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

Mosquito population dynamics is shaped by the interaction among larval density, season, and land use

Understanding how variation in key abiotic and biotic factors interact at spatial scales relevant for mosquito fitness and population dynamics is crucial for predicting current and future mosquito distributions and abundances, and the transmission potential for human pathogens. However, studies investigating the effects of environmental variation on mosquito traits have investigated environmental factors in isolation or in laboratory experiments that examine constant environmental conditions that often do not occur in the field. To address these limitations, we conducted a semi-field experiment in Athens, Georgia using the invasive Asian tiger mosquito (Aedes albopictus). We selected nine sites that spanned natural variation in impervious surface and vegetation cover to explore effects of the microclimate (temperature and humidity) on mosquitoes. On these sites, we manipulated conspecific larval density at each site. We repeated the experiment in the summer and fall. We then evaluated the effects of land cover, larval density, and time of season, as well as interactive effects, on the mean proportion of females emerging, juvenile development time, size upon emergence, and predicted per capita population growth (i.e., fitness). We found significant effects of larval density, land cover, and season on all response variables. Of most note, we saw strong interactive effects of season and intra-specific density on each response variable, including a non-intuitive decrease in development time with increasing intra-specific competition in the fall. Our study demonstrates that ignoring the interaction between variation in biotic and abiotic variables could reduce the accuracy and precision of models used to predict mosquito population and pathogen transmission dynamics, especially those inferring dynamics at finer-spatial scales across which transmission and control occur.

Overcompensation of transient and permanent death rate increases in age-structured models with cannibalistic interactions

There has been renewed interest in understanding the mathematical structure of ecological population models that lead to overcompensation, the process by which a population recovers to a higher level after suffering a permanent increase in predation or harvesting. Here, we apply a recently formulated kinetic population theory to formally construct an age-structured single-species population model that includes a cannibalistic interaction in which older individuals prey on younger ones. Depending on the age-dependent structure of this interaction, our model can exhibit transient or steady-state overcompensation of an increased death rate as well as oscillations of the total population, both phenomena that have been observed in ecological systems. Analytic and numerical analysis of our model reveals sufficient conditions for overcompensation and oscillations. We also show how our structured population partial integrodifferential equation (PIDE) model can be reduced to coupled ODE models representing piecewise constant parameter domains, providing additional mathematical insight into the emergence of overcompensation.

Predation risk effects on larval development and adult life of Aedes aegypti mosquito

Biological control is one of the methods available for control of Aedes aegypti populations. We used experimental microcosms to evaluate the effects of actual predation and predation risk by dragonfly larvae (Odonata) on larval development, adult longevity, and adult size of Ae. aegypti. We used six treatments: control, removal, variable density cues (Cues VD), fixed density cues (Cues FD), variable density predator (Predator VD), and fixed density predator (Predator FD) (n = 5 each). Predator treatments received one dragonfly larva. Cue treatments were composed of crushed Ae. aegypti larvae released into the microcosm. For the FD treatments, we maintained a larval density of 200 individuals. The average mortality of Ae. aegypti larvae in the Predator VD treatment was used as the standard mortality for the other treatments. Mosquitoes from the Predator VD and Cues VD treatments developed faster, and adults were larger and had greater longevity compared to all other treatments, likely due to the higher food availability from larval density reduction. High larval density negatively affected larval developmental time, adult size, and longevity. Males were less sensitive to density-dependent effects. Results from this study suggest that the presence of predators may lead to the emergence of adult mosquitoes with greater fitness, causing an overall positive effect on Ae. aegypti population growth rates.

The size of larval rearing container modulates the effects of diet amount and larval density on larval development in Aedes aegypti

Mass-rearing of mosquitoes under laboratory conditions is an important part of several new control techniques that rely on the release of males to control mosquito populations. While previous work has investigated the effect of larval density and diet amount on colony productivity, the role of the size of the container in which larval development takes place has been relatively ignored. We investigated the role of container size in shaping life history and how this varied with density and food availability in Aedes aegypti, an important disease vector and target of mass-rearing operations. For each treatment combination, immature development time and survival and adult body size and fecundity were measured, and then combined into a measure of productivity. We additionally investigated how larval aggregation behaviour varied with container size. Container size had important effects on life history traits and overall productivity. In particular, increasing container size intensified density and diet effects on immature development time. Productivity was also impacted by container size when larvae were reared at high densities (1.4 larva/ml). In these treatments, the productivity metric of large containers was estimated to be significantly lower than medium or small containers. Regardless of container size, larvae were more likely to be observed at the outer edges of containers, even when this led to extremely high localized densities. We discuss how container size and larval aggregation responses may alter the balance of energy input and output to shape development and productivity.

