Search

Background The public health impact of the coronavirus disease 2019 (COVID-19) pandemic has motivated a rapid search for potential therapeutics, with some key successes. However, the potential impact of different treatments, and consequently research and procurement priorities, have not been clear. Methods Using a mathematical model of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission, COVID-19 disease and clinical care, we explore the public-health impact of different potential therapeutics, under a range of scenarios varying healthcare capacity, epidemic trajectories; and drug efficacy in the absence of supportive care. Results The impact of drugs like dexamethasone (delivered to the most critically-ill in hospital and whose therapeutic benefit is expected to depend on the availability of supportive care such as oxygen and mechanical ventilation) is likely to be limited in settings where healthcare capacity is lowest or where uncontrolled epidemics result in hospitals being overwhelmed. As such, it may avert 22% of deaths in high-income countries but only 8% in low-income countries (assuming R = 1.35). Therapeutics for different patient populations (those not in hospital, early in the course of infection) and types of benefit (reducing disease severity or infectiousness, preventing hospitalization) could have much greater benefits, particularly in resource-poor settings facing large epidemics. Conclusions Advances in the treatment of COVID-19 to date have been focused on hospitalized-patients and predicated on an assumption of adequate access to supportive care. Therapeutics delivered earlier in the course of infection that reduce the need for healthcare or reduce infectiousness could have significant impact, and research into their efficacy and means of delivery should be a priority.

All animals, including carnivores, adapt their daily activity duration and distribution to satisfy food demands, breed, or avoid mortality risk. We used the kernel density method to estimate daily movement activity levels and movement activity patterns of jaguars in Hato Piñero, in Venezuelan Western Llanos, based on 3,656 jaguar detection time records from two and a half years of camera trapping. Jaguars were active for 11.7 h per day on average and exhibited mostly nocturnal and crepuscular activity pattern, however, with marked differences between sex/age/reproductive groups. Reproductive females had the highest daily activity level (13.2 h/day), followed by adult males (10.9 h/day), non-reproductive females (10.5 h/day), and cubs (8.7 h/day). Activity patterns also differed, with males and reproductive females having activity peaks at the same hours after sunset and before sunrise, cubs in the night and after sunrise, while non-reproductive females were most active during night hours. This study was the first to document the effect of sex, age, and reproductive status on daily level and activity pattern in the jaguar.

BackgroundThe unprecedented public health impact of the COVID-19 pandemic has motivated a rapid search for potential therapeutics, with some key successes. However, the potential impact of different treatments, and consequently research and procurement priorities, have not been clear.Methods and FindingsWe develop a mathematical model of SARS-CoV-2 transmission, COVID-19 disease and clinical care to explore the potential public-health impact of a range of different potential therapeutics, under a range of different scenarios varying: i) healthcare capacity, ii) epidemic trajectories; and iii) drug efficacy in the absence of supportive care. In each case, the outcome of interest was the number of COVID-19 deaths averted in scenarios with the therapeutic compared to scenarios without. We find the impact of drugs like dexamethasone (which are delivered to the most critically-ill in hospital and whose therapeutic benefit is expected to depend on the availability of supportive care such as oxygen and mechanical ventilation) is likely to be limited in settings where healthcare capacity is lowest or where uncontrolled epidemics result in hospitals being overwhelmed. As such, it may avert 22% of deaths in high-income countries but only 8% in low-income countries (assuming R=1.35). Therapeutics for different patient populations (those not in hospital, early in the course of infection) and types of benefit (reducing disease severity or infectiousness, preventing hospitalisation) could have much greater benefits, particularly in resource-poor settings facing large epidemics.ConclusionsThere is a global asymmetry in who is likely to benefit from advances in the treatment of COVID-19 to date, which have been focussed on hospitalised-patients and predicated on an assumption of adequate access to supportive care. Therapeutics that can feasibly be delivered to those earlier in the course of infection that reduce the need for healthcare or reduce infectiousness could have significant impact, and research into their efficacy and means of delivery should be a priority.

Testing and isolation have been crucial for controlling the COVID-19 pandemic. Venezuela has one of the weakest testing infrastructures in Latin America and the low number of reported cases in the country has been attributed to substantial underreporting. However, the Venezuelan epidemic seems to have lagged behind other countries in the region, with most cases occurring within the capital region and four border states. Here, we describe the spatial epidemiology of COVID-19 in Venezuela and its relation to the population mobility, migration patterns, non-pharmaceutical interventions and fuel availability that impact population movement. Using a metapopulation model of SARS-CoV-2 transmission dynamics, we explore how movement patterns could have driven the observed distribution of cases. Low within-country connectivity most likely delayed the onset of the epidemic in most states, except for those bordering Colombia and Brazil, where high immigration seeded outbreaks. NPIs slowed early epidemic growth and subsequent fuel shortages appeared to be responsible for limiting the spread of COVID-19 across the country.

