Differences in ecomorphology and microhabitat use of four saproxylic larvae (Diptera,Syrphidae) in Scots pine stump rot holes

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ADAPTIVE PHENOTYPIC PLASTICITY IN GROWTH,DEVELOPMENT, AND BODY SIZE IN THE YELLOW DUNG FLY

Life-history theory predicts that age and size at maturity of organisms should be influenced by time and food constraints on development. This study investigated phenotypic plasticity in growth, development, body size, and diapause in the yellow dung fly, Scathophaga stercoraria. Full-sib families were allowed to develop under predator-free field conditions. The time before the onset of winter was varied and each brood was split into three environments differing in the amount of dung a set number of larvae had as a resource. When resources were abundant and competition was minimal, individuals of both sexes grew to larger body sizes, took longer time to mature, and were able to increase their growth rates to attain large body sizes despite shorter effective development periods later in the season. In contrast, limited larval resources and strong competition constrained individuals to mature earlier at a smaller adult size, and growth rates could not be increased but were at least maintained. This outcome is predicted by only two life-history optimality models, which treat mortality due to long development periods and mortality due to fast growth as independent. Elevated preadult mortality indicated physiological costs of fast growth independent of predation. When larval resources were limited, mortality increased with heritable variation in development time for males, and toward the end of the season mortality increased as larval resources became more abundant because this induced longer development periods. Sexual and fecundity selection favoring large body size in this species is thus opposed by larval viability selection favoring slower growth in general and shorter development periods when time and resources are limited; this overall combination of selective pressures is presumably shaping the reaction norms obtained here. Flexible growth rates are facilitated by low genetic correlations between development time and body size, a possible consequence of selection for plasticity. Heritable variation was evident in all traits investigated, as well as in phenotypic plasticity of these traits (genotype X interactions). This is possibly maintained by unpredictable spatiotemporal variation in dung abundance, competition, and hence selection.     

Effects of nutrition and density in Culex pipiens

Mosquito larvae face numerous biotic and abiotic challenges that affect their development and survivorship, as well as adult fitness. We conducted two experiments under semi‐natural conditions to evaluate the effects of intraspecific competition, nutrient limitation and sub‐lethal doses of malathion on individual life history traits in adult Culex pipiens (Diptera: Culicidae). In the first experiment, larvae of Cx. pipiens were reared at different intraspecific densities and exposed to sub‐lethal doses of malathion. In the second experiment, different intraspecific densities of Cx. pipiens larvae were reared under conditions of low or high larval nutrients, and subsequent adults were fed on either water or 10% sucrose solution. Malathion treatment had relatively minor effects compared with density, which had significant negative effects on development rate, survivorship to adulthood, body size (wing length) and longevity. As larval density increased, a sex ratio distortion in survivorship to adulthood emerged, in which a bias towards males was apparent. Nutrient‐rich larval environments alleviated, in part, the effects of increasing density and extended the lifespan of mosquitoes fed on water and 10% sucrose. Density‐dependent alterations in adult longevity attributable to the larval environment are complex and show contrasting results depending on interactions with other environmental factors. This study suggests that larval resource availability and competition influence Cx. pipiens population growth correlates and have lasting effects on traits that relate to a mosquito's ability to vector pathogens.     

Status and Breeding Sites of Three Presumed Endangered Scottish Saproxylic Syrphids (Diptera, Syrphidae)

From 1987 to 1999 efforts were made to understand the status and breeding sites of three presumed endangered flies in Britain: Blera fallax (Linnaeus), Hammerschmidtia ferruginea (Fallén) and Callicera rufa Schummel (Diptera, Syrphidae). Historical data on flight periods, localities and breeding sites were collated from the literature and captured specimens in museums and other collections. Using these data, life cycles were investigated, and cited and other localities searched for adults and early stages. Looking for early stages was more productive than looking for adults. B. fallax is the most endangered. It has declined in abundance, is restricted to two localities and, in 1999, breeding sites were destroyed at one of these localities. In contrast C. rufa is widespread and not uncommon throughout northern Scotland. H. ferruginea is not as endangered as B. fallax but adverse factors such as habitat destruction affect most of its sites.     

