Dan M. Johnson

Behavioral ecology of larval dragonflies and damselflies.

During the past decade, larval dragonflies and damselflies (Insecta: Odonata) have been the subjects for very productive ecological research. Descriptive field work, enclosure experiments and laboratory behavior studies have identified fish predation, intraguild predation (especially mutual predation among odonates, including cannibalism) and interference competition as particularly strong interactions influencing larval odonate assemblages. Behavioral differences among species suggest evolutionary adaptations for coexistence with different predators, and for winning intraspecific aggressive encounters.

Behavior and Ecological Interactions of Larval Odonata

Enallagma aspersum and E. traviatum (Odonata: Coenagrionidae) are the most abundant larval odonates in Bays Mountain Park (Sullivan County, Tennessee, USA), although their spatial distributions are essentially nonoverlapping. E. traviatum coexists with insectivorous fish in a small lake, whereas E. aspersum is restricted to a small fishless pond nearby. Behavioral observations revealed that E. aspersum larvae were more active than E. traviatum, and tended to occupy more conspicuous positions. E. aspersum also enganged in more confrontations than E. traviatum, especially at higher density. In laboratory experiments with juvenile bluegills (Lepomis macrochirus) as predators, E. aspersum larvae were more vulnerable to predation than E. traviatum. Red—spotted newts (Notophthalmus viridescens) also preyed on E. aspersum disproportionately. Field enclosure experiments revealed that dry mass of individual E. aspersum larvae was density dependent, and that increased density of E. aspersum or addition of E. traviatum produced similar reductions. Competition was asymmetrical, as E. aspersum appeared to have no significant effect on E. traviatum. The substantial increase in confrontations among E. aspersum larvae at higher density, and the lack of evidence for prey depletion, suggest that interference may be the mechanism of competition. Our results suggest that the distribution of E. aspersum larvae may be limited by fish predation, but although competitive interactions were detected, we have no evidence that larval competition influences the distribution of either species.

Competition among Larval Dragonflies: A Field Enclosure Experiment

Tetragoneuria cynosura and Celithemis elisa dominate the larval dragonfly assemblage of Bays Mountain Lake, Tennessee, USA, where they coexist in the extensive submersed macrophyte and allochthonous detritus habitats despite relatively high overlap in both seasonal occurrence and diet. Field enclosure experiments, designed to determine the intensity of intraspecific and interspecific competition at approximately natural densities, were conducted during September 1981 and April 1982. Survival rate for both species was dependent on intraspecific density in September, and that for C. elisa was also affected by the presence of T. cynosura. These effects are attributed to interference (encounter) competition rather than to exploitation (consumption) competition. The mechanism of competition seems to be predation by larger larvae on smaller larvae. No evidence of either exploitation or interference competition was found in the April experiment.

Direct and indirect effects of predators on the dominant invertebrates of two freshwater littoral communities

SummaryTwo congeneric damselfly species, Enallagma traviatum and E. aspersum, dominate the littoral macroinvertebrates of Bays Mountain Lake and of the adjacent fish-free Ecology Pond, respectively (northeastern Tennessee, USA). Extending previous experimental studies, we test seven hypotheses concerning the role of fish (bluegill sunfish, Lepomis macrochirus) and larvaldragonfly (Anax junius) predation, competitive effects on damselflies, and the interaction between competition and predation, in determining invertebrate dominance in these communities. Three types of experiments were conducted: an enclosure experiment within Ecology Pond, an outdoor replicated tub experiment, and a laboratory behavior experiment. The in-situ enclosure experiment showed that E. traviatum larvae were more susceptible to Anax predation than were E. aspersum larvae; a tendency toward greater vulnerability to fish of E. aspersum compared with E. traviatum was not statistically significant. The outdoor tub experiment confirmed both of these trends with statistically significant results. In the tubs, both predators inhibited feeding of both zygopterans (as indicated by reduced fecal mass), particularly for E. aspersum in the presence of fish. This effect appears to have been primarily indirect, mediated through exploitation of the zooplankton. We also detected competitive effects of E. traviatum on E. aspersum: E. traviatum reduced the emergence and increased the exposure above the substrate of E. aspersum. In the absence of predators, E. traviatum inhibited feeding of E. aspersum via interference. In the laboratory behavior experiment, predators inhibited crawling by E. aspersum. E. aspersum was more exposed than was E. traviatum; it swam and crawled more than did E. traviatum, considerably increasing these movements at night. Over all, E. traviatum consistently appeared to be the more cryptic of the two species, and E. aspersum appeared to be much more active. Our results suggest an explanation for the clear difference in structure between communities like Bays Mountain Lake and Ecology Pond: predaceous fish eliminate large invertebrate predators and shift the community toward cryptic forms at relatively low densities, reflecting the effects of both predation and exploitation competition. In the absence of fish, large invertebrate predators are less able to deplete littoral invertebrates but may favor the more active forms, perhaps because these are better able to avoid invertebrate predators.