Survival-Larval Density Relationships in the Field and Their Implications for Control of Container-Dwelling Aedes Mosquitoes

Population density can affect survival, growth, development time, and adult size and fecundity, which are collectively known as density-dependent effects. Container Aedes larvae often attain high densities in nature, and those densities may be reduced when larval control is applied. We tested the hypothesis that density-dependent effects on survival are common and strong in nature and could result in maximal adult production at intermediate densities for Aedes aegypti, Aedes albopictus, and Aedes triseriatus. We surveyed naturally occurring densities in field containers, then introduced larvae at a similar range of densities, and censused the containers for survivors. We analyzed the survival-density relationships by nonlinear regressions, which showed that survival-density relationships vary among seasons, sites, and species. For each Aedes species, some sites and times yielded predictions that larval density reduction would yield the same (compensation), or more (overcompensation), adults than no larval density reduction. Thus, larval control targeting these Aedes species cannot always be assumed to yield a reduction in the number of adult mosquitoes. We suggest that mosquito control targeting larvae may be made more effective by: Imposing maximum mortality; targeting populations when larval abundances are low; and knowing the shape of the survival-density response of the target population.

Non-linear relationships between density and demographic traits in three Aedes species

Understanding the relationship of population dynamics to density is central to many ecological investigations. Despite the importance of density-dependence in determining population growth, the empirical relationship between density and per capita growth remains understudied in most systems and is often assumed to be linear. In experimental studies of interspecific competition, investigators often evaluate the predicted outcomes by assuming such linear relationships, fitting linear functions, and estimating parameters of competition models. In this paper, we experimentally describe the shape of the relationship between estimated population rate of change and initial density using laboratory-reared populations of three mosquito species. We estimated per capita growth rate for these experimental populations over a 30-fold range of larval densities at a standard resource abundance. We then compared fits of linear models and several different nonlinear models for the relationship of estimated rate of change and density. We find that that the relationship between density and per capita growth is strongly non-linear in Aedes aegypti (Linnaeus), Aedes albopictus (Skuse), and Aedes triseriatus (Say) mosquitoes. Components of population growth (survivorship, development time, adult size) are also nonlinearly related to initial density. The causes and consequences of this nonlinearity are likely to be important issues for population and community ecology.

Cost-benefit evaluation of management strategies for an invasive amphibian with a stage-structured model

Management strategies for invasive populations should be designed to maximise efficacy and efficiency, i.e. to accomplish their goals while operating with the least resource consumption. This optimisation is often difficult to achieve in stage-structured populations, because costs, benefits and feasibility of removing individuals may vary with stage. We use a spatially-explicit stage-structured model to assess efficacy of past, present and alternative control strategies for invasive guttural toads, Sclerophrys gutturalis, in Cape Town. The strategies involve removal of variable proportions of individuals at different life-history stages and spatial scales. We also quantify the time necessary to implement each strategy as a proxy of financial resources and we correct strategy outcomes by implementation of time to estimate efficiency. We found that the strategy initially pursued in Cape Town, which did not target any specific stage, was less efficient than the present strategy, which prioritises adult removal. The initial strategy was particularly inefficient because it did not reduce the population size despite allocating consistent resources to remove eggs and tadpoles. We also found that such removal might be detrimental when applied at high levels. This counter-intuitive outcome is due to the ‘hydra effect’: an undesired increase in population size caused by removing individuals before overcompensatory density dependence. Strategies that exclusively remove adults ensure much greater management efficiency than those that also remove eggs and tadpoles. Available management resources should rather be allocated to increase the proportion of adult guttural toads that are removed or the spatial extent at which this removal is pursued.

