Cannibalism and early instar survival in a larval damselfly

Effects of temperature and resource level on interspecific interactions in two species of Odonata larvae

Identifying how temperature and food resources affect interactions between species is important for understanding how climate change will shape community structure in the future. Here, we tested how temperature and resource density affect survival and growth in the larval stage of two coexisting odonates: the damselfly Lestes sponsa and the dragonfly Sympetrum vulgatum. We performed a laboratory experiment at two temperatures (21 and 24°C) with two resource densities. We estimated the timing of egg hatching of individual egg clutches and thereafter the larval growth rate-, survival- and size-mediated priority effects under interspecific conditions. Eggs of both species hatched slightly faster at 24°C, and S. vulgatum eggs started hatching approximately 1 day earlier than L. sponsa eggs. However, this earlier hatching did not result in a size-mediated priority effect, that is, a higher predation on the later hatching L. sponsa. Nevertheless, L. sponsa larvae were significantly larger than S. vulgatum at hatching. Growth rate and survival were significantly higher: (1) at 24°C compared with 21°C, (2) at high compared with low-resource density and (3) in L. sponsa compared with S. vulgatum. Several significant interaction effects between resource density and temperature and between temperature and species were found. At high temperature, L. sponsa had a higher growth rate than S. vulgatum, but no difference in growth rate between species was found at low temperature. Additionally, a high-resource density resulted in a higher growth rate in both species, but only under high temperature. There was a negative relationship between growth rate and survival in both species, suggesting that the higher growth rate of larvae was to some degree driven by intraguild predation and/or cannibalism. Our results imply that resource levels interact with temperature to affect interactions between the species.

Non-target effects of methoprene and larvicidal surface films on invertebrate predators of mosquito larvae

Mosquito larvicides are used across a variety of aquatic habitats, although when applied they likely affect other aquatic organisms. The removal or impairment of top insect predators via larvicides could be beneficial to mosquitoes by allowing their populations to rebound once pesticide levels dissipate. Our goal was to determine if two larvicide types, growth regulators (IGRs) and surface films (SFs), harm non-target aquatic insect communities, and if these chemicals influence the ability of predatory aquatic insects to regulate mosquitoes. We surveyed aquatic sites before and after IGR and SF-application, then compared changes in insect community structure. Evenness was lower in SF treated habitats, and when analyzing prey/controphic taxa only, evenness and diversity changed in untreated reference areas, suggesting that differences measured were due to other environmental factors, not larvicide presence. A field experiment was then conducted by exposing specific predatory aquatic insects to varying doses of IGRs and SFs and then placing them in mesocosms containing mosquito larvae. Surface films were directly lethal to adult dytiscids at recommended and high concentrations. Although we found no significant differences in mosquito emergence among all treatment levels, there was a trend of negative controls (no predator mesocosms) and SF-treated predators allowing the most mosquitoes to emerge compared to positive controls (predators not exposed to larvicides) and IGR-treated predators. Thus, these larvicides may have minimal effects on mosquito larvae predators, but the direct effects of surface films on insects that interact with the water's surface require further investigation.

Warming and predation risk only weakly shape size-mediated priority effects in a cannibalistic damselfly

Differences in hatching dates can shape intraspecific interactions through size-mediated priority effects (SMPE), a phenomenon where bigger, early hatched individuals gain advantage over smaller, late hatched ones. However, it remains unclear to what extent and how SMPE are affected by key environmental factors such as warming and predation risk imposed by top predators. We studied effects of warming (low and high temperature) and predation risk (presence and absence of predator cues of perch) on SMPE in life history and physiological traits in the cannibalistic damselfly Ischnura elegans. We induced SMPE in the laboratory by manipulating hatching dates, creating following groups: early and late hatchlings reared in separate containers, and mixed phenology groups where early and late hatchlings shared the same containers. We found strong SMPE for survival and emergence success, with the highest values in early larvae of mixed phenology groups and the lowest values in late larvae of mixed phenology groups. Neither temperature nor predator cues affected SMPE for these two traits. The other life history traits (development rate and mass at emergence) did not show SMPE, but were affected by temperature and predator cues. A tendency for SMPE was found for protein content, in the high temperature treatment. The other physiological traits (phenoloxidase activity and fat content) showed fixed expressions across treatments, indicating decoupling between physiology and life history. The results underline that SMPEs are trait-dependent, and only weakly or not affected by temperature and predation risk.