Correlated responses to artificial body size selection in growth, development, phenotypic plasticity and juvenile viability in yellow dung flies

Most life history traits are positively influenced by body size, whereas disadvantages of large body size are poorly documented. To investigate presumed intrinsic costs of large size in the yellow dung fly (Scathophaga stercoraria; Diptera: Scathophagidae), we established two replicates each of three body size laboratory selection lines (small, control and large; selection on males only), and subjected flies of the resulting extended body size range to various abiotic stresses. Response to selection was symmetrical in the small and large lines (realized h(2) = 0.16-0.18). After 24 generations of selection body size had changed by roughly 10%. Female size showed a correlated response to selection on male size, whereas sexual size dimorphism did not change. Development time also showed a correlated response as, similar to food limited flies, small line flies emerged earlier at smaller body size. At the lowest larval food limit possible, flies of all lines emerged at the same small body size after roughly the same development time; so overall phenotypic plasticity in body size and development time strongly increased following selection. Juvenile mortality increased markedly when food was extremely limited, large line flies showing highest mortality. Winter frost disproportionately killed large (line) flies because of their longer development times. Mortality at high temperatures was high but size-selective effects were inconsistent. In all environments the larger males suffered more. Initial growth rate was higher for males and at unlimited food. Small line individuals of both sexes grew slowest at unlimited larval food but fastest at limited larval food, suggesting a physiological cost of fast growth. Overall, extension of the natural body size range by artificial selection revealed some otherwise cryptic intrinsic juvenile viability costs of large size, mediated by longer development or faster growth, but only in stressful environments.     

The quantitative genetics of two life history trade-offs in the yellow dung fly in abundant and limited food environments

The trade-offs between body size and development time and between egg size and egg number (clutch size) are central to life history theory, but evidence for them, particularly in terms of genetic correlations, is equivocal. For the yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), we investigated variation in phenotypic and genetic variances and covariances, i.e. heritabilities and genetic correlations, of these life history traits (plus diapause) in benign and stressful larval field or adult laboratory food environments. We found both trade-offs to be weak, as evidenced by low phenotypic and genetic correlations, but stronger in the food limited environments. Broad sense heritabilities were generally significant for all traits considered, whereas the narrow sense heritabilities for egg and clutch size were nil. With regard to the question of how environmental stress affects heritabilities, we found a whole range of responses within one single species depending on the traits considered. All three possible patterns occurred, i.e. increased h² due to increased V_G or decreased decreased h² due to increased and no change in h² due to increased V_G and V_P. These can be explained by the particular ecological circumstances yellow dung flies face in their natural environment. Nevertheless, the majority of patterns was consistent with the idea that stressful conditions amplify phenotypic differences between genotypes. Such variable responses of traits even within one organism underscores the complexity of this issue and may well explain the multiple patterns found in various organisms.Co-ordinating editor: Leimar     

Phenotypic and genetic integration of personality and growth under competition in the sheepshead swordtail,Xiphophorus birchmanni

Competition for resources including food, physical space, and potential mates is a fundamental ecological process shaping variation in individual phenotype and fitness. The evolution of competitive ability, in particular social dominance, depends on genetic (co)variation among traits causal (e.g., behavior) or consequent (e.g., growth) to competitive outcomes. If dominance is heritable, it will generate both direct and indirect genetic effects (IGE) on resource‐dependent traits. The latter are expected to impose evolutionary constraint because winners necessarily gain resources at the expense of losers. We varied competition in a population of sheepshead swordtails, Xiphophorus birchmanni, to investigate effects on behavior, size, growth, and survival. We then applied quantitative genetic analyses to determine (i) whether competition leads to phenotypic and/or genetic integration of behavior with life history and (ii) the potential for IGE to constrain life history evolution. Size, growth, and survival were reduced at high competition. Male dominance was repeatable and dominant individuals show higher growth and survival. Additive genetic contributions to phenotypic covariance were significant, with the G matrix largely recapitulating phenotypic relationships. Social dominance has a low but significant heritability and is strongly genetically correlated with size and growth. Assuming causal dependence of growth on dominance, hidden IGE will therefore reduce evolutionary potential.     

