Predation yields greater population performance: What are the contributions of density- and trait-mediated effects?

1. Population responses to extrinsic mortality can yield no change in number of survivors (compensation) or an increase in number of survivors (overcompensation) when the population is regulated by negative density-dependence. This intriguing response has been the subject of theoretical studies, but few experiments have explored how the source of extrinsic mortality affects the response. 2. This study tests abilities of three functionally diverse predators, alone and combined, to induce (over)compensation of a prey population. Larval Aedes aegypti (Diptera: Culicidae) were exposed to predation by Mesocyclops longisetus (Crustacea: Copepoda), Anopheles barberi (Diptera: Culicidae), Corethrella appendiculata (Diptera: Corethrellidae), all three in a substitutive design, or no predation. 3. The number of survivors to adulthood, female size and development time, and a composite index of performance (r') were analysed. Predator treatment did not have a significant effect on total number of survivors, nor on number of males, suggesting mortality by predation was compensatory. Predation significantly affected number of female survivors, with a trend of more females produced with predation, though no post hoc tests were significant. Predation significantly increased female development rate and r' relative to no-predator control. 4. A sensitivity analysis indicated that the change in the number of female adults produced was the largest contributing factor to the differences in r' among cohorts. While predation did not significantly increase overall production of adults, it did release survivors from density-dependent effects sufficiently to increase population performance. This study provides an empirical test of mechanisms by which predation may yield positive effects on a population of victims, a phenomenon predicted to occur across many taxa and food webs.

Effects of larval density on a natural population of Culex restuans (Diptera: Culicidae): No evidence of compensatory mortality.

1. We investigated the effects of strong density-dependence on larval growth, development, and survival of the mosquito Culex restuans (Theobald). We tested the hypothesis that density reduction early in larval development could result in as many or more individuals surviving to adulthood (compensation or overcompensation, respectively), or increased reproductive performance via rapid development and greater adult size. 2. In a field study of a natural population of C. restuans we tested for the effects of a 75% lower density on percent survivorship to adulthood, number of adults, development time, adult size, adult longevity, and size dependent fecundity. 3. We found no evidence for compensation or overcompensation in adult production, nor for effects of lower density on percent survivorship. Low density yielded significant increases in adult size, adult longevity, and size-dependent fecundity, and a decrease in development time. 4. Estimated per capita population growth rate was significantly greater in the low-density treatment than in the high-density treatment. We infer this difference resulted from greater per capita resources increasing female size and fecundity, and reducing development time. Greater per capita population growth could therefore result from early mortality of larvae, meaning that the hydra effect, which predicts greater equilibrium population with, as opposed to without, extrinsic mortality, may be possible for these mosquitoes.

How can mortality increase population size? A test of two mechanistic hypotheses.

Overcompensation occurs when added mortality increases survival to the next life-cycle stage. Overcompensation can contribute to the Hydra effect, wherein added mortality increases equilibrium population size. One hypothesis for overcompensation is that added mortality eases density dependence, increasing survival to adulthood ("temporal separation of mortality and density dependence"). Mortality early in the life cycle is therefore predicted to cause overcompensation, whereas mortality later in the life cycle is not. Another hypothesis for overcompensation is that threat of mortality (e.g., from predation) causes behavioral changes that reduce overexploitation of resources, allowing resource recovery, and increasing production of adults ("prudent resource exploitation"). Behaviorally active predation cues alone are therefore predicted to cause overcompensation. We tested these predictions in two experiments with larvae of two species of Aedes. As predicted, early mortality yielded greater production of adults, and of adult females, and greater estimated rate of population increase than did later mortality. Addition of water-borne predation cues usually reduced browsing on surfaces in late-stage larvae, but contrary to prediction, resulted in neither significantly greater production of adult mosquitoes nor significantly greater estimated rate of increase. Thus we have strong evidence that timing of mortality contributes to overcompensation and the Hydra effect in mosquitoes. Evidence that predation cues alone can result in overcompensation via prudent resource exploitation is lacking. We expect the overcompensation in response to early mortality will be common in organisms with complex life cycles, density dependence among juveniles, and developmental control of populations.