Phenological Shifts in a Warming World Affect Physiology and Life History in a Damselfly

Under climate warming, temperate ectotherms are expected to hatch earlier and grow faster, increase the number of generations per season, i.e., voltinism. Here, we studied, under laboratory conditions, the impact of artificial warming and manipulated hatching dates on life history (voltinism, age and mass at emergence and growth rate) and physiological traits (phenoloxidase (PO) activity at emergence, as an indicator of investment in immune function) and larval survival rate in high-latitude populations of the damselfly Ischnura elegans. Larvae were divided into four groups based on crossing two treatments: early versus late hatching dates and warmer versus control rearing temperature. Damselflies were reared in groups over the course of one (univoltine) or two (semivoltine) growth seasons, depending on the voltinism. Warming temperature did not affect survival rate. However, warming increased the number of univoltine larvae compared to semivoltine larvae. There was no effect of hatching phenology on voltinism. Early hatched larvae reared under warming had elevated PO activity, regardless of their voltinism, indicating increased investment in immune function against pathogens. Increased PO activity was not associated with effects on age or mass at emergence or growth rate. Instead, life history traits were mainly affected by temperature and voltinism. Warming decreased development time and increased growth rate in univoltine females, yet decreased growth rate in univoltine males. This indicates a stronger direct impact of warming and voltinism compared to impacts of hatching phenology on life history traits. The results strengthen the evidence that phenological shifts in a warming world may affect physiology and life history in freshwater insects.

The Biology of Aging in Insects: From Drosophila to Other Insects and Back

An enormous amount of work has been done on aging in Drosophila melanogaster, a classical genetic and molecular model system, but also in numerous other insects. However, these two extensive bodies of work remain poorly integrated to date. Studies in Drosophila often explore genetic, developmental, physiological, and nutrition-related aspects of aging in the lab, while studies in other insects often explore ecological, social, and somatic aspects of aging in both lab and natural populations. Alongside exciting genomic and molecular research advances in aging in Drosophila, many new studies have also been published on aging in various other insects, including studies on aging in natural populations of diverse species. However, no broad synthesis of these largely separate bodies of work has been attempted. In this review, we endeavor to synthesize these two semi-independent literatures to facilitate collaboration and foster the exchange of ideas and research tools. While lab studies of Drosophila have illuminated many fundamental aspects of senescence, the stunning diversity of aging patterns among insects, especially in the context of their rich ecology, remains vastlyunderstudied. Coupled with field studies and novel, more easily applicable molecular methods, this represents a major opportunity for deepening our understanding of the biology of aging in insects and beyond.

Geometric morphometrics of endophytic oviposition traces of Odonata (Eocene, Argentina)

The insertion of the Odonata ovipositor in the plant tissue generates a scar that surrounds the eggs (trace). In insects, individual egg traces are known to vary in size, but their variation in individual shape is mostly unknown. Twenty-four specimens were obtained from the Laguna del Hunco (Lower Eocene, Chubut) and Río Pichileufú (Middle Eocene, Río Negro), Argentina, which had 1346 oviposition traces (MEF Collection). For the first time, a study of the shape and size of a large number of individual Odonata endophytic egg traces was carried out using traditional (general and mixed linear models) and geometric morphometrics (Fourier elliptical series) to elucidate whether there are changes in size or shape of the individual endophytic egg traces associated with the substrate used at the time of oviposition, if the Lower Eocene traces have varied in relation to those of the Middle Eocene, and if the ichnological classification (Paleoovoidus arcuatus, P. bifurcatus and P. rectus) reflects such variations. We found differences in size (p < 0.05), but not in shape, in relation to the variables studied. This could reflect that the shape of Odonata eggs (inferred from the traces), unlike their size, could have a strong evolutionary constraint already observed since the Eocene.

