The Geographic Spread of “El mal de las caderas” in Capybaras (Hydrochaeris hydrochaeris)

The aim of this work is to describe an epidemiological model for a capybara (Hydrochaeris hydrochaeris) population. The model considers a tabanid (“mutuca”) population (Diptera: tabanidae), as a vector for the disease called “mal de las caderas” in Estero del Ibera, Corrientes, Argentina. The study of this problem has ecological and economical importance since the meat and the hide of the capybara can be an exploitation resource. At first, a threshold value is determined as a function of the model parameters, obtaining a critical carrying capacity which determines the disease propagation or eradication. Then as the carrying capacity condition for the disease existence is satisfied, the existence of traveling wave solution is studied. Independent speeds are considered for the susceptible capybaras, the noninfected insect, and the disease. The speed of propagation for this model is obtained as function of model parameters followed by a discussion of strategies for controlling the spread of the disease. N.A. Maidana is a fellowship Fapesp and partially supported by Grant Fapesp (temático).     

Can mechanism help explain insect host choice?

Evolutionary theory predicts that herbivorous insects should lay eggs on plants in a way that reflects the suitability of each plant species for larval development. Empirical studies, however, often fail to find any relationship between an adult insect's choice of host-plant and offspring fitness, and in such cases, it is generally assumed that other 'missing' factors (e.g. predation, host-plant abundance, learning and adult feeding sites) must be contributing to overall host suitability. Here, I consider an alternative theory - that a fitness cost inherent in the olfactory mechanism could constrain the evolution of insect host selection. I begin by reviewing current knowledge of odour processing in the insect antennal lobe with the aid of a simple schematic: the aim being to explain the workings of this mechanism to scientists who do not have prior knowledge in this field. I then use the schematic to explore how an insect's perception of host and non-host odours is governed by a set of processing rules, or algorithm. Under the assumptions of this mechanistic view, the perception of every plant odour is interrelated, and seemingly bad host choices can still arise as part of an overall adaptive behavioural strategy. I discuss how an understanding of mechanism can improve the interpretation of theoretical and empirical studies in insect behaviour and evolution.     

Relative contributions to seed production by floral visitors of slickspot peppergrass, Lepidium papilliferum (Brassicaceae)

Assessing the relative contributions to seed production made by different types of floral visitors is fundamental to understanding the evolution of floral morphology and the influence that particular pollinator taxa have on plant fitness and reproduction. This 3-year study examined the pollinator activity and the seed production in three populations of Lepidium papilliferum, a threatened mustard endemic to sagebrush-steppe habitat in southwest Idaho. Relative amounts of time visitor taxa spent foraging on flowers, visitation rates (number of flowers visited per unit time during a foraging bout), and pollination effectiveness (fruit set per single visit to a virgin flower) were recorded for each of 12 insect taxa that visited L. papilliferum flowers. Relative contributions to seed production were calculated as the product of relative interaction frequencies (the relative number of flowers visited by each taxon—the “quantity” component of pollination) and pollination effectiveness (fruit set per single visit to a virgin flower—the “quality” component of pollination). Despite the superiority of some insect taxa in terms of pollination effectiveness, estimates of relative pollinator contributions to seed production were influenced primarily by an insect taxon’s interaction frequency with flowers. Pollinator assemblages varied widely both spatially and temporally, which suggest that L. papilliferum is not under strong selective pressure to shift from its generalist pollination strategy toward greater specialization. For this threatened plant, reliance on a diverse assemblage of insect pollinators may serve as an important buffer against disruption in reproductive success caused by fluctuations in population sizes of individual pollinator taxa.     