Sex‐specific compensatory growth in the larvae of the greater wax moth Galleria mellonella

Deficiency of food resources in ontogeny is known to prolong an organism's developmental time and affect body size in adulthood. Yet life‐history traits are plastic: an organism can increase its growth rate to compensate for a period of slow growth, a phenomenon known as ‘compensatory growth’. We tested whether larvae of the greater wax moth Galleria mellonella can accelerate their growth after a fast of 12, 24 or 72 h. We found that a subgroup of female larvae showed compensatory growth when starved for 12 h. Food deficiency lasting more than 12 h resulted in longer development and lower mass gain. Strength of encapsulation reactions against a foreign body inserted in haemocoel was the weakest in females that showed compensatory growth, whereas the strongest encapsulation was recorded in the males and females that fasted for 24 and 72 h. More specifically, we found sex‐biased immune reactions so that females had stronger encapsulation rates than males in one group that fasted for 72 h. Overall, rapidly growing females had a short larval development period and the shortest adult lifespan. These results suggest that highly dynamic trade‐offs between the environment, life‐history traits and sex lead to plasticity in developmental strategies/growth rates in the greater wax moth.     

Larval development and poor trophic resource availability: Local adaptations and plasticity in a widespread amphibian species

Theory predicts that, in organisms with complex life cycles, if the earlier-stage limiting factor induces weak later-stage phenotypes, the development of the later-stage trait should evolve to reduce carry-over effects. Local adaptations could thus favour decoupling of later stages. However, decoupling is not always possible. In this study, we used a widespread amphibian, the European fire salamander (Salamandra salamandra), to assess the role of local adaptations to environmental stressful conditions experienced at the larval stage. We exposed 150 larvae from different altitudes to two conditions: rich food and poor food condition. Conditions in early life stages can affect an individual's traits, either as a direct effect or mediated through outcomes in successive life stages. To distinguish between effects of rearing conditions and local adaptation, we searched for a causal model. The causal model detected effects of both food treatment and population origin (altitude) on all life stages. Larvae reared under rich food condition metamorphosed earlier, had higher growth rates and reached smaller size at metamorphosis. Significant differences occurred between larvae of different origin: low-altitude individuals performed poorly under the poor food treatment. Moreover, larvae from higher altitudes were slower with rich food and faster with poor food compared to those from lower altitudes. Our results underline that environmental conditions and local adaptation can interplay in determining the plasticity of larval stages, still adaptations can maximize the growth efficiency of early stages in oligotrophic environments, leading to divergent pathways across populations and environmental conditions.     

Lack of lipogenesis in parasitoids: a review of physiological mechanisms and evolutionary implications

The ability of organisms to adapt to fluctuating food conditions is essential for their survival and reproduction. Accumulating energy reserves, such as lipids, in anticipation of harsh conditions, will reduce negative effects of a low food supply. For Hymenoptera and Diptera, several parasitoid species lack adult lipogenesis, and are unable to store excess energy in the form of lipid reserves. The aim of this review is to provide a synthesis of current knowledge regarding the inability to accumulate lipids in parasitoids, leading to new insights and prospects for further research. We will emphasize physiological mechanisms underlying lack of lipogenesis, the evolution of this adaptation in parasitoids and its biological implications with regard to life history traits. We suggest the occurrence of lack of lipogenesis in parasitoids to be dependent on the extent of host exploitation through metabolic manipulation. Currently available data shows lack of lipogenesis to have evolved independently at least twice, in parasitic Hymenoptera and Diptera. The underlying genetic mechanism, however, remains to be solved. Furthermore, due to the inability to replenish adult fat reserves, parasitoids are severely constrained in resource allocation strategies, in particular the trade-off between survival and reproduction.     