Intraspecific interference among larvae in a semivoltine dragonfly population

SummaryThis study focuses on ways that the size distribution of individuals influences the types and intensities of competitive interactions within a population of aquatic arthropod predators. Three field experiments and one laboratory experiment were designed to test for feeding interference, interference mortality, and dispersal effects within and between larval size classes of the primarily semivoltine dragonfly Tetragoneuria cynosura in Bays Mountain Lake. One field experiment documented the temporal pattern of colonization of large-mesh cylinders by the small, first-year-class larvae during a 30-day period; the results are consistent with passive (density-independent) colonization. A second field experiment examined the effect of large, second-year-class larvae at densities of 1 or 3 per cylinder (14 or 42 m-2) on colonization by small larvae; this colonization was inhibited at the high density of large larvae. In the laboratory experiment, when larvae of the two size-classes were together in the same aquarium, small larvae moved around less than when by themselves (dispersal inhibition). Thus the inhibition of colonization observed in the field may result from interference mortality, rather than from a flight response to the presence of larger conspecifics.To evaluate this interpretation, the third field experiment measured the in-situ functional response of large larvae to each other and to their small conspecific prey. Results suggest a type 1 (linear) functional response, with feeding inteference among large larvae. Moreover, the interference mortality inflicted by larger larvae on smaller conspecifics was apparently more intense on larger individuals within the small size-class. Taken together, the three field experiments and a statistical power analysis show how colonization and interference interact to determine the local density of small larvae, and why such interference effects are difficult to detect experimentally in the field.

Variability and stability of a dragonfly assemblage

SummaryUsing 12 years of monthly sweep-net data from 9–12 permanent sampling stations, we evaluated the variability and stability of the dragonfly assemblage in Bays Mountain Lake (northeastern Tennessee, USA). To do this, we adopted the view that a stable assemblage (i.e. one capable of recovering quickly from disturbances) should have low variability (i.e. high persistence of taxa, relatively constant densities, and high rank concordance), except with disturbances more intense and frequent than those in this system. Moreover, a stable assemblage should contain populations that exhibit density dependence and should tend to remain within a restricted range of densities (boundedness), shifting toward a narrow density interval between generations (attraction). To test some specific predictions derived from these views, we analyzed 12-year sequences of larval population sizes just before the onset of emergence for the 13 dominant dragonfly taxa in the lake. Most but not all of the 13 dominant taxa persisted during the 12-year period. Variabilities of taxon densities, measured as standard deviations across generations of log-transformed population sizes, were representative of the broad range for other invertebrates but somewhat higher than those of terrestrial vertebrates. There were fewer than three significant abundance trends over the 12-year period, and rank concordance between generations was high (W=0.716). Density dependence was detected among some of the dragonfly density sequences by five different methods. Using techniques presented in the companion paper, we found strong indications of both boundedness and attraction in the whole assemblage. We conclude tentatively that an assemblage consisting of at least 11 of the 13 dominant dragonfly taxa at Bays Mountain Lake has low-to-moderate variability and is stable, but that the complete 29-species assemblage is probably not stable at the scale of this single lake. We emphasize the need for coupling such long-term descriptive analyses with studies of responses to experimental disturbances.

Interactions among coexisting larval Odonata: an in situ experiment using small enclosures

Field experiments using small replicated enclosures focused on interactions between larval populations of Epitheca cynosura and Ladona deplanata (Odonata: Anisoptera) — two species that emerge in early spring. The presence of Epitheca reduced the total biomass of Ladona, but Ladona had no significant effect on Epitheca. These early-emerging species reduced the biomass of small instars of late-emerging Anisoptera which colonized enclosures during the experiments; and the late-emerging Anisoptera seem to have inhibited colonization by Zygoptera larvae. Results are consistent with the importance of predatory (cannibalism or mutual predation) interactions in this community.