The American bullfrog (Lithobates catesbeianus Shaw, 1802), an amphibian species native to eastern North America, is considered one of the 100 most harmful invasive species in the world. Previous studies document several feral populations in the Amazon and Andean regions of Ecuador. However, only few adults have been reported in the Coast region, despite some evidence suggesting its introduction 31 years ago. Using visual and auditory cues, we explored a 490-hectare wetland area at Santay Island, a protected sanctuary and a Ramsar site on the Ecuadorian Coast. Bullfrogs were detected in seven out of 15 sampled ponds in all types of habitats except for mangroves. The low abundance of adults and juveniles suggests a recently established population. This is the first record of a feral population inside a protected area or Ramsar site in Ecuador. In accordance with the Ramsar Convention mission of preserving wetlands, we propose two strategies to manage bullfrogs at Santay Island.

The worldwide decline in carnivore populations has been attributed to various human impacts. However, our understanding of the mechanisms behind these declines is insufficient to predict the timing and location of local extinctions. We collected data on presence/absence and time since extirpation of jaguars across Venezuela. To test if human impacts or ecosystem productivity better explain the observed spatial variation in probability of jaguar occurrence we compared logistic regression models fit with different combinations of anthropogenic and environmental variables. Similarly, we modelled the time since extirpation, using a multiple regression approach. Our study supported the hypothesis that jaguar extirpations and distribution are determined by a joint effect of anthropogenic factors and environmental variables, mainly those related with ecosystem productivity. Human population density and habitat alterations exerted strong negative effects on jaguar populations, while annual precipitation, mean temperature, forest cover, primary productivity, and other vegetation indices had positive effects. The strength of human impact is shaped by ecosystem productivity: jaguars disappear faster in dry, unproductive areas, and survive better in humid, productive areas even when human densities are higher. We estimated that jaguars in Venezuela have been extirpated from approximately 26% of the territory of Venezuela; present jaguar range covers approximately 66% of the country. We demonstrate that human population density alone cannot adequately explain past extirpations nor predict future jaguar declines. We conclude that the predicted future growth of the human population will not necessarily determine jaguar declines, and proper management and conservation programs could potentially prevent jaguar extirpations.

Density is crucial for understanding large carnivore ecology and conservation, but estimating it has proven methodologically difficult. We conducted 1 year of camera trapping to estimate jaguar (Panthera onca) density and population structure in the Los Llanos region of Venezuela on the Hato Pinero ranch, where hunting is prohibited and livestock are excluded from half of ranch lands. We identified 42 different jaguars and determined their sex, age class, and reproductive status. We estimated adult jaguar densities with spatial capture-recapture models, using sex/reproductive state and session as covariates. Models without temporal variation received more support than models that allowed variation between sessions. Males, reproductive females, and nonreproductive females differed in their density, baseline detectability, and movement. The best estimate of total adult jaguar population density was 4.44 individuals/100 km2. Based on reproductive female density and mean number of offspring per female, we estimated cub density at 3.23 individuals/100 km2 and an overall density of 7.67 jaguars/100 km2. Estimated jaguar population structure was 21% males, 11% nonreproductive females, 26% reproductive females, and 42% cubs. We conclude that extending the sampling period to 1 year increases the detectability of females and cubs and makes density estimates more robust as compared to the more common short studies. Our results demonstrate that the Venezuelan Llanos represent important jaguar habitat, and further, they emphasize the importance of protected areas and hunting restrictions for carnivore conservation.

Broad scale population estimates of declining species are desired for conservation efforts. However, for many secretive species including large carnivores, such estimates are often difficult. Based on published density estimates obtained through camera trapping, presence/absence data, and globally available predictive variables derived from satellite imagery, we modelled density and occurrence of a large carnivore, the jaguar, across the species’ entire range. We then combined these models in a hierarchical framework to estimate the total population. Our models indicate that potential jaguar density is best predicted by measures of primary productivity, with the highest densities in the most productive tropical habitats and a clear declining gradient with distance from the equator. Jaguar distribution, in contrast, is determined by the combined effects of human impacts and environmental factors: probability of jaguar occurrence increased with forest cover, mean temperature, and annual precipitation and declined with increases in human foot print index and human density. Probability of occurrence was also significantly higher for protected areas than outside of them. We estimated the world’s jaguar population at 173,000 (95% CI: 138,000–208,000) individuals, mostly concentrated in the Amazon Basin; elsewhere, populations tend to be small and fragmented. The high number of jaguars results from the large total area still occupied (almost 9 million km2) and low human densities (< 1 person/km2) coinciding with high primary productivity in the core area of jaguar range. Our results show the importance of protected areas for jaguar persistence. We conclude that combining modelling of density and distribution can reveal ecological patterns and processes at global scales, can provide robust estimates for use in species assessments, and can guide broad-scale conservation actions.

