Density-dependent population dynamics in larvae of the dragonfly Pachydiplax longipennis: a field experiment

Summary Several features of dragonfly population biology suggest that population regulation occurs in the larval stage. This study was designed to determine if density-dependent interactions among larval odonates can affect survival, growth and emergence. First-instar larvae of the dragonfly Pachydiplax longipennis were raised in outdoor experimental ponds at initial densities of 38, 152, and 608 larvae · m^-2, under two levels of food availability. Food availability was supplemented in half the pools by volumetric addition of zooplankton every other day. Pools in the low food treatment did not receive the zooplankton supplement.There was a strong negative effect of density on the mean growth rate of survivors, which included both emerging tenerals and individuals overwintering in the larval stage. A higher proportion emerged from low density than high density pools. Metamorphs from high density populations were smaller and emerged slightly later than those from low density, but the absolute number of metamorphs did not differ significantly among density treatments. Food supplementation significantly increased the proportion of overwintering larvae. There were no significant food-by-density interactions, indicating that food and density acted independently on larval population dynamics. Density-dependent mechanisms can clearly contribute to odonate population regulation, especially by controlling the number of larvae which emerge and the average age at reproduction. Population-level responses to density may be a result of interference among larvae.     

Lizards reduce food consumption by spiders: mechanisms and consequences

Summary To determine the effect of lizards on webspider populations, we conducted a long-term field experiment in the Bahamas. Numbers of spider individuals were about 3 times higher in lizard-removal enclosures than in control enclosures with natural densities of lizards. Dietary analyses showed that lizards ate spiders and that lizard and spider diets overlapped substantially. Lizards reduced biomass of prey consumed by spiders; details indicated that they reduced biomass of large (> 4 mm) prey consumed by spiders more than biomass of small (4 mm) prey. Similarly, lizards reduced biomass of large aerial arthropods caught in sticky traps but not biomass of small aerial arthropods. We found no evidence that the lizard effect on prey consumption by spiders was caused by a spatial shift from areas with high aerial arthropod abundance to areas with low aerial arthropod abundance. Lizards reduced adult female cephalothorax width and fecundity of spiders. In a separate experiment, food-supplemented spiders were more fecund than control spiders. This study indicates that the interaction between lizards and spiders includes both predation and competition for food.     

Effects of zooplankton availability and foraging mode on cannibalism in three dragonfly larvae

Summary Cannibalism is likely to operate as a form of population control in dragonfly larvae. I performed aquarium experiments to investigate the effect of foraging activity and zooplankton availability on cannibalism in three dragonfly larvae. Large Cordulia aenea larvae showed low activity, and large Leucorrhinia dubia larvae showed high activity irrespective of zooplankton availability. In contrast, large Coenagrion hastulatum larvae changed from high activity in the absence to low activity in the presence of zooplankton. Small Cordulia aenea larvae were active in the absence of large conspecifics irrespective of zooplankton availability. In the presence of large conspecifics they showed a reduced activity when zooplankton were present. Small L. dubia larvae showed high activity and small Coenagrion hastulatum larvae low activity irrespective of pressence or absence of zooplankton and large conspecifics. In all three species cannibalism was highest in the absence of zooplankton. In the absence of zooplankton cannibalism was low in Coenagrion hastulatum compared to the other two species. On the contrary, in the presence of zooplankton, cannibalism did not differ between the three species.     

The population dynamics of pike,Esox lucius,and perch,Perca fluviatilis,in a simple predator-prey system