Timing and magnitude of climatic extremes differentially elevate mortality but enhance recovery in a fish population

The countervailing effects of disturbances (e.g., high mortality and enhanced recovery) on population dynamics can occur through demographic processes under rapidly increasing climatic extremes. Across an extreme-event gradient, we mechanistically demonstrated how dramatic changes in streamflow have affected the population persistence of endangered salmon in monsoonal Taiwan over a three-decade period. Our modeling indicated that the dynamics of the age-structured population were attributed to demographic processes, in which extensive mortality was characterized as a function of climatic extremes and vulnerability in the young stage of fish. In the stochastic simulations, we found that the extensive mortality and high proportion of large fish resulted from extreme flooding, which caused high values of postimpact population recovery. Our empirical evidence suggests that the magnitudes and timing of disturbance can explain the population persistence when facing climatic extremes and thereby challenges the understanding of the mechanistic drivers of these countervailing phenomena under changing environmental conditions.

Quantifying the nature and strength of intraspecific density dependence in Arctic mosquitoes

Processes that change with density are inherent in all populations, yet quantifying density dependence with empirical data remains a challenge. This is especially true for animals recruiting in patchy landscapes because heterogeneity in habitat quality in combination with habitat choice can obscure patterns expected from density dependence. Mosquitoes (Diptera: Culicidae) typically experience strong density dependence when larvae compete for food, however, effects vary across species and contexts. If populations experience intense intraspecific density-dependent mortality then overcompensation can occur, where the number of survivors declines at high densities producing complex endogenous dynamics. To seek generalizations about density dependence in a widespread species of Arctic mosquito, Aedes nigripes, we combined a laboratory experiment, field observations, and modeling approaches. We evaluated alternative formulations of discrete population models and compared best-performing models from our lab study to larval densities from ponds in western Greenland. Survivorship curves from the lab were the best fit by a Hassell model with compensating density dependence (equivalent to a Beverton-Holt model) where peak recruitment ranged from 8 to 80 mosquitoes per liter depending on resource supply. In contrast, our field data did not show a signal of strong density dependence, suggesting that other processes such as predation may lower realized densities in nature, and that expected patterns may be obscured because larval abundance covaries with resources (cryptic density dependence). Our study emphasizes the importance of covariation between the environment, habitat choice, and density dependence in understanding population dynamics across landscapes, and demonstrates the value of pairing lab and field studies.

The ecological causes and consequences of hard and soft selection

Interactions between natural selection and population dynamics are central to both evolutionary-ecology and biological responses to anthropogenic change. Natural selection is often thought to incur a demographic cost that, at least temporarily, reduces population growth. However, hard and soft selection clarify that the influence of natural selection on population dynamics depends on ecological context. Under hard selection, an individual's fitness is independent of the population's phenotypic composition, and substantial population declines can occur when phenotypes are mismatched with the environment. In contrast, under soft selection, an individual's fitness is influenced by its phenotype relative to other interacting conspecifics. Soft selection generally influences which, but not how many, individuals survive and reproduce, resulting in little effect on population growth. Despite these important differences, the distinction between hard and soft selection is rarely considered in ecology. Here, we review and synthesize literature on hard and soft selection, explore their ecological causes and implications and highlight their conservation relevance to climate change, inbreeding depression, outbreeding depression and harvest. Overall, these concepts emphasise that natural selection and evolution may often have negligible or counterintuitive effects on population growth-underappreciated outcomes that have major implications in a rapidly changing world.

Comparing sterile male releases and other methods for integrated control of the tiger mosquito in temperate and tropical climates

The expansion of mosquito species worldwide is creating a powerful network for the spread of arboviruses. In addition to the destruction of breeding sites (prevention) and mass trapping, methods based on the sterile insect technique (SIT), the autodissemination of pyriproxyfen (ADT), and a fusion of elements from both of these known as boosted SIT (BSIT), are being developed to meet the urgent need for effective vector control. However, the comparative potential of these methods has yet to be explored in different environments. This is needed to propose and integrate informed guidelines into sustainable mosquito management plans. We extended a weather-dependent model of Aedes albopictus population dynamics to assess the effectiveness of these different vector control methods, alone or in combination, in a tropical (Reunion island, southwest Indian Ocean) and a temperate (Montpellier area, southern France) climate. Our results confirm the potential efficiency of SIT in temperate climates when performed early in the year (mid-March for northern hemisphere). In such a climate, the timing of the vector control action was the key factor in its success. In tropical climates, the potential of the combination of methods becomes more relevant. BSIT and the combination of ADT with SIT were twice as effective compared to the use of SIT alone.