Long-term demographic data are valuable for assessing the effect of anthropogenic impacts on endangered species and evaluating recovery programs. Using a 2-state open robust design model, we analyzed mark-recapture data from green turtles Chelonia mydas sighted be - tween 1979 and 2009 on Aves Island, Venezuela, a rookery heavily impacted by human activities before it was declared a wildlife refuge in 1972. Based on the encounter histories of 7689 nesting females, we estimated the abundance, annual survival, and remigration intervals for this popula- tion. Female survival varied from 0.14−0.91, with a mean of 0.79, which is low compared to sur- vival of other populations from the Caribbean (mean = 0.84) and Australia (mean = 0.95), even though we partially corrected for tag loss, which is known to negatively bias survival estimates. This supports prior suggestions that Caribbean populations in general, and the Aves Island popu- lation in particular, may be more strongly impacted than populations elsewhere. It is likely that nesters from this rookery are extracted while foraging in remote feeding grounds where hunting still occurs. Despite its relatively low survival, the nesting population at Aves Island increased dur- ing the past 30 years from approx. 500 to >1000 nesting females in 2009. Thus, this population, like others in the Caribbean and the Atlantic, seems to be slowly recovering following protective man- agement. Although these findings support the importance of long-term conservation programs aimed at protecting nesting grounds, they also highlight the need to extend management actions to foraging grounds where human activities may still impact green turtle populations.

Harlequin toads (Atelopus) are among the most severely impacted amphibians by the emergence of chytridiomycosis, a fungal disease caused by the pathogen Batrachochytrium dendrobatidis (Bd). Many species disappeared while others suffered drastic contractions of their geographic distribution to lower altitudes. A diminished virulence of Bd in warm habitats was proposed to explain the survival of lowland populations of harlequin toads (i.e. thermal refuge hypothesis). To understand the mechanisms that allow some populations to reach an endemic equilibrium with this pathogen, we estimated demographic and epidemiological parameters at one remnant population of Atelopus cruciger in Venezuela using mark-recapture data from 2007-2013. We demonstrated that Bd is highly virulent for A. cruciger, increasing the odds of dying of infected adults four times in relation to uninfected ones and reducing the life expectancy of reproductive toads to a few weeks. Despite an estimated annual loss of 18% of the reproductive population due to Bd-induced mortality, this population has persisted in an endemic equilibrium for the last decade through the large recruitment of healthy adults every year. Given the high vulnerability of harlequin toads to Bd in lowland populations, thermal refuges need to be redefined as habitats of reduced transmission rather than attenuated virulence.

Chytridiomycosis is an emerging disease that has driven some amphibian species to extinction while leaving others apparently unharmed. Its causative agent, Batrachochytrium dendrobatidis (Bd), now persists endemically in many amphibian communities. Understanding host species response to Bd infection is critical for managing chytridiomycosis because the epidemiology of this disease is host-specific. Dendropsophus meridensis is an endangered hylid frog endemic to the Venezuelan Andes. This species is sympatric with the American bullfrog Lithobates catesbeianus, an introduced species known to act as a reservoir for Bd. High prevalence of infection and high zoospore burdens in wild populations of D. meridensis in the Venezuelan Andes suggested some tolerance for Bd. However, experimental exposure of post-metamorphic frogs resulted in 53% mortality, a value that represents a 14-fold increase in the odds of dying compared to control frogs. Repeated diagnostics using real-time polymerase chain reaction assays demonstrated that individuals that died accumulated a higher number of zoospores than those that survived, although this value was lower than the mean zoospore burdens observed in natural populations. Given the susceptibility of D. meridensis to a strain of Bd isolated from a nearby population of bullfrogs, we emphasize the need to limit the dispersion of this invasive species.

Rhinella marina is a Neotropical toad that has been introduced widely worldwide. Its toxic effects to frog-eating predators threaten the native and domestic fauna of some regions where it has been introduced. Despite previous studies suggesting two genetically distinct cryptic species within R. marina, one east and one west of the Andes, its taxonomic status remained unresolved due to the absence of morphological complementary evidence. For the first time, data from two mitochondrial genes (ND3 and CR) and 23 morphometric landmarks are combined to evaluate the taxonomic status of this species. Our results support the hypothesis of two separate evolutionary lineages within R. marina and demonstrate that these lineages have significantly diverged in skull shape. We identified two distinct morphotypes, one eastern and one Andean western, with no overlapping morphospaces. The geographic pattern of genetic variation was consistent with a stable structured population with no evidence of recent demographic or geographic expansions. The concordance between the observed geographic patterns in morphometric and genic traits calls for the recognition of two species under R. marina name.

Atelopus is among the most threatened of all amphibian genera. Most species of harlequin frogs disappeared more than two decades ago and only a few still exist. From ten critically endangered Atelopus species endemic to Venezuela, Atelopus cruciger is the only one that can be located at present. To assess the status of remnant populations of A. cruciger and to provide the demographic data for designing in situ management programs, we estimated: (1) the population size; (2) the apparent survival; and (3) the recruitment rates of one remnant population using mark-recapture data. The adult population size varied (69117), and this variation was not related to that of abundance indices based on visual counts at the river margins. Thus, caution is recommended when using visual counts as an index of abundance in Atelopus, because capture rates differ significantly among months and between seasons. Despite the observed variations, this population appears to be stable. Previous reports suggest that species of Atelopus are long-lived. For populations of long-lived species to remain approximately constant, recruitment must be low. Our mark-recapture study, however, showed that adults tend to remain in the population for approximately 15 mo, but an average of 165 new frogs are recruited every year. Although immigration and emigration are possibilities, the site fidelity and the absence of nearby streams suggests that movement in and out of the study area is less important than births and deaths. Under the proposed hypothesis of a short life expectancy/high recruitment, the risk of extinction must be lower than previously thought.