Synopsis The population dynamics and predator-prey relationship of pike, Esox lucius, and perch, Perca fluviatilis, were examined in simple fish communities in two adjacent shallow lakes, Lochs Kinord and Davan, Deeside, Scotland. Few perch survive to age 3 but Z is low for fish > 3 years and perch live up to 17 years. Population fecundity remained relatively high and constant in perch because of the multi-age spawning stock and the presence of older more fecund perch. Growth rates of perch in both lochs are relatively high as a consequence of low stock abundance. The N, B, and P of adult perch were unusually low. The age range of pike, and N, B, P, and growth were in the range of values reported elsewhere. There was little variation in the strength of pike year classes and the importance of cannibalism and low occurrence of alternative prey in the lochs suggest that the populations were self-regulating. Cannibalism by adults was responsible for most of mortality in perch larvae, and predation by pike and adult perch was responsible for the majority of juvenile losses. This conclusion is supported by the high biomass ratios of pike:juvenile perch of 1.0–30.8. While the number of adult fish was almost equal, the biomass of adult pike was 2–3 × that of perch in Kinord and 6 × in Davan. In L. Kinord, where year class strength was stable, high predation pressure from perch and pike reduced perch abundance rather than eliminated year classes. Perch year classes fluctuated in abundance in L. Davan and were eliminated in the first summer in two sampling years. The pike, and particularly the perch populations, have features characteristic of fish communities in unperturbed ecosystems: namely, a wide range of age classes, stability in biomass with variation dampened by longevity, and low production.     

Intraguild predation, cannibalism, and microhabitat use in Calopteryx virgo and Somatochlora metallica larvae: a laboratory experiment

Intraguild predation (IGP) and cannibalism among co-occurring lotic odonate species was studied in Central Finland. A laboratory experiment was performed to assess the microhabitat use and cannibalism between intermediate and late instars of Calopteryx virgo larvae and predation by larger Somatochlora metallica larvae on the intermediate C. virgo instars. The experiment was run in small running-water aquaria where the larvae were able to divide their mutual habitat vertically by clinging onto artificial perches or crawling on the bottom. Life span of the small C. virgo larvae and attack rate on them were compared between the cannibalism and IGP treatments. The effect of predation on the activity, habitat use and spatial distribution of the small C. virgo larvae was examined. The IGP rate was 36%. The prey larvae spent the most of their time on the perches, whereas the S. metallica preferred the substrate. The large C.␣virgo larvae did not cannibalise smaller conspecifics. The presence of a predator (S. metallica) had no effect on the habitat use or activity of the prey (C. virgo) larvae. Habitat use differed more between those species than between conspecifics of different size classes of C. virgo. The spatial distribution between S.␣metallica and C. virgo showed a completely random pattern, whereas the two size classes of C. virgo aggregated in the vegetation. Absence of cannibalism and behavioural observations indicate that C. virgo may have a low tendency for intraspecific aggressions.     

Behaviour and growth of dragonfly larvae along a permanent to temporary water habitat gradient

Abstract.  1. Freshwaters form a gradient from small temporary waters to large permanent waters. Identifying and examining traits that restrict the distribution of species along this gradient are crucial to the understanding of community structure in these habitats.
2. Using dragonfly larvae species, differences in traits important for growth and survival were studied.
3. The traits were studied in a series of laboratory experiments using two pairs of dragonfly species that coexist in the Namibian semi-desert. One species pair was from the most temporary part of the water permanence gradient and the other species pair from an intermediate part of the gradient.
4. As predicted, activity, capture rate, and growth rate were significantly greater in the two temporary water species. Contrary to the prediction made in the work reported here, species differences in microhabitat selection were not related to the species' habitat origin. Cannibalism did not differ between species.
5. The results lend support to the hypothesis that selection has favoured certain combinations of trait values and that these traits are important for a successful life in temporary and permanent waters.     

Morphological and behavioral defenses in dragonfly larvae: trait compensation and cospecialization

Many animals have two basic traits for avoiding being killedby a predator: behavioral modification and morphological defense.We examined the relationship between antipredator behavior andmorphological defense in larvae of three closely related dragonflyspecies within the genus Leucorrhinia. The three species differwith regard to their morphological defense as expressed in thelength of the larval abdominal spines. Results showed that longerabdominal spines provided protection against an attacking fishpredator (perch) because the probability of being rejected afteran attack was significantly higher in the species with the longestabdominal spines. In contrast to other studies, the specieswith the strongest morphological defense did not show the leastbehavioral predator avoidance. Instead, the species with intermediatemorphological defense showed the least predator behavioral avoidance.The results suggest that the Leucorrhinia system is a mixtureof trait cospecialization (a positive correlation between antipredatorbehavior and morphological defense) and trait compensation (anegative correlation between antipredator behavior and morphologicaldefense). Differences in the relationship between morphologicaland behavioral defense between species might be related to abundancepatterns of the three species in lakes with and without fishpredators.     

