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Linkages between trophic variability and distribution of Pteronarcys spp. (Plecoptera: Pteronarcyidae) along a stream continuum.

INTRODUCTION

The concept of functional feeding groups (FFGs) among stream macroinvertebrates (sensu Merritt and Cummins, 1996) is an important tenet of stream ecology. Although FFG classifications are often linked to the type of food an organism consumes (e.g., shredders - vascular plant tissue, collectors - fine particulate organic matter [FPOM]), FFGs are primarily based on the macroinvertebrate's mode of food acquisition (sensu Cummins, 1987). For example, filter feeders that obtain the majority of their production from animal consumption (e.g., Benke and Wallace, 1980) are nonetheless placed in the filter feeder, and not the predator, FFG. The River Continuum Concept (RCC) (Vannote et al., 1980) is largely linked to the concept of FFGs. The RCC predicts that shredders will have their highest biomass in forested headwater streams (roughly stream orders 1-3) due to the relatively high input of allochthonous coarse particulate organic matter (CPOM). As the amount of CPOM in a stream decreases with increasing stream size, shredders will likewise progressively decrease from headwaters to medium sized streams (orders 4-6) to larger rivers (orders [greater than] 6).

Some aquatic insects are known to change their diet because of differential instar preferences (Crosby, 1975; Fuller and Stewart, 1979) or changing food availability (Richardson and Gaufin, 1971; Lechleitner and Kondratieff, 1983). As Cummins (1973) suggests, many taxa are trophic generalists that are greatly influenced by local food availability. Because FFGs are tied to a consumer's food, trophic shifting and generalism can distort relative FFG biomasses. Hence, "blind faith" assumptions of an organism's FFG based solely on published classifications (e.g., Merritt and Cummins, 1996) may refute the RCCs predictions. The fact that FFGs are primarily founded at the generic and not at the species level (see Merritt and Cummins, 1996) can also skew the RCCs predictions because some species may not feed like the majority of their congeners and therefore should not be placed in the same FFG. These potential problems must be considered whenever doing system level comparisons of FFGs, especially when observed biomass and production of specific taxa do not coincide with RCC predictions.

Pteronarcys spp. (Plecoptera: Pteronarcyidae) are large stoneflies generally characterized as CPOM shredders (McDiffett, 1970; Merritt and Cummins, 1996), although Angradi (1993) observed Pteronarcys californica (Newport) feeding primarily on FPOM. In addition to detritus, some species consume varying amounts of animal material and algae (Richardson and Gaufin, 1971; Shapas and Hilsenhoff, 1976; Lechleitner and Kondratieff, 1983; Freilich, 1991). Therefore, the genus has additionally been characterized as facultative predators and scrapers (Merritt and Cummins, 1996). Pteronarcys occurs from second- through seventh-order streams in the Little Tennessee River (LTR) drainage basin in western North Carolina, constituting a fairly large percentage of the invertebrate biomass through this range ([less than]1-47%, Grubaugh et al., 1996). Pteronarcys displays its highest biomass in the seventh-order LTR (Grubaugh et al., 1996), which has very little allochthonous CPOM standing crop (see below). Greater biomass of this "shredder" taxon at downstream sites is inconsistent with RCC predictions. Our objective is to investigate whether: (1) Pteronarcys exclusively shreds allochthonous material throughout the drainage basin, which would disagree with the RCC at the medium and large order stream sites (collecting stations 5b-7, see below), or (2) their diet shifts with stream order, possibly as the result of a species composition change along the continuum.