Food quality effects on instar‐specific life histories of a holometabolous insect

1. It is a long‐standing challenge to understand how changes in food resources impact consumer life history traits and, in turn, impact how organisms interact with their environment. To characterize food quality effects on life history, most studies follow organisms throughout their life cycle and quantify major life events, such as age at maturity or fecundity. From these studies, we know that food quality generally impacts body size, juvenile development, and life span. Importantly, throughout juvenile development, many organisms develop through several stages of growth that can have different interactions with their environment. For example, some parasitoids typically attack larger instars, whereas larval insect predators typically attack smaller instars. Interestingly, most studies lump all juvenile stages together, which ignores these ecological changes over juvenile development.
2. We combine a cross‐sectional experimental approach with a stage‐structured population model to estimate instar‐specific vital rates in the bean weevil, Callosobruchus maculatus across a food quality gradient. We characterize food quality effects on the bean weevil's life history traits throughout its juvenile ontogeny to test how food quality impacts instar‐specific vital rates.
3. Vital rates differed across food quality treatments within each instar; however, their effect differed with instar. Weevils consuming low‐quality food spent 38%, 37%, and 18% more time, and were 34%, 53%, and 63% smaller than weevils consuming high‐quality food in the second, third, and fourth instars, respectively. Overall, our results show that consuming poor food quality means slower growth, but that food quality effects on vital rates, growth and development are not equal across instars. Differences in life history traits over juvenile ontogeny in response to food quality may impact how organisms interact with their environment, including how susceptible they are to predation, parasitism, and their competitive ability.

     

Investment into Defensive Traits by Anuran Prey (Lithobates pipiens) Is Mediated by the Starvation-Predation Risk Trade-Off

Prey can invest in a variety of defensive traits when balancing risk of predation against that of starvation. What remains unknown is the relative costs of different defensive traits and how prey reconcile investment into these traits when energetically limited. We tested the simple allocation model of prey defense, which predicts an additive effect of increasing predation risk and resource availability, resulting in the full deployment of defensive traits under conditions of high risk and resource saturation. We collected morphometric, developmental, and behavioural data in an experiment using dragonfly larvae (predator) and Northern leopard frog tadpoles (prey) subject to variable levels of food availability and predation risk. Larvae exposed to food restriction showed limited response to predation risk; larvae at food saturation altered behaviour, development, and growth in response to predation risk. Responses to risk varied through time, suggesting ontogeny may affect the deployment of particular defensive traits. The observed negative correlation between body size and activity level for food-restricted prey – and the absence of a similar response among adequately-fed prey – suggests that a trade-off exists between behavioural and growth responses when energy budgets are limited. Our research is the first to demonstrate how investment into these defensive traits is mediated along gradients of both predation risk and resource availability over time. The interactions we demonstrate between resource availability and risk level on deployment of inducible defenses provide evidence that both internal condition and extrinsic risk factors play a critical role in the production of inducible defenses over time.     

Effects of food quality on trade‐offs among growth,immunity and survival in the greater wax moth Galleria mellonella

The resources available to an individual in any given environment are finite, and variation in life history traits reflect differential allocation of these resources to competing life functions. Nutritional quality of food is of particular importance in these life history decisions. In this study, we tested trade‐offs among growth, immunity and survival in 3 groups of greater wax moth (Galleria mellonella) larvae fed on diets of high and average nutritional quality. We found rapid growth and weak immunity (as measured by encapsulation response) in the larvae of the high‐energy food group. It took longer to develop on food of average nutritional quality. However, encapsulation response was stronger in this group. The larvae grew longer in the low‐energy food group, and had the strongest encapsulation response. We observed the highest survival rates in larvae of the low‐energy food group, while the highest mortality rates were observed in the high‐energy food group. A significant negative correlation between body mass and the strength of encapsulation response was found only in the high‐energy food group revealing significant competition between growth and immunity only at the highest rates of growth. The results of this study help to establish relationships between types of food, its nutritional value and life history traits of G. mellonella larvae.     

