Variation In Larval Density Research Articles

Larval density can be used to predict genetic modifiers of glucagon signaling in Drosophila melanogaster.

Obesity is a growing concern. 42.3% of people in the U.S were considered obese between 2017-2018. Much is still unknown about the genetic components that contribute to weight gain. In humans, the hormone glucagon is a major contributor to the body's energy regulation as it signals for the breakdown of lipids. Treatments targeting the glucagon pathway have helped patients with both weight loss and appetite suppression. Understanding the genetic modifiers of glucagon signaling and its downstream pathways could enable the development of a wider variety of effective therapeutics. In this study, we blocked the glucagon pathway in Drosophila melanogaster by reducing the expression of the fly ortholog of the glucagon receptor (AKHR). We then crossed our model to the Drosophila Genetic Reference Panel (DGRP) and looked for natural variation in fat content. We used variation in larval density to identify candidate modifier genes through a genome-wide association study. We then tested these modifier genes by increasing or decreasing their expression in the AKHR model. We screened these candidates initially with the same density assay used in the original study to narrow down to four candidate genes that substantially impacted the density of the larvae: THADA, AmyD, GluRIIC, and CG9826. We further characterized these candidates using biochemical assays to analyze stored metabolites such as triglycerides, glucose, glycogen, and protein under control, high sugar, and high fat conditions to see if the larvae are resistant to environmental changes. Our results indicate consistency between the results of the density assay and direct measurement of metabolite levels. In particular, THADA and AmyD are highlighted as interesting genes for additional study. We hope to improve our understanding of the glucagon signaling pathway, obesity, and lipid metabolism. We also aim to provide candidate genes that can be regarded as future therapeutic targets.

Declines in lake whitefish larval densities after dreissenid mussel establishment in Lake Huron

Lake whitefish (Coregonus clupeaformis) is an ecologically and commercially significant species across the Laurentian Great Lakes. Over the past 20 years, lake whitefish population abundance has substantially declined across lakes Huron and Michigan, being driven by reduced recruitment of juvenile fish into the population. However, the life stage at which the recruitment bottleneck is occurring and what factors are contributing to the declines remain unknown. One hypothesis is that dreissenid mussels reduced zooplankton availability to larval lake whitefish, leading to poor growth and survival of this critical life stage. Here, we present results of a larval fish survey conducted at the Fishing Islands spawning region in Lake Huron and examine whether declines in juvenile recruitment are linked to reduced larval fish density. Larval fish were collected annually during two time periods: (1) a historical time period before dreissenid mussel establishment (1976–1986); and (2) a contemporary time period after dreissenid mussels became established (2017–2019, 2021). We found significant declines in larval densities and growth between historical and contemporary time periods. Following dreissenid establishment, larval densities and growth were on average only 23% and 55% of historical values, respectively. Moreover, year class strength at the juvenile stage (age 4) was positively related to larval density. Several explanatory variables contributed to annual variation in larval densities, with dreissenid mussels and water levels having the most consistent effect. Our results suggest that juvenile recruitment is being limited at the larval stage, owing to overall lower larval production and potentially exacerbated by slower growth.

Tracking gastrointestinal nematode risk on cattle farms through pasture contamination mapping

Gastrointestinal nematode (GIN) parasites in grazing cattle are a major cause of production loss and their control is increasingly difficult due to anthelmintic resistance and climate change. Rotational grazing can support control and decrease reliance on chemical intervention, but is often complex due to the need to track grazing periods and infection levels, and the effect of weather on larval availability. In this paper, a simulation model was developed to predict the availability of infective larvae of the bovine GIN, Ostertagia ostertagi, at the level of individual pastures. The model was applied within a complex rotational grazing system and successfully reproduced observed variation in larval density between fields and over time. Four groups of cattle in their second grazing season (n = 44) were followed throughout the temperate grazing season with regular assessment of GIN faecal egg counts, which were dominated by O. ostertagi, animal weight and recording of field rotations. Each group of cattle was rotationally grazed on six group-specific fields throughout the 2019 grazing season. Maps and calendars were produced to illustrate the change in pasture infectivity (density of L3 on herbage) across the 24 separate grazing fields. Simulations predicted differences in pasture contamination levels in relation to the timing of grazing and the return period. A proportion of L3 was predicted to persist on herbage over winter, declining to similar intensities across fields before the start of the following grazing season, irrespective of contamination levels in the previous year. Model predictions showed good agreement with pasture larval counts. The model also simulated differences in seasonal pasture infectivity under rotational grazing in systems that differed in temperature and rainfall profiles. Further application could support individual farm decisions on evasive grazing and refugia management, and improved regional evaluation of optimal grazing strategies for parasite control. The integration of weather and livestock movement is inherent to the model, and facilitates consideration of climate change adaptation through improved disease control.

