium primarily uses cobble substrates as its periphyton source, but must contend with large emigration costs once local periphyton resources decline (RJH and JCC, unpublished data). Average mass of a G. intermedium case is 1283 higher than that of the larva bearing it (RJH, unpublished data), so given this mass and the distance required for emigration from an aggregation of conspecifics, locomotive costs to reach higher-quality patches could be substantial. As a result, the energy gained through case grazing could outweigh the costs of emigration. Conspecific case grazing also could be energetically expensive to both the case grazer and case bearer. The mean duration of case-grazing bouts increased from 2 min, when periphyton abundance was high, to 14 min when no such resource was available. In a separate study, several case-grazing bouts lasted .40 min (RJH and J. Meisbauer, University of Wisconsin-La Crosse, unpublished data). Bearer costs could arise from locomotive constraints imposed by reductions in mobility and increased drag. Moreover, case-grazer costs could increase further if the grazer falls off the bearer, and the cost to right the case in the event of an upsidedown landing is high. Larvae unable to right themselves in the experimental flumes typically abandoned their cases and entered the drift. The risk of predation and cost of case reconstruction associated with case abandonment can be high
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(Wiley and Warren 1992, Dobson et al. 2000). Such risks also could explain why fewer G. intermedium larvae engaged in case grazing when resources were high on cobbles. Similar experiments conducted over a range of resource levels could provide insights into the positive and negative aspects of case- vs benthic-substrate grazing. Acknowledgements We thank T. Kalish, T. Cavanaugh, and many undergraduate students from the UW-La Crosse Department of Biology for field and laboratory assistance, and W. Richardson, M. Sandheinrich, and J. Saros for comments on the manuscript. We also thank the UW-La Crosse River Studies Center, the College of Science and Allied Health Undergraduate Research Grant Programs, the Graduate Student Research Program, the Wisconsin Laboratory Association, and the University of Wisconsin System’s Institute on Race and Ethnicity for support of the research. Finally, we sincerely appreciate advice from Jack Feminella, Rob Baker, and the anonymous reviewers, which greatly improved the manuscript. Literature Cited ARENS, W. 1989. Comparative functional morphology of the mouthparts of stream animals feeding on epilithic algae. Archiv fu¨r Hydrobiologie Supplement 83:253–354. BECKER, G. 2001. Larval size, case construction and crawling velocity at different substratum roughness in three scraping caddis larvae. Archiv fu¨r Hydrobiologie 151:317–334. BERGEY, E. A. 1999. Crevices as refugia for stream diatoms: effect of crevice size on abraded substrates. Limnology and Oceanography 44:1522– 1529. BERGEY, E. A., AND V. H. RESH. 1994a. Effects of burrowing by a stream caddisfly on case-associated algae. Journal of the North American Benthological Society 13:379–390. BERGEY, E. A., AND V. H. RESH. 1994b. Interactions between a stream caddisfly and the algae on its case: factors affecting algal quantity. Freshwater Biology 31:153–163. CAVANAUGH, J. C. 2001. Conspecific cases as alternative grazing surfaces for Glossosoma intermedium. MSc Thesis, University of Wisconsin-La Crosse, La Crosse, Wisconsin. COX, E. J., AND R. WAGNER. 1989. Does Agapetus fuscipes cultivate algae in its case? Hydrobiologia 175:117–120.
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DEUSCHLE, D. R. 2001. Effects of deposited sediment on a keystone grazer, Glossosoma intermedium (Trichoptera:Glossosomatidae), and its associated macroinvertebrate community in southwest Wisconsin streams. MSc Thesis, University of Wisconsin-La Crosse, La Crosse, Wisconsin. DOBSON, M., K. POYNTER, AND H. CARISS. 2000. Case abandonment as a response to burial by Potamophylax cingulatus (Trichoptera: Limnephilidae) larvae. Aquatic Insects 22:99–107. ELZINGA, C. H. 2000. An application of the Lotka–Volterra model to competition between two lotic grazers. PhD Dissertation, University of Michigan, Ann Arbor, Michigan. FEMINELLA, J. W., AND C. P. HAWKINS. 1995. Interactions between stream herbivores and periphyton: a quantitative analysis of past experiments. Journal of the North American Benthological Society 14:465–509. FULLER, R. L., T. J. FRY, AND J. A. ROELOFS. 1986. Influence of different food types on the growth of Simulium vittatum (Diptera) and Hydropsyche betteni (Trichoptera). Journal of the North American Benthological Society 7:197–204. GELLER, J. B. 1989. Grazing ecology and life-history variation of some rocky shore gastropods (Prosobranchia) in northern California. PhD Dissertation, University of California, Berkeley, California. GILLER, P. S., AND B. MALMQVIST. 1998. The biology of streams and rivers. Oxford University Press, Oxford, UK. HART, D. D. 1983. The importance of competitive interactions within stream populations and communities. Pages 99–136 in J. R. Barnes and G. W. Minshall (editors). Stream ecology—applications and testing of general ecological theory. Plenum Press, New York. HART, D. D., AND V. H. RESH. 1980. Movement patterns and foraging ecology of a stream caddisfly larva. Canadian Journal of Zoology 58:1174–1185. HART, D. D., AND C. T. ROBINSON. 1990. Resource limitation in a stream community: phosphorus enrichment effects on periphyton and grazers. Ecology 71:1494–1502. HERSHEY, A. E., A. L. HILTNER, M. A. J. HULLAR, M. C. MILLER, J. R. VESTAL, M. A. LOCK, S. RUNDLE, AND B. J. PETERSON. 1988. Nutrient influence on a stream grazer: Orthocladius microcommunities respond to nutrient input. Ecology 69:1383–1392. HILL, W. R. 1992. Food limitation and interspecific competition in snail-dominated streams. Canadian Journal of Fisheries and Aquatic Sciences 49: 1257–1267. HILLEBRAND, H., AND M. KAHLERT. 2001. Effect of grazing and nutrient supply on periphyton biomass and nutrient stoichiometry in habitats of dif-
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