Performance of pine looper Bupalus piniarius larvae under population build-up conditions

In the first year of an outbreak, Bupalus piniarius larvae, encounter intense crowding. In the later stages of larval development, they are forced to feed on the non-preferred current-year needles of Scots pine or even on alternative hosts. It was hypothesized that larval feeding on a non-preferred resource (current-year needles) will negatively affect B. piniarius performance. It was also hypothesized that larval mutual interference (crowding without competition for food) will have negative additive effects. These hypotheses were tested in laboratory and field experiments. Fourth instar larvae were reared singly and under crowded conditions in cohorts of ten. Larvae in both situations were reared on control branches (containing both mature and current-year shoots) and branches containing only current-year shoots. Crowded larvae were reared also on Norway spruce, an alternative host. Crowding and feeding on a non-preferred resource had opposite effects on B. piniarius larval performance. Crowding in the late larval instars enhanced larval performance while absence of the preferred resource had negative effects. Larval growth rate was higher and development time was shorter for larvae exposed to crowded conditions than for solitary larvae. There was, however, no difference between the groups in final pupal weights or survival. Survival was 25% lower for larvae feeding on non-preferred current-year needles and pupal weights 9% lower, compared with results for larvae feeding on mature needles. Larvae feeding on Norway spruce suffered greatly extended development time, 82% lower survival, and resulted in 60% lighter pupae compared with conspecifics on Scots pine. It was concluded that not only quantity but also quality of the available food resource is critical for B. piniarius development.     

Optimum body size: effects of food quality and temperature,when reproductive growth rate is restricted,with examples from aphids

Most models of optimum energy partitioning predict variability in adult size, although not always explicitly. Increase in size is usually attributed to an increase in the growth rate or decline in mortality. The model presented shows that this may not always be the case. Even when mortality is kept constant in organisms with overlapping generations, a constraint on the maximum reproductive growth rate may lead, when the rate of overall growth increases, to either an increase or a decline in the optimum adult body size. It is shown that adult size could be a consequence of the differential responses of life history traits to changes in temperature and food quality. This is clearly advantageous for short lived organisms, like aphids, each generation of which only experience a very small part of the great seasonal range in conditions. This hypothesis complements Iwasa's (1991) explanation of the phenotypic plasticity observed in long lived organisms. The predictions are illustrated with empirical data from aphids. The model presented, which has been verified against a very large data set, indicates that for aphids the adult weight observed at a particular combination of temperature and food quality is that at which the population growth rate, r_m, is maximized. We conclude that predictions about adult size from models based on the partitioning of energy are more likely to apply to organisms that scramble for resources, i.e., “r” selected species. The size of organisms that contest for resources is more likely to be determined by competitive status and avoidance of natural enemies.     

Resource availability modulates the effect of body size on reproductive development

Within-species variation in animal body size predicts major differences in life history, for example, in reproductive development, fecundity, and even longevity. Purely from an energetic perspective, large size could entail larger energy reserves, fuelling different life functions, such as reproduction and survival (the “energy reserve” hypothesis). Conversely, larger body size could demand more energy for maintenance, and larger individuals might do worse in reproduction and survival under resource shortage (the “energy demand” hypothesis). Disentangling these alternative hypotheses is difficult because large size often correlates with better resource availability during growth, which could mask direct effects of body size on fitness traits. Here, we used experimental body size manipulation in the freshwater cnidarian Hydra oligactis, coupled with manipulation of resource (food) availability to separate direct effects of body size from resource availability on fitness traits (sexual development time, fecundity, and survival). We found significant interaction between body size and food availability in sexual development time in both males and females, such that large individuals responded less strongly to variation in resource availability. These results are consistent with an energy reserve effect of large size in Hydra. Surprisingly, the response was different in males and females: small and starved females delayed their reproduction, while small and starved males developed reproductive organs faster. In case of fecundity and survival, both size and food availability had significant effects, but we detected no interaction between them. Our observations suggest that in Hydra, small individuals are sensitive to fluctuations in resource availability, but these small individuals are able to adjust their reproductive development to maintain fitness.     