Variation in the distribution of yellowfin sole Limanda aspera larvae in warm and cold years in the eastern Bering Sea

AbstractMultiyear periods of relatively cold temperatures (2007–2013) and warm temperatures (2001–2005 and 2014–2018) altered the eastern Bering Sea ecosystem, affecting ocean currents and wind patterns, plankton community, and spatial distribution of fishes. Yellowfin sole Limanda aspera larvae were collected from the inner domain (≤50 m depth) of the eastern Bering Sea among four warm years (2002, 2004, 2005, 2016), an average year (2006), and three cold years (2007, 2010, 2012). Spatial distribution and density of larvae among those years was analyzed using generalized additive models that included timing of sea‐ice retreat, areal coverage of water ≤0°C, and water temperature as covariates. Analyses indicated a combination of temperature effects on the location and timing of spawning, and on egg and larval survival, may explain the variation in larval density and distribution among years. During warm years, higher density and wider spatial distribution of larvae may be due to earlier spawning, an expansion of the spawning area, and higher egg and larvae survival due to favorable temperatures. Larval distribution contracted shoreward, and density was lower during cold conditions and was likely due to fish spawning closer to shore to remain in preferred temperatures, later spawning, and increased mortality. Predicted drift trajectories from spawning areas showed that larvae would reach nursery grounds in most years. Years when the drift period was longer than the pelagic phase of the larvae occurred during both warm and cold conditions indicating that settlement outside of nursery areas could happen during either temperature condition.

Validating a biophysical parasite model with fish farm pen and plankton trawl data

Biophysical models of parasite dispersal are being increasingly used as a method for screening marine aquaculture developments, whether in establishment of new sites or expansion of existing sites, or planning of farmed fish health management strategies on local or regional spatial scales. How well these models reflect reality, however, is often brought into question, due to the difficulties in validating their outputs. Larval parasitic sea lice can spend up to around 14 d in the water column, as a result potentially travelling several 10s of km between farms. Lice distribution in the water column is typically patchy and low density. Furthermore, infection can occur from lice carried by wild fish. Combined with rapid population turnover and larval exchange between farms, this causes difficulties in attributing links between juvenile lice and their sources. We sought to validate a biophysical model of sea lice dispersal using plankton trawl abundance data and farm site juvenile lice counts. Unusually high farm lice abundances over the study period allowed model predictions of larval density to be compared with trawled samples, in addition to mapping the link between parent and offspring lice counts found on farm sites. We compared the prediction of the larval dispersal model with a site neighbourhood-based metric of infection pressure. Our results validate the ability of the model to predict variation in larval density over time and space and suggest an exponential relationship between estimated infection pressure and observed site juvenile count.

Water temperature drives variability in salmonfly abundance, emergence timing, and body size

AbstractFreshwater organisms are disproportionately impacted by climate change and human disturbance, resulting in shifts in species' distributions and life histories. We coupled contemporary and historical datasets documenting physical and ecological variables over four decades to quantify changes in the abundance, emergence timing, and body size of salmonflies (Pteronarcys californica) in the Madison River in southwest Montana. In contemporary surveys, water temperature was the main driver of salmonfly abundance, emergence timing, and body size. Salmonfly densities were negatively correlated with summer water temperature, which explained 60% of variation in larval density among sites, whereas substrate type played a negligible role. Emergence occurred 20 days earlier, and male and female exuvia length were 13.8% and 11.3% shorter, respectively, at the warmest site relative to the coolest site (4°C difference). These patterns were supported by historical data. For example, a 1.2°C increase in mean annual water temperature in the Madison River between 1977 and 2017 coincided with evidence for upstream range contraction. Between 1973 and 2017, emergence timing varied widely among years, occurring up to 41 days earlier in years when spring water temperatures were relatively warm. As climate change progresses, we predict that salmonflies could be extirpated from an additional 28 km of currently occupied habitat, representing a 22.6% reduction in suitable habitat along the Madison River. This study provides evidence of long‐term biological change of an aquatic insect and highlights the importance of combining spatial and historical datasets to better understand species' responses to environmental stressors across both space and time.