Slow, fast and in between: habitat distribution and behaviour of larvae in nine species of libellulid dragonfly

1. Activity and microhabitat use are important factors determining species performance in habitats that differ in permanence and species composition of top predators. This study examined the relationship between the distribution across a gradient of habitat permanence and an associated transition in the composition of top predators and the behaviour of species of larval dragonflies. It also assessed the relationship between larval behaviour, body size and the duration of the larval stage. In laboratory mesocosms the mobility of the different species was measured, as was the extent to which they associated with artificial vegetation. 2. Species mobility was positively related to their natural occurrence in habitats in which invertebrates or small‐bodied fish were the top predators, and negatively related with the frequency with which species co‐existed with large‐bodied fish, the permanence of the habitat and the length of the larval stage. 3. Rather than falling into strict low and high mobility categories, habitat generalists that occurred across the habitat gradient, co‐existing with different top predators, had variable mobility levels. In these generalists, mobility was positively related to how frequently they were found in natural habitats in which invertebrates were the top predators. 4. The extent to which species utilized the artificial vegetation in mesocosms was associated with the length of the larval period but was not associated with mobility or species habitat distribution in the field.     

Polymorphic microsatellite loci to study population dynamics in a dragonfly,the libellulid Trithemis arteriosa (Burmeister, 1839)

One of the most widely distributed dragonfly species in Africa is the red‐veined‐dropwing Trithemis arteriosa. It is an indicator for permanent water bodies, which are freshwater ecosystems of high environmental value especially in arid regions. For studies to determine population structures, assess species viability and monitor environmental changes, a panel of 10 polymorphic microsatellite loci was developed. The number of alleles per locus ranged from four to 12, with an observed heterozygosity ranging from 0.149 to 0.843.     

The development of dragonfly larvae (Odonata: Anisoptera) from two streams in north-eastern Victoria, Australia

Developmental stages of larvae of eight species of Anisoptera from the Kiewa River and Middle Creek, Victoria, were determined and their phenology investigated. The last six to nine instars of each species were distinguished by size frequency and scatter plots, using labium width, metafemur and wing-pad measurements, and the early instars were estimated from Dyar's Law. This suggested between 11 to 14 instars. Four species appeared to be univoltine and four species semivoltine.     

Wax covers in larvae of two Scymnus species: do they enhance coccinellid larval survival?

We tested the protective function of larval wax covers in the two ladybird beetle species, Scymnus nigrinus and S. interruptus, against cannibalism, predation and ant aggression, and its importance for the distribution of both species in the field. Cannibalism was generally very low and not influenced by the presence or absence of the wax cover, or by larval size. Fourth-instar larvae of three ladybird species, Adalia bipuncata, Exochomus quadripustulatus and Harmonia quadripunctata, consumed Scymnus larvae-which are much smaller-regularly, independent of the presence or absence of waxes. By contrast, first-instar larvae of the three species had generally little success when attacking Scymnus spp. larvae. Wax-covered S. interruptus larvae survived significantly more attacks by the predacious carabid beetle Platynus dorsalis than larvae without wax cover. Wax-covered S. interruptus larvae and S. nigrinus larvae survived attacks by workers of the ant species, Lasius niger and Formica polyctena, respectively, significantly more often than larvae without wax covers. We show that, in the field, Scymnus larvae have higher densites in ant-attended resources than in unattended ones and conclude that both Scymnus species benefit from the ability to feed in ant-attended aphid colonies by a reduced predation risk.     