Biological invasions have a great impact on biodiversity and ecosystem functioning worldwide. Kalanchoe daigremontiana is a noxious invasive plant in arid zones. Besides being toxic for domestic animals and wildlife, this species inhibits the growth of native plants. Its rapid proliferation in Cerro Saroche National Park (Vene- zuela) is of great concern because this area hosts several species endemic to the scarce arid zones in the Caribbean. The traits of K. daigremontiana that contribute to its invasive success are unknown. Based on empirical data, we derived a stage structured, stochastic and density-depen- dent model, to identify characteristics relevant for its establishment. Sensitivity analyses revealed that the establishment of K. daigremontiana depends exclusively on plantlet recruitment. Because asexual plantlets repro- duce in less than 1 year populations are able to increase rapidly during the initial phases of invasion, when extinc- tion risks are higher. Sexual seedlings, on the contrary, require a minimum of 3 years to reproduce. As a result, seedling recruitment contributes little to the transient dynamics of the population and therefore cannot warrant the successful establishment of the species. Simulations of various management strategies show that eradication through plant removal may only be achieved if harvest begins shortly after introduction. If a rapid response is not possible, reducing the survival and growth rates of plantlets through biological control is an alternative option. Thus, a strict control of dispersal of plantlets by humans and a continuous monitoring of new invasions should be the first priority for reducing further impact on native species.

Amphibian declines worldwide have been linked to the fungal disease chytridiomycosis. Its causative agent (Batrachochytrium dendrobatidis, hereafter Bd), however, also infects many nondeclining species. Experimental infections have shown species-specific and temperature-dependent frog responses to Bd infection. Although Bd infection may be eliminated by housing amphibians at temperatures above those tolerated by the fungus, the question of whether frogs can eliminate infection under more favorable conditions remains unanswered. Repeated diagnostics using real-time polymerase chain reaction (rt-PCR) assays of postmetamorphic individuals at 28, 38, 45, 53, and 62 days after exposure demonstrated that Hypsiboas crepitans is able to clear infection within a few weeks at 23°C. Thus, we demonstrate a temperature-independent and likely immunological mechanism for the clearance of Bd in a resistant amphibian species. Future studies are needed to determine the generality of this mechanism among amphibians and to describe the immune factors affecting different outcomes of Bd exposure including resistance to infection, tolerance of infection, and clearance of infection.

Many recent amphibian declines have been associated with chytridiomycosis, a cutaneous disease caused by the chytrid fungus Batrachochytrium dendrobatidis , but increasing evidence suggests that this pathogen may coexist with some species without causing declines. In the Venezuelan Andes, the disappearance of three anuran species during the late eighties was attributed to B. dendrobatidis. Recently, this pathogen was found to be prevalent in this region on the introduced American bullfrog, Lithobates catesbeianus. As a first step toward assessing the risk of amphibian communities to B. dendrobatidis in this region, we conducted a broad survey across multiple habitat types and an altitudinal gradient spanning over 2000 m. We diagnosed 649 frogs from 17 species using real time and conventional PCR assays, and recorded relevant abiotic characteristics of host habitats. Infection was detected in 10 native species of pond, stream and terrestrial habitats from 80–2600 m, representing nine new host records. L. catesbeianus was the most important reservoir with 79.9% of individuals infected and an average of 2299 zoospores. Among native frogs, Dendropsophus meridensis, an endangered species sympatric with L. catesbeianus, showed the highest infection prevalence and mean zoospore load (26.7%; 2749 zoospores). We did not detect clinical signs of disease in infected hosts; however, species such as D. meridensis may be at risk if environmental stress exacerbates vulnerability or pathogen loads. While surveillance is an effective strategy to identify highly exposed species and habitats, we need to understand species-specific responses to B. dendrobatidis to stratify risk in amphibian communities.

The harlequin frog, Atelopus mucubajiensis, is a critically endangered species that was believed to be extinct until 2004, when a single adult female was found in the Venezuelan Andes. Its disappearance after 1994 has been attributed to an increase in the prevalence of the fungal pathogen, Batrachochytrium dendrobatidis, in populations of these frogs. Using histology and Real-Time PCR assays, we tested this adult female positive for B. dendrobatidis. We report for the first time, to our knowledge, infection in a live specimen of A. mucubajiensis. The presence of this pathogen in remaining individuals poses a problem for the long-term persistence of this critically endangered species, as new epidemic episodes of chytridiomycosis could jeopardize the survival of remnant populations.