Sibling rivalry between seeds within a fruit: Some population genetic models

Competition between seeds within a fruit for parental resources is described using one-locus-two-allele models. While a “normal” allele leads to an equitable distribution of resources between seeds (a situation which also corresponds to the parental optimum), the “selfish” allele is assumed to cause the seed carrying it to usurp a higher proportion of the resources. The outcome of competition between “selfish” alleles is also assumed to lead to an asymmetric distribution of resources, the “winner” being chosen randomly. Conditions for the spread of an initially rare selfish allele and the optimal resource allocation corresponding to the evolutionarily stable strategy, derived for species with n-seeded fruits, are in accordance with expectations based on Hamilton’s inclusive fitness criteria. Competition between seeds is seen to be most intense when there are only two seeds, and decreases with increasing number of seeds, suggesting that two-seeded fruits would be rarer than one-seeded or many-seeded ones. Available data from a large number of plant species are consistent with this prediction of the model. Based on a talk given at the Haldane Centenary Symposium held on 6 November 1992 at Ahmedabad as part of the 58th Annual Meeting of the Indian Academy of Sciences.     

Non-host volatiles mediate associational resistance to the pine processionary moth

An increasing body of evidence indicates that the association between different plant species may lead to a reduction in insect herbivory, i.e. associational resistance. This might be due to a top–down regulation of herbivores by increased numbers of natural enemies or to a disruptive bottom–up influence of lower host plant accessibility. In particular, the richer plant communities release more diverse plant odours that may disturb olfactory-guided host choice and mating behaviour of insect herbivores, i.e. the “semiochemical diversity hypothesis”. However, this hypothesis has been rarely tested experimentally in natural habitats, notably forest ecosystems. We tested the effects of non-host volatiles (NHV) on mate and host location by the pine processionary moth (PPM) at the scale of individual pine trees with branches of non-host tree (birch) at their base. Pheromone trap catches and the numbers of larval nests were both reduced by non-host presence under treated pine trees, confirming an associational resistance mediated by NHV. In both males and females, the antenna could detect several birch volatiles, including methyl salicylate (MeSa). MeSa inhibited the attraction of the PPM male to pheromone traps, as did bark and leaf chips from birch trees. Our test of three doses of MeSa at the habitat scale (50 m forest edges) showed that the reduction in the numbers of male PPM captured in traps and in larval nests was MeSa dose-dependent. These results show that odours released by deciduous non-host trees can reduce herbivory by a forest defoliator in conifers, providing support to the “semiochemical diversity hypothesis” as a mechanism of associational resistance.     

Resolving the limitation – regulation debate

Many recent ecological studies have demonstrated that animal populations are limited by their food. Examples are presented here to refute the view that natural populations are regulated by negative feedback mortality factors. Additionally, several incorrect statements in a recent publication are discussed, specifically (1) that there is no difference between the concepts of regulation and limitation; (2) that the debate is about what causes the time it takes a population to reach the carrying capacity of its habitat, not what sets that carrying capacity; (3) that the results of a laboratory experiment using a closed population with fixed amounts of food represents what happens in natural open populations with varying supplies of food; (4) that a thermostat analogy can be used, assuming that an “equilibrium” is controlling natural populations “from above” instead of the original steam analogy which says the varying input of a resource “from below” is the controlling factor.     

Evolution of polyembryony: Consequences to the fitness of mother and offspring

Polyembryony, referring here to situations where a nucellar embryo is formed along with the zygotic embryo, has different consequences for the fitness of the maternal parent and offspring. We have developed genetic and inclusive fitness models to derive the conditions that permit the evolution of polyembryony under maternal and offspring control. We have also derived expressions for the optimal allocation (evolutionarily stable strategy, ESS) of resources between zygotic and nucellar embryos. It is seen that (i) Polyembryony can evolve more easily under maternal control than under that of either the offspring or the ‘selfish’ endosperm. Under maternal regulation, evolution of polyembryony can occur for any clutch size. Under offspring control polyembryony is more likely to evolve for high clutch sizes, and is unlikely for low clutch sizes (<3). This conflict between mother and offspring decreases with increase in clutch size and favours the evolution of polyembryony at high clutch sizes, (ii) Polyembryony can evolve for values of “x” (the power of the function relating fitness to seed resource) greater than 0.5758; the possibility of its occurrence increases with “x”, indicating that a more efficient conversion of resource into fitness favours polyembryony. (iii) Under both maternal parent and offspring control, the evolution of polyembryony becomes increasingly unlikely as the level of inbreeding increases, (iv) The proportion of resources allocated to the nucellar embryo at ESS is always higher than that which maximizes the rate of spread of the allele against a non-polyembryonic allele.     