Optimal reproductive phenology under size-dependent cannibalism

Intra-cohort cannibalism is an example of a size-mediated priority effect. If early life stages cannibalize slightly smaller individuals, then parents face a trade-off between breeding at the best time for larval growth or development and predation risk from offspring born earlier. This game-theoretic situation among parents may drive adaptive reproductive phenology toward earlier breeding. However, it is not straightforward to quantify how cannibalism affects seasonal egg fitness or to distinguish emergent breeding phenology from alternative adaptive drivers. Here, we devise an age-structured game-theoretic mathematical model to find evolutionary stable breeding phenologies. We predict how size-dependent cannibalism acting on eggs, larvae, or both changes emergent breeding phenology and find that breeding under inter-cohort cannibalism occurs earlier than the optimal match to environmental conditions. We show that emergent breeding phenology patterns at the level of the population are sensitive to the ontogeny of cannibalism, that is, which life stage is subject to cannibalism. This suggests that the nature of cannibalism among early life stages is a potential driver of the diversity of reproductive phenologies seen across taxa and may be a contributing factor in situations where breeding occurs earlier than expected from environmental conditions.

Abundance and traits link predator ontogeny to prey communities

Function and abundances shape species interactions and thus ecological communities. While communities are often summarized as the mean function of each species, intraspecific variation in traits and thus function is an important driver of community composition. Ontogeny is a common source of intraspecific variation, but while age-related functional changes can alter species interactions, so too can the effects of those functions on the density of the focal organism. For instance, ontogenetic variation can trigger higher levels of cannibalism, reducing abundances and altering interspecific interactions. I manipulate ontogenetic variation in damselfly larvae to show that intraspecific variation can impact communities through two distinct mechanisms. First, within-species differences affect population sizes, and thus indirectly shape communities (indirect effect). In particular, ontogenetic variation resulted in smaller damselfly populations, likely because of increased cannibalism rates, and thus ontogenetically diverse populations had a smaller total effect on their prey. Second, trait variation can affect communities by creating differences in the strength of per capita species interactions (direct effect). In this case, damselfly populations with greater age variation had smaller per capita effects on prey communities. I conclude that ontogeny of a single species can directly and indirectly shape community composition.

Predators weaken prey intraspecific competition through phenotypic selection

Predators have a key role shaping competitor dynamics in food webs. Perhaps the most obvious way this occurs is when predators reduce competitor densities. However, consumption could also generate phenotypic selection on prey that determines the strength of competition, thus coupling consumptive and trait-based effects of predators. In a mesocosm experiment simulating fish predation on damselflies, we found that selection against high damselfly activity rates - a phenotype mediating predation and competition - weakened the strength of density dependence in damselfly growth rates. A field experiment corroborated this finding and showed that increasing damselfly densities in lakes with high fish densities had limited effects on damselfly growth rates but generated a precipitous growth rate decline where fish densities were lower - a pattern expected because of spatial variation in selection imposed by predation. These results suggest that accounting for both consumption and selection is necessary to determine how predators regulate prey competitive interactions.

Dragonflies community assembly in artificial habitats: Glimpses from field and manipulative experiments