Chytridiomycosis is an emerging fungal disease of amphibians responsible for mass mortalities and population declines globally. One hypothesis for its recent emergence is anthropogenic introduction of the causative agent Batrachochytrium dendrobatidis through trade in amphibians for pets, food and biocontrol. In this study, we examined histological samples from apparently healthy American bullfrogs Rana catesbeiana that have been introduced into the Venezuelan Andes. B. dendrobatidis was present in 96% (46/48) of the individuals examined. In contrast to cases of chytridiomycosis outbreaks, the majority (44/46) of frogs had few, small lesions consistent with little or no clinical disease and no unusual mortality was observed. These findings have implications for amphibian declines in Venezuela and elsewhere. First, the high prevalence of Batrachochytrium but lack of clinical signs or chytridiomycosis-related mortality suggests that R. catesbeiana may be a good reservoir for this parasite in Venezuela. Second, the presence of this emerging pathogen in an introduced amphibian species suggests that trade and introduction of amphibians should be monitored and controlled to halt the spread of this pathogen nationally and internationally.

Molecular data on Lutzomyia are very scarce, despite the fact that this genus includes all the species involved in the transmission of leishmaniasis in America. We examine the genetic relationships among eight morphologic groups within the Lutzomyia genus and two Brumptomyia species, using nine enzyme loci and the last 285 basepairs of the mitochondrial cytochrome b gene. The structure of the genetic variation among the species analyzed indicated a closer genetic relationship among members of a morphologic group than between members of different groups. The lower levels of variation recorded among these groups compared with that between Brumptomyia and Lutzomyia suggest a subgeneric status for all of these groups, including Psychodopygus. A maximum likelihood tree for the allozyme data and a neighbor-joining consensus tree for the mitochondrial DNA sequences showed a general agreement with morphologic groups, with only minor differences. Nyssomyia, Verrucarum and Micropygomyia formed separate monophyletic groups. Lutzomyia could not be separated from Psathyromyia, and both Migonei species, L. dubitans and L. migonei, grouped in different clades according to the host species they are found on. This study was supported by Consejo Nacional de Investigacions Científicas y Tecnológicas (CONICIT) (No. 96–0001370), the World Health Organization-World Bank (No. 021–007), the Canadian International Development Agency (CIDA), and the Canadian Bureau for International Education (CBIE).

Lantana camara, a native plant from tropical America, is considered one of the most harmful invasive species worldwide. Several studies have identified potentially invasible areas under scenarios of global change, on the assumption that niche is conserved during the invasion process. Recent studies, however, suggest that many invasive plants do not conserve their niches. Using Principal Components Analyses (PCA), we tested the hypothesis of niche conservatism for L. camara by comparing its native niche in South America with its expressed niche in Africa, Australia and India. Using MaxEnt, the estimated niche for the native region was projected onto each invaded region to generate potential distributions there. Our results demonstrate that while L. camara occupied subsets of its original native niche in Africa and Australia, in India its niche shifted significantly. There, 34% of the occurrences were detected in warmer habitats nonexistent in its native range. The estimated niche for India was also projected onto Africa and Australia to identify other vulnerable areas predicted from the observed niche shift detected in India. As a result, new potentially invasible areas were identified in central Africa and southern Australia. Our findings do not support the hypothesis of niche conservatism for the invasion of L. camara. The mechanisms that allow this species to expand its niche need to be investigated in order to improve our capacity to predict long-term geographic changes in the face of global climatic changes.

The detection of cryptic species by biochemical methods indicates that within the phlebotomine fauna morphological data are not always adequate for species diagnosis. Cellulose acetate and polyacrylamide enzyme electrophoresis methods were compared for their effectiveness in identifying 13 species of Venezuelan phlebotomine sand flies and resolving alleles. Eight diagnostic loci unambiguously separated these 13 species of sand flies. Although acrylamide was as effective as cellulose acetate in species separation, differences were detected in the resolution of some alleles. Cellulose acetate identified more alleles at Ak and Fum, and resolved better at Pgm, whereas acrylamide identified more alleles at Gpi, Mdh, and Me. Therefore, erroneous species diagnoses may occur, if diagnostic loci detected by one technique are used by a second technique without adequate reference standards.

The epidemiology of the visceral leishmaniasis in the Americas is associated with both a natural and a domestic cycle. The existence of reproductively isolated populations of Lutzomyia longipalpis (Lutz & Neiva), and the scarcity of records of this species from natural habitats in areas where it has been associated with domestic habitats indicated that natural populations could be genetically distinct from domestic ones. Therefore, we compared the genetic structure and estimated the gene flow between L. longipalpis from domestic and peridomestic habitat and from an adjacent undisturbed natural environment along a 1.2-km transect. The analyses were performed on elec- trophoretic data from eight isozyme loci. The absence of fixed differences in the diagnostic loci Ak and Hk indicated that all specimens belonged to one of the two cryptic species identified in Venezuela. The average number of alleles per locus ranged from 2.0 to 2.9 and the average heterozygosity ranged from 7.8 to 13.4%. No differences were detected in the genetic structure of this species from domestic or peridomestic habitats and those trapped as far as 1.2 km from human dwellings. Nm, estimated from Wright's Fst, indicated that at least 208 individuals per generation migrated between the peridomestic habitat and a 1.2-km distant point to maintain the observed similarities in allelic frequencies. This high rate of gene flow indicated that this species has high migration rates between domestic and natural environments, and has the potential to transport for Leishmania from natural to domestic environments.

