Resource Limitation,Controphic Ostracod Density and Larval Mosquito Development

Aquatic environments can be restricted with the amount of available food resources especially with changes to both abiotic and biotic conditions. Mosquito larvae, in particular, are sensitive to changes in food resources. Resource limitation through inter-, and intra-specific competition among mosquitoes are known to affect both their development and survival. However, much less is understood about the effects of non-culicid controphic competitors (species that share the same trophic level). To address this knowledge gap, we investigated and compared mosquito larval development, survival and adult size in two experiments, one with different densities of non-culicid controphic conditions and the other with altered resource conditions. We used Aedes camptorhynchus, a salt marsh breeding mosquito and a prominent vector for Ross River virus in Australia. Aedes camptorhynchus usually has few competitors due to its halo-tolerance and distribution in salt marshes. However, sympatric ostracod micro-crustaceans often co-occur within these salt marshes and can be found in dense populations, with field evidence suggesting exploitative competition for resources. Our experiments demonstrate resource limiting conditions caused significant increases in mosquito developmental times, decreased adult survival and decreased adult size. Overall, non-culicid exploitation experiments showed little effect on larval development and survival, but similar effects on adult size. We suggest that the alterations of adult traits owing to non-culicid controphic competition has potential to extend to vector-borne disease transmission.     

Local Competition Between Foraging Relatives: Growth and Survival of Bruchid Beetle Larvae

Kin selection theory states that when resources are limited and all else is equal, individuals will direct competition away from kin. However, when competition between relatives is completely local, as is the case in granivorous insects whose larval stages spend their lives within a single seed, this can reduce or even negate the kin-selected benefits. Instead, an increase in competition may have the same detrimental effects on individuals that forage with kin as those that forage with non-kin. In a factorial experiment we assessed the effects of relatedness and competition over food on the survival and on fitness-related traits of the bruchid beetle Callosobruchus maculatus. Relatedness of competitors did not affect the survival of larvae. Larval survival substantially decreased with increasing larval density, and we found evidence that beetles maturing at a larger size were more adversely affected by competition, resulting in lower survival rates. Furthermore, females showed a reduction in their growth rate with increasing larval density, emerging smaller after the same development time. Males increased their growth rate, emerging earlier but at a similar size when food was more limited. Our results add to the growing number of studies that fail to show a relationship between relatedness and a reduction in competition between relatives in closed systems, and emphasize the importance of the scale at which competition between relatives occurs.     

Intraspecific competition effects on Aethina tumida (Coleoptera: Nitidulidae)

Two kinds of experiments were conducted with Aethina tumida Murray larvae over four temperatures: "consumption" experiments, in which larvae and diet were weighed to determine food consumption rates under conditions of unlimited food and few conspecifics; and "competition" experiments, in which varying numbers of larvae were presented with the same amount of honey and pollen diet, and larval weight at final instar was used to determine competition effects. In consumption experiments temperature, diet and their interaction all had significant effects on the ratio of larval weight to the weight of food consumed, which was higher at 24 degrees C than at any other temperature. In competition experiments, three relationships were examined and modeled: that between the number of larvae per experimental unit and the average weight of those larvae; that between average larval and adult weights; and that between average adult weight and survivorship to adult (emergence rate). An exponential decay function was fit to the relationship between number of larvae per experimental unit and their average weight. Average adult weight was linearly correlated with larval weight. Likewise, emergence rates for adults < 11.6 mg in weight were linearly correlated with adult weights, but no significant relationship was observed for heavier adults. Using these relationships, the reproductive potential for A. tumida were estimated for a frame of honey and pollen. Information on resource acquisition by A. tumida will be useful in evaluating the impact of different factors on beetle population dynamics, such as bee hygienic behavior or control strategies used by the beekeeper.     