Several factors act on community structure, so determining species composition and abundance patterns. Core processes operating at local scales, such as species-environment matching and species interactions, shape observed assemblages. Artificial habitats (simplified structure) are useful systems for assessing the main factors affecting community composition and disentangling their assembly rules. Drinking troughs (brickwork tanks for free-ranging cattle watering) are widespread in Italy and represent a suitable aquatic habitat for colonization by various aquatic organisms. Dragonflies larvae are usually found in drinking troughs and often exhibit strong species interactions and striking community assembly patterns. Our primary aim was to search for Odonata communities exhibiting non-random co-occurrence/segregation patterns in drinking troughs. We performed null-model analyses by measuring a co-occurrence index (C-score) on larval Odonata assemblages (13 species from 28 distinct troughs). Overall, we found a non-random structure for the studied dragonfly assemblages, which, given their fast generation time, must have been generated by short-term ecological processes (i.e. interspecific interactions). We thus analyzed potential competition/predation among and within ecological guilds. From the field data, we speculated that interactions within the sprawlers' guild is likely among the main drivers structuring the studied assemblages, especially the effect of intraguild predation between C. erythraea and Sympetrum spp larval stages. We then experimentally tested these interactions in laboratory and demonstrated that intraguild predation among larvae at different development stages may result in an effective exclusion/negative impact on density pattern, representing one of the processes to take into consideration when studying dragonfly assemblages.

Temperature Effect on the Development of Tropical Dragonfly Eggs

Physiological constraints in insects are related to several large-scale processes such as species distribution and thermal adaptation. Here, we fill an important gap in ecophysiology knowledge by accessing the relationship between temperature and embrionary development time in four dragonfly species. We evaluated two questions (1) what is the effect of temperature on the development time of Odonata eggs, and (2) considering a degree-day relationship, could a simple linear model describe the dependence of embrionary development time on temperature or it is better described by a more complex non-linear relation. Egg development time of Erythrodiplax fusca (Rambur), Micrathyria hesperis Ris, Perithemis mooma Kirby, and Miathyria simplex (Rambur) (Odonata: Libellulidae) were evaluated. We put the eggs at different temperatures (15, 20, 25, and 30°C) and counted the number of hatched larvae daily. A nonlinear response of the development to the temperature was found, differing from the expected pattern for standard degree-day analysis. Furthermore, we observed that there is a similar process in the development time and hatching synchronization between species, with all species presenting faster egg development at high temperatures. Species-specific differences are more evident at lower temperatures (15°C), with no egg development in M. simplex. Only E. fusca was relatively insensitive to temperature changes with similar hatching rates in all treatments.

The Rate of Seasonal Changes in Temperature Alters Acclimation of Performance under Climate Change

How the ability to acclimate will impact individual performance and ecological interactions under climate change remains poorly understood. Theory predicts that the benefit an organism can gain from acclimating depends on the rate at which temperatures change relative to the time it takes to induce beneficial acclimation. Here, we present a conceptual model showing how slower seasonal changes under climate change can alter species' relative performance when they differ in acclimation rate and magnitude. To test predictions from theory, we performed a microcosm experiment where we reared a mid- and a high-latitude damselfly species alone or together under the rapid seasonality currently experienced at 62°N and the slower seasonality predicted for this latitude under climate change and measured larval growth and survival. To separate acclimation effects from fixed thermal responses, we simulated growth trajectories based on species' growth rates at constant temperatures and quantified how much and how fast species needed to acclimate to match the observed growth trajectories. Consistent with our predictions, the results showed that the midlatitude species had a greater capacity for acclimation than the high-latitude species. Furthermore, since acclimation occurred at a slower rate than seasonal temperature changes, the midlatitude species had a small growth advantage over the high-latitude species under the current seasonality but a greater growth advantage under the slower seasonality predicted for this latitude under climate change. In addition, the two species did not differ in survival under the current seasonality, but the midlatitude species had higher survival under the predicted climate change scenario, possibly because rates of cannibalism were lower when smaller heterospecifics were present. These findings highlight the need to incorporate acclimation rates in ecological models.