Grape variety affects larval performance and also female reproductive performance of the European grapevine moth Lobesia botrana (Lepidoptera: Tortricidae)

For insect herbivores, the quality of the larval host plant is a key determinant of fitness. Therefore, insect populations are supposed to be positively correlated with the nutritional quality of their host plant. This study aimed to determine if and how different varieties of grapes (including the wild grape Lambrusque) affect both larval and adult performance of the polyphagous European grapevine moth Lobesia botrana (Denis & Schiffermüller). Significant differences were found in larval development time, but not in pupal mass, adult emergence rate, or sex ratio. Although the fecundity of females is not different among varieties, females fed on some varieties produced eggs of different sizes which are correlated to their fertility. Thus, females adapt resource allocation to eggs depending on their diet as larvae. Using a fitness index, the average reproductive output was found to be highest for females reared on cv. Chardonnay. Females reared on wild grape produced a fitness index identical to the cultivated grapes. However, Lambrusque and Gewurztraminer separate themselves from the cultivated varieties according to our discriminant analyses. It is emphasized, through this study, that cultivars fed on by larvae should be considered in the population dynamics of L. botrana and that egg number is insufficient to determine host plant quality.     

A trade-off between female lifespan and larval diet breadth at the interspecific level in Lepidoptera

A prediction arising from several evolutionary diet breadth models is that, in insect herbivores whose adults practise adaptive host plant selection based on larval performance, female adult lifespan should be negatively correlated with larval diet breadth. In one category of models, female adult lifespan drives evolutionary changes in larval diet breadth; in the other category, larval diet breadth drives evolutionary changes in female adult lifespan. Applying the method of independent contrasts to a biologically and phylogenetically diverse array of Lepidoptera, we ask whether larval diet breadth—as measured by the number of larval food plant species reported in the literature—is negatively correlated with female adult lifespan at the interspecific level. We show that these two life history variables are indeed inversely related. Next, we relax the assumption, common to all of the models, that the female adult is the life stage responsible for the distribution of progeny among different host plants. By introducing into our data set three species whose females are incapable of flight (due to either aptery or brachyptery), and whose larvae are the dispersive stage, the negative correlation between female adult lifespan and larval diet breadth is lost, when using the independent contrasts method. We interpret this effect as supporting the models’ common prediction. Ours is the first reported evidence of a lifespan/diet breadth trade-off at the interspecific level among insects, and it confirms the findings of a previous study in which the degree of habitat specialisation among arthropods was inversely related to proxy measures of the degree of search time constraint. In one of our “diet breadth drives changes in lifespan” models, the females’ type of egg maturation strategy (as measured by the ovigeny index) is predicted to be positively correlated with larval diet breadth, and it mediates a female adult lifespan/larval diet breadth trade-off; however, we found no convincing support for such a role.     

Habitat structure and animal movement: the behaviour of bumble bees in uniform and random spatial resource distributions

Foraging organisms (like bumble bees) move between resource points (like flowers) whose natural distributions vary enormously: from hyperdispersed to random to clumped. These differences in habitat structure may significantly influence the fitness of both plant and pollinator. To examine the effect of habitat structure on pollinator movement and fitness, we observed captive worker bumble bees collecting nectar from artificial flowers containing equal volumes of reward and arranged in two spatial configurations: a hexagonal array with constant distances between flowers (“constant”), and an “exploded hexagonal” array, with variable distances between flowers (“variable”). The mean nearest-neighbour distance was the same in both arrays, as was the general hexagonal appearance. The experiment therefore compares how resource dispersion, independent of nearest-neighbour distance, influences bee behaviour. Bees in the variable array showed decreased directionality, higher revisitation frequencies, and greater inter-flower flight distances than shown in the constant array. As a consequence, bees in the variable array had a 19% lower gross rate of nectar collection. Our results suggest that wild-foraging bees should prefer regularly spaced flowers (when all else, including mean nearest-neighbour distance, is equal), and that plants can decrease self-pollination by regular spacing between flowers, inflorescences, or individuals. Received: 16 January 1996 / Accepted: 30 June 1997     