Fish-Mediated Alternative Life-History Strategies in the Dragonfly Epitheca cynosura

We investigated the potential impact of predation on life-history traits of aquatic insects by examining various factors affecting the voltinism pattern of Epitheca (Tetragoneuria) cynosura (Say) in Bays Mountain Lake, Sullivan County, Tennessee, USA. We observed the growth of individual Epitheca protected from competition and predation in small (0.03 m2) in situ enclosures. Approximately 70% of the larvae followed univoltine development; this contrasts sharply with earlier studies in Bays Mountain Lake where only 25% of emerging imagoes were thought to be univoltine. We also conducted a study of the diets of large sunfish to identify the predator with the greatest potential impact on Epitheca population densities. Redear sunfish (Lepomis microlophus) consumed large numbers of Epitheca during mid-July, and the dragonflies consumed were larger than expected based on size-distributions found in the lake. In fact, those eaten were often from sizes identified as potentially following univoltine development, suggesting that predation from large fish could reduce the success of the univoltine strategy. We also conducted an experiment in small (0.15 m2) in situ enclosures to examine competition between small sunfish and Epitheca. We found that while small sunfish had little influence on larval survival, larvae grew more slowly in their presence than in their absence. Thus small sunfish potentially hindered the ability of larvae to develop in a univoltine manner. Lastly, we conducted an experiment in large (24 m2) enclosures to test hypotheses generated by the studies above. Small sunfish produced a statistically significant reduction in the ratio of univoltine to semivoltine Epitheca larvae; and both large and small fish decreased the number of successfully emerging univoltine Epitheca. These studies show that in the absence of potentially deleterious biotic interactions, most Epitheca larvae in Bays Mountain Lake follow univoltine development; but competition and predation from sunfish result in lowered success of the univoltine strategy.

Predation by Damselfly Naiads on Cladoceran Populations: Fluctuating Intensity

Ishnura verticalis naiads (Odonata: Zygoptera) were reared among simulated aquatic plants in eight large pools. Simocephalus serrulatus (Crustacea: Cladocera) was the principle prey. The intensity of damselfly predation on Simocephalus was estimated using two independent procedures: (1) a mathematical model predicating the potential density of prey at the end of successive 4—day intervals, and (2) exclosure nets which permit random samples of the prey population to grow, free from predation, during each 4—day interval. Potential densities minus observed density yield estimates of Simocephalus mortality during each interval. These estimates fluctuate dramatically, suggesting an intermittent influence of the predators. Peaks in prey mortality tend to follow periods during which large number of naiads molted. It is hypothesized that changes in feeding behavior associated with developmental phenomena are responsible for fluctuations in the intensity of predation by damselfly naiads.

Predation, Density Dependence, and Life Histories of Dragonflies: A Field Experiment in a Freshwater Community

To characterize interactions among fish and dragonfly predators that influence population dynamics and community structure, we conducted enclosure/exclosure experiments (April-October) using 1.8-m diameter cylinders at 1-m depth in the littoral zone of Bays Mountain Lake, Tennessee. A nearly natural invertebrate assemblage colonized allochthonous detritus through 1.8-mm mesh screen from April to May when further colonization was restricted by 0.5-mm mesh. Treatments introduced in mid May consisted of all combinations of two densities of two predators--small sunfish (Lepomis macrochirus), 0 or 4 individuals/m2; senior-year-class dragonfly larvae (Epitheca cynosura), 0 or 15 individuals/m2--acting on cohorts of junior-year-class E. cynosura hatching from eggs stocked at two densities, 90 or 900/m2. Treatments were assigned randomly in each of six spatiotemporal blocks: two in 1987 and four in 1988. Junior-year-class Epitheca hatching from eggs stocked at different densities experienced strong density-dependent survival early; a 10:1 ratio of Egg Density treatments in May declined to 2:1 by mid July and 1:1 by October. This result occurred even in treatments without predators, where it is attributed to intra-cohort cannibalism. Sunfish predation reduced numbers of junior-year-class Epitheca surviving to October, but predation by senior-year-class Epitheca had little effect. Surviving junior-year-class larvae grew rapidly; at low larval densities (low Egg Density and/or Fish treatments which led to low larval density), more than 90% reached the final instar by October; in enclosures with higher densities, less than 80% did so (p < 0.03). Fish predation that reduces larval dragonfly densities early in the life cycle has long-term effects by promoting faster density-dependent development and thus shortening larval development times.