Intercohort size structure dynamics of fire salamander larvae in ephemeral habitats: a mesocosm experiment

The size structure of a larval population facilitates interaction asymmetries that, in turn, influence the dynamics of size-structure. In species that exhibit conspicuous aggressive interactions, the competitive effects of the smaller individuals may be overlooked. We manipulated initial size differences between two larval cohorts and young-cohort density of Salamandra infraimmaculata in mesocosms to determine: (1) whether young individuals function primarily as prey or as competitors of older and larger individuals; (2) the resulting dynamics of size variation; and (3) recruitment to the postmetamorph population. Intercohort size differences generally remained constant over time at low young-cohort densities, but reduced over time at high densities due to retardation of the old-cohort growth rate. This suggests a competitive advantage to the young cohort that outweighs the interference advantage of older cohorts previously documented in this species. The increase in mortality from desiccation due to high young-cohort density was an order of magnitude greater in the old cohort than in the young-cohort, further indicating size-dependent vulnerability to competition. However, the conditions least favorable to most of the old-cohort larvae (large size difference and high young-cohort density) promoted cannibalism. Among cannibals, mortality and time to metamorphosis decreased and sizes at metamorphosis increased substantially. Thus, a balance between the competitive advantage to young cohorts, and the interference and cannibalism advantage to old cohorts shapes larval size-structure dynamics. Larval densities and individual expression of cannibalism can shift this balance in opposite directions and alter relative recruitment rates from different cohorts.

Negative frequency-dependent selection or alternative reproductive tactics: maintenance of female polymorphism in natural populations

Background

Sex-limited polymorphisms have long intrigued evolutionary biologists and have been the subject of long-standing debates. The coexistence of multiple male and/or female morphs is widely believed to be maintained through negative frequency-dependent selection imposed by social interactions. However, remarkably few empirical studies have evaluated how social interactions, morph frequencies and fitness parameters relate to one another under natural conditions. Here, we test two hypotheses proposed to explain the maintenance of a female polymorphism in a species with extreme geographical variation in morph frequencies. We first elucidate how fecundity traits of the morphs vary in relation to the frequencies and densities of males and female morphs in multiple sites over multiple years. Second, we evaluate whether the two female morphs differ in resource allocation among fecundity traits, indicating alternative tactics to maximize reproductive output.

Results

We present some of the first empirical evidence collected under natural conditions that egg number and clutch mass was higher in the rarer female morph. This morph-specific fecundity advantage gradually switched with the population morph frequency. Our results further indicate that all investigated fecundity traits are negatively affected by relative male density (i.e. operational sex ratio), which confirms male harassment as selective agent. Finally, we show a clear trade-off between qualitative (egg mass) and quantitative (egg number) fecundity traits. This trade-off, however, is not morph-specific.

Conclusion

Our reported frequency- and density-dependent fecundity patterns are consistent with the hypothesis that the polymorphism is driven by a conflict between sexes over optimal mating rate, with costly male sexual harassment driving negative frequency-dependent selection on morph fecundity.

Alternative trait combinations and secondary resource partitioning in sexually selected color polymorphism

Resource partitioning within a species, trophic polymorphism is hypothesized to evolve by disruptive selection when intraspecific competition for certain resources is severe. However, in this study, we reported the secondary partitioning of oviposition resources without resource competition in the damselfly Ischnura senegalensis. In this species, females show color polymorphism that has been evolved as counteradaptation against sexual conflict. One of the female morphs is a blue-green (andromorph, male-like morph), whereas the other morph is brown (gynomorph). These female morphs showed alternative preferences for oviposition resources (plant tissues); andromorphs used fresh (greenish) plant tissues, whereas gynomorphs used decaying (brownish) plants tissues, suggesting that they chose oviposition resources on which they are more cryptic. In addition, the two-color morphs had different egg morphologies. Andromorphs have smaller and more elongated eggs, which seemed to adapt to hard substrates compared with those of gynomorphs. The resource partitioning in this species is achieved by morphological and behavioral differences between the color morphs that allow them to effectively exploit different resources. Resource partitioning in this system may be a by-product of phenotypic integration with body color that has been sexually selected, suggesting an overlooked mechanism of the evolution of resource partitioning. Finally, we discuss the evolutionary and ecological consequences of such resource partitioning.