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.

Site Occupancy by Aedes aegypti in a Subtropical City is Most Sensitive to Control during Autumn and Winter Months

The Aedes aegypti mosquito inhabits most tropical and subtropical regions of the globe, where it transmits arboviral diseases of substantial public health relevance, such as dengue fever. In subtropical regions, Ae. aegypti often presents an annual abundance cycle driven by weather conditions. Because different population states may show varying responses to control, we are interested in studying what time of the year is most appropriate for control. To do so, we developed two dynamic site-occupancy models based on more than 200 weeks of mosquito trapping data from nearly 900 sites in a subtropical Brazilian city. Our phenomenological, Markovian models, fitted to data in a Bayesian framework, accounted for failure to detect mosquitoes in two alternative ways and for temporal variation in dynamic rates of local extinction and colonization of new sites. Infestation varied from nearly full cover of the city area in late summer, to between 10% and 67% of sites occupied in winter depending on the model. Sensitivity analysis reveals that changes in dynamic rates should have the greatest impact on site occupancy during autumn and early winter months, when the mosquito population is declining. We discuss the implications of this finding to the timing of mosquito control.

Monitoring, modeling and harvest management of non-native invasive green iguanas on Grand Cayman, Cayman Islands

The green iguana (Iguana iguana) was most likely introduced as a pet and became overabundant during the last 20 years on Grand Cayman. Because negative impacts were unmanageable (e.g., damage to buildings and other infrastructure), a harvest management strategy was developed and implemented, and over 874,252 green iguanas were removed between October 2018 and August 2019. Distance sampling surveys were conducted to estimate abundance in February 2019 and annually in August 2014–2019. Abundance estimates were used to develop a Bayesian state-space logistic model, generate the posterior distributions of population and harvest management parameters, and make future predictions of abundance for August 2020–2030. Abundance increased over fivefold between August 2014 and 2018, from an average of 254,162 to 1,319,939 green iguanas. However, after harvesting for 5 months, abundance declined to an average of 600,113 green iguanas in February 2019; and after 11 months, abundance declined to an average of 103,020 green iguanas in August 2019. Maximum population growth rate averaged 1.552, carrying capacity averaged 1,611,013, equilibrium abundance averaged 805,506, maximum sustainable total harvest averaged 628,491, and maximum sustainable harvest rate averaged 0.776. With harvest rates between 0.600 and 0.800, predicted abundance averaged 28,751 green iguanas for August 2020–2030. However, harvest mortality may have unforeseen outcomes due to the release from density dependence and overcompensation through high survival and fecundity rates. Because natural resource managers have partial control over harvesting and incomplete understanding of green iguana population dynamics, monitoring and modeling are essential to assess population response and guide harvest management decisions.

Finding the sweet spot: What levels of larval mortality lead to compensation or overcompensation in adult production?

Extrinsic mortality impinging on negatively density-dependent populations can result in no change in the number of survivors (compensation) or an increase (overcompensation) by releasing the population from density-dependent effects on survivorship. The relationship between the level of extrinsic mortality (i.e., percentage of mortality) and the level and likelihood of overcompensation is theoretically important, but rarely investigated. We tested the hypothesis that overcompensation occurs below a threshold value of extrinsic mortality that is related to density-dependent mortality rate, and that additive extrinsic mortality occurs above this threshold. This hypothesis predicts that survivorship vs. extrinsic mortality will: 1) be best described by a two-segmented model with a threshold; 2) have a slope >0 below the threshold; and 3) have a slope=-1 above the threshold. We also tested whether mortality imposed by real predators and random harvest have equivalent effects on adult production, and whether magnitude of overcompensation is related to species sensitivity to density-dependence. These hypotheses were tested in the container mosquitoes Aedes aegypti, A. albopictus, A. triseriatus, and Culex pipiens (Diptera: Culicidae). Cohorts of 150 larvae were exposed to random harvest of 0-70% two days after hatch or to predation by 1-3 Mesocyclops longisetus (Crustacea: Copepoda). Overcompensation occurred in A. aegypti in a pattern consistent with predictions. Aedes triseriatus showed strong overcompensation but no evidence of a threshold, whereas A. albopictus and C. pipiens had survival consistent with compensatory mortality but no evidence of a threshold. Compared to random harvest, mortality from predation yielded greater adult production in A. aegypti and A. albopictus, lesser adult production in C. pipiens, and no difference in adult production in A. triseriatus. Our results are largely consistent with our hypothesis about overcompensation, with the caveat that thresholds for additive mortality appear to occur at very high levels of extrinsic mortality. Magnitudes of overcompensation for the three Aedes were inversely related to survival in the 0% mortality treatment, consistent with our hypothesis that overcompensation is related to sensitivity to density-dependence. A broad range of extrinsic mortality levels can yield overcompensation, which may have practical implications for attempts to control pest populations.