Host plant preference and performance of the sibling species of butterflies <Emphasis Type="Italic">Leptidea sinapis</Emphasis> and <Emphasis Type="Italic">Leptidea reali</Emphasis>: a test of the trade-off hypothesis for food specialisation

A large proportion of phytophagous insect species are specialised on one or a few host plants, and female host plant preference is predicted to be tightly linked to high larval survival and performance on the preferred plant(s). Specialisation is likely favoured by selection under stable circumstances, since different host plant species are likely to differ in suitability—a pattern usually explained by the “trade-off hypothesis”, which posits that increased performance on a given plant comes at a cost of decreased performance on other plants. Host plant specialisation is also ascribed an important role in host shift speciation, where different incipient species specialise on different host plants. Hence, it is important to determine the role of host plants when studying species divergence and niche partitioning between closely related species, such as the butterfly species pair Leptidea sinapis and Leptidea reali. In Sweden, Leptidea sinapis is a habitat generalist, appearing in both forests and meadows, whereas Leptidea reali is specialised on meadows. Here, we study the female preference and larval survival and performance in terms of growth rate, pupal weight and development time on the seven most-utilised host plants. Both species showed similar host plant rank orders, and larvae survived and performed equally well on most plants with the exceptions of two rarely utilised forest plants. We therefore conclude that differences in preference or performance on plants from the two habitats do not drive, or maintain, niche separation, and we argue that the results of this study do not support the trade-off hypothesis for host plant specialisation, since the host plant generalist Leptidea sinapis survived and performed as well on the most preferred meadow host plant Lathyrus pratensis as did Leptidea reali although the generalist species also includes other plants in its host range. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Pollination biology of <Emphasis Type="Italic">Disanthus cercidifolius</Emphasis> var. <Emphasis Type="Italic">longipes</Emphasis>, an endemic and endangered plant in China

Disanthus cercidifolius Maxim. var. longipes H.T. Chang usually has two inflorescences growing in opposite directions in the axillae, but occasionally three inflorescences grow paratactically. The typical flowering process could be divided into 4 periods: “Pre-dehiscence”, “Initial dehiscence”, “Full dehiscence” and “Withering”. Both the natural population and the planted population had a flowering peak of 15–35 days after the first flower bloomed. There were significant differences between the time courses of flowering of the two populations. Out-crossing is the main breeding system in this species. And autogamy decreases the risk of reproductive failure of this species. The main insect pollinators of D. cercidifolius var. longipes are Episyrphus balteatus de Geer, Scaptodrosophila coracina Kikkawa and Peng, Polistes olivaceus de Geer, Apis cerana Fabricius, Nezara viridula L. and Coccinella septempunctata L., and so on. Among the insects, S. coracina and E. balteatus are the most important and efficient pollinators, but others are inefficient pollinators. Though wind pollination is not efficient, it guarantees reproduction when insect pollinators are not available. “Mass flowering” is an adaptive behavior and reproductive strategy of this species, and “few fruiting” could be caused by the lack of pollinators.     

Age-biased parasitism and density-dependent distribution of fleas (Siphonaptera) on a desert rodent

Parasites often confront conflicting demands when evaluating and distributing themselves among host individuals, in order to attain maximum reproductive success. We tested two alternative hypotheses about host preference by fleas in relation to the age of their rodent host. The first hypothesis suggests that fleas select adult over juvenile rodents because the latter represent a better nutritional resource (the “well-fed host” hypothesis), whereas the second hypothesis suggests that fleas prefer the weaker and less resistant juveniles because they are easier to colonise and exploit (“poorly fed host” hypothesis). We sampled fleas (Synosternus cleopatrae) on the gerbil (Gerbillus andersoni) in 23 different plots in the Negev desert and found an unequal distribution of fleas between adult and juvenile hosts. Furthermore, flea distribution changed as a function of flea density—from juvenile-biased flea parasitism (the “poorly fed host” hypothesis) at low densities to adult-biased flea parasitism (the “well-fed host” hypothesis) at high densities. Other factors that influenced flea preference were soil temperature and the presence of ticks. These results suggest that host selection is not an explicit alternative choice between adults and juveniles (“well-fed host” versus “poorly fed host” hypotheses), but rather a continuum where the distribution between adults and juveniles depends on host, parasite, and environmentally related factors.     