Within-clutch variation in yolk testosterone as an adaptive maternal effect to modulate avian sibling competition: evidence from a comparative study

In many species, embryos are exposed to maternal hormones in utero, in the egg, or in the seed. In birds, mothers deposit substantial testosterone into their eggs, which enhances competitive ability of offspring. These maternal testosterone concentrations vary systematically within clutches in different patterns and may enable mothers to adaptively fine-tune competitive hierarchies within broods. We performed a comparative analysis to investigate this hypothesis using a broad set of avian species. We expected species with small size differences among siblings (arising from small hatching asynchrony or slow growth rates) to aim for survival of the whole brood in good years and therefore compensate last-hatching eggs with relatively more testosterone. We expected species with large size differences among siblings (large hatching asynchrony or fast growth rates) to produce surplus young as insurance against failed offspring and to facilitate elimination of redundant surplus young by bestowing last-hatching eggs with relatively less testosterone. As predicted, we found that maternal testosterone compensation to last-hatching eggs is stronger when size differences among siblings become smaller. Maternal testosterone compensation to last-hatching eggs also correlated negatively with hatching asynchrony and growth rates. These findings provide evidence for correlated evolution of several maternal effects that together support different maternal reproductive strategies.

Higher temperature variability increases the impact of Batrachochytrium dendrobatidis and shifts interspecific interactions in tadpole mesocosms

The emergence of amphibian chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis (Bd) has led to the decline and extinction of numerous amphibian species. Multiple studies have observed links between climatic factors and amphibian declines apparently caused by Bd. Using outdoor experimental mesocosms, we tested the response of red-legged frog (Rana aurora) tadpoles to increased variation in temperature, a component of climate linked to amphibian declines, and Bd exposure. We included tadpoles of a sympatric competitor species, Pacific chorus frog (Pseudacris regilla), in a fully factorial design to test the effects of Bd and temperature on interspecific interactions. We found that higher variation in temperature had numerous effects in mesocosms, including interacting with Bd presence to decrease the condition of R. aurora, shifting the relative performance of competing P. regilla and R. aurora, and accelerating the development of P. regilla relative to R. aurora. Our results demonstrate that increased variation in temperature can affect amphibians in multiple ways that will be contingent on ecological context, including the presence of Bd and competing species.

Effects of size and size structure on predation and inter-cohort competition in red-eyed treefrog tadpoles

Individual and relative body size are key determinants of ecological performance, shaping the strength and types of interactions within and among species. Size-dependent performance is particularly important for iteroparous species with overlapping cohorts, determining the ability of new cohorts to invade habitats with older, larger conspecifics. We conducted two mesocosm experiments to examine the role of size and size structure in shaping growth and survival in tadpoles of the red-eyed treefrog (Agalychnis callidryas), a tropical species with a prolonged breeding season. First, we used a response surface design to quantify the competitive effect and response of two tadpole size classes across three competitive environments. Large tadpoles were superior per capita effect competitors, increasing the size difference between cohorts through time at high resource availability. Hatchlings were better per biomass response competitors, and maintained the size difference between cohorts when resource availability was low. However, in contrast to previous studies, small tadpoles never closed the size gap with large tadpoles. Second, we examine the relationship between body size, size structure, and predation by dragonfly nymphs (Anax amazili) on tadpole survival and growth. Hatchlings were more vulnerable to predation; predator and large competitor presence interacted to reduce hatchling growth. Again, the size gap between cohorts increased over time, but increased marginally more with predators present. These findings have implications for understanding how variation in resources and predation over the breeding season will shape population size structure through time and the ability of new cohorts to invade habitats with older conspecifics.

A geographic cline induced by negative frequency-dependent selection

Background

Establishment of geographic morph frequency clines is difficult to explain in organisms with limited gene flow. Balancing selection, such as negative frequency-dependent selection (NFDS), is instead suggested to establish a morph frequency cline on a geographic scale at least theoretically. Here we tested whether a large-scale smooth cline in morph frequency is established by NFDS in the female-dimorphic damselfly, Ischnura senegalensis, where andromorphs and gynomorphs are maintained by NFDS.