Crayfish assemblage shifts in a large drought-prone wetland: the roles of hydrology and competition

1. Faster growing, larger and/or more aggressive crayfish species are predicted to dominate permanent waterbodies. We tested this prediction using a 9 year dataset for two species of crayfish (Procambarus alleni and Procambarus fallax) co‐existing in a sub‐tropical flowing slough in southern Florida. Using a series of laboratory and mesocosm experiments we also compared life history traits and performance of the respective species to test mechanisms that could explain dominance shifts in the local crayfish assemblages. 2. Over the 9‐year period, P. alleni densities were the greatest in shallower, shorter‐hydroperiod areas bordering the slough, while P. fallax densities were higher in deeper, longer‐hydroperiod central areas. These areas were separated by 0.8–2 km of continuous wetland with no apparent barriers to movement between them. 3. Density of P. fallax was not strongly affected by any measures of hydrological variation, while P. alleni density increased with more severe drought conditions. Following the strongest droughts, P. alleni colonized areas in the centre of the slough where they had been absent or scarce in wetter years. 4. We conducted experiments to compare growth rates, drought tolerance, and competitive dominance of these species. P. alleni survived drought conditions better, had higher growth rates, and was the dominant competitor for space and food. While drought probably limits P. fallax in the drier slough habitats, neither drought sensitivity nor interspecific competition with P. fallax can explain decreases of P. alleni with wetter conditions. 5. Our results indicate that a competition‐colonization tradeoff cannot explain the crayfish compositional dynamics in this wetland because P. alleni is both the best competitor and the best at surviving in and colonizing areas with the strongest droughts. Future attention should focus on the potential for selective effects of predators that co‐vary with hydrology. 6. The traits (large size, fast growth, competitive dominance) exhibited by P. alleni, which is absent in long‐hydroperiod wetlands, are those exhibited by dominant crayfish in permanent lakes and streams containing fish. Although these traits make crayfish less vulnerable to fish in some lakes and streams, life‐history models of community structure across permanence gradients suggest the opposite traits should be favoured for co‐existence with fish.     

Putting prey back together again: integrating predator-induced behavior,morphology, and life history

The last decade has seen an explosion in the number of studies exploring predator-induced plasticity. Recently, there has been a call for more comprehensive approaches that can identify functional relationships between traits, constraints on phenotypic responses, and the cost and benefits of alternative phenotypes. In this study, we exposed Helisoma trivolvis, a freshwater snail, to a factorial combination of three resource levels and five predator environments (no predator, one or two water bugs, and one or two crayfish) and examined ten traits including behavior, morphology, and life history. Each predator induced a unique suite of behavioral and morphological responses. Snails increased near-surface habitat use with crayfish but not with water bugs. Further, crayfish induced narrow and high shells whereas water bugs induced wide shells and wide apertures. In terms of life history, both predators induced delayed reproduction and greater mass at reproduction. However, crayfish induced a greater delay in reproduction that resulted in reduced fecundity whereas water bugs did not induce differences in fecundity. Resource levels impacted the morphology of H. trivolvis; snails reared with greater resource levels produced higher shells, narrower shells, and wider apertures. Resource levels also impacted snail life history; lower resources caused longer times to reproduction and reduced fecundity. Based on an analysis of phenotypic correlations, the morphological responses to each predator most likely represent phenotypic trade-offs. Snails could either produce invasion-resistant shells for defense against water bugs or crush-resistant shells for defense against crayfish, but not both. Our use of a comprehensive approach to examine the responses of H. trivolvis has provided important information regarding the complexity of phenotypic responses to different environments, the patterns of phenotypic integration across environments, and the potential costs and benefits associated with plastic traits.     

Genotypic and phenotypic variation in transmission traits of a complex life cycle parasite

Characterizing genetic variation in parasite transmission traits and its contribution to parasite vigor is essential for understanding the evolution of parasite life‐history traits. We measured genetic variation in output, activity, survival, and infection success of clonal transmission stages (cercaria larvae) of a complex life cycle parasite (Diplostomum pseudospathaceum). We further tested if variation in host nutritional stage had an effect on these traits by keeping hosts on limited or ad libitum diet. The traits we measured were highly variable among parasite genotypes indicating significant genetic variation in these life‐history traits. Traits were also phenotypically variable, for example, there was significant variation in the measured traits over time within each genotype. However, host nutritional stage had no effect on the parasite traits suggesting that a short‐term reduction in host resources was not limiting the cercarial output or performance. Overall, these results suggest significant interclonal and phenotypic variation in parasite transmission traits that are not affected by host nutritional status.