No detectable role for predators mediating effects of aquatic habitat size and permanence on populations and communities of container­dwelling mosquitoes.

General theory from aquatic ecology predicts that smaller aquatic habitats have shorter hydroperiods favouring species that are better resource competitors and complete development quickly. Larger habitats are predicted to have longer hydroperiods enabling longer-lived predators to persist. Habitats with long hydroperiods and predators are predicted to favour slower-developing, predator resistant species, rather than competitive species.In a field experiment, we manipulated independently habitat size and hydroperiod in water-filled containers, to test these hypotheses about processes structuring aquatic communities. We used human-made containers that are dominated by mosquitoes that vary in desiccation resistance, competitive ability, and predation resistance.Habitat size and drying had significant effects on abundances of larvae of the common species in these communities. There was sorting of species by habitat size and by drying, with species that are better competitors relatively more abundant in smaller, more ephemeral habitats, and predator resistant, slower-developing species relatively more abundant in larger or permanently flooded habitats. There were no detectable effects of habitat size or drying on the dominant predator.Habitat size and its interaction with drying affected inputs of eggs to containers. Habitat size also affected relative abundances of the two dominant species in the egg population.Although habitat size and hydroperiod significantly affected composition of these communities, these impacts did not appear to be mediated through effects on predator abundance. Species specific differences in habitat size and drying regime preferences, and habitat-dependent larval performance appear to be the main forces shaping these communities.

The roles of history: age and prior exploitation in aquatic container habitats have immediate and carry-over effects on mosquito life history.

Per-capita resource availability in aquatic habitats is influenced directly by consumer density via resource competition and indirectly via delayed resource competition when temporally non-overlapping cohorts of larvae exploit the same resources. In detritus-based systems, resources are likely to be influenced by the age of the aquatic habitat, as detritus changes in quality over time and may be replenished by new inputs.For aquatic insects that exploit detritus-based habitats, feeding conditions experienced during immature stages can influence fitness directly via effects on development and survivorship, but also indirectly by influencing adult traits such as fecundity and longevity.Larval habitat age and prior resource exploitation were manipulated in a field experiment using the container mosquito Aedes triseriatus.It was found that A. triseriatus from older habitats had greater larval survival, faster development and greater adult longevity. Exploitation of larval habitats by a prior cohort of larvae had a significant negative effect on subsequent cohorts of larvae by delaying development.It is suggested that extended conditioning of detritus probably resulted in conversion of recalcitrant resources to more available forms which improved the quality of the habitat.In a parallel study, evidence was found of carry-over effects of habitat age and prior exploitation on adult longevity for A. triseriatus and Aedes japonicus collected from unmanipulated aquatic habitats.These results indicate the importance of detritus dynamics and the discontinuous nature of resource competition in these mosquito-dominated aquatic systems.

Modafinil Activates Phasic Dopamine Signaling in Dorsal and Ventral Striata.

Modafinil (MOD) exhibits therapeutic efficacy for treating sleep and psychiatric disorders; however, its mechanism is not completely understood. Compared with other psychostimulants inhibiting dopamine (DA) uptake, MOD weakly interacts with the dopamine transporter (DAT) and modestly elevates striatal dialysate DA, suggesting additional targets besides DAT. However, the ability of MOD to induce wakefulness is abolished with DAT knockout, conversely suggesting that DAT is necessary for MOD action. Another psychostimulant target, but one not established for MOD, is activation of phasic DA signaling. This communication mode during which burst firing of DA neurons generates rapid changes in extracellular DA, the so-called DA transients, is critically implicated in reward learning. Here, we investigate MOD effects on phasic DA signaling in the striatum of urethane-anesthetized rats with fast-scan cyclic voltammetry. We found that MOD (30-300 mg/kg i.p.) robustly increases the amplitude of electrically evoked phasic-like DA signals in a time- and dose-dependent fashion, with greater effects in dorsal versus ventral striata. MOD-induced enhancement of these electrically evoked amplitudes was mediated preferentially by increased DA release compared with decreased DA uptake. Principal component regression of nonelectrically evoked recordings revealed negligible changes in basal DA with high-dose MOD (300 mg/kg i.p.). Finally, in the presence of the D2 DA antagonist, raclopride, low-dose MOD (30 mg/kg i.p.) robustly elicited DA transients in dorsal and ventral striata. Taken together, these results suggest that activation of phasic DA signaling is an important mechanism underlying the clinical efficacy of MOD.

