Threat of predation negates density effects in larval gray treefrogs

While density-dependence is central to most theory regarding population regulation and community structure, specific mechanisms that modify its effects in the absence of changes in consumer-resources ratios (e.g., thinning) are not well understood. To determine if the threat of predation alters effects of density, we investigated the interaction between density of larval treefrogs (Hyla chrysoscelis) and the non-lethal presence of a predatory fish (Enneacanthus obesus). A significant density by fish interaction was consistent for all response variables (e.g., larval survivorship, mass, and time to metamorphosis) driven by a complete lack of density effects in the presence of predators, while predator-free tanks showed classic density-dependent responses. Given that female H. chrysoscelis strongly avoid ovipositing in ponds containing fish, certain larval adaptations are apparently not constrained by maternal behavior and suggest redundancy in response to predators. Our data suggest that non-lethal effects of predators can determine larval performance irrespective of larval density, and that the non-lethal effects of predators can be strong whether lethal effects are strong or weak.     

Experimental studies on the larval development of the shrimpsCrangon crangon andC. allmanni

Larvae of the shrimpsCrangon crangon L. andC. allmanni Kinahan were reared in the laboratory from hatching through metamorphosis. Effects of rearing methods (larval density, application of streptomycin, food) and of salinity on larval development were tested only inC. crangon, influence of temperature was studied in both species. Best results were obtained when larvae were reared individually, with a mixture ofArtemia sp. and the rotiferBrachionus plicatilis as food. Streptomycin had partly negative effects and was thus not adopted for standard rearing techniques. All factors tested in this study influenced not only the rates of larval survival and moulting, but also morphogenesis. In both species, in particular inC. crangon, a high degree of variability in larval morphology and in developmental pathways was observed. Unsuitable conditions, e.g. crowding in mass culture, application of antibiotics, unsuitable food (rotifers, phytoplankton), extreme temperatures and salinities, tend to increase the number of larval instars and of morphological forms. The frequency of moulting is controlled mainly by temperature. Regression equations describing the relations between the durations of larval instars and temperature are given for bothCrangon species. The number of moults is a linear function of larval age and a power function of temperature. There is high variation in growth (measured as carapace length), moulting frequency, morphogenesis, and survival among hatches originating from different females. The interrelations between these different measures of larval development in shrimps and prawns are discussed.     

Density-dependent injury in larval salamanders

Summary The effects of initial larval density, food level, and pond drying regime on intraspecific aggression of larval Ambystoma talpoideum were studied in an artificial pond experiment. Aggression was measured by the frequency of injury of feet, limbs, tail, and the extent of tail loss. Initial larval density had a significant effect on the frequency of foot, limb, and tail loss but not on the extent of tail loss. More larvae reared at medium and high densities sustained injuries than larvae reared at low densities but injuries were not more extensive. Food level had no effect on the four measures of injury. Pond drying regime had no effect on foot loss, limb loss, or extent of tail loss but more larvae reared in constant water level ponds had tail loss than in drying ponds. The frequency of limb and tail loss was negatively correlated to density-dependent survival which was the result of intraspecific predation or cannibalism. These results indicate that substantive levels of body injury, particularly tail loss, can occur at high natural larval densities and may result in a subsequent reduction of growth and survival.     

Evolution of increased adult longevity in Drosophila melanogaster populations selected for adaptation to larval crowding

In holometabolous animals such as Drosophila melanogaster, larval crowding can affect a wide range of larval and adult traits. Adults emerging from high larval density cultures have smaller body size and increased mean life span compared to flies emerging from low larval density cultures. Therefore, adaptation to larval crowding could potentially affect adult longevity as a correlated response. We addressed this issue by studying a set of large, outbred populations of D. melanogaster, experimentally evolved for adaptation to larval crowding for 83 generations. We assayed longevity of adult flies from both selected (MCUs) and control populations (MBs) after growing them at different larval densities. We found that MCUs have evolved increased mean longevity compared to MBs at all larval densities. The interaction between selection regime and larval density was not significant, indicating that the density dependence of mean longevity had not evolved in the MCU populations. The increase in longevity in MCUs can be partially attributed to their lower rates of ageing. It is also noteworthy that reaction norm of dry body weight, a trait probably under direct selection in our populations, has indeed evolved in MCU populations. To the best of our knowledge, this is the first report of the evolution of adult longevity as a correlated response of adaptation to larval crowding.     

