The life cycle of the dragonfly Cordulegaster dorsalis was studied over one year by systematic sampling of larvae in three intermittent headwater streams in southwestern British Columbia. We determined that larvae normally take three years to reach maturity, emerging throughout July and August. There is limited evidence suggesting a split cohort development, with early emergence after two years. Additionally, we tested whether larval instars were distributed randomly or if they occupied different sediment microhabitats. Smaller animals tend to be associated with smaller grained organic sediments, although there was high variation between the streams

Classen, Marie-Claire

Marczak, Laurie B.

Richardson, John S.

English

The Canadian Field-Naturalist (E-Journal)

347Dragonfly larvae are often conspicuous and impor-tant predators in stream ecosystems. In British Colum-bia, the family Cordulegastridae is represented by asingle species, Cordulegaster dorsalis Hagen. Cord-ulegastrids occur in lotic waters, usually in smallstreams, where larvae live as shallow burrowers in sedi-ment (Cannings 2002). In southwestern coastal BritishColumbia, larvae of C. dorsalis are often found insmall headwater streams that are subject to extremeepisodic drying. In the interior of the province, thespecies is less common and is mainly associated withsmall, spring-fed streams. Adults of C. dorsalis nor-mally fly from mid-May to early September, peaking inJuly (Cannings 2002). Published accounts of the lifehistory, habitat associations or ecological requirementsof C. dorsalis in its larval stage are sparse.Kennedy (1917) estimated C. dorsalis larvae in cen-tral California may require as many as four years todevelop. The larval development of cordulegastridsin Europe ranges from 2 to 5 years and may includesplit cohorts (Ferreras-Romero and Corbet 1999). Splitcohort timing has been documented in other species inthe genus (C. boltonii [Donovan] in Ferreras-Romeroand Corbet 1999). Corbet (1999: 230) maintains it ispossible to identify a winter critical size, below whichlarvae in the first year of development will enter dia-pause during the subsequent summer period, requiringanother year’s growth to reach metamorphosis. Suchdifferential growth rates within a single cohort may beproduced by staggered egg-laying times, food limita-tion or variability in the environment. Given the tran-sitory nature of the streams used by C. dorsalis, it ispossible that it exhibits plasticity in growth rates andemergence times in response to continual shrinkingand rewetting of the streams.Dragonfly larvae increase in size by three to fourorders of magnitude as they develop. Microhabitatselection may have much to do with the physical con-straints of size. Different species of burrowing odo-nates prefer certain sediment-size categories (Corbet1999). C. boltonii has sediment-size preferences thatvary with specific larval size classes (Prodon 1976unpublished dissertation referenced in Corbet 1999).In contrast, C. maculata Sélys in Virginia is a habitatgeneralist, occuring in equal densities in both silts andsands (Burcher and Smock 2002). In general, smalllarvae of C. boltonii prefer much finer sediments thando larger larvae – larvae tend to select a particle sizethat offers the least obstruction to burrowing (Corbet1999).We report life-history observations (morphologicalmeasurements) of larval C. dorsalis over a one yearperiod in three small headwater streams subject to latesummer drying. We observed that large and small lar-vae did not occur together within pools. At the sametime, we observed surprisingly high densities of odo-nate larvae in small, shrinking pools during late sum-mer. We tested whether small and large larval instarsoccupy different sediment microhabitats and report life-history observations (morphological measurements) oflarval C. dorsalis over a one-year period in three smallheadwater streams subject to late summer drying. Life History Phenology and Sediment Size Association of the Dragonfly Cordulegaster dorsalis (Odonata: Cordulegastridae) in an Ephemeral Habitat in Southwestern British Columbia LAURIE B. MARCZAK1, JOHN S. RICHARDSON1, and MARIE-CLAIRE CLASSEN21 Department of Forest Sciences, University of British Columbia, 3041-2424 Main Mall, Vancouver, British Columbia V6T 1Z4Canada; Corresponding author: e-mail: laurie@interchange.ubc.ca2 Department of Biological Sciences, University of Alberta, CW405, Biological Sciences Centre, University of Alberta,Edmonton, Alberta T6G 2E9 CanadaMarczak, Laurie B., John S. Richardson, and Marie-Claire Classen. 