Results

We found a large-scale latitudinal cline in the morph frequency: andromorph frequency ranged from 0.05 (South) to 0.79 (North). Based on the empirical data on the numbers of eggs, the number of ovariole, abdomen length and latitude, the potential fitness of andromorphs was estimated to be lower than that of gynomorphs in the south, and higher in the north, suggesting the gene-by-environment interaction. From the morph-specific latitudinal cline in potential fitness, the frequency of andromorphs was expected to shift from 0 to 1 without NFDS, because a morph with higher potential fitness wins completely and the two morphs will switch at some point. In contrast, NFDS led to the coexistence of two morphs with different potential fitness in a certain geographic range along latitude due to rare morph advantage, and resulted in a smooth geographic cline of morph frequency.

Conclusion

Our results provide suggestive evidence that the combination of NFDS and gene-by-environment interaction, i.e., multi-selection pressure on color morphs, can explain the geographic cline in morph frequency in the current system.

Density and intercohort priority effects on larval Salamandra salamandra in temporary pools

Priority effects, i.e., effects of an early cohort on the performance of a later cohort, are generally studied between, and not within, species. The paucity of intraspecific assessments does not reflect a lack of ecological importance, but the technical problem associated with differentiating between conspecific cohorts. Here, we examine priority and density-dependent effects on larval Salamandra salamandra infraimmaculata. Larvae deposited by their mother early in the season have increased risk of desiccation, as rains at the beginning of the season are less frequent and unpredictable. However, breeding later may incur a high cost through conspecific priority effects, including cannibalism and competition. In an outdoor artificial pool experiment, we established densities of 0, 1, 2, 4 or 6 newly born larvae per pool ( approximately 30 l), and 40 days later, added a second cohort of three newly born larvae to each pool. We differentiated between cohorts using natural individual-specific markings. For the early cohort, increasing density decreased survival and size at metamorphosis, and increased time to metamorphosis. For the late cohort, survival was 100% in pools without early-cohort larvae, but ranged between 13 and 33% in the presence of early-cohort larvae. Time to metamorphosis was significantly longer in the presence of low vs high densities of early-cohort larvae. Results suggest that early-cohort larvae are mainly subjected to exploitative competition and cannibalism mediated by food limitation, and that late-cohort larvae are subjected to cannibalism and interference due to size asymmetry between cohorts. The strong priority effects suggest that Salamandra females could increase their fitness by adjusting the number of larvae they deposit in specific pools to avoid cannibalism and intraspecific competition.

Differences in growth and behaviour influence asymmetric predation among early-instar dragonfly larvae

Libellulid dragonflies lay large egg clutches. When eggs of two or more species are deposited at the same time and patch, one can expect a high density of early-instar larvae. Thus, interspecific interactions should be evident. We studied the interaction of two species, Sympetrum fonscolombii and Orthetrum cancellatum, which typically co-occur in French rice fields. We experimentally simulated the laying of egg clutches of both species at the same time and site. Survival of O. cancellatum was reduced compared with that of controls without S. fonscolombii and also with that of S. fonscolombii in the two-species treatment. At the end of the experiment, mean head width of S. fonscolombii was greater than that of O. cancellatum, which may be one reason for the differential survival. In a second experiment we observed the behaviour of pairs of early-instar larvae of both species in different size combinations. Small larvae of S. fonscolombii reduced locomotory activity in the presence of larger O. cancellatum. In contrast, small O. cancellatum did not do so in the presence of larger S. fonscolombii. This behavioural difference may also cause asymmetric interspecific predation.

Unpredictable offspring survivorship in the damselfly, Megaloprepus coerulatus, shapes parental behavior, constrains sexual selection, and challenges traditional fitness estimates

Evolutionary biologists typically assume that the number of eggs fertilized or developing embryos produced is correlated with an individual's fitness. Using microsatellite markers, we document for the first time estimates of realized fitness quantified as the number of offspring surviving to adulthood in an insect under field conditions. In a territorial damselfly whose males defend tree hole oviposition sites, patterns of offspring survivorship could not be anticipated by adults. Fewer than half of the parents contributing eggs to a larval habitat realized any reproductive success from their investment. The best fitness correlate was the span over which eggs in a clutch hatched. Among parents, female fecundity and male fertilization success were poor predictors of realized fitness. Although body size was correlated with female clutch size and male mating success, larger parents did not realize greater fitness than smaller ones. The uncoupling of traditional fitness surrogates from realized fitness provides strong empirical evidence that selection at the larval stage constrains selection on mated adults.