Nutritional availability and larval density dependence in Aedes aegypti

1. Density dependence is the effect of density on population growth. Density dependence is an aggregate term for a suite of complex interactions between animals and their environment. 2. Mechanistic studies of density dependence in mosquito ecology are sparse, and the role of environmental factors is poorly understood. 3. Two empirical study designs were compared to consider the interaction between nutritional availability and density in Aedes aegypti. First, larvae were fed per capita. Second, larvae were fed a fixed amount of food unadjusted for the number of individuals; therefore, at higher densities, individuals received less per capita. 4. Survivorship, wing length, and development rate were lower at high densities when larvae were fed a fixed, unadjusted amount of food. The opposite was observed when food was adjusted per capita, suggesting that high densities may be beneficial for larval development when per capita nutrition is held constant 5. These results demonstrate that negative associations between Ae. aegypti larval density and larval development are a manifestation of decreased per capita nutrient uptake at high densities. 6. Population regulation is a proportional response to environmental variability in Ae. aegypti. Increased survivorship at high densities when larvae were fed per capita demonstrates that nutritional availability is not the only mechanism of density dependence in mosquitoes. Further studies should characterise density dependence in mosquitoes by using mechanistic study designs across diverse environmental conditions.     

Consequences of larval intraspecific competition to stonefly growth and fecundity

Summary We conducted experiments in replicated circular streams to measure the effect of intraspecific larval density on growth rates, size at emergence, timing of emergence, and fecundity of two species of predatory stoneflies (Megarcys signata and Kogotus modestus, Perlodidae). Early instars of both species showed no significant effect of intraspecific larval density on mean growth rates, despite the observation that in the absence of competitors stoneflies ate on average, significantly more prey (Baetis bicaudatus, Ephemeroptera, Baetidae) than in the presence of competitors. However, larval size of stoneflies held at higher densities (two per chamber) diverged over time, resulting in a greater size variability (coefficient of variation) among Kogotus than in treatments with low densities of stoneflies (one per chamber). The effect of doubling the density of early-instar Megarcys larvae was also asymmetrical, resulting in one larger and one smaller individual. In contrast, doubling the density of last-instar stoneflies whose feeding rates declined significantly prior to emergence had few measurable consequences, except that male Megarcys, which continued to feed throughout the last instar, had lower average feeding rates in high-density than in low-density chambers, and emerged at a significantly smaller mean size. We conclude that competition between early-instar stonefly larvae results in an asymmetry of body sizes, but that competitive effects are reduced as larvae slow or cease feeding before emergence. Since larger females of both stonefly species produced more eggs, the probable cost to females of early-instar larval competition was a reduction in their potential contribution of offspring to the next generation. The cost of attaining a smaller body size for male stoneflies is unknown; but if, as in many other insects, larger males have greater reproductive success, larval competition may increase the opportunity for sexual selection among males. This hypothesis remains to be tested experimentally.     

Unexpected larval habit of Listronotus bonariensis (Coleoptera: Curculionidae) raises questions about population dynamics analysis and management