A comparative study of the egg-laying behaviour and larval development of Pieris rapae L. and P. brassicae L. on the same host plants

Summary Pieris rapae and P. brassicae feed on the same host plants and have synchronized seasons. P. brassicae, whose larvae are twice the size of P. rapae, lays eggs in clusters of 40–100 eggs whereas P. rapae lays single eggs. In this paper we examine how egg clustering may be advantageous for P. brassicae. The larval development of each species was studied, and found not to differ significantly. P. brassicae larvae were observed to migrate from their host plant after defoliating it. A comparison of the efficiency of host plant utilization by the two pierid species was undertaken by measuring the effect of larval feeding on the growth of their host plants (kale and brussel sprouts). The results show that egg clustering is advantageous for larval fitness in terms of host resource exploitation, and we suggest that P. brassicae is adapted for ovipositing on clumped vegetation, while P. rapae is selected for exploiting isolated plants.     

Volatiles that encode host-plant quality in the grapevine moth

Plant volatiles are signals used by herbivorous insects to locate host plants and select oviposition sites. Whether such volatiles are used as indicators of plant quality by adult insects in search of host plants has been rarely tested. We tested whether volatiles indicate plant quality by studying the oviposition of the grapevine moth Lobesia botrana on the grapevine plant Vitis vinifera. Host plants were infected with a variety of microorganisms, and larval fitness was correlated to the infected state of the substrate. Our results show an oviposition preference for volatiles that is significantly correlated with the fitness of the substrate. The chemical profiles of the bouquets from each V. vinifera–microorganism system are clearly differentiated in a PCA analysis. Both the volatile signal and the quality of the plant as larval food were affected by the introduction of microorganisms. Our study represents a broad approach to the study of plant–insect interactions by considering not only the direct effect of the plant but also the effect of plant–microorganism interactions on insect population dynamics.     

Law of the Minimum Paradoxes

The “Law of the Minimum” states that growth is controlled by the scarcest resource (limiting factor). This concept was originally applied to plant or crop growth (Justus von Liebig, 1840, Salisbury, Plant physiology, 4th edn., Wadsworth, Belmont, 1992) and quantitatively supported by many experiments. Some generalizations based on more complicated “dose-response” curves were proposed. Violations of this law in natural and experimental ecosystems were also reported. We study models of adaptation in ensembles of similar organisms under load of environmental factors and prove that violation of Liebig’s law follows from adaptation effects. If the fitness of an organism in a fixed environment satisfies the Law of the Minimum then adaptation equalizes the pressure of essential factors and, therefore, acts against the Liebig’s law. This is the the Law of the Minimum paradox: if for a randomly chosen pair “organism–environment” the Law of the Minimum typically holds, then in a well-adapted system, we have to expect violations of this law.     

Workers, sexuals, or both? Optimal allocation of resources to reproduction and growth in annual insect colonies

Understanding decisions about the allocation of resources into colony growth and reproduction in social insects is one of the challenging issues in sociobiology. In their seminal paper, Macevicz and Oster predicted that, for most annual insect colonies, a bang–bang strategy should be favoured by selection, i.e. a strategy characterised by an “ergonomic phase” with exponential colony growth followed by a “reproductive phase” with all resources invested into the production of sexuals. Yet, there is empirical evidence for the simultaneous investment into the production of workers and sexuals in annual colonies (graded control). We, therefore, re-analyse and extend the original model of Macevicz and Oster. Using basic calculus, we can show that sufficiently strong negative correlation between colony size and worker efficiency or increasing mortality of workers with increasing colony size will favour the evolution of graded allocation strategies. By similar reasoning, graded control is predicted for other factors limiting colony productivity (for example, if queens’ egg laying capacity is limited).     