Effect of larval density and substrate quality on the wing geometry of Stomoxys calcitrans L. (Diptera: Muscidae)

BackgroundIn insects, oviposition decisions may lead to egg deposition in substrates with different larval density and nutritional levels. Individuals developing in such substrates may present plasticity in their phenotype. Here, we investigated the effect of two factors related to oviposition decisions, namely larval density and substrate quality, on the wing size and wing shape of the stable fly, Stomoxys calcitrans L. (Diptera: Muscidae).MethodsWe reared S. calcitrans larvae at different densities (5, 15 and 25) and on different substrates (camel, cow, donkey and sheep dung). For each fly that emerged, we recorded body weight, and detached, slide-mounted and photographed the right wing. Next, we collected 15 landmarks on each photographed wing, and applied geometric morphometric analysis to assess variation in wing size and wing shape of S. calcitrans across the different larval densities and substrate types.ResultsWe observed that wing size and wing shape of S. calcitrans were affected by larval density and the nature of the developmental substrate. Flies reared in a group of 5 had larger wing centroid size, wing length, wing width, wing area and wing loading compared with those reared in a group of 25. Also, flies developed in donkey and sheep dung had larger wing centroid size, wing length, wing width, wing area and wing loading in comparison with those grown in camel and cow dung. Canonical variate analysis followed by discriminant analysis revealed significant wing shape variation in S. calcitrans across the different densities and substrates. Wing size had a significant but weak positive effect on wing shape.ConclusionsThis study demonstrates the high sensitivity of S. calcitrans wings to variation in larval density and developmental substrate, and that use of landmark-based geometric morphometric analysis could improve our understanding of how flies of veterinary importance respond to environmental variability.

Estimate zebra mussel veliger density from the riverbanks, lotic stretch and reservoir, in two Mediterranean rivers (E Spain)

Larval detection is the most commonly used procedure for monitoring the presence of Zebra mussel,Dreissena polymorha. Different filtering techniques carried out on the banks of the Ebro (EB) and Júcar (JU) rivers (Eastern Spain) in different water conditions are compared. Low volume water filtration from the banks is a method sensitive enough to detect variations in larval density due to water temperature and flow rates. The EB densities were lower and related to the temperature of the water. In JU, different spawning strategies and a much extended period of larval releasing into the water occurred in two consecutive years. The presence of larvae in both rivers was constant for many months of the year, during which various reproductive cycles occurred, facilitated by high water temperatures during mid-spring to late fall. Shoreline surveys can be easily used for allowing the early warning of the presence of this dangerous invasive species, mitigating damages and preventing its spread into new areas. However, it is important to choose a suitable location for capturing the larvae on the banks of reservoirs and in flowing rivers.

Characterizing spatial structures of larval fish assemblages at multiple scales in relation to environmental heterogeneity in the Strait of Georgia (British Columbia, Canada)

Spatial structures of larval fish in the Strait of Georgia (British Columbia, Canada) were quantified in the springs of 2009 and 2010 to investigate linkages to environmental heterogeneity at multiple scales. By applying a multiscale approach, principal coordinate neighborhood matrices, spatial variability was decomposed into three predefined scale categories: broad scale (>40 km), medium scale (20∼40 km), and fine scale (<20 km). Spatial variations in larval density of the three dominant fish taxa with different early life histories (Pacific herring (Clupea pallasii), Pacific hake (Merluccius productus), and northern smoothtongue (Leuroglossus schmidti)) were mainly structured at broad and medium scales, with scale-dependent associations with environmental descriptors varying interannually and among species. Larval distributions in the central-southern Strait were mainly associated with salinity, temperature, and vertical stability of the top 50 m of the water column on the medium scale. Our results emphasize the critical role of local estuarine circulation, especially at medium spatial scale, in structuring hierarchical spatial distributions of fish larvae in the Strait of Georgia and suggest the role of fundamental differences in life-history traits in influencing the formation and maintenance of larval spatial structures.