Interactions among juveniles of two freshwater crayfish species and a predatory fish

Two freshwater crayfish species, Astacus astacus L. and Pacifastacus leniusculus Dana, co-occur in some Swedish lakes. Observational studies indicate that the introduced, North American species P. leniusculus may gradually replace the native A. astacus, but the mechanism behind the replacement is not known. This study examined the direct effects of interspecific competition between the crayfish, and indirect effects of competitive interactions and fish (European perch, Perca fluviatilis L.) predation. Three different experiments with young-of-the-year (YOY) crayfish were performed. P. leniusculus was strongly dominant over similar-sized A. astacus in interference competition for shelter in a laboratory experiment. However, in a 35-day experiment in outdoor pools, A. astacus growth and survival were about equally affected by interactions with conspecifics and P. leniusculus. In contrast, P. leniusculus was significantly more affected by intraspecific competition than by competition with A. astacus, suggesting asymmetric competition between the two species. The presence of perch in outdoor ponds with mixed-species groups of the two crayfish species resulted in considerably higher predation rates on A. astacus than on P. leniusculus. Both species showed strong antipredator responses to perch by increasing refuge use. I suggest that higher perch predation rates on A. astacus originate from P. leniusculus being the superior species in interspecific competition for shelter. Because of displacement from refuges, A. astacus individuals become more exposed to the predator. This indirect effect of interactions among the two cray-fish species and the predator may be important in the observed in situ replacement of A. astacus by P. leniusculus.     

Evaluating the importance of predation on subtidal benthic assemblages in sandy habitats around rocky reefs

It was proposed that predation could explain differences in the structure of the benthic macrofaunal assemblages in subtidal sandy sediments close to compared with those far from rocky reefs. This hypothesis was tested using experimental exclusion cages and partial cages at two sites at two distances at two different rocky reefs. Undisturbed uncaged assemblages of macrofauna close to the rocky reefs were generally different from those in the partial cages and full cages. However, caging artifacts could not be detected and there were no strong correlations between the macrofauna and the proportions of different grain size and organic content. The structure of the macrofaunal assemblages close to the rocky reefs was, nevertheless, different from those far from the reefs and the sediments were finer far from than close to the rocky reefs. The results indicated that factors other than predation or grain size caused the differences in the macrofauna. For the spatial and temporal scales used in this study, it was clear that, although predation maybe intense, on its own it cannot explain the differences in the structure of the assemblages close and far from rocky reefs. The importance of adequate replication on caging experiments is discussed and it is suggested that alternative ways need to be found to test predictions about the influence of predation on soft sediment benthic assemblages.     

Population regulation of a tropical damselfly in the larval stage by food limitation,cannibalism, intraguild predation and habitat drying

The relative importance of intraspecific, interspecific, and seasonal causes of larval mortality were investigated for aquatic larvae of the giant damselfly Megaloprepus coerulatus in Panama. These larvae live in water-filled holes in fallen and living trees, where they and three other common odonate species are the top predators. By mid wet season, M. coerulatus larvae were found in nearly half of all tree holes that harbored odonates. Although M. coerulatus were typically, but not always, eliminated from holes inhabited by larger hetero-specifics, M. coerulatus were more likely to encounter conspecifics than other odonate species. Hole with less than 11 of water rarely contained more than a single larva. In large holes where M. coerulatus was the only odonate species present, multiple larvae coexisted at a density of one larva per 1–21 of water. There the absence of 2–4 of the 5 larval size classes, despite a continuous input of eggs, suggested that cannibalism was a common cause of mortality. The size of the final instar, which determined adult body size, was correlated positively with tree hole volume for male, but not female, larvae. Experiments showed that when two larvae were placed together in 0.4–1 holes with abundant tadpole prey, the larger larva killed the smaller one. Often the larva that was killed was not eaten. Small larvae were more tolerant of each other than were pairs of medium or large larvae. Before killing occurred, the presence of larger larvae reduced the growth of smaller individuals, relative to controls. Obligate killing was density-dependent. In 3.0–1 holes with ad libitum prey, conspecific killing occurred until the larval density stabilized at one larva per 1–1.5 I, similar to the density found in large holes under field conditions, For M. coerulatus, cannibalism functions to reduce the number of potential competitors for food in addition to providing nutrition. When interactions between paired larvae in small holes were experimentally prevented, competition for food reduced the growth of one or both larvae relative to controls. Holes that were watered during the dry season supported larval densities similar to those in the wet season. Thus, dry season mortality could not be attributed to a decrease in available prey. Rather, M. coerulatus larvae could not survive more than 1 month of complete drying. Because the dry season typically lasts more than 6 weeks, habitat drying is a secondary source of mortality, affecting second- or third-generation larvae that fail to emerge before tree holes dry out completely.     