2006. Life history phenology and sediment size associ-ation of the dragonfly Cordulegaster dorsalis (Odonata: Cordulegastridae) in an ephemeral habitat in southwesternBritish Columbia. Canadian Field Naturalist 120(3): 347–350.The life cycle of the dragonfly Cordulegaster dorsalis was studied over one year by systematic sampling of larvae in threeintermittent headwater streams in southwestern British Columbia. We determined that larvae normally take three years toreach maturity, emerging throughout July and August. There is limited evidence suggesting a split cohort development, withearly emergence after two years. Additionally, we tested whether larval instars were distributed randomly or if they occupieddifferent sediment microhabitats. Smaller animals tend to be associated with smaller grained organic sediments, althoughthere was high variation between the streams.Key Words: Odonata, dragonfly, Cordulegaster dorsalis, ephemeral stream, life history, phenology, British Columbia.11_05052_dragonfly.qxd  11/1/07  11:10 AM  Page 347MethodsStudy SiteThis study took place in the 5157 ha MalcolmKnapp Research Forest, located in Maple Ridge,British Columbia. The forest is located in the Pacificcoastal rain forest, with a maritime climate of cool drysummers and warm wet winters (Naiman et al. 2000).We sampled Cordulegaster dorsalis larvae for lifehistory characteristics and sediment associations inthree streams within the forest (I, F and G streams;49º18'40"N, 122º32'40"W), which were part of an on-going riparian management experiment (see Kiffney etal. (2003) for site descriptions). Both F and G streamsretained a 10 m forest buffer while I stream flowedthrough a 5-year-old clearcut. The dominant vegetationsurrounding these creeks included Red Alder (Alnusrubra) and Vine Maple (Acer circinatum) with a canopycomposed largely of Western Hemlock (Tsuga hetero-phylla), Western Redcedar (Thuja plicata) and Dou-glas-fir (Pseudotsuga menziesii). Vegetation adjacentto I stream consisted of shrubs, particularly Salmon-berry (Rubus spectabilis) and Huckleberry (Vacciniumovatum). Life-history characteristicsWe surveyed approximately 200 m each of I, F,and G streams in August, October and December of2002 and April, July and August of 2003, collectinglarvae using dip nets and hand sorting. We measuredhead width (Hdw max. distance between the lateralmargins of the compound eyes), total body length (Tbl;max. distance between the mouthparts and the end ofthe cerci measured along the dorsal surface) and wing-pad length to the nearest 0.1 mm with vernier-scalecallipers. Using regression analysis we determined thescaling coefficient (reported as the slope of the line) forhead width and body length. Size ranges of late instarclasses were determined from the relationship betweenwingpad length and head width. Year class separationwas analysed using wingpad to head width relation-ships together with visual assessment of size frequen-cy distributions (after Ferreras-Romero and Corbet1999). The level of significance was set at P = 0.05.SedimentsThe habitat associations of different size classes oflarvae were characterized from August 2002 throughApril 2003. Sediment samples were collected at eachpoint of larval capture and sieved into six size frac-tions (63 µm, 250 µm, 500 µm, 1 mm, 2 mm, 4 mm).Each sediment fraction was air-dried and weighed,then ashed at 550°C for 2 h and reweighed to obtainweights of both inorganic and organic fractions. Cumu-lative percent curves were used to determine the medi-an particle size of each sample. We used linear regres-sion to investigate the data for relationships betweenlarval head width, median sediment size and organicfraction. 348 THE CANADIAN FIELD-NATURALIST Vol. 120FIGURE 1. Life history patterns of Cordulegaster dorsalis shown for 2002-2003 — in G Creek (n = 175). Continuous linesascending from left to right indicate inferred boundaries between successive hatching cohorts. The emergence periodduring late summer is shown (dotted line). Size ranges of the last three instars are indicated. The broken line indicatesan alternate interpretation and represents the inferred boundary between the two components that could result fromcohort-splitting in the two-year-old hatching cohort, thus distinguishing larvae that grow fast (above line) and slowly(below line). Numbers below columns indicate sample sizes for each sampling date. Widths of black bars for eachinterval of headwidth on a given date represent the relative fraction of the total sample that fell in that interval; totalpossible width for any given sampling date is represented by the black legend box labelled 100%.11_05052_dragonfly.qxd  11/1/07  11:10 AM  Page 348Results