Larval competition for patchy resources in Chrysomya megacephala (Dipt., Calliphoridae): implications of the spatial distribution of immatures

In the present study, a single procedure was established to investigate the effect of the spatial distribution of immatures in patchy resources, on the outcome of larval competition for food, in experimental populations of Chrysomya megacephala . A theoretical model of intraspecific competition was extended and applied to experimental data on survival to adulthood for 20 larval densities, to obtain the theoretical mean number of individuals that will survive, considering a hypothetical previous random adult oviposition in a system of homogeneous patches. The survival curve obtained suggests that the larval competition for food in C. megacephala is of the scramble/exploitative type, which corroborates results from previous studies, although the latter did not consider the correlation between local and global abundances. The present model allows that experimental data could be perfectly applicable, and it incorporates fundamental assumptions about the spatial context of competition for patchy resources in blowflies, and may be applied to the optimization of mass rearing techniques and to the maintenance of insect colonies under experimental conditions.     

Larval aggregation and competition for food in experimental populations of Chrysomya putoria (Wied.) and Cochliomyia macellaria (F.) (Dipt., Calliphoridae)

In blowflies, larval aggregation in patches of food can be both intra- and interspecific, depending upon the degree to which competitors are clumped among the patches. In the present study, the implications of spatial aggregation for larval competition was investigated in experimental populations of the introduced blowfly Chrysomya putoria and the native Cochliomyia macellaria , using data from survival to adulthood in a range of single- and double-species larval cultures. The reduction in C. macellaria survival rate in the presence of C. putoria suggests that the former species is the inferior competitor. The results on survival to adulthood for both species in single- and double-species cultures can be explained in the light of the relationship between the level of intra- and interspecific aggregation and the efficiency of the larval feeding process. The possible implications of these results for the population biology of both species in natural environments are discussed.     

Extraspecific imposition of social grouping among surgeonfishes (Pisces: Acanthuridae)

The Manini is a widely distributed surgeonfish that is often abundant on reefs. It has two modes of existence, scattered individuals or dense column-forming schools, which may be seen on the same day at different places on the reef. In Hawaii schooling is a tactic used for getting to the bottom to feed when the bottom is held by territory-holding food competitors. Mixed-species "flocks" are an extension of this tactic. In atoll situations habitat partitioning exists, with the Maninis feeding in the intertidal area away from competitors; there, schooling is anti-predator behaviour.     

Sex-specific reaction norms to intraspecific larval competition in the mosquito Aedes aegypti

As the relationship between a given life‐history trait and fitness is not necessarily the same for the two sexes, an ‘intersexual ontogenetic conflict’ may arise. We analysed the phenotypic reaction to intraspecific larval competition of the mosquito, Aedes aegypti, asking: (i) Do both sexes pay the cost of competition with the same life‐history traits and are they equal competitors? (ii) Is there a specific cost of competition beyond sharing food resources? We found that competition incurs a specific cost that was expressed differently by the two sexes. Indeed, each sex maintained the more important life‐history trait(s) for their fitness (developmental time for males and body weight and size for females) at the expense of other traits, thus minimizing the effects of competition on their fitness. The competition exerted by females was estimated as being more intense, probably linked with the greater importance of body size for their fitness.     

Food calling by captive bonobos (Pan paniscus): An experiment

We examined (i) whether bonobos display a specific food-calling behavior when discovering a hidden food resource, (ii) whether the presence of competitors affects this behavior, and (iii) whether food quantity or gender influences its appearance. We carried out experiments (n = 108) within a captive group of eight bonobos at the Animal Park Planckendael (Mechelen,Belgium). We hid highly preferred food items (n = 7 or 25) in their enclosure and recorded vocal behavior and interactions between discoverer and group members. As a control, we gave the same number of items to the individuals when isolated from the group, a situation without potential food competition (n = 38). The only vocalization frequently uttered by the discoverer was the food peep. They uttered food peeps significantly more often when no food competition was possible. The amount of food had no significant influence on whether food peeps were uttered. The same applies to the individuals’ identity or gender. Although the costs of food calling behavior seemed much higher for males, both sexes uttered food calls to the same extent. We hypothesize thai males signal food presence in order to attract potential mates and are willing to give up the discovered food resource in return for sex: sex for food exchange. In contrast, females may vocalize to attract coalition partners. Through these coalitions, they can monopolize food resources vis-à-vis males. It is also possible that females have less reason to suppress food calk, since they are dominant to males. This study suggests that bonobos are able to give shaded signals about their environment and have the potential to communicate this information in order to promote their sexual strategy.     