Listronotus bonariensis (Kuschel) is a pest of agriculturally important graminaceous species, with mining larvae that kill the stems of the host plants. In this study, larval populations were measured in spring and summer in irrigated dairy grassland comprising Lolium perenne L. (cv. Nui) with and without the endophyte Epichloë festucae var. lolii Latch, M.J. Chr. and Samuels and Poa annua L.. Larvae were extracted from tillers taken from the swards of these two grass species and extracted from turves, and L. bonariensis population densities were estimated from tiller and turf larval counts on a m⁻² basis. Over the study period, the total number of larvae and larval densities extracted from turves was on average 2× greater than indicated from tillers. In most seasons, larval densities from turves were significantly higher than those from the tillers, though there was no correlation between tiller and turf larval densities. Mean head capsule widths of larvae emerging from turf samples showed significant seasonal effects compared with tillers, while mean head capsule widths of all four instars were significantly greater when extracted from tillers compared with turves. There was a significant endophyte effect on head capsule widths of larvae collected in summer, but the effect was not consistent across instars or source. Conversely, no significant endophyte effect on head capsule width was found in spring populations from either tillers or turves. This study shows that in irrigated dairy pasture, a high proportion of L. bonariensis larvae can live externally of tillers, presumably among the organic matter around the base of grasses in irrigated dairy pasture, and that density estimates based only on tiller populations will have significantly underestimated actual numbers. Having a precise indication of larval population densities is essential when developing life tables or determining economic damage threshold levels.     

Effect of density on larval development and female productivity ofTisbe holothuriae (Copepoda,Harpacticoida) under laboratory conditions

The harpacticoid copepodTisbe holothuriae has been cultivated in the Helgoland laboratory for more than 20 years. The effects of density on the larval development and the female productivity were studied by comparing two culture systems: (1) enclosed system, and (2) running-water system. In both systems, a nutritious mixed diet ofDunaliella tertiolecta, Skeletonema costatum, and granulatedMytilus edulis was offered. Larval mortality, larval development and female productivity are found to be significantly dependent on both the population density and specificity of the culture system. Increasing density causes higher larval mortality, longer larval development time, and a reduction in female productivity. In comparison with the enclosed system, the running-water system shows decisive advantages: larval mortality is about 20% lower, the rate of larval development is about two days shorter, and there is a very high rate of nauplii production. The sex ratio exhibits high variations, but in general, there is no clear relationship between sex ratio and population density. Nevertheless, when reared in the running-water system, a relatively high percentage of females (>45%) was found at lower densities.     

Influence of larval experience on preference of a subterranean insect Delia antiqua on Allium hosts

Numerous studies have reported that larval experience can affect subsequent host plants selection and future oviposition decisions of many different species, but the investigation of pre‐imaginal experiences on host preference of adults has rarely been tested for soil‐dwelling insects. In this study, we present evidence that larval feeding experience can affect adult host preference in the onion maggot, Delia antiqua. By rearing D. antiqua on different host plants, we were able to examine the role of the natal host of different generations and the effect of larval density on host‐choice behaviour. We also performed bioassays by means of switched host treatment to evaluate the host‐selection principle. Choice bioassays among the three host species demonstrated that D. antiqua females preferred to oviposit on their natal host in each generation and host‐switching treatments. Additionally, increasing larval density could intensify this ovipositional preference on the natal host. The overall results showed that host preference of female D. antiqua is determined by larval experience and density. These findings also add support for the controversial Hopkins’ host‐selection principle.     

Effect of larval fish and nutrient enrichment on plankton dynamics in experimental ponds

The hypotheses that larval fish density may potentially affect phytoplankton abundance through regulating zooplankton community structure, and that fish effect may also depend on nutrient levels were tested experimentally in ponds with three densities of larval walleye, Stizostedion vitreum (0, 25, and 50 fish m^–3), and two fertilizer types (inorganic vs organic fertilizer). A significant negative relationship between larval fish density and large zooplankton abundance was observed despite fertilizer types. Larval walleye significantly reduced the abundances of Daphnia, Bosmina, and Diaptomus but enhanced the abundance of various rotifer species (Brachionus, Polyarthra, and Keratella). When fish predation was excluded, Daphnia became dominant, but Daphnia grazing did not significantly suppress blue-green algae. Clearly, larval fish can be an important regulator for zooplankton community. Algal composition and abundance were affected more by fertilizer type than by fish density. Inorganic fertilizer with a high N:P ratio (20:1) enhanced blue-green algal blooms, while organic fertilizer with a lower N:P ratio (10:1) suppressed the abundance of blue-green algae. This result may be attributed to the high density of blue-green algae at the beginning of the experiment and the fertilizer type. Our data suggest that continuous release of nutrients from suspended organic fertilizer at a low rate may discourage the development of blue-green algae. Nutrient inputs at a low N:P ratio do not necessarily result in the dominance of blue-green algae.     