Mark-recapture studies indicate that Bufo marinus populations introduced into Australia tend to attain densities much higher than those reported in native habitats in South America. Moreover, its high rate of spread into northern Australia is raising concerns about the local fauna. The major ecological factors contributing to the observed differences in density are unknown. A population model based on empirical data from both continents was derived to explore the relationship between the life history parameters and population densities of this species, and to identify those parameters most likely to explain the differences in densities observed between continents. Model predictions seem consistent with the empirical data. The model predicted equilibrium densities in Australia one order of magnitude higher than those reported in South America. Moreover, a sensitivity analysis showed that adult equilibrium densities were much more sensitive to variations in adult survival than to variations in any of the other parameters at high densities. Coincidentally, it is adult survival for which the biggest differences between continents have been detected, suggesting that the higher toad densities reported in Australia are likely to be the result of higher adult survival in that continent. On the other hand, the high sensitivity of adult equilibrium densities to adult survival also has important implications for control of this species in Australia.

SUMMARYWe analysed the patterns of tick distribution on 2274 adult toads from Venezuela and Brazil, to explore whether these ectoparasites have any impact on the survival of Bufo marinus. A maximum-likelihood analysis showed that aggregation levels of ticks decreased significantly with the mean intensity of infection. This decline could be attributed to a density-dependent reduction of ticks within toads, density-dependent tick-induced toad mortality and/or density-dependent tick-induced changes in toad susceptibility. However, the relationship between the rate of change in tick loads and tick burdens from recaptured toads indicated that neither the loss of ticks within toads nor the toad susceptibility to further infection were dependent upon tick burdens. Therefore, we can indirectly infer that density-dependent tick-induced toad mortality is responsible for the observed decline in aggregation levels with tick age and burdens. On the other hand, a significant negative relationship between tick burdens and the size-specific weight of toads suggested that ticks may also have a significant impact on the patterns of weight deposition of adult toads. This evidence suggests that these ectoparasites may play an important role in regulating the densities of B. marinus in native habitats.

Patterns of energy allocation in the cane toad,Bufo marinus, were explored by analysing the seasonal variations in fat bodies and gonads in two populations subjected to different rainfall regimes and water permanency. In both populations, fat deposition occurred shortly after the rainy season due to an increase in feeding activity. In contrast, the timing of reproduction varied between sites according to the availability of suitable water bodies. Nevertheless, females with mature oocytes were predominant during the dry season, even in populations that bred throughout the year in permanent ponds. This suggests that seasonal variations in the nutritional condition of adults may also result in a tendency towards cyclicity in the reproduction ofB. marinusin tropical regions. Analyses of allometric relationships of reproductive and storage tissues with body weights indicated that the fractions of weights in reproductive tissues did not vary either with the size of females or between populations. On the contrary, not only did larger toads have proportionally larger fat bodies, but those from wet sites had greater fractions of weight allocated to fat bodies than those from dry sites. This suggests that reproductive output may be fixed in this species and, therefore, excess energy is allocated to fat bodies. Given that the initiation of ovarian cycles depends on the restoration of proper nutritional levels, energy stored in fat bodies during breeding may allow females to resume a new ovarian cycle shortly after spawning and thus maximize the number of reproductive events.

Rhinella marina is a Neotropical toad that has been introduced widely worldwide. Its toxic effects to frog-eating predators threaten the native and domestic fauna of some regions where it has been introduced. Despite previous studies suggesting two genetically distinct cryptic species within R. marina, one east and one west of the Andes, its taxonomic status remained unresolved due to the absence of morphological complementary evidence. For the first time, data from two mitochondrial genes (ND3 and CR) and 23 morphometric landmarks are combined to evaluate the taxonomic status of this species. Our results support the hypothesis of two separate evolutionary lineages within R. marina and demonstrate that these lineages have significantly diverged in skull shape. We identified two distinct morphotypes, one eastern and one Andean western, with no overlapping morphospaces. The geographic pattern of genetic variation was consistent with a stable structured population with no evidence of recent demographic or geographic expansions. The concordance between the observed geographic patterns in morphometric and genic traits calls for the recognition of two species under R. marina name.