and DiscussionDuring August 2003 when sample sizes were lar-gest, head width of F-0 (7.5 mm – 9.2 mm) and F-1(6 mm – 7.8 mm) larvae overlapped, but wingpadlengths were discrete (3 mm – 5.5 mm in F-2, 6.9 mm– 10 mm in F-1 and 11.9 mm – 15.5 mm in F-0). Totalbody length and head width were correlated (R2 = 0.86,P < 0.001, TBL = 4.49 HW – 0.59). In G Creek, larvae of Cordulegaster dorsalis – de-rived from eggs that likely hatched in early summer –grew through late summer and early autumn (Figure1). At the beginning of their second year most larvaehad a head width between 2 mm and 5 mm. At the endof their second year of growth, larvae achieved head-widths between 5 mm and 8 mm. Emergence occurredat head widths greater than ca. 8 mm during July andAugust of the third year. The pattern of development that we observed in GCreek (where sample size was greatest, n = 180) indi-cated that, in general, C. dorsalis larvae require threeyears to achieve metamorphosis. It is possible to inter-pret these data as evidence of a split-cohort in the sec-ond year with a “fast” group of larvae emerging fol-lowing accelerated late summer to early fall growthat the end of their first year (Figure 1, dashed line).This interpretation would suggest a winter critical sizeof ca. 6 mm (Hdw; Figure 1). Sample sizes were toosmall in I Creek (n = 30) and F Creek (n = 21) to bol-ster a split-cohort interpretation; the Hdw of collectedlarvae generally support a three-year development.Many Odonata retain a synchronized emergenceperiod through the use of photoperiod cues (e.g. Aoki1999; Burcher and Smock 2002). Some species ofodonates appear to have both “slow” and “fast” com-ponents of the same cohort (Martin et al. 1991; John-son et al. 1995; Ferreras-Romero and Corbet 1999).We present two possible development schedules forC. dorsalis: a split cohort with a “fast” componentemerging after two years and a “slow” component re-quiring three years to complete development. Cordule-gastridae are noted for their protracted larval develop-ment, a pattern that might be anticipated of shallowburrowers occupying small, cool streams (Corbet1999). Two years would represent a remarkably shortdevelopment time within this family, particularly giventhe known four-year development time for cordule-gastrids in northern California (Kennedy 1917). Amore complete record of larval growth rates is requiredbefore evidence of a slow and fast development in C.dorsalis can be dismissed; in the meantime, the datamost strongly support a simple three-year developmentschedule. During our surveys for larval development, we ob-served that small pools seldom contained both largeand small larvae. Larvae were most abundant at medi-an particle sizes between 1.5 mm and 2.5 mm. Thehead width of larvae was negatively associated withpercent total organic content in F Creek (Figure 2a; P < 0.01; R2 = 0.451) but no relationship was evidentin either G or I creeks. Similarly, the head width oflarvae was positively associated with median particlesize in F Creek (Figure 2b; P < 0.05; R2 = 0.446) whileno relationship was evident in either G or I creeks. Itshould be noted that median particle and percent2006 MARKZAK, RICHARDSON, AND CLASSEN: DRAGONFLY IN EPHEMERAL HABITAT 349FIGURE 2. Relationship between larval head width of Cordulegaster dorsalis and sediment characteristics — (A) percentageof organic matter; and (B) median particle size. Regressions were significant for F Creek (closed circles) but not forG Creek (open squares) or I Creek (open triangles).11_05052_dragonfly.qxd  11/1/07  11:10 AM  Page 349organic matter are not independent measures; there isa negative correlation (r = –0.699). Post-hoc poweranalysis suggested that these results must be interpretedcautiously (power = 0.5134 organic; 0.5072 medianparticle). All odonate larvae are opportunistic predators thatwill eat almost any type of prey they can detect andsubdue (Burcher and Smock 2002). Since larger lar-vae will eat smaller larvae when they encounter them,it seems plausible that some form of habitat partition-ing occurs to minimize intraspecific competition andcannibalism. How larvae are distributed at the micro-habitat scale will largely determine the frequency oftheir interactions with conspecifics, affecting the prob-ability of agonistic interactions. The presence or ab-sence of other predators, such as fish, may also affectthe microdistribution of larvae. Suhling (1999) demon-strated that small larvae of the gomphid, Onychogom-phus uncatus (Charpentier), were significantly moreabundant in sand where fish predation was also lowestwhile medium-sized and large larvae were most abun-dant