Lack of preference for low-predation-risk habitats in larval damselflies explained by costs of intraspecific interactions

Many studies indicate prey organisms select microhabitats with high structural complexity as a way of reducing risk of predation. We used laboratory experiments to show that damselfly larvae, Ischnura verticalis, suffer higher predation rates from pumpkinseed sunfish in low-density vegetation. However, larvae do not preferentially occupy microhabitats with high vegetation density in either the presence or absence of sunfish; when given a choice, the number of larvae per stem of vegetation was equal across all densities of vegetation. That larvae do not congregate in dense vegetation may reflect costs of aggressive interactions. Results from laboratory experiments indicated larval interactions increase conspicuous behaviours (most notably swimming) and consequently increase fish predation. A subsequent experiment indicated that frequency of larval interactions increases with increased vegetation density when number of larvae/stem is constant. Thus, larval microhabitat selection may reflect a trade-off between reduced risk of predation in areas of high vegetation density, caused by reduced fish foraging ability, and increased aggressive larval interactions, due to decreased proximity of larvae. Copyright 2000 The Association for the Study of Animal Behaviour.

Dwarfs and Giants: Cannibalism and Competition in Size-Structured Populations

Cannibals and their victims often share common resources and thus potentially compete. Smaller individuals are often competitively superior to larger ones because of size-dependent scaling of foraging and metabolic rates, while larger ones may use cannibalism to counter this competition. We study the interplay between cannibalism and competition using a size-structured population model in which all individuals consume a shared resource but in which larger ones may cannibalize smaller conspecifics. In this model, intercohort competition causes single-cohort cycles when cannibalism is absent. Moderate levels of cannibalism reduce intercohort competition, enabling coexistence of many cohorts. More voracious cannibalism, in combination with competition, produces large-amplitude cycles and a bimodal population size distribution with many small and few giant individuals. These coexisting ``dwarfs'' and ``giants'' have very different life histories, resulting from a reversal in importance of cannibalism and competition. The population structure at time of birth determines whether individuals suffer severe cannibalism, with the few survivors reaching giant sizes, or whether they suffer intense intracohort competition, with all individuals remaining small. These model results agree remarkably well with empirical data on perch population dynamics. We argue that the induction of cannibalistic giants in piscivorous fish is a population-dynamic emergent phenomenon that requires a combination of size-dependent cannibalism and competition.

The ontogenetic switch between odonate life history stages: effects on fitness when time and food are limited

During the course of ontogeny, odonates switch from being aquatic larvae to being terrestrial adults. Ontogenetic niche shift theory proposes that such shifts are adaptive and have evolved to maximize a growth rate (size) to mortality rate ratio. Individuals should therefore switch from one niche to the other at an optimal size or state. Since the majority of odonates are seasonal breeders, the extent to which the switch is optimal will depend upon the time and the resources available during postembryonic development. We collected a cohort of larvae that varied in how close they were to eclosion and reared them on either a high-nutrition or a low-nutrition diet. We then determined the relative influence of both time and nutritional constraints on survival and development rate, as well as the body size, size-corrected flight muscle mass and fat reserves of individuals at eclosion. Damselflies in both high- and low-nutrition treatments responded to a short development period by developing faster and reducing their body size, but did not change their proportional investment in fat reserves and flight muscle. Reduced larval nutrition resulted in decreased body size, flight muscle mass and fat reserves at eclosion. However, it had no effect on survival to eclosion, or development rate. We discuss these results in terms of the influence that time and nutritional constraints have on odonate development patterns and fitness. Copyright 1999 The Association for the Study of Animal Behaviour.