Hidden treatments in ecological experiments: re-evaluating the ecosystem function of biodiversity

Interactions between biotic and abiotic processes complicate the design and interpretation of ecological experiments. Separating causality from simple correlation requires distinguishing among experimental treatments, experimental responses, and the many processes and properties that are correlated with either the treatments or the responses, or both. When an experimental manipulation has multiple components, but only one of them is identified as the experimental treatment, erroneous conclusions about cause and effect relationships are likely because the actual cause of any observed response may be ignored in the interpretation of the experimental results. This unrecognized cause of an observed response can be considered a “hidden treatment.” Three types of hidden treatments are potential problems in biodiversity experiments: (1) abiotic conditions, such as resource levels, or biotic conditions, such as predation, which are intentionally or unintentionally altered in order to create differences in species numbers for “diversity” treatments; (2) non-random selection of species with particular attributes that produce treatment differences that exceed those due to “diversity” alone; and (3) the increased statistical probability of including a species with a dominant negative or positive effect (e.g., dense shade, or nitrogen fixation) in randomly selected groups of species of increasing number or “diversity.” In each of these cases, treatment responses that are actually the result of the “hidden treatment” may be inadvertently attributed to variation in species diversity. Case studies re-evaluating three different types of biodiversity experiments demonstrate that the increases found in such ecosystem properties as productivity, nutrient use efficiency, and stability (all of which were attributed to higher levels of species diversity) were actually caused by “hidden treatments” that altered plant biomass and productivity. Received: 16 December 1996 / Accepted: 2 March 1997     

The Transparent Goby, Aphia MinutaReview of Biology and Fisheries of a Paedomorphic European Fish

The transparent goby, Aphia minuta, is one of the few pelagic species of the family Gobiidae. Its geographic range covers the eastern Atlantic Ocean, from Gibraltar to Norway and the Mediterranean basin including the Azov and Black seas. However, it does not occur along the north African coasts. The species is characterized by a unique larval morphology and precocious sexual maturation, i.e. a type of heterochrony known as progenesis. The transparent goby is an annual species that lives less than 1 year, with a maximum size of less than 60 mm. The breeding season is quite long and spawning takes place at least twice during its short lifespan. Just after spawning, all breeders quickly die probably as a consequence of the degeneration of the intestinal epithelium by apoptosis, marking the end of the entire cohort. Spending most of its life in the water column, it is a planktotrophic feeder, relying mainly on small copepods. In turn the transparent goby is preyed on by several species of fish and larger cephalopods. During ontogenesis, t hree phases characterize the species: a “pelagic phase”, composed of larval stages hatched from demersal eggs that inhabit shallow coastal waters; an “aggregated phase”, composed of juveniles that gather in schools in shallow waters during winter; finally, a “demersal phase”, composed of adults that in spring migrate offshore with a more dispersed distribution in proximity of the bottom. Despite its small size, this species is largely exploited by local small-scale fisheries developed mainly in the western and central Mediterranean. The fishing fleets, harboured in several sites in Spain and Italy, are generally composed of small vessels. Most of them use very selective gear such as purse-seine nets to catch schools of the transparent goby during winter. For these artisanal fisheries, the transparent goby represents, although seasonally, a very important source of income, yielding locally up to more than 100 tons per fishing season and 75 kg/vessel/day. This review critically examines the published literature on the biology and fisheries of the transparent goby, aiming to provide useful tools for appropriate management and for a sustainable exploitation of this important resource.     

Pioneer neurons: A basis or limiting factor of lophotrochozoa nervous system diversity?

It has been demonstrated by us and other authors that first nervous cells in developing larvae from various trochozoan groups differentiate at the periphery. These pioneer neurons are distinguished by the set of characters. They are located outside the forming central ganglia; outgrowing fibers of central neurons use their processes as a “scaffolding” transmitter expression in these neurons is transient. On the one hand, pioneer neurons mark the “frame” of the adult nervous system and thus play a limiting role. On the other hand, pioneering navigation provides possible mechanisms for evolutional plasticity of the nervous system in adults. In addition, pioneer neurons can underlie functional adaptation of trochophore animals, which minimizes fitness decrease during the transition from the larval to the adult form during metamorphosis.