METHODS

Study sites. - The study was conducted along a 35-km section of a river continuum in the Blue Ridge province of the southern Appalachian Mountains. The continuum includes Ball Creek at the Coweeta Hydrologic Laboratory, Coweeta Creek, and the LTR upstream of Fontaria Reservoir. Five sampling sites were established along the fourth- through seventh-order reaches of the continuum and are numbered by stream order, with site 5a upstream of 5b. Geomorphic parameters and thermal regime change with stream order along the continuum (Table 1). At the uppermost sites (4 and 5a; Lower Ball Creek and Coweeta Creek, respectively, at the Coweeta Hydrologic Laboratory), an extensive canopy of mixed hardwoods and rhododendron (Rhododendron maximum L.) effectively shades the stream year-round. Stream width is narrow (5.5 m and 7.2 m, respectively), allochthonous inputs are prevalent, and leaf packs and debris dams are common within the stream channel. [TABULAR DATA FOR TABLE 1 OMITTED] Farther downstream (site 5b; Coweeta Creek at the Coweeta Creek campground), canopy cover diminishes as the stream widens (13 m). Leaf packs become a less common feature of the channel. Large cobble is colonized by the submerged macrophyte Podostemum ceratophyllum (Michaux). At the lowermost sites (6 and 7; Little Tennessee River at Prentiss and Iotla, respectively), the stream channel becomes much wider (25 and 60 m, respectively) and the canopy cover becomes a minor feature of stream area. Leaf packs and debris dams are sparse and occur only in small slackwater areas along the shore. Bedrock, boulder and cobble substrata are extensively colonized by thick stands of P. ceratophyllum at both downstream sites.

Dietary analysis. - Pteronarcys nymphs at all sites were collected with a kicknet (1-mm-mesh opening) on 25 August and 13 December 1993, and were preserved in Kahle's solution (Wiggins, 1996). The foreguts of five similar sized individuals from each site and date were dissected (after Cummins, 1973). A sample of each animal's gut contents was individually sonicated in a beaker for 30 sec and was then drawn down under vacuum onto a 0.45 [[micro]meter] pore Metricel membrane filter. The filters were dried at 60 C for 20 min and then cleared with several drops of immersion oil. Each filter was permanently mounted on a slide with a drop of Permount[R] and a cover slip. Slides were scanned at 100x to identify food items as either animal material, amorphous detritus, vascular plant detritus, diatoms, or filamentous algae. Projected areas of individual food particles within each category were measured by digitizing 10 randomly selected fields of view for each insect using a SummaSketch[R] digitizer interfaced to a desktop computer. Total area of diatoms consumed was obtained by digitizing 100 diatoms at 200x and converting that measurement to a mean size at 100X. The total number of diatoms was then counted at 100X and multiplied by the mean size. Although the amorphous detritus category is a catchall for all unrecognizable organic matter (Shapas and Hilsenhoff, 1976), in this study it is presumed to be dominated by allochthonous material. Therefore, this, along with vascular plant detritus, will hereafter be combined as detrital material.

Projected areas of particles were used to calculate proportions of the various food types in the guts from each site and date. A Mann-Whitney nonparametric one-way analysis of variance (ANOVA) was run on SYSTAT Version 5.2 (SYSTAT, Inc., 1992) to determine whether the percentage of each of the four food categories differed between collection dates for each site. A Jonckheere-Terpstra test for ordered alternatives (Daniel, 1978) was performed to test the a priori hypotheses that the amount of detrital material consumed progressively decreased while the amount of diatoms consumed progressively increased at higher order sites.

[TABULAR DATA FOR TABLE 2 OMITTED]

The amount of production attributable to each food type was computed for each site and date (see Table 2 for an example of this procedure). We did not calculate assimilation efficiency (AE) or net production efficiency (NPE) for these pteronarcids and have assumed these numbers to be similar to those published for other aquatic insects. The AE of detrital material was taken from McDiffett's (1970) calculations for Pteronarcys scotti Ricker. AEs for all other food items were taken from Benke and Wallace (1980) while NPE was also taken from McDiffett's (1970) calculations for P scotti.

Species identification. - Although several dichotomous keys exist for Pteronarcys spp. nymphs (Ricker, 1952; Tarter, 1976; Surdick and Kim, 1976), none are applicable to all potential species inhabiting the LTR drainage basin and free of subjective couplets. Therefore, we conducted an allozyme electrophoretic analysis on individuals from each site in order to definitively identify the number of species inhabiting the LTR continuum. A total of 62 nymphs ([greater than or equal to]10/site) were collected from the five sites and analyzed at five presumptive allozyme loci: Aat-1, Aat-2, Dia, Mdh, and Pgm. These loci were chosen because Wright and White (1992 and M. M. White, Ohio University, pers. comm.) found them to be diagnostic and therefore good characters to discriminate among the five Pteronarcys species (Pteronarcys biloba Newman, P. dorsata (Say), P. pictetii Hagen, P. proteus Newman and P. scotts) which potentially occur in the LTR drainage basin.