How do Nutritional Stress and La Crosse Virus Infection Interact? Tests for Effects on Willingness to Blood Feed and Fecundity in Aedes albopictus (Diptera: Culicidae).

Evolutionary theory predicts that vector-borne pathogens should have low virulence for their vector because of selection against pathogens that harm the vector sufficiently to reduce transmission. Environmental factors such as nutritional stress can alter vector-pathogen associations by making the vectors more susceptible to pathogens (condition-dependent competence) and vulnerable to the harm caused by pathogen replication (condition-dependent virulence). We tested the hypotheses of condition-dependent competence and condition-dependent virulence by examining the interactive effects of short-term sugar deprivation and exposure to La Crosse virus (LACV) in female Aedes albopictus (Skuse). We predicted that infection status interacts with sugar deprivation to alter willingness to blood feed and fecundity in the second gonotrophic cycle (condition-dependent virulence). Sugar deprivation had no effect on body infection or disseminated infection rates. Infection status, sugar treatment, and their interaction had no effect on fecundity. Mosquitoes that had intermittent access to sugar were significantly more willing to take a second bloodmeal compared with those that had continuous access to sugar. Infection status and the interaction with sugar treatment had no effect on blood-feeding behavior. Thus, we found no evidence of short-term sugar deprivation leading to condition-dependent competence for, or condition-dependent virulence of, LACV in Ae. albopictus.

Impact of inter- and intra-specific competition among larvae on larval, adult, and life-table traits of Aedes aegypti and Aedes albopictus females.

Few studies have taken a comprehensive approach of measuring the impact of inter- and intra-specific larval competition on adult mosquito traits. In this study, the impact of competition Aedes aegypti and A. albopictus was quantified over the entire life of a cohort.Competitive treatments affected hatch-to-adult survivorship and development time to adulthood of females for both species, but affected median wing length of females only for A. albopictus. Competitive treatments had no significant effect on the median adult female longevity nor were there any effects on other individual traits related to bloodfeeding and reproductive success.Analysis of life table traits revealed no effect of competitive treatment on net reproductive rate (R0) but there were significant effects on cohort generation time (Tc) and cohort rate of increase (r) for both species.Inter-specific and intra-specific competition among Aedes larvae may produce individual and population-level effects that are manifest in adults; however, benign conditions may enable resulting adults to compensate for some impacts of competition, particularly those affecting blood feeding success, fecundity, and net reproductive rate, R0. The effect of competition, therefore, affects primarily larva - to - adult survivorship and larval development time, which in turn impact the cohort generation time, Tc and ultimately cohort rate of increase, r.The lack of effects of larval rearing environment on adult longevity suggests that effects on vectorial capacity due to longevity may be limited if adults have easy access to sugar and blood meals.

Bone-remodeling transcript levels are independent of perching in end-of-lay white leghorn chickens.

Osteoporosis is a bone disease that commonly results in a 30% incidence of fracture in hens used to produce eggs for human consumption. One of the causes of osteoporosis is the lack of mechanical strain placed on weight-bearing bones. In conventionally-caged hens, there is inadequate space for chickens to exercise and induce mechanical strain on their bones. One approach is to encourage mechanical stress on bones by the addition of perches to conventional cages. Our study focuses on the molecular mechanism of bone remodeling in end-of-lay hens (71 weeks) with access to perches. We examined bone-specific transcripts that are actively involved during development and remodeling. Using real-time quantitative PCR, we examined seven transcripts (COL2A1 (collagen, type II, alpha 1), RANKL (receptor activator of nuclear factor kappa-B ligand), OPG (osteoprotegerin), PTHLH (PTH-like hormone), PTH1R (PTH/PTHLH type-1 receptor), PTH3R (PTH/PTHLH type-3 receptor), and SOX9 (Sry-related high mobility group box)) in phalange, tibia and femur. Our results indicate that the only significant effect was a difference among bones for COL2A1 (femur > phalange). Therefore, we conclude that access to a perch did not alter transcript expression. Furthermore, because hens have been used as a model for human bone metabolism and osteoporosis, the results indicate that bone remodeling due to mechanical loading in chickens may be a product of different pathways than those involved in the mammalian model.