Use of food and space by tunneler dung beetles (Coleoptera; Scarabaeinae) during reproduction

At two temperate pasturelands in northern Mexico, we explored possible competition for food and space under pats during the simultaneous nesting periods of the univoltine species Dichotomius colonicus (Say), Phanaeus quadridens (Say), and Copris sierrensis Matthews. To simulate unlimited resources, 50 5-kg cow dung pats were placed at regular distance intervals in each pastureland. After building trenches around the pats, the number and depth of each nest, as well as larval development status, were documented once for a period of 1-8 mo. Analyses of variance and association tests were used to make a between-site comparison of dung pat occupation, nests occupied per species, nests per dung pat, and nest depth below each pat. The proportion of pats occupied by each species differed significantly between sites. C. sierrensis colonizing most pats at one site and D. colonicus at the other. There were no differences between sites in the frequency of pats occupied by more than one species. The association test and Ochiai index showed that each species colonized dung pats independently. The results suggest that pat occupation depended on their location by beetles and the relative abundance of each species. The species tended to dig nests at different depths, possibly reducing interspecific competition for space. It can therefore be concluded that, when food resources seem to be unlimited, they are shared following a "lottery dynamic" model if there is spatial differentiation among species.     

Better to Be in Bad Company than to Be Alone? Aedes Vectors Respond Differently to Breeding Site Quality in the Presence of Others

This study focuses on two competing species, Aedes aegypti and Aedes albopictus (Diptera: Culicidae), both invasive mosquitoes of the New World. Context-specific competition between immature forms inside containers seems to be an important determinant of the coexistence or displacement of each species in different regions of the world. Here, competition experiments developed at low density (one, two or three larvae) and receiving four different resource food concentration, were designed to test whether Ae. albopictus and Ae. aegypti respond differently to competition, and whether competition can be attributed to a simple division of resources. Three phenotypic traits - larval development, adult survival under starvation and wing length - were used as indicators of performance. Larvae of neither species were limited by resource concentration when they were alone, unlike when they developed with competitors. The presence of conspecifics affected Ae. aegypti and Ae. albopictus, inducing slower development, reduced survival and wing length. The response to resource limitation was different when developing with heterospecifics: Ae. aegypti developing with one heterospecific showed faster development, producing smaller adults with shorter lives, while in the presence of two competitors, development increased and adults lived longer. Aedes albopictus demonstrated a better performance when developing with heterospecifics, with no loss in their development period and improved adult survival. Overall, our results suggest that response to competition can not simply be attributed to the division of resources, and that larvae of both species presented large phenotypic plasticity in their response to the presence or absence of heterospecifics and conspecifics.     

Larval competition in serially transferred populations of Drosophila melanogaster

Summary Experiments showed that larval competition for food is not always the result of decreases in the amount of food available per larva as population density increases. The feeding period during which the larvae try to attain the minimum survival weight may be restricted when food quantity is not limiting. The scramble type competition involves both exploitation and interference components, in degrees which vary with population density.     

An experimental investigation of a new ideal free distribution model

Summary We investigate a new continuous input ideal free distribution model which removes the assumption that resources are consumed as soon as they enter a patch. The model makes predictions about the standing crop of resources and allows consideration of the effects of simultaneous exploitation and interference competition. Using a group of cichlid fish competing for food items, we show that consistent with the model, standing crops can vary in continuous input situations. As predicted, higher standing crops are associated with increased intake rates. Furthermore, with greater numbers of competitors, standing crops are higher, suggesting that there is interference as well as exploitation competition in our system. An experiment to investigate the effects of fish density on the level of movement revealed that the reported interference competition could not be attributed to increased fish movement at higher density.     