Intraspecific competition among first instars of the parasitic wasp Biosteres longicaudatus

Summary Females of the solitary endoparasitic wasp Biosteres longicaudatus sometimes deposit >1 egg (superparasitism) in each larva of the Caribbean fruit fly host, Anastrepha suspensa. As host density increases, there is an inverse relationship between the level of superparasitism and the number of progeny produced/female. Larval parasitoid competition in superparasitized hosts causes an abrupt decline from >1 to 1 or <1 parasitoid/host 24–36 h before the surviving parasitoid larva molts to the 2nd instar. The mechanism by which supernumaries are eliminated was investigated by indirect, in vivo and direct, in vitro methods. There is no apparent competition between parasitoid eggs of the same age. Parasitoid first instars utilize their heavily sclerotized mandibles to eliminate competitors, some of which are subsequently encapsulated by the host. First instars in vitro produce a substance that kills conspecifics. Presumably, this substance is secreted into the surrounding medium. One of each pair of parasitoid first instars, evenly matched for age and size, may live up to 6.4 days longer and grow to 0.13 mm larger than the other. Thus, B. longicaudatus, like other solitary endoparasitoids eliminates competitors by both combat and interference competitions. The latter case, presumably involves allelochemical toxins against conspecifics in the absence of physical encounter.     

Diversity of foraging strategies and responses to predator interference in seed-eating carabid beetles

The prediction of pest regulation by multi-predator communities often remains challenging because of variable and opposite effects of niche complementarity and predator interference. Carabid communities are regulating weeds in arable fields and include a mix of species ranging from granivores to predators that are obligate omnivores. It is not clear from field studies whether granivore and obligate omnivore species either contribute equally or are complementary in the process of weed suppression, and little is known about the impact of potential predator interference within carabid communities on weed suppression. We compared the weed seed foraging strategy of the granivore Harpalus affinis and the obligate omnivore Poecilus cupreus. Using no-choice test experiments, we compared their activity and seed acceptance for four weed species through a scoring of the proportion of tested individuals consuming weeds, their latency before the consumption of the first seed and the total number of seeds consumed. We then evaluated their seed acceptance for dandelion seed Taraxacum officinale under predator interference by using chemical cues of carabids and tested the impact of three treatments, namely cues of intraspecific competition, interspecific competition and intraguild predation. We found that the obligate omnivore P. cupreus was highly active, had a low latency before consuming its first seed but had an interest in only two of the four weed species. P. cupreus seed acceptance remained unchanged in the presence of predator cues. By contrast, H. affinis was slow to start its seed consumption, accepted equally seeds of the four weed species and significantly increased its seed consumption in the presence of cues mimicking intraguild predation. These findings indicate that the two species differ in their foraging strategies, and as such, could have different contributions to weed seed suppression. This novel result calls for further studies documenting the foraging strategy of carabid species that thrive in arable fields as this could significantly improve our understanding of the delivery of weed seed regulation.     