Direct observations and analyses of selection occurring in natural populations are rare. The biology of the bruchid beetle,Acanthoscelides alboscutellatus, on its host plant,Ludwigia alternifolia, provides an anusual opportunity to study the process of selection on the morphology of an organism under field conditions.A. alboscutellatus larvae mature within the variably dehiscent fruit ofL. alternifolia. At eclosion, adults are confined within indehiscent fruit but are not confined within dehiscent fruit. Beetles can escape from indehiscent fruit only by forcing their bodies through the fruit's apical pore (a circular opening in the top of the fruit). Thus, during the eclosion stage of this beetle's life cycle the relationship between body size and differential fitness appears to be clearly defined.We examined entrapment ofA. alboscutellatus within indehiscentL. alternifolia fruit in a natural population. Only 8.8% of the beetles that attempted to escape were successful. Smaller beetles were trapped within a narrower range of pore diameters than were larger beetles; and trapped beetles had only limited abilities to enlarge fruit pore diameter. These data suggest (1) that escape from indehiscent fruit is regulated by the relationship between adult body diameter and fruit pore diameter and (2) that adult beetles may experience strong selection for small body diameter (size) within idehiscent fruit.

The genus Atelopus (Anura: Bufonidae) has suffered one of the most drastic declines recorded in the Neotropics. Nine of 10 Venezuelan species are categorized as Critically Endangered on the IUCN Red List. None of these species had been observed since the mid 1980s until recently, when an adult female of Atelopus mucubajiensis and several adults of A. cruciger were rediscovered. To assess the current distribution and status of A. cruciger we explored 15 locations where it was formerly known. Two populations were detected. Historical and current distribution maps were constructed based on these field explorations and data from museum collections. Using PCR assays we detected infection with Batrachochytrium dendrobatidis in one morbid and three live specimens. Our surveys suggest A. cruciger populations from lower altitudes on the northern slope of the Cordillera de La Costa are recovering, although the presence of B. dendrobatidis could jeopardize their long-term survival unless active conservation strategies are adopted.

†Background and Aims The genus Melocactus comprises 36 species of globose cacti with the most derived traits in the Cereeae tribe. It is the proper study system to examine what are the most derived reproductive strategies within that tribe. This study aims to characterize the reproductive biology and to estimate the mating system parameters of two Andean melocacti, Melocactus schatzlii and M. andinus. †Methods The reproductive attributes of the two species were described, including floral morphology, anthesis patterns, floral rewards, floral visitors and visitation patterns. Levels of self-compatibility and autonomous selfpollination were estimated by hand-pollination experiments. Mating system estimates were obtained by conducting progeny array analyses using isozymes. †Key Results The flowers of the two species present the typical hummingbird-pollination syndrome. Despite their morphological resemblance, the two species differ in flower size, pollen and ovule production and anthesis pattern. Their main pollinator agents are hummingbirds, four species in M. schatzlii and one species in M. andinus. Both cacti are self-compatible and capable of self-pollination without the aid of pollen vectors. Population-level outcrossing rate was higher for M. schatzlii (tm ¼ 0.9) than for M. andinus (tm ¼ 0.4). At the family level, outcrossing rates for most mothers of M. schatzlii were higher (tm . 0.8) than for M. andinus (tm , 0.5). †Conclusions Although the two cacti are capable of selfing, M. schatzlii is a predominantly outcrossing species, while M. andinus behaves as a mixed-mating cactus. Hummingbirds are the only pollinators responsible for outcrossing and gene flow events in these species. In their absence, both melocacti set seeds by selfing. Based on its low population size, restricted distribution in Venezuela, low rates of floral visits, and high levels of inbreeding, M. andinus is considered to be an endangered species deserving further study to define its conservation status.

Molecular data on Lutzomyia are very scarce, despite the fact that this genus includes all the species involved in the transmission of leishmaniasis in America. We examine the genetic relationships among eight morphologic groups within the Lutzomyia genus and two Brumptomyia species, using nine enzyme loci and the last 285 basepairs of the mitochondrial cytochrome b gene. The structure of the genetic variation among the species analyzed indicated a closer genetic relationship among members of a morphologic group than between members of different groups. The lower levels of variation recorded among these groups compared with that between Brumptomyia and Lutzomyia suggest a subgeneric status for all of these groups, including Psychodopygus. A maximum likelihood tree for the allozyme data and a neighbor-joining consensus tree for the mitochondrial DNA sequences showed a general agreement with morphologic groups, with only minor differences. Nyssomyia, Verrucarum and Micropygomyia formed separate monophyletic groups. Lutzomyia could not be separated from Psathyromyia, and both Migonei species, L. dubitans and L. migonei, grouped in different clades according to the host species they are found on.

Wildlife diseases are increasingly recognized as a major threat to biodiversity. Chytridiomycosis is an emerging infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis ( Bd ). Using a mathematical model and simulations, we study its effects on a generic riparian host population with a tadpole and adult life stage. An analytical expression for the basic reproduction quotient, Q o , of the pathogen is derived. By sampling the entire relevant parameter space, we perform a statistical assessment of the importance of all considered parameters in determining the risk of host extinction, upon exposure to Bd . We find that Q o not only gives a condition for the initial invasion of the fungus, but is in fact the best predictor for host extinction. We also show that the role of tadpoles, which in some species tolerate infections, is ambivalent. While tolerant tadpoles may provide a reservoir for the fungus, thus facilitating its persistence or even amplifying its outbreaks, they can also act as a rescue buffer for a stressed host population. Our results have important implications for amphibian conservation efforts.