Scale‐dependent relationships between suspension‐feeding stream insects and water velocity in spatially heterogeneous flow environments

Summary Valuable insights about relationships between consumers and their resources have emerged from studies that investigated how these interactions vary with spatial scale. We conducted field studies to examine the scale-dependence of patterns of spatial variation in the densities of larval blackflies (Simulium tribulatum) and water velocity, a key variable determining resource supply rates to these passive suspension-feeders. We sought to answer two main questions: (i) How do patterns of spatial variation in water velocity and larval density change with scale? (ii) Does the relationship between larval density and water velocity vary with scale? We deployed artificial substrates on the bed of riffles and pools to assess spatial variation in water velocity and larval density at three spatial scales spanning four orders of magnitude: (i) among microhabitats within single substrates (centimetre scale); (ii) among substrates within individual riffles or pools (metre scale); and (iii) among riffles and pools (tens of metres scale). We used semi-cylindrical substrates that allowed us to estimate water velocities 1 mm above the substrate surface using a validated computational fluid dynamic model and measurements made with a 10-mm-diameter propeller flowmeter. On natural substrates (stones), we quantified fine-scale velocity variation (measured 1 mm above the stone using hot-film anemometry) and larval density at the two smallest scales. We used hierarchical regression models to partition variation in larval density and water velocity among spatial scales, and to measure the strength of relationships between larval density and water velocity at each scale. The distribution of spatial variation across scales was similar for both water velocity and larval density. The proportion of total variation differed markedly among scales, but variation did not increase monotonically with increasing spatial scale. The greatest proportional variation in velocity and density occurred at the smallest (among microhabitats) and largest (among riffles and pools) scales. The similar cross-scale distributions of variance for water velocity and larval density were not accompanied by consistent density–velocity relationships at all scales. At the smallest scale (i.e. among microhabitats on a single substrate), where spatial variation in velocity was large, there was a consistently strong positive relationship between larval density and water velocity. At the among-cylinder scale (within riffles or pools), where flow variation was smaller, the density–velocity relationship was strongly positive in pools but weakly negative in riffles. At the largest scale (among riffles and pools), the density–velocity relationship was positive among pools, and between riffles and pools, but was negative among riffles. Our results support the hypothesis that dispersal limitations may constrain the ability of larvae to reach preferred feeding sites at intermediate (among substrate) spatial scales. We believe the creative interplay of modelling, experiments and descriptive studies holds great promise for illuminating the causes and consequences of scale-dependence in resource tracking and habitat selection.

Allelochemical resistance traits of muskmelon (Cucumis melo) against the fruit fly (Bactrocera cucurbitae) in a hot arid region of India

Host plant resistance is an important component for management of the melon fruit fly, Bactrocera cucurbitae (Coquillett), owing to difficulties associated with its chemical and biological control. Various biochemical traits including total sugar, reducing sugar, non-reducing sugar, tannins, phenols, alkaloids, flavinoid and pH contents of fruit were studied on 11varieties/ genotypes of muskmelon, Cucumis melo L., in relation to resistance against B. cucurbitae under field conditions. Significant differences were found in tested varieties/ genotypes for fruit infestation and larval density per fruit. AHMM/BR-1, RM-50 and AHMM/BR-8 were the most resistant; MHY-5, Durgapura Madhu and Pusa Sarabati were moderately resistant; AHMM/BR-13, Pusa Madhuras and Arka Jeet were susceptible; whereas Arka Rajhans and GMM-3 were the highly susceptible varieties/ genotypes to fruit fly in both seasons, 2011 and 2012. The larval density per fruit increased with an increase in percent fruit infestation and there was a significant positive correlation (r = 0.97) between percent fruit infestation and larval density per fruit. Total sugar, reducing sugar, non-reducing sugar and pH were lowest in resistant and highest in susceptible varieties/ genotypes, whereas tannins, phenols, alkaloids and flavinoid contents were highest in resistant and lowest in susceptible varieties/ genotypes. Total alkaloid and pH contents explained 97.96% of the total variation in fruit fly infestation and 92.83% of the total variation in larval density per fruit due to alkaloids and total sugar contents.