Pteronarcids were collected on 5 November 1995 from cobble riffle habitats at each site using a kicknet (1-mm-mesh opening). Live individuals were transported back to the laboratory on ice and then stored at -80 C until electrophoretic analysis. Thin thoracic slices from each stonefly ([approximately equal to]8 [[micro]liter]) were placed in individual 1.5-ml microcentrifuge tubes and crushed in 8 [[micro]liter] of crushing buffer (10 ml di[H.sub.2]O, Img NADP, 10 [[micro]liter] [Beta]-mercaptoethanol) using a glass rod. The samples were then centrifuged at 10,000 rpm for 90 sec. Electrophoresis was performed on cellulose acetate plates using the method of Hebert and Beaton (1993). All gels were run at room temperature ([approximately equal to]25 c) at 200 V for 30 min. The electrophoretic running conditions and stain recipes for each enzyme are listed in Table 3. Allelic designations were confirmed by running presumed homologues side by side on the same gel (i.e., line-up gels, sensu Richardson et al., 1986). Alleles and loci (for Aat) were numbered by decreasing electrophoretic mobility.

The criteria used to determine reproductive isolation, and therefore species distinctness, was the presence of differentially fixed alleles between Pteronarcys individuals. If two groups of individuals are fixed for different alleles at a locus, then we can assume there is no gene flow between them and they are therefore "good" species, as defined by the biological species concept. Allele frequencies were calculated for all presumptive loci and species. [TABULAR DATA FOR TABLE 3 OMITTED] Deviations from Hardy-Weinberg equilibrium expected allele frequencies were measured using the [[Chi].sup.2] goodness-of-fit test. All calculations were performed by BIOSYS-1 (Version 1.6, Swofford and Selander, 1981).

Voucher specimens from both the dietary analysis and the species identification are stored in the University of Georgia Natural History Museum.

RESULTS

Dietary analysis. - A total [greater than]7700 food particles were digitized or counted for the 50 individuals sampled in this study. Overall, 80.0% of gut contents was comprised of detrital material, 13.3% of diatoms, 5.8% of animal material and 1.0% of filamentous algae. All sampled animals contained detrital material, 76% contained animal material, 70% contained diatoms and 32% contained filamentous algae. At sites 4, 5a and 5b, pteronarcids consumed a significantly lower percentage of detrital material (P [less than or equal to] 0.05) and a significantly higher percentage of animal material in August than in December (Table 4). Pteronarcids [TABULAR DATA FOR TABLE 4 OMITTED] also consumed a significantly higher percentage of diatoms and a significantly lower percentage filamentous algae in August than in December at sites 5b and 7, respectively (Table 4).

On each date, Pteronarcys significantly consumed progressively more diatoms with increasing stream size (August, P [less than] 0.005; December, P [less than] 0.005) while significantly consuming progressively less detrital material (August, P = 0.005; December, P [less than] 0.005) (Table 4). Detritus accounted for [greater than]90% of their diet at site 4 and ca. 50% at site 7, while diatoms accounted for [less than]1% of their diet at site 4 and 46% at site 7 (the latter three values are means for all individuals as none of these food items were consumed at significantly different proportions between dates). In August, detritus was responsible for more of Pteronarcys' production than algae at every site [ILLUSTRATION FOR FIGURE 1 OMITTED]. In December, detritus was responsible for [greater than]85% of the production at upstream sites (4, 5a and 5b) and algae were responsible for [greater than]75% of production at the downstream site 7 [ILLUSTRATION FOR FIGURE 1 OMITTED]. Although animal material contributed little to Pteronarcys production in December [ILLUSTRATION FOR FIGURE 1 OMITTED]), in August animals accounted for [greater than]25% of production at sites 4 and 5a, ca. 50% of production at site 5b and [greater than]75% of production at site 6 [ILLUSTRATION FOR FIGURE 1 OMITTED].
TABLE 5. - Sample sizes and allele frequencies at each collecting
station and locus for each putative Pteronarcys species collected
from the Little Tennessee River drainage basin