What does not kill them makes them stronger: larval environment and infectious dose alter mosquito potential to transmit filarial worms.

For organisms with complex life cycles, larval environments can modify adult phenotypes. For mosquitoes and other vectors, when physiological impacts of stressors acting on larvae carry over into the adult stage they may interact with infectious dose of a vector-borne pathogen, producing a range of phenotypes for vector potential. Investigation of impacts of a common source of stress, larval crowding and intraspecific competition, on adult vector interactions with pathogens may increase our understanding of the dynamics of pathogen transmission by mosquito vectors. Using Aedes aegypti and the nematode parasite Brugia pahangi, we demonstrate dose dependency of fitness effects of B. pahangi infection on the mosquito, as well as interactions between competitive stress among larvae and infectious dose for resulting adults that affect the physiological and functional ability of mosquitoes to act as vectors. Contrary to results from studies on mosquito-arbovirus interactions, our results suggest that adults from crowded larvae may limit infection better than do adults from uncrowded controls, and that mosquitoes from high-quality larval environments are more physiologically and functionally capable vectors of B. pahangi. Our results provide another example of how the larval environment can have profound effects on vector potential of resulting adults.

Attracted to the enemy: Aedes aegypti prefers oviposition sites with predator-killed conspecifics.

Oviposition habitat choices of species with aquatic larvae are expected to be influenced by both offspring risk of mortality due to predation, and offspring growth potential. Aquatic predators may indirectly influence growth potential for prey by reducing prey density and, for filter-feeding prey, by increasing bacterial food for prey via added organic matter (feces, partially eaten victims), creating the potential for interactive effects on oviposition choices. We tested the hypothesis that the mosquito Aedes aegypti preferentially oviposits in habitats with predatory Toxorhynchites larvae because of indirect effects of predation on chemical cues indicating bacterial abundance. We predicted that A. aegypti would avoid oviposition in sites with Toxorhynchites, but prefer to oviposit where bacterial food for larvae is abundant, and that predation by Toxorhynchites would increase bacterial abundances. Gravid A. aegypti were offered paired oviposition sites representing choices among: predator presence; the act of predation; conspecific density; dead conspecific larvae; and bacterial activity. A. aegypti preferentially oviposited in sites with Toxorhynchites theobaldi predation, and with killed conspecific larvae, but failed to detect preferences for other treatments. The antibiotic tetracycline eliminated the strongest oviposition preference. Both predation by Toxorhynchites and killed larvae increased bacterial abundances, suggesting that oviposition attraction is cued by bacteria. Our results show the potential for indirect effects, like trophic cascades, to influence oviposition choices and community composition in aquatic systems. Our results suggest that predators like Toxorhynchites may be doubly beneficial as biocontrol agents because of the attraction of ovipositing mosquitoes to bacterial by-products of Toxorhynchites feeding.

Sexually dimorphic body size and development time plasticity in Aedes mosquitoes (Diptera: Culicidae).

BACKGROUND: Sexual size dimorphism (SSD) in insects often accompanies a sexual difference in development time, sexual bimaturism (SBM). GOAL: To determine whether three Aedes mosquito species have similar plasticity in SSD, attain sexual dimorphism through similar strategies, and whether SSD and SBM are associated. ORGANISMS: Aedes albopictus, Aedes aegypti, and Aedes triseriatus (Diptera: Culicidae). METHODS: In four different food availability environments, we quantified plastic responses of relative growth rate (RGR), development time, and adult body size in individually reared males and females. RESULTS: Food availability affected RGR differently for the sexes for all three species. The RGR of males and females differed significantly in the 0.1 g/L food treatment. This difference did not account for observed SSD. Food levels over which the largest changes in RGR were observed differed among the species. Male and female adult mass and development time were jointly affected by food availability in a pattern that differed among the three species, so that degree of SSD and SBM changed differentially with food availability for all three species. Development time was generally less sexually dimorphic than mass, particularly in A. albopictus. At lower food levels, A. aegypti and A. triseriatus had accentuated dimorphism in development time. These results, combined with our knowledge of mosquito life history, suggest that a direct benefit of SBM is improbable for mosquitoes and that the observed intersexual differences in development time are more likely byproducts of selection for SSD.