Chytridiomycosis impacts predator-prey interactions in larval amphibian communities

Despite ecologists increasingly recognizing pathogens as playing significant roles in community dynamics, few experimental studies have quantified patterns of disease impacts on natural systems. Amphibians are experiencing population declines, and a fungal pathogen (Batrachochytrium dendrobatidis; Chytridiomycota) is a suspected causal agent in many declines. We studied the effects of a pathogenic fungus on community interactions between the gray treefrog, Hyla chrysoscelis, and eastern newts, Notophthalmus viridescens. Recent studies have characterized chytridiomycosis as an emerging infectious disease, whose suspected rapid range expansion and widespread occurrence pose a significant risk for amphibian populations worldwide. We reared larvae in outdoor polyethylene experimental tanks and tested the effects of initial larval density, predator presence, and fungal exposure on Hyla recruitment and predator-prey interactions between Hyla and Notophthalmus. Newts reduced treefrog survival, and high intraspecific density decreased metamorphic body mass independent of B. dendrobatidis. The presence of fungi reduced treefrog body mass at metamorphosis by 34%, but had no significant main effect on survival or larval period length. B. dendrobatidis differentially affected larval development in the presence of predators; Hyla developed slower when reared with the pathogen, but only when newts were present. This significant predator-by-pathogen interaction suggests that the impact of chytridiomycosis on larval amphibians may be exacerbated in complex communities. Our data suggest that B. dendrobatidis effects on host life history may be complex and indirect. Direct measurements of the community-level effects of pathogens offer an important opportunity to understand a significant threat to global biodiversity—declining amphibian populations.     

Characteristics of Anopheles arabiensis larval habitats in Tubu village,Botswana

Documented information on the ecology of larval habitats in Botswana is lacking but is critical for larval control programs. Therefore, this study determined the characteristics of these habitats and the influences of biotic and abiotic factors in Tubu village, Botswana. Eight water bodies were sampled between January and December, 2013. The aquatic vegetation and invertebrate species present were characterized. Water parameters measured were turbidity (NTU), conductivity (μS/cm), oxygen (mg/l), and pH. Larval densities of Anopheles arabiensis mosquitoes and their correlation with abiotic factors were determined. Larval breeding was associated with ‘short’ aquatic vegetation, a variety of habitats fed by both rainfall and flood waters and sites with predators and competitors. The monthly mean (± SE_mean) larval density was 8.16±1.33. The monthly mean (±SE_mean) pH, conductivity, oxygen, and turbidity were 7.65±0.13, 1152.834±69.171, 5.59±1.33, and 323.421±33.801, respectively. There was a significant negative correlation between larval density and conductivity (r = ‐0.839; p < 0.01), while a significant positive correlation occurred between turbidity and larval density (r = 0.685; p < 0.05). Oxygen (r = 0.140; p > 0.05) and pH (r = 0.252; p > 0.05) were not correlated with larval density. Floods and diversified breeding sites contributed to prolonged and prolific larval breeding. ‘Short’ aquatic vegetation and predator‐infested waters offered suitable environments for larval breeding. Turbidity and conductivity were good indicators for potential breeding places and can be used as early warning indices for predicting larval production levels.     

Reduction of black vine weevil larval growth by vesicular-arbuscular mycorrhizal infection

Larvae ofOtiorhynchus sulcatus (black vine weevil) were reared at three densities (zero, two or eight), on plants ofTaraxacum officinale (dandelion) with and without infection by the mycorrhizal fungusGlomus mosseae. On plants without the fungus, survival of larvae to late final instar was 84%, but this was only 43% on infected plants. The differential survival of larvae was evident in their effects on plant biomass. Significant interactions were found between larval density and infection, indicating that mycorrhizal presence mitigated the effects of herbivory at the low larval density. Infection byG. mosseae thus conferred some degree of resistance in roots to this insect and the consequences of this for horticultural and natural situations are discussed.     