                                  Station

                  4             5a        5b          6          7

P. sp. A
(N)               11            10        10          3
Aat-2
1               1.000         1.000     1.000       1.000
Dia
2               1.000         1.000     1.000       1.000
Pgm
1                                       0.050
2               0.091         0.150     0.250       0.500
3               0.909         0.800     0.650       0.500
4                             0.050     0.050

P. sp. B
(N)                                                   2
Aat-2
2                                                   1.000
Dia
2                                                   1.000
Pgm
3                                                   1.000

P. sp. C
(N)                                                   8         18
Aat-2
1                                                   0.625     0.361
2                                                   0.375     0.639
Dia
1                                                   1.000     1.000
Pgm
3                                                   0.063     0.028
4                                                             0.028
5                                                   0.938     0.944


Allozyme analysis. - Among the five presumptive loci examined, Mdh was monomorphic while Aat-1 was not consistently scorable. These two loci were therefore not included in the analysis. The genotypes of all 62 assayed individuals were resolved for the other three loci, however. These loci were very informative, suggesting that three putative species [Pteronarcys species A (= P. scotti, we suspect), Pteronarcys species B (= P. biloba, we suspect), and Pteronarcys species C (= P. dorsata or P. pictetii, we suspect)] exist within the LTR drainage basin. Each species had one fixed allelic difference with each other species (Table 5), thereby indicating reproductive isolation among the three taxa. Also, no observed allele frequencies deviated significantly from Hardy-Weinberg expectations for any species (P [greater than] 0.05) (which is not the case when P. sp. B individuals are lumped into P. sp. A), suggesting that each population within the LTR continuum is panmictic and each species is real. Although these taxa could be genetically divergent populations of the same species, we feel it is more parsimonious to consider them distinct species instead of trying to explain why one species is divided into three sympatric reproductively isolated subpopulations. For P. sp. A, the frequency of allele 2 at Pgm progressively increased down the stream continuum while the frequency of allele 3 progressively decreased (Table 5), which may suggest clinal variation, and therefore likely selection, at this locus.

Of the 62 pteronarcids collected, 34 were Pteronarcys sp. A, two were P. sp. B, and 26 were P. sp. C. Pteronarcys sp. A was the exclusive taxon at sites 4, 5a and 5b, with additional individuals occurring at site 6 (Table 5). Pteronarcys sp. B was only collected from site 6. Pteronarcys sp. C also occurred at site 6 and was the exclusive taxon at site 7.

DISCUSSION

We anticipated identifying the Pteronarcys individuals from the LTR drainage basin by comparing our observed genotypes to those Wright and White (1992) found in their comprehensive biochemical systematic analysis of Pteronarcys spp. (White, pers. comm.). Unfortunately, positive identifications were impossible because the suite of genotypes from the three LTR drainage basin species did not coincide with the genotypes of any three species (out of the potential five species known from western North Carolina) Wright and White observed. This failure can probably be attributed to one of four phenomena: (1) we may not have encountered the same genotypes Wright and White observed due to sampling error caused by relatively small sample sizes (n = 12 per species in their study, n = 2 for P. sp. B in our study), (2) differential resolution of alleles may have resulted from our use of different support media or buffers (i.e., electromorph splittings; see Richardson et al., 1986), (3) one or more of these three species may exhibit geographic population genetic differences (i.e., different populations have different genotypes) due to limited gene flow or differential selection, or (4) one or more of these species may be represented by more than one morphologically identical yet reproductively isolated species (i.e., cryptic species), essentially meaning that Wright and White (1992) did not assay all Pteronarcys species. Numerous aquatic insects exhibit significant intraspecific allelic differences which suggest either limited gene flow or differential selection (Sweeney et al., 1987; Funk et al., 1988; Sweeney et al., 1991; Jackson and Resh, 1992), and several cryptic aquatic insect species have been discovered using genetic markers (Funk et al., 1988; Sweeney and Funk, 1991; Jackson and Resh, 1992). Wright and White (1992) similarly found substantial allozyme differences between two P. biloba populations, thereby suggesting this too may represent either geographic genetic differentiation or a species complex with one or more cryptic species. Therefore, one of these two explanations is likely the reason our observed genotypes did not coincide with Wright and White's. A thorough analysis of Pteronarcys spp. population genetic structures over a large geographic range may resolve this discrepancy and aid in future biochemical identifications. Such a study could also: (1) clarify the amount of gene flow and thus the dispersal capabilities of these taxa, and (2) confirm whether a cline exists at the Pgm locus for P sp. A and accurately define its location (e.g., is it controlled by longitudinal stream gradients or latitudinal gradients?).