in gravel and stones, respectively. In our study, smaller larvae appeared to be associ-ated with finer sediments with a higher organic frac-tion. There was a trend supporting larval size sortingrelative to sediment microhabitats; however, these re-sults must be interpreted conservatively since thispattern was evident in only one of the three streamssurveyed and overall sample sizes were low. In previ-ous habitat surveys of this genus, larvae of C. biden-tata Sélys were most abundant at a mean grain sizeof 2.04 mm while C. heros Theischinger were mostabundant at 2.79 mm (Lang et al. 2001). This is con-sistent with our finding that larvae of C. dorsalis weremost abundant within medium sand sediments with amedian grain size between 1.5 mm and 2.5 mm. Although C. dorsalis larvae prefer smaller sedimentsas a group, our evidence is inconclusive regardingmicrohabitat partitioning amongst larval size classes.There are several alternative explanations for the sizesorting we observed between small pools. Larvae maybecome trapped in these shrinking habitats in a nearlyrandom fashion with respect to size, and the apparentsorting occurs post hoc through intraspecific predation.Smaller larvae would only survive in pools that have alow proportion of late instar larva when these habitatsbecome isolated. Alternatively, the observed size sort-ing of large and small larvae may relate to a factor thatis strongly correlated with sediment grain sizes suchas current speed or oxygen levels. AcknowledgmentsRobert Cannings provided helpful commentary andreview. The authors would like to thank Trent Hooverand Conan Phelan for assistance with fieldwork, andmembers of the Stream and Riparian Research (StaRR)lab at the University of British Columbia for comment.The authors acknowledge funding support from NSERCand the Forest Investment Initiative Account.Literature CitedAoki, T. 1999. Larval development, emergence and seasonalregulation in Asiagomphus pryeri (Selys) (Odonata: Gom-phidae). Hydrobiologia 394: 179-192.Burcher, C. L., and L. A. Smock. 2002. Habitat distribution,dietary composition and life history characteristics ofodonate nymphs in a blackwater coastal plain stream.American Midland Naturalist 148: 75-89. Cannings, R. A. 2002. Introducing the dragonflies of BritishColumbia and the Yukon. Royal British Columbia Muse-um, Victoria. Corbet, P. S. 1999. Dragonflies: behaviour and ecology ofOdonata. Cornell University Press, New York.Ferreras-Romero, M., and P. S. Corbet. 1999. The lifecycle of Cordulegaster boltonii (Donovan, 1807) (Odona-ta: Cordulegastridae) in the Sierra Morena Mountains(southern Spain). Hydrobiologia 405: 39-48.Johnson D. M., T. H. Martin, M. Mahato, L. B. Crowder,and P. H. Crowley. 1995. Predation, density dependence,and life histories of dragonflies: A field experiment in afreshwater community. Journal of the North AmericanBenthological Society 14(4): 547-562.Kennedy, C. H. 1917. Notes on the life history and ecologyof the dragonflies (Odonata) of central California andNevada. Proceedings of the U.S. National Museum 52:483-635.Kiffney, P. M., J. S. Richardson, and J. P. Bull. 2003.Responses of periphyton and insects to experimentalmanipulation of riparian buffer width along forest streams.Journal of Applied Ecology 40: 1060-1076.Lang, C., H. Müller, and J. A. Waringer. 2001. Larval habi-tats and longitudinal distribution patterns of Cordulegasterheros Theischinger and C. bidentata Sélys in an Austrianforest stream (Anisoptera: Cordulegastridae). Odonatolog-ica 30: 395-409.Martin, T. H., D. M. Johnson, and R. D. Moore. 1991. Fish-mediated alternative life-history strategies in the dragonflyEpitheca cynosura. Journal of the North American Ben-thological Society 10: 271-279.Naiman, R. J., R. E. Bilby, and P. A. Bisson. 2000. Riparianecology and management in the Pacific coastal rain forest.BioScience 50: 996-1011.Suhling, F. 1999. Effects of fish on the microdistribution ofdifferent larval size groups of Onychogomphus uncatus(Odonata: Gomphidae). Archiv für Hydrobiologie 144:229-244.Received 14 September 2005Accepted 15 March 2007350 THE CANADIAN FIELD-NATURALIST Vol. 12011_05052_dragonfly.qxd  11/1/07  11:10 AM  Page 350

Life History Phenology and Sediment Size Association of the Dragonfly Cordulegaster dorsalis (Odonata: Cordulegastridae) in an Ephemeral Habitat in Southwestern British Columbia

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Life History Phenology and Sediment Size Association of the Dragonfly Cordulegaster dorsalis (Odonata: Cordulegastridae) in an Ephemeral Habitat in Southwestern British Columbia

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