Evolutionary responses of Drosophila melanogaster life history to differences in larval density

The study examined the effects of evolution at two different larval densities on pre-adult and adult fitness traits. Five replicate selection lines each were cultured at either 50 or 150 larvae per vial, avoiding selection on development time, age at breeding or for adaptation to adult density, one or more of which factors has been a confounding variable in previous studies. Low density selection lines evolved extended development times at both growth densities. The extended development times were associated with greater adult body size at the lower growth density only, and particularly in females. The lines did not differ significantly in larval competitive ability at either growth density. At neither growth density did the early adult fertility of females or the lifespan of either sex differ between the lines from the two selection regimes, but at the lower growth density the late fertility of low density line females was significantly enhanced. The results suggest that larval density does have important effects on the expression and resolution of life history trade-offs in Drosophila melanogaster, but that these may be somewhat different from those reported in previous studies.     

Effects of larval movement behavior and density on emergence success and adult body size in a commensal midge

Changes in larval density and movement behavior of a commensal midge, Nanocladius (Plecopteracoluthus) sp. #4, were monitored for 26 weeks in recirculating laboratory streams. Adults were captured at emergence, sexed, and weighed to assess the effect of larval density and movement behavior on emergence success and adult size. The density of midges on hosts declined with time and coincided with a springtime increase in larval movement frequency. Midges residing on hosts with high spring densities emerged significantly less than midges on hosts with low densities. Resident midge density on hosts did not influence the likelihood of successful colonization by commensals and colonizers showed no preference for initial attachment site on hosts. However, colonizing midges emerged significantly less than resident midges. Similarly, successful emergers changed tube positions significantly less often as larvae relative to non-emergers. There was no difference in adult body weight of resident midges and colonists/movers, but adult males which emerged from commensal-laden (high density) hosts were significantly smaller than males from low density hosts. These data indicate larval density and movement behavior may have strong fitness consequences for commensal midges.     

Diet quality mitigates intraspecific larval competition in Drosophila suzukii

The invasive frugivore Drosophila suzukii (Matsumura) (Diptera: Drosophilidae) utilizes a wide range of host plants and damages important fruit crops, including blueberries, cherries, blackberries, raspberries, and strawberries. Field infestations of D. suzukii often exceed one larva per berry, suggesting that intraspecific competition may frequently occur. Because dietary resources are also likely to vary across the host range of D. suzukii, we designed a laboratory assay to measure larval performance across diets of varying quality: a standard artificial diet, a fruit‐based medium, a low‐protein, and a low‐carbohydrate diet. We manipulated egg density across these diets to provide increasing levels of competition and measured larval performance by observing survival to pupation and adulthood, and development times for both life stages. Although increasing density generally negatively impacted D. suzukii performance across diets, the magnitude of these impacts varied by diet type. Drosophila suzukii performance was generally similar in fruit and standard diets, although larval development was more rapid in fruit diets at lower densities. Even at low densities (5 or 10 eggs per arena), survival was reduced and development time increased in low‐protein diets relative to standard and fruit diets. At the two highest larval densities (20 or 40 eggs per arena), survivorship was reduced in low‐carbohydrate diets as compared to standard and fruit diets. There is evidence that larvae compensated in both low‐quality diets by extending development time, which could have consequences for population dynamics. Population models for use in D. suzukii management may need to account for both host nutritional quality and relative competition to accurately predict turnover and geographic expansion.     