Although the dietary analysis and the species analysis were conducted on different animals 2 yr apart, each shows a changing trend through the LTR continuum which may be linked. Specifically, the Pteronarcys community apparently shifts through the LTR continuum, and this shift may be coupled with a change in food consumption. If this is the case, P sp. A individuals predominantly consumed detritus in the upper reaches of the continuum (sites 4, 5a and 5b), where leaf packs and debris dams are prevalent, whereas P. sp. C individuals consumed mixed amounts of detritus and diatoms in the low reaches (site 7), where the canopy cover is minimal and we assume the diatom abundance is relatively high (Minshall, 1978). The distributions of P. sp. A and P. sp. C may be influenced by the availability of each of these food resources, or each species may simply be opportunistic omnivores, both able to shift their diet as available food resources change. However, the latter seems to be the most likely explanation because pteronarcids throughout the continuum fed on more animal material in August, presumably when allochthonous CPOM is relatively scarce, yet they consume very little animal material shortly after leaf fall in December. For future studies, this question could be addressed by conducting dietary and allozyme analyses on the same individuals.

Pteronarcys in the LTR drainage basin had its highest biomass at the downstream site 7 (Grubaugh et al., 1996), which also corresponds to its highest ingestion of diatoms. Diatoms have a greater AE than detrital material (Benke and Wallace, 1980), which suggests higher food quality in the lower reaches of this continuum than in the upper reaches. However, we are uncertain whether P. sp. C individuals scrape diatoms at this site or whether they coincidentally consume attached algae while shredding Podostemum and/or CPOM.

"Shredder" biomass in mid- and downstream reaches of the Coweeta Creek, LTR continuum is dominated by Pteronarcys spp. (Grubaugh et al., 1996). Although the proportional contribution of shredders to total invertebrate biomass decreases along this gradient, shredder biomass remained higher than would be anticipated based on the RCC predictions of Vannote et al. (1980). Pteronarcys biomass was adjusted and reassigned to the "shredder" category based only on the portion of detrital material found in their guts (using the mean of all individuals combined), assuming that detritus was from shredding activities. The downstream decrease in shredder biomass with increasing stream size was much more evident and provided a much better fit to the data after biomasses were adjusted based on Pteronarcys gut contents [ILLUSTRATION FOR FIGURE 2 OMITTED]. Our results clearly show both longitudinal changes in food resources and sequential shifts in Pteronarcys species along the LTR continuum, as well as a temporal shift in animal consumption at the three uppermost sites. Similarly, Lechleitner and Kondratieff (1983) found that P. dorsata in the Little River (Virginia) shifted their diets temporally, from detritus in winter to algae in summer, as food resources changed. These results underscore the need for caution when assigning functional feeding groups without examining patterns of food consumption. Although FFG assignment based on dietary analysis alone can be misleading or uninformative (e.g., Palmer et al., 1993), fine-grained resolution of feeding habits may be required in some cases for omnivorous species which occur over a broad spectrum of stream sizes with different energy bases. Such resolution is very important with taxa such as Pteronarcys which often comprise a large portion of benthic biomass.

Acknowledgments. - We thank Sue Eggert for her field assistance, digitizing advice, and comments on an earlier version of this manuscript. We also appreciate the help of Sara Baer in the field, Phil Dixon for statistical advice, and Matt White for sharing unpublished data. Randy Fuller, Carl von Ende and two anonymous reviewers improved an earlier version of this manuscript. Data collection and analysis was supported by a U.S. Forest Service grant to JBW and National Science Foundation LTER grant #BSR 91-11661. Allozyme analysis and manuscript preparation were supported by Financial Assistance Award Number DE-FC09-96SR18546 between the U.S. Department of Energy and the University of Georgia.

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Author:Plague, Gordon R.; Wallace, Bruce J.; Grubaugh, Jack W.
Publication:The American Midland Naturalist
Date:Apr 1, 1998
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