Oviposition habitat selection by container-dwelling mosquitoes: responses to cues of larval and detritus abundances in the field.

Insects' oviposition responses to resource and larval densities can be important factors determining distributions and competitive interactions of larvae. Aedes albopictus (Skuse) and Aedes aegypti (L.) (Diptera: Culicidae) show aggregated distributions of larvae in the field, larval interactions that are affected by detritus resources, and oviposition responses to resource and density cues in the laboratory. We conducted field experiments testing whether these species choose oviposition sites in response to chemical cues indicating detritus resource quantity and quality or larval abundances.In experiment 1, both species showed interactive responses to water conditioned with high or low quantities of senescent live oak leaves and density combinations of A. albopictus and A. aegypti larvae. Aedes aegypti preferred high-detritus containers when conspecifics were absent. Aedes albopictus tended to prefer high-detritus containers when larval density was low. We found no evidence of interspecific differences in oviposition preferences.In experiment 2, A. albopictus preferred high detritus over low or no detritus, and rapidly-decaying, high-quality detritus over low-quality detritus.Oviposition choices by these Aedes are mainly determined by resource quantity and quality, with larval densities having minor, variable effects. Oviposition responses of these species are unlikely to lead to resource partitioning. Aggregated distributions of these species in the field are unlikely to be products of oviposition choices based on larval densities.

Intrinsic and extrinsic drivers of succession: Effects of habitat age and season on an aquatic insect community.

1. Classical studies of succession, largely dominated by plant community studies, focus on intrinsic drivers of change in community composition, such as interspecific competition and changes to the abiotic environment. They often do not consider extrinsic drivers of colonization, such as seasonal phenology, that can affect community change. 2. We investigated both intrinsic and extrinsic drivers of succession for dipteran communities that occupy ephemeral pools, such as those in artificial containers. By initiating communities at different times in the season and following them over time, we compared the relative importance of intrinsic (i.e., habitat age) vs. extrinsic (i.e., seasonal phenology) drivers of succession. 3. We placed water-filled artificial containers in a deciduous forest with 20 containers initiated in each of three months. Containers were sampled weekly to assess community composition. Repeated-measures mixed-effects analysis of community correspondence analysis (CA) scores enabled us to partition intrinsic and extrinsic effects on succession. Covariates of temperature and precipitation were also tested. 4. Community trajectories (as defined by CA) differed significantly with habitat age and season, indicating that both intrinsic and extrinsic effects influence succession patterns. Comparisons of AICcs showed that habitat age was more important than season for species composition. Temperature and precipitation did not explain composition changes beyond those explained by habitat age and season. 5. Quantification of relative strengths of intrinsic and extrinsic effects on succession in dipteran and other ephemeral communities enables us to disentangle processes that must be understood for predicting changes in community composition.

She's a femme fatale: low-density larval development produces good disease vectors.

Two hypotheses for how conditions for larval mosquitoes affect vectorial capacity make opposite predictions about the relationship of adult size and frequency of infection with vector-borne pathogens. Competition among larvae produces small adult females. The competition-susceptibility hypothesis postulates that small females are more susceptible to infection and predicts frequency of infection should decrease with size. The competition-longevity hypothesis postulates that small females have lower longevity and lower probability of becoming competent to transmit the pathogen and thus predicts frequency of infection should increase with size. We tested these hypotheses for Aedes aegypti in Rio de Janeiro, Brazil, during a dengue outbreak. In the laboratory, longevity increases with size, then decreases at the largest sizes. For field-collected females, generalised linear mixed model comparisons showed that a model with a linear increase of frequency of dengue with size produced the best Akaike's information criterion with a correction for small sample sizes (AICc). Consensus prediction of three competing models indicated that frequency of infection increases monotonically with female size, consistent with the competition-longevity hypothesis. Site frequency of infection was not significantly related to site mean size of females. Thus, our data indicate that uncrowded, low competition conditions for larvae produce the females that are most likely to be important vectors of dengue. More generally, ecological conditions, particularly crowding and intraspecific competition among larvae, are likely to affect vector-borne pathogen transmission in nature, in this case via effects on longevity of resulting adults. Heterogeneity among individual vectors in likelihood of infection is a generally important outcome of ecological conditions impacting vectors as larvae.