Competition between perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus): the advantage of turning night into day

1. The outcome of interspecific competition for food resources depends both on the competitors’ sensory abilities and on environmental conditions. In laboratory experiments we tested the influence of daylight and darkness on feeding behaviour and specific growth rate (SGR) of two species with different sensory abilities. 2. We used perch (Perca fluviatilis) as a visually orientated, and ruffe (Gymnocephalus cernuus) as a mechano‐sensory oriented predator and tested their growth rates and behaviour under conditions of interspecific and intraspecific competition. Three different foraging conditions were used: food supplied (i) only during the day, (ii) only during the night or (iii) during both day and night. 3. In perch neither SGR nor feeding behaviour were influenced substantially by interspecific competition during daylight. During darkness their foraging behaviour changed markedly and their access to the food source as well as their SGR were negatively affected by the presence of ruffe. 4. Ruffe's foraging behaviour did not change during either day or night with interspecific competition. During the night ruffe's SGR was higher with interspecific competition, probably because of a release from intraspecific competition and the competitive inferiority of perch during the night. 5. Because of its seonsory abilities ruffe feeds predominantly at night, thereby reducing competitive interference from perch.     

Wall-making behavior as a proximate mechanism to generate variation in larval competition in Callosobruchus maculatus (Coleoptera: Bruchidae)

Variation from contest to scramble in larval competition types was observed among laboratory lines derived from a geographic strain of Callosobruchus maculatus. In contest competition, only one adult can emerge from a small bean because the successful larva monopolizes resources. In scramble competition, however, multiple adults can emerge from the bean because larvae share resources. To explain the variation in competition types, we used six lines of the geographic strain to test the hypothesis that the larval competition type is determined by the larval behavior of building walls, which prevent larvae from interfering with each other, allowing multiple adults to emerge from a single bean. We also investigated the proportions of wall-making in contest-scramble hybrid lines to test whether the formation of a wall structure was genetically determined. Results support our hypothesis that wall-making behavior determines the type of larval competition within a geographic strain, and that the behavior is genetically determined. Scramble-type lines exhibited higher frequencies of wall-making than contest-type lines when two larvae of the same line infested a bean. Larval competition type and the tendency towards wall formation in contest-scramble hybrid lines ranged intermediate of parental lines. We concluded that the variation in larval competition type is determined by the variation in larval wall-making behavior among laboratory lines derived from the geographic strain. We will discuss the evolution of scramble-type larvae in C. maculatus based on our results.     

Herbivore overlap and competition in Kenya rangelands

Small herbivores such as lagomorphs, rodents and orthopterans are important consumers in grasslands. Techniques of data collection by microscope analysis of stomach contents and quantitative analysis of results are presented for assessing dietary overlap and potential competition for food resources among different classes of herbivores. Example analysis of data for small herbivores and ungulates from an area south of Nairobi National Park, Kenya, are presented. In the average growing season, diet overlap between angulates and small herbivores is low, and therefore, potential for competition slight. Considering sesonal variation in food resources availability and extreme shortages documented for occasional seasons, it is probable that periodic competition occurs. Hares are potentially the greatest competitors with ungulates in such conditions, because of their wide niche breadth and spatial overlap with ungulates. Assessment of the degree and implications of periodic competitive interactions requires careful analysis of niche breadth and overlap on the food resource gradient based upon a spatial and temporal evaluation of habitat utilization.     