Temporal and Spatial Distribution of Larval Fish Assemblage in Different Ecological Habitat in Johor Strait, Peninsular Malaysia

Fish larval diversity is very important for management of the fisheries resources. Fish larval density, family richness, Shannon Wiener index and evenness were determined by analyzing samples collected from the seagrass-mangrove ecosystem of Gelang Patah, Johor Strait, Peninsular Malaysia between October 2007 and September 2008. Five stations were selected namely upper estuary (S1), middle estuary (S2), lower estuary (S3), seagrass beds (S4) and outside seagrass beds (S5). In total, 24 fish larval families were identified from the investigated area. Among them, 14 occurred in upper estuary, 17 in middle estuary, 16 in lower estuary, 20 in seagrass beds and 16 in outside seagrass beds. Overall five (Clupeidae, Blenniidae, Terapontidae, Gobiidae and Sillaginidae) were the most dominant in study areas. Shannon-Wiener index varied significantly within monsoon and intermonsoon seasons peaking in the months October-January and May-August. The highest density of larval fishes was recorded at seagrass station (S4) and the spatial variations in larval density were significant (p<0.05) between seagrass and other four sampling sides. None of the diversity indices showed significant among-stations except only family richness was significantly (p<0.05) higher in seagrass beds than upper estuary.

Across-Shelf Transport of Bivalve Larvae: Can the Interface between a Coastal Current and Inshore Waters Act as an Ecological Barrier to Larval Dispersal?

Using an integrated physical and biological approach, we examined across-shelf advection and exchange and the associated transport of bivalve larvae in the presence of a strong coastal current separated from the coast by a stratified inshore environment. We tested the hypothesis that the interface of the coastal current and inshore waters can act as an ecological barrier to across-shelf transport of larvae but can be overcome by wind- or tidally-induced transport. Our study region in the Gulf of Maine encompasses a coastal current that diverges from the coast as it moves downshelf. The region inshore of this current is home to several species that exhibit limited recruitment in spite of extensive upshelf larval sources. Analysis of surface water temperatures and wind velocities revealed episodic decreases in temperature along the coast correlated with alongshelf (but not upwelling) winds, indicating wind-forced onshore movement of the cold coastal current. Such wind-driven onshore migrations are more common along the northern portion of the study region where the coastal current is near the coast, tidal currents are strong, and wind directions are more conducive to onshore migration, but rarer further south where the interface between inshore waters and the coastal current is further offshore and suitable wind events are less common. The distribution of bivalve larvae was consistent with the physical measurements. There was little across-shelf variation in larval abundance where the current abuts the coast, indicating strong across-shelf exchange of larvae, but strong across-shelf variation in larval density where the stratified inshore waters separate the current from the coast, indicating weak across-shelf transport of larvae. Our results suggest that the interface between the coastal current and inshore waters may constitute a major ecological barrier to larval dispersal in the southern part of the region that may only be overcome by rare, strong wind-forced events.

Sex‐specific effects of developmental environment on reproductive trait expression in Drosophila melanogaster

Variation in the expression of reproductive traits provides the raw material upon which sexual selection can act. It is therefore important to understand how key factors such as environmental variation influence the expression of reproductive traits, as these will have a fundamental effect on the evolution of mating systems. It is also important to consider the effects of environmental variation upon reproductive traits in both sexes and to make comparisons with the environment to which the organism is adapted. In this study, we addressed these issues in a systematic study of the effect of a key environmental factor, variation in larval density, on reproductive trait expression in male and female Drosophila melanogaster. To do this, we compared reproductive trait expression when flies were reared under controlled conditions at eight different larval densities that covered a 20-fold range. Then, to place these results in a relevant context, we compared the results to those from flies sourced directly from stock cages. Many reproductive traits were surprisingly insensitive to variation in larval density. A notable exception was nonlinear variation in female fecundity. In contrast, we found much bigger differences in comparisons with flies from stock cages—including differences in body size, latency to mate, copulation duration, fecundity, and male share of paternity in a competitive environment. For a number of traits, even densities of 1000 larvae per vial (125 larvae per mL of food) did not phenocopy stock cage individuals. This study reveals novel patterns of sex-specific sensitivity to environmental variation that will influence the strength of sexual selection. It also illustrates the importance of comparisons with the environment to which individuals are adapted.