Different demographic responses of three species of container Aedes (Diptera: Culicidae) larvae to timing of extrinsic mortality.

Mortality imposed on a population can interact with negatively density-dependent mortality to produce overcompensation, wherein added mortality results in more survivors. Experimental mortality can cause overcompensation in mosquito larvae, which would be counterproductive if it resulted from mosquito control in nature. We tested for different demographic responses to mortality among 3 container Aedes species when impacted by density dependence. We imposed 48.2% mortality on cohorts of larvae 2, 6, or 8 days after hatching and compared adult production, development times, and female size to those variables for controls without mortality. Mortality significantly increased adult production compared to controls, but the 3 species varied in the details of that response. Aedes albopictus (Skuse) produced more adults with mortality on day 2 primarily because of greater production of males. Aedes triseriatus (Say) yielded more adults with mortality on day 2 primarily because of greater production of females. Aedes aegypti (L.) adult production was not significantly affected by mortality, but development times for both sexes were significantly shorter with mortality on day 8. There were no effects of mortality on female wing length. None of our mortality treatments yielded significant reductions of adults for any species. These species responses to mortality are not the same, despite their similar ecologies and life histories. Thus, we cannot assume that killing almost half the larvae present in a dense population will reduce adult production, nor can we assume that different Aedes species will respond to mortality in the same way.

Amphetamine augments vesicular dopamine release in the dorsal and ventral striatum through different mechanisms.

Amphetamine has well-established actions on pre-synaptic dopamine signaling, such as inhibiting uptake and degradation, activating synthesis, depleting vesicular stores, and promoting dopamine-transporter reversal and non-exocytotic release. Recent in vivo studies have identified an additional mechanism: augmenting vesicular release. In this study, we investigated how amphetamine elicits this effect. Our hypothesis was that amphetamine enhances vesicular dopamine release in dorsal and ventral striata by differentially targeting dopamine synthesis and degradation. In urethane-anesthetized rats, we employed voltammetry to monitor dopamine, electrical stimulation to deplete stores or assess vesicular release and uptake, and pharmacology to isolate degradation and synthesis. While amphetamine increased electrically evoked dopamine levels, inhibited uptake, and up-regulated vesicular release in both striatal sub-regions in controls, this psychostimulant elicited region-specific effects on evoked levels and vesicular release but not uptake in drug treatments. Evoked levels better correlated with vesicular release compared with uptake, supporting enhanced vesicular release as an important amphetamine mechanism. Taken together, these results suggested that amphetamine enhances vesicular release in the dorsal striatum by activating dopamine synthesis and inhibiting dopamine degradation, but targeting an alternative mechanism in the ventral striatum. Region-distinct activation of vesicular dopamine release highlights complex cellular actions of amphetamine and may have implications for its behavioral effects.

An empirical test of the aggregation model of coexistence and consequences for competing container-dwelling mosquitoes.

We investigated the aggregation model of coexistence as a potential mechanism explaining patterns of coexistence between container mosquitoes Aedes albopictus and Aedes aegypti in southern Florida, USA. Aedes aegypti coexists with the invasive A. albopictus in many locations despite being an inferior resource competitor under most conditions. In agreement with aggregation theory we observed significant intraspecific aggregation of A. albopictus in all six field sites sampled in southern Florida in 2009. Quantitative results suggest that larval distributions of A. albopictus across containers are sufficiently aggregated to permit persistence of the inferior competitor A. aegypti. We tested whether observed levels of A. albopictus aggregation would significantly improve A. aegypti population performance in a controlled laboratory competition experiment manipulating A. albopictus aggregation while holding mean densities constant. We quantified A. aegypti's estimated rate of population change for replicate, multi-container cohorts in response to increasing A. albopictus aggregation across the cohorts. Aedes albopictus aggregation treatments produced J statistics for aggregation that spanned the range observed in the field study. We demonstrate a positive linear relationship between intraspecific aggregation of the superior competitor A. albopictus and estimated rate of population change for cohorts of the inferior A. aegypti. Thus, aggregation of A. albopictus at levels comparable to those observed in nature appears to be sufficient to reduce significantly the competitive impact of A. albopictus on multi-container cohorts of A. aegypti, and may therefore contribute to local coexistence of these competitors.