The natural regulation of buffalo populations in East Africa: The food supply as a regulating factor,and competition*

Evidence is presented to show that both the quality and quantity of food available to a buffalo population falls below the minimum maintenance requirements of that population at certain times of the year. In the Serengeti grasslands there was a shortage of the only good quality component, grass leaf, in the dry season, with the result that the animals consumed an increasing proportion of poor quality food such as grass stem. By the end of the dry season the diet had dropped in quality below the minimum maintenance level. In areas such as Mt. Meru where there was a more continuous growing season, the high density of animals kept the standing crop of leaf at a low level. During the cooler dry season the growth of leaf became insufficient in quantity for the maintenance requirements of the population. These two quite different situations suggested that food shortage was a more general phenomenon in eastern Africa. Various measurements of feeding behaviour were made. Total grazing time per 24 h did not differ between seasons but ruminating time may have increased as the season became drier and could have been a response to the more fibrous food. Analysis for cycles of activity showed that there was more temporal organization during the dry season. These changes in activity cycles appeared to be related to the increase in energy expenditure produced by heat stress and sweating. Old animals with poor teeth did not compensate for the poor food supply by changing their feeding behaviour. There was a positive relationship between annual rainfall and mean crude density in different areas of eastern Africa, indicating that regulation was taking place. Since rainfall determined the amount of available food, it could have operated through the food supply. On a finer scale it was found that the extent of the preferred riverine habitats was also related to density. Thus rainfall, the extent of riverine habitat and perhaps soil moisture were three limiting factors that determined mean density and all could have taken effect through the food supply. As a result of initial selective grazing the amount of available leaf declined as the dry season progressed to the extent that by the end of the season the proportion of this component in the diet fell to a very low level. The impact of the population on its limited food supply indicated that intra-specific competition was acting as the cause of regulation. Measurements of wildebeest eating the same food in the same habitats as buffalo showed that inter-specific competition was also taking place. A small proportion (7“) of the wildebeest population could have reduced the buffalo population by approximately 18^o,‘, from its potential population size. The buffalo population in the Serengeti was regulated by adult mortality which was caused by undernutrition as a result of food shortage. This food shortage was caused by intra- and inter-specific competition. The effect of predation and disease was to hasten the response of the population to changes in the food supply. The limiting factors determining the mean level of the available food were, amongst others, rainfall, soil moisture and the extent of the preferred riverine habitat. The effect of interspecific competition could result in a complex regulation of populations through their food supply. There appears to be no foundation for hypotheses which invoke over-utilization or damage as a consequence of regulation through food.     

Timing of resource input and larval competition between invasive and native container-inhabiting mosquitoes (Diptera: Culicidae).

Container-inhabiting mosquito species are subject to both intraspecific and interspecific competition during larval development in resource-limited habitats. The arrival of an invasive species, Aedes albopictus, in the U.S. has altered competitive interactions among container-inhabiting mosquito species and, in some cases, has led to displacement of these species. Resource enrichment of container habitats has been shown to alleviate competitive interactions and to promote species co-existence; however, the importance of the timing of enrichment has yet to be explored. Larval competition between Ae. albopictus and a native species, Ochlerotatus triseriatus, was explored when resources were added either gradually or in a single pulse. Replacement series experiments revealed that Ae. albopictus was able to outcompete and displace Oc. triseriatus via resource monopolization when all resources were made available simultaneously; however, when the same resource amount was added over time, survival was high for both species, leading to co-existence. Timing of resource input also had an effect in monospecific treatments, indicating that intraspecific competition impacts survival as well. Duration of larval development was influenced by both species presence and by timing of resource input for Oc. triseriatus. These results indicate competitive outcome is condition-specific and that timing of resource input can determine whether a dominant invasive competitor displaces a native species, or if the two species are able to co-exist. Both intraspecific and interspecific competition occur at different temporal scales due to species-specific differences in larval developmental time. Timing of resource availability in container habitats can impact mosquito survival via competitive interactions, which can ultimately influence vector population size and behavior, possibly influencing vector-borne disease transmission.     