Comparison of Larval Fish Density Between Seagrass Beds and Outside Seagrass Beds of the Southwestern Johor, Peninsular Malaysia

Temporal variation of fish larval density and composition between seagrass and outside seagrass beds of the southwestern Johor, Malaysia were investigated between October 2007 and September 2008. Fish larvae were sampled monthly by using a bongo net with 500 μm mesh size and 30 min sub-surface tow. In situ environmental variables were also recorded during the sampling works. The fish larval assemblage comprised of 20 families from the seagrass beds and 16 families recorded from the outside seagrass beds station. In total, 3738 larvae (2,801 from seagrass and 937 from non-seagrass area) were collected. Total larva density was at 79 individuals per 100 m3 and 34 individuals per 100 m3 for seagrass beds and outside seagrass station respectively. Larval abundance varied significantly within monsoon and inter-monsoon seasons, with peaks in February-March and May-July. Top five families were Blenniidae, Clupeidae, Gobiidae, Sillaginidae and Terapontidae and they occurred consistently throughout the year. Larvae belonging to family Clupeidae (47.94% in seagrass and 42.03% in outside seagrass) and Terapontidae (17% in seagrass and 24% in outside seagrass) were the most abundant family in the study areas. The highest density of total larval fishes was recorded at the seagrass ecosystem. The spatial variations in larval density were not significantly (p>0.05) different between the seagrass beds and open sea station.

Investigating phenotypic structures and allelochemical compounds of the fruits of Momordica charantia L. genotypes as sources of resistance against Bactrocera cucurbitae (Coquillett) (Diptera: Tephritidae)

Plant resistance is an important component of integrated management of the melon fruit fly, Bactrocera cucurbitae (Coquillett) owing to difficulties associated with its chemical control. Various biophysical traits including fruit length, fruit diameter, fruit toughness, number of the longitudinal ribs, height of the longitudinal ribs, number of small ridges, depth of small ridges and pericarp thickness, and biochemical traits including total chlorophyll, pH, tannin, flavanol, phenol, ash and silica contents of fruit were studied on six genotypes of bitter gourd ( Momordica charantia L.) in relation to resistance against B. cucurbitae under field conditions in Pakistan. Faisalabad-long and Col-II were the most resistant whereas Chaman and Col-Vehari were the most susceptible among the tested genotypes. Fruit length, fruit diameter, number of the longitudinal ribs and number of small ridges had significant positive correlations whereas fruit toughness, depth of small ridges, height of the longitudinal ribs and pericarp thickness had significant negative correlations with the percent fruit infestation and larval density. Maximum variation in fruit infestation and larval density was explained by fruit toughness (63.4 and 49.2% respectively) followed by fruit diameter (23.2 and 22.4% respectively) and number of the longitudinal ribs (8.2 and 11.6% respectively) whereas the remainder of the physical traits explained <2.0% variation in fruit infestation. Total chlorophyll and pH were lowest in resistant and highest in susceptible genotypes whereas tannin, flavanol, phenol, ash and silica contents were highest in resistant and lowest in susceptible genotypes. Tannin and flavanol contents explained 96.5% of the total variation in fruit infestation and 97.7% of the total variation in larval density whereas rest of the biochemical traits explained <0.2% variation in fruit infestation.

Measures of larval lake whitefish length and abundance as early predictors of year-class strength in Lake Michigan