Seasonal variation in competition and coexistence of Aedes mosquitoes: stabilizing effects of egg mortality or equalizing effects of resources?

Theory shows that fluctuation of environmental conditions can produce temporal niches for inferior competitors that mitigate effects of interspecific competition and facilitate long-term persistence of poor competitors. In south Florida, the mosquitoes Aedes albopictus and Aedes aegypti often co-occur in water-filled containers despite A. albopictus being competitively superior to A. aegypti. We tested the hypothesis that seasonal fluctuation in environmental conditions reduces or reverses competitive asymmetry between the species and contributes to persistence of the poorer competitor via stabilizing or equalizing effects. During the Florida wet and dry seasons, we manipulated mosquito egg exposure to desiccation before inducing hatching and allowing surviving larvae to compete for 59 days. The effect of season also incorporated seasonal fluctuations in resource input to experimental containers. For both species, composite index of population performance (λ') was greater in the dry season than in the wet season, indicating strong seasonal effects on population dynamics. Aedes albopictus was not affected by competition in either season. Aedes aegypti was negatively affected by interspecific competition in the wet season. Aedes aegypti egg survival was unaffected by exposure to the different experimental environments. There was a small reduction in A. albopictus egg survival in the wet season, but this reduction was unrelated to effects on λ', indicating fluctuation in the egg environment did not contribute to dry season release from competition. Detritus resource inputs were over three times greater in the dry season than in the wet season. Given the relatively small effect of environment on egg survival, these results suggest that seasonal differences in population performance are driven primarily by per-capita food availability. Large inputs of detritus in the dry season appear to reduce competition and produce similar responses in both species. This result suggests that seasonal variation contributes to coexistence of A. albopictus and A. aegypti as a fitness-equalizing factor.

Predation resistance does not trade off with competitive ability in early-colonizing mosquitoes.

The tradeoff between colonization and competitive ability has been proposed as a mechanism for ecological succession, and this tradeoff has been demonstrated in multiple successional communities. The tradeoff between competitive ability and predation resistance is also a widely-described phenomenon; however, this tradeoff is not usually postulated as a cause of ecological succession. Early successional species that arrive before predator colonization could be either (1) less vulnerable to predation than their successors, by virtue of being poor competitors (direct competition-predation tradeoff); or (2) equally or more vulnerable to predation, because they normally colonize ahead of predators in succession and therefore are not evolutionarily adapted to avoid predators that they rarely encounter (no competition-predation tradeoff). To test these alternative hypotheses, we established water-filled containers in an oak-hickory forest. We allowed half of the containers to be naturally colonized by early-successional Culex mosquitoes, mid-successional Aedes mosquitoes, and the mosquito predator Toxorhynchites rutilus. In the other half of the containers, we prevented Aedes colonization via systematic removal of Aedes eggs, but allowed Culex and T. rutilus to colonize. The numbers of mature Culex larvae and pupae, and later the total number of Culex, were significantly greater in containers where Aedes had been removed, which suggests that Culex are competitively suppressed by Aedes. Toxorhynchites rutilus abundance and colonization rate were unaffected by the removal of Aedes, and densities of both Culex and Aedes decreased significantly with T. rutilus abundance in both treatments. In-laboratory bioassays showed that Culex were significantly more vulnerable to predation by T. rutilus than were Aedes. These data are consistent with the hypothesis that Culex and Aedes demonstrate a direct colonization-competition tradeoff, and are inconsistent with the hypothesis of a direct competition-predation tradeoff.