Carry-over effects of the larval environment on post-metamorphic performance in two hylid frogs

Life history theory and empirical studies suggest that large size or earlier metamorphosis are suitable proxies for increased lifetime fitness. Thus, across a gradient of larval habitat quality, individuals with similar phenotypes for these traits should exhibit similar post-metamorphic performance. Here we examine this paradigm by testing for differences in post-metamorphic growth and survival independent of metamorphic size in a temperate (spring peeper, Pseudacris crucifer) and tropical (red-eyed treefrog, Agalychnis callidryas) anuran reared under differing larval conditions. For spring peepers, increased food in the larval environment increased post-metamorphic growth efficiency more than predicted by metamorphic phenotype and led to increased mass. Similarly, red-eyed treefrogs reared at low larval density ended the experiment at a higher mass than predicted by metamorphic phenotype. These results show that larval environments can have delayed effects not captured by examining only metamorphic phenotype. These delayed effects for the larval environment link larval and juvenile life history stages and could be important in the population dynamics of organisms with complex life cycles.     

Eugregarine infection within the digestive tract of larval Antarctic krill, Euphausia superba

The infection rate of eugregarine parasites, Cephaloidophora pacifica, within the digestive tract of larval Antarctic krill, Euphausia superba, was examined using samples collected from the Indian sector of the Southern Ocean. Immature and mature eugregarine gamont stages were found at all larval stages older than Calyptopis I stage. Eugregarine infection in 14.0% (N = 108) of the first feeding stage (Calyptopis I) suggested that krill larvae are at risk from being infected during physiological transition from non-feeding to feeding stages. Eugregarine prevalence and intensity of infection at the three calyptopis stages increased with stage/krill length. Statistical analysis showed that the intensity of C. pacifica infection also increased with host density. Thus, krill density is probably a key determinant of the intensity of infection. We found gamont stage eugregarines in the host hind-gut, blocking the passage of food. Eugregarine infections in larval krill may have a negative impact on digestion and absorption in the host digestive tract.     

Modeling the effects of Aedes aegypti’s larval environment on adult body mass at emergence

Mosquitoes vector harmful pathogens that infect millions of people every year, and developing approaches to effectively control mosquitoes is a topic of great interest. However, the success of many control measures is highly dependent upon ecological, physiological, and life history traits of mosquito species. The behavior of mosquitoes and their potential to vector pathogens can also be impacted by these traits. One trait of interest is mosquito body mass, which depends upon many factors associated with the environment in which juvenile mosquitoes develop. Our experiments examined the impact of larval density on the body mass of Aedes aegypti mosquitoes, which are important vectors of dengue, Zika, yellow fever, and other pathogens. To investigate the interactions between the larval environment and mosquito body mass, we built a discrete time mathematical model that incorporates body mass, larval density, and food availability and fit the model to our experimental data. We considered three categories of model complexity informed by data, and selected the best model within each category using Akaike’s Information Criterion. We found that the larval environment is an important determinant of the body mass of mosquitoes upon emergence. Furthermore, we found that larval density has greater impact on body mass of adults at emergence than on development time, and that inclusion of density dependence in the survival of female aquatic stages in models is important. We discuss the implications of our results for the control of Aedes mosquitoes and on their potential to spread disease.     

Prey to predator size ratio influences foraging efficiency of larval Aeshna juncea dragonflies

We investigated foraging behaviour of larval dragonflies Aeshna juncea in order to examine the significance of prey density and body size in predator-prey dynamics. A. juncea were offered separately three size-classes of Daphnia magna at low and high densities. The data were collected with direct observations of the foraging individuals. We found that large A. juncea larvae could better enhance their intake of prey biomass as prey size and prey density increased than their smaller conspecifics. However, increasing feeding efficiency of both larval instars was constrained by declining attack success and search rate with increasing prey size and density. With small D. magna, in contrast to large A. juncea, small A. juncea increased their searching efficiency as prey density increased keeping D. magna mortality rate at a constant level. In a predator-prey relationship this indicates stabilizing potential and feeding thresholds set by both prey density and prey-predator size ratio. Attack success dropped with prey size and density, but did not change in the course of the foraging bout. For both A. juncea sizes prey handling times increased as more medium and large prey were eaten. The slope of the increase became steeper with increasing prey-predator size ratio. These observations indicate that components of the predator-prey relationship vary with prey density, contrary to the basic assumptions of functional response equations. Moreover, the results suggest that the effects of prey density change during the ontogeny of predators and prey.