Risk-taking behaviour in foraging young-of-the-year perch varies with population size structure

I investigated if risk-taking behaviour of young-of-the-year (YOY) perch Perca fluviatilis was connected with population-specific predation patterns in four lakes in northern Sweden. The lakes differ in perch size distribution, according to earlier fishing surveys. Thus, the most intense predation pressure by cannibals is assumed to occur at different prey-size windows in the four lakes. In an aquarium study, I observed groups of perch, and registered time spent foraging in an open habitat and number of prey attacks in the presence of a predator. Perch from Ängersjön, with the highest proportion of large fish in the population, spent more time in the open area than those from Fisksjön that has a dense population of mainly small perch. The Ängersjön perch also made more prey attacks than did perch from Fisksjön and Bjännsjön. Relative differences in predation risk in the four lakes were estimated as cannibalistic attack rates, on a range of sizes of YOY perch, calculated from population size distributions. Principal component analysis on predation risk patterns resulted in two components, of which PC1 explained 79.1% of the variation. High scores of PC1 indicated low cannibalistic attack rates on smaller perch (10–20 mm) and high rates on larger fish (≥30 mm), while low scores indicated the opposite. The level of risk-taking behaviour in the aquarium study positively correlated with lake-specific PC1 scores. The perch with the most cautious behaviour in the aquaria originated from the population with the highest predation pressure on early stages. The boldest perch came from the lake with low predation on the smallest, but with higher predation on larger YOY perch. Thus, the influence of predation risk on behaviour patterns in perch may depend on the timing of the highest exposure to predators.     

Intraguild predation and interference competition on the endangered dragonfly <Emphasis Type="Italic">Aeshna viridis</Emphasis>

We examined the effects of intraguild predation (IGP) and interference competition on an endangered dragonfly, Aeshna viridis Eversm. (Odonata: Anisoptera). A. viridis is rare in Europe due to the decrease in suitable habitats harboring the macrophyte Stratiotes aloides L. Stratiotes plants are the principal oviposition substrate for A. viridis females and protect the larvae of A. viridis from fish predation. In our study lakes A. viridis larvae are sympatric with larvae of Aeshna grandis and Aeshna juncea. The susceptibility of A. viridis larvae to IGP by similar-sized larvae of A. grandis and A. juncea was tested in a laboratory predation experiment. Microhabitat use of A. viridis and A. grandis was studied in the laboratory to determine the possible effects of interference competition on the spatial distribution of A. viridis larvae. Our results show that at least in laboratory conditions, A. viridis is susceptible to IGP and interference competition. In competition, A. grandis larvae dominated the middle and outer portion of S. aloides rosettes whereas A. viridis stayed in the inner parts. When A. grandis larvae were absent, A. viridis colonized the middle and outer parts of the rosettes. We conclude that asymmetric predation between odonate larvae of equal size can be intense, and that both IGP and interference competition affect A. viridis. Although natural habitat complexity diminishes their impact, these interactions may nevertheless influence the distribution of A. viridis in S. aloides waters and restrict its microhabitat use in S. aloides rosettes.     

Competition and the distribution of spring peeper larvae

Studies of tadpole distributions have shown that despite overlapping affinities for semipermanent and permanent ponds, distributions of the spring peeper (Pseudacris crucifer) and the green frog (Rana clamitans) tend to be nonoverlapping. Because spring peepers are believed to be poor competitors, I hypothesized that competition from green frog larvae limits the distribution of spring peeper larvae. I stocked field enclosures with a constant density of spring peeper larvae, and one of four densities of green frog larvae (a target-neighbor design). Increased green frog density had a small effect on metamorphic size and no effects on survivorship, larval period or growth rates of spring peepers. In contrast to these small interspecific effects, green frogs had a large effect on their own performance. Intraspecific competition resulted in a 50% decline in growth rate and an 11% decline in metamorphic size. These results suggest that the species are segregated in resource use, or that compared with green frogs, spring peepers are better able to cope with depressed resource densities. In either case, this field experiment provides no evidence that interspecific competition from green frogs limits distributions of spring peepers. Other factors such as predation and breeding site choice by adults may contribute to the absence of spring peeper larvae from many semipermanent and permanent ponds.