The delimitation of cryptic species within the main vector of the American visceral leishmaniasis, Lutzomyia longipalpis, remains a topic of controversy. An analysis of genetic variability based on 8 enzymatic loci revealed fixed differences in 2 diagnostic loci, adenylate kinase (Ak) and hexokinase (Hk), between sympatric and allopatric populations at 4 localities in Venezuela. The absence of heterozygotes for these 2 loci within 1 locality indicates, for the first time, the presence of 2 sympatric reproductively isolated populations or cryptic species within L. longipalpis. Significant differences were also detected between these cryptic species in the allele frequencies of glucose-6-phosphate isomerase (Gpi) and malate dehydrogenase, decarboxylating (Me). One species showed mean heterozygosities that ranged between 6.6% and 6.7%, with 1.6-1.9 alleles detected per locus, while the other had mean heterozygosities that ranged from 4.3% to 6.3%, with 1.3-1.6 alleles per locus. Comparisons of isozyme profiles with published data suggests that 1 species is similar to the L. longipalpis described in Colombian and Brazilian populations, whereas the other has not been previously reported.

Patterns of genetic variation for the tick Amblyomma dissimile were analyzed from a total of 200 ticks collected on 12 toads (Bufo marinus), 14 snakes (Boa constrictor), and 8 lizards (Iguana iguana) at 11 localities. The analyses were performed on electrophoretic data from 8 isozyme loci. Mean heterozygosity per locus was 6% (+/-3.1) per population. Differences in allelic frequencies among ticks from different individual hosts were the major source of genetic variability in this study. Host species was a smaller source of genetic variation. Genetic distances between localities varied according to which host species was present in each locality, and these appeared to be related to the extent of habitat overlap between host species. The smallest genetic distances between samples from different host species were recorded for I. iguana and B. constrictor. In contrast, the genetic distances between tick samples from B. marinus and either of the reptile species were significantly larger than between tick samples from this amphibian species. Ecological variables or the geographic distance did not explain the local patterns of differentiation observed in A. dissimile. Major genetic differences between island and mainland sites (0.03702) suggested an association between genetic distances and geographic isolation. The consistency between patterns of genetic variation and those of host home range overlap suggests that host dispersion is the main force structuring the genetic variation within this tick species.

Some evidence suggests that bats may provide an alternative blood source for Lutzomyia longipalpis, the main vector of American visceral leishmaniasis. Feeding trials were conducted to determine whether L. longipalpis feeds on captive bats. The high feeding success indicated that L. longipalpis is capable of feeding on at least four species of bats. Implications for the epidemiology of leishmaniases are discussed.

Genetic markers are described for 2 species of reptile and amphibian ticks, Amblyomma dissimile and Amblyomma rotundatum, by allozyme electrophoresis. Fixed allelic differences in 4 out of the 8 examined loci allowed the unequivocal separation of both of these species. A strong correlation was found between these genetic markers and the relative size of the spurs in coxae IV but not with the punctuation pattern of the scutum. Moreover, no overlap was found in the distribution of relative spur sizes between samples of both species. The percent polymorphic loci and the mean percent heterozygosity per locus for A. rotundatum was considerably lower than that for A. dissimile. Differences in the amount of genetic variability may be related to their modes of reproduction.

The cane toad (Bufo marinus) has received considerable attention because of its rapid spread in Australia and the potential threat it may represent to native species. Although the introduction of pathogens from native populations is now being considered to control this species, population estimates based on comparable methods that demonstrate that native populations are in fact less dense than the introduced ones are not available. Accurate population estimates are necessary to evaluate potential techniques for the control of the cane toad. We estimated population densities of cane toads over a wide range of habitat types and climate conditions by means of mark-recapture data. The capture history and location of toads each night were analysed to explore the validity of some of the assumptions of mark-recapture models. Because migrations, deaths and recruitment over three nights appear to be unimportant, populations may be legitimately regarded as closed for that period. However, cane toads seem highly sensitive to disturbance effects due to trapping andlor handling. Consequently, density estimates based on removal methods seem the most reliable because they are not sensitive to handling and trapping effects. Similarly, analyses of residuals of regressions between 1-night counts and density estimates suggested that toad nightly activity is affected by the air temperature during sampling. However, only 60% of the variation in estimated densities can be predicted by 1-night counts and air temperature. Estimates of population density over a wide range of habitats in South America were one order of magnitude lower than estimates in Australia. We speculate on the possible factors that may account for the lower densities in populations in the native range of the species.

The population dynamics and stability properties of the sorghum midge Contarinia sorghicola and its predominant parasitic wasp Aprostocetus diplosidis were explored with a modified version of the Nicholson-Bailey model using experimental data, that included age-structure effects, density dependence and a type II functional response. A Leslie matrix was used to model the non-linear transitions between age-classes within the population of each species and the interaction between these populations. The stability and persistence of the interaction between these two species depended on the presence of refuges resulting from mutual interference of the parasitoids