Many lake whitefish stocks in Lake Michigan have experienced substantial declines in growth and condition since the 1990s. Reduced growth and condition could result in reduced quality or quantity of eggs produced by spawning females, which in turn could negatively impact recruitment. We evaluated the potential for reduced recruitment by measuring early life stage density and length, and we discuss the utility of these measures as early indicators of lake whitefish year-class strength. Overall, mean larval density (number per 1000 m 3 ± SE) in Lake Michigan was greater in 2006 (373.7 ± 28.3) than in 2005 (16.6 ± 24.8); whereas, mean length (mm ± SE) of larval lake whitefish was smaller in 2006 (12.87 ± 0.07) than in 2005 (14.38 ± 0.13). The ratio of zooplankton to fish density did not show an expected relationship with larval fish density or length. Rather, variation in larval density was best explained by a multiple-regression model that included larval length, spring wind intensity, and adult stock density as predictor variables. Our results suggest that the density of larval lake whitefish is not directly regulated by temperature or zooplankton density at the time of emergence, but that a potential for density-dependent regulation exists when larval emergence rates are high. We conclude that the observed declines in growth and condition of adult lake whitefish are not resulting in substantial reductions in recruitment.

Crowding and disease: effects of host density on response to infection in a butterfly–parasite interaction

Abstract. 1. Hosts experiencing frequent variation in density are thought to benefit from allocating more resources to parasite defence when density is high (‘density‐dependent prophylaxis’). However, high density conditions can increase intra‐specific competition and induce physiological stress, hence increasing host susceptibility to infection (‘crowding‐stress hypothesis’).2. We studied monarch butterflies (Danaus plexippus) and quantified the effects of larval rearing density on susceptibility to the protozoan parasite Ophryocystis elektroscirrha. Larvae were inoculated with parasite spores and reared at three density treatments: low, moderate, and high. We examined the effects of larval density on parasite loads, host survival, development rates, body size, and wing melanism.3. Results showed an increase in infection probability with greater larval density. Monarchs in the moderate and high density treatments also suffered the greatest negative effects of parasite infection on body size, development rate, and adult longevity.4. We observed greater body sizes and shorter development times for monarchs reared at moderate densities, and this was true for both unparasitised and parasite‐treated monarchs. We hypothesise that this effect could result from greater larval feeding rates at moderate densities, combined with greater physiological stress at the highest densities.5. Although monarch larvae are assumed to occur at very low densities in the wild, an analysis of continent‐wide monarch larval abundance data showed that larval densities can reach high levels in year‐round resident populations and during the late phase of the breeding season. Treatment levels used in our experiment captured ecologically‐relevant variation in larval density observed in the wild.

Long lasting impact of forest harvesting on the diversity of herbivorous insects

Many of the protected forest areas in Uganda have been subject to logging in the past. It is known that logging changes communities, but how long these changes last is unclear. Most of the studies on butterflies and moths have looked at the effects of logging relatively shortly (<15 years) after the logging took place. In this study we investigated community of herbivorous lepidopteran larvae and its temporal dynamics in a natural forest and three differently managed forest compartments after 40 years of regeneration. We collected samples of larvae from the leaves of Neoboutonia macrocalyx Pax. between April 2006 and March 2008 in Kibale National Park, Western Uganda. Herbivory, density of larvae, and species richness were significantly lower in logged compartments than in natural forest. Furthermore, the community composition differed significantly between the logged compartments and the natural forest. There was seasonal variation in larval density, species richness and diversity. In species richness and diversity the variation was synchronous in all the study areas, but larval density did not vary synchronously across the compartments, probably due to the impact of logging on the environment. We also observed changes in the community composition during different seasons in all the study areas. We attribute the long term impact of logging to the hindered regeneration of logged compartments and recommend restoration activities to help to return the logged areas closer to the natural state.

Seasonal variation in larval density of Limnoperna fortunei (Bivalvia, Mytilidae) in the Iguaçu and Paraná rivers, in the region of Foz do Iguaçu, Paraná, Southern Brazil

The larval density of the invasive mussel Limnoperna fortunei (Bivalvia, Mytilidae) was monitored from January 2005 to February 2006 at two points along the Paraná and the Iguaçu Rivers, near Foz do Iguaçu, PR, Brazil. The results indicated two density peaks in the Paraná River: a lower peak between March and June and a higher peak in October. In the Iguaçu River, the values were much lower, yet they also showed a peak between September and October. A reduction in the reproductive activity was observed in both the rivers between July and August. The average larval density at the collection point in the Paraná reached 948.5 larvae/m³, with a peak of 2,999.5 larvae/m³ in October. The present study represented the first quantification of the larval density of L. fortunei in the lower reaches of the Iguaçu River, downstream of the Iguaçu falls.