The impact of larval predators and competitors on the morphology and fitness of juvenile treefrogs

Studies of phenotypic plasticity typically focus on traits in single ontogenetic stages. However, plastic responses can be induced in multiple ontogenetic stages and traits induced early in ontogeny may have lasting effects. We examined how gray treefrog larvae altered their morphology in four different larval environments and whether different larval environments affected the survival, growth, development, and morphology of juvenile frogs at metamorphosis. We then reared these juveniles in terrestrial environments under high and low intraspecific competition to determine whether the initial differences in traits at metamorphosis affected subsequent survival and growth, whether the initial phenotypic differences converged over time, and whether competition in the terrestrial environment induced further phenotypic changes. Larval and juvenile environments both affected treefrog traits. Larval predators induced relatively deep tail fins and short bodies, but there was no impact on larval development. In contrast, larval competitors induced relatively short tails and long bodies, reduced larval growth, and slowed larval development. At metamorphosis, larval predators had no effect on juvenile growth or relative morphology while larval competitors produced juveniles that were smaller and possessed relatively shorter limbs and shorter bodies. After 1 month of terrestrial competition among the juvenile frogs, the initial differences in juvenile morphology did not converge. There were no differences in growth due to larval treatment but there were differences in survival. Individuals that experienced low competition as tadpoles experienced near perfect survival as juvenile frogs but individuals that experienced high competition as tadpoles suffered an 18% decrease in survival as juvenile frogs. There were also morphological responses to juvenile competition, but these changes appear to be due, at least in part, to allometric effects. Collectively, these results demonstrate that larval environments can have profound impacts on the traits and fitness of organisms later in ontogeny.     

A study of trophic niche partitioning between larval populations of reproductively isolated whitefish (Coregonus sp.) ecotypes

The abundance, spatio-temporal distribution, and feeding of larvae were compared from two reproductively isolated dwarf and normal ecotypes of whitefish ( Coregonus sp.) to test the hypothesis that larval ecotypes should reduce competition by using different resources. Contrary to a priori expectations, trophic niche partitioning between larval populations was much less pronounced than previously reported for the adult stages of fish ecotypes, presumably due to the lack of competition acting at this stage, and related to non-limiting food resources. Because this study was conducted in a single year and a single lake, we cannot however, strictly rule out the possibility that resource-based competition and resulting niche partitioning may occur at the larval stage in these species complexes. Nevertheless, this suggests for the first time, that resource-based selection at the larval stage may be relatively unimportant compared to that occurring at older life-history stages in driving the divergence and the development of reproductive isolation in sympatric fish ecotypes.     

Facultative gregarious development in a solitary koinobiont parasitoid,Ephedrus californicus: implications of larval ecology and host constraints

In solitary parasitoids, the mandibulate first instars behave aggressively towards potential competitors so that generally only one larva survives per host. A ‘failure of competition’ may result in facultative gregarious development, however. We used Ephedrus californicus Baker (Hymenoptera: Braconidae: Aphidiinae), a solitary koinobiont parasitoid of aphids, to test two hypotheses in the laboratory that could explain facultative gregarious development. Gregarious development increased with the intensity of parasitism, with two (rarely three) parasitoids successfully developing in a single aphid. In heavily superparasitized hosts, interference between surviving larvae often caused abnormal pupation behaviour and inability to emerge from the mummy. The hypothesis that the survival of more than one larva per host is dependent on differences in larval age was not supported. The total body size in terms of dry mass of two males or two females developing together in the same host was higher than that of same‐sex counterparts developing singly. Females were larger than males with which they shared a host. Hypotheses about the evolutionary transition from a solitary to a gregarious lifestyle in parasitoid Hymenoptera have focused on lethal fighting between first instars but have ignored other constraints including immature mortality during later development and limiting host resources. Especially in species that pupate inside the dead host, specific requirements for pupation and emergence may determine whether one or several offspring per host can develop to adult.