The ecological and epidemiological consequences of reproductive interference between the vectors Aedes aegypti and Aedes albopictus

Vector ecology is integral to understanding the transmission of vector-borne diseases, with processes such as reproduction and competition pivotal in determining vector presence and abundance. The arbovirus vectors Aedes aegypti and Aedes albopictus compete as larvae, but this mechanism is insufficient to explain patterns of coexistence and exclusion. Inviable interspecies matings-known as reproductive interference-is another candidate mechanism. Here, we analyse mathematical models of mosquito population dynamics and epidemiology which include two Aedes-specific features of reproductive interference. First, as these mosquitoes use hosts to find mates, reproductive interference will only occur if the same host is visited. Host choice will, in turn, be determined by behavioural responses to host availability. Second, females can become sterilized after mis-mating with heterospecifics. We find that a species with an affinity for a shared host will suffer more from reproductive interference than a less selective competitor. Costs from reproductive interference can be 'traded-off' against costs from larval competition, leading to competitive outcomes that are difficult to predict from empirical evidence. Sterilizations of a self-limiting species can counterintuitively lead to higher densities than a competitor suffering less sterilization. We identify that behavioural responses and reproductive interference mediate a concomitant relationship between vector ecological dynamics and epidemiology. Competitors with opposite behavioural responses can maintain disease where human hosts are rare, due to vector coexistence facilitated by a reduced cost from reproductive interference. Our work elucidates the relative roles of the competitive mechanisms governing Aedes populations and the associated epidemiological consequences.

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 Do Trait-Mediated Non-lethal Effects of Predation Affect Population-Level Performance of Mosquitoes?

Non-lethal, trait-mediated effects of predation impact prey behavior and life-history traits. Studying how these effects in turn influence prey demography is crucial to understand prey life-history evolution. Mosquitoes are important vectors that claim several million lives every year worldwide by transmitting a range of pathogens. Several ecological factors affect life-history traits of both larval and adult mosquitoes, creating effects that cascade to population-level consequences. Few studies have comprehensively explored the non-lethal effects of predation and its interactions with resources and competition on larval, adult, and population traits of mosquitoes. Understanding these interactions is important because the effects of predation are hypothesized to rescue prey populations from the effects of density-dependence resulting from larval competition. Aedes aegypti larvae reared at two different larval densities and subjected to three non-lethal predator treatments were monitored for survival, development time, and adult size through the larval stages to adult eclosion, and adult females were monitored for survival and reproduction through their first gonotrophic cycle. Intraspecific competition increased larval development time, yielded small-bodied adults, and reduced fecundity in individuals exposed to predatory chemical cues as larvae. Exposure to cues from a living predator affected both body size and latency to blood feed in females. Analysis of life-table traits revealed significant effects of competition on net reproductive rate (R ₀) of mosquitoes. The interaction between competition and predator treatments significantly affected the cohort rate of increase (r) and the index of performance (r'). The index of performance, which estimates rate of population change based on the size-fecundity relationship, was significantly and positively correlated with r, but overestimated r slightly. Lack of significant effect of predator treatments and larval density on cohort generation time (T _c) further suggests that the observed effects of treatments on r and r' were largely a consequence of the effects on R ₀. Also, the significant effects of treatment combinations on larval development time, adult body size and fecundity were ultimately manifested as effects on life-table traits estimated from adult survival and reproduction.