The host specificity of Gyrodactylus salaris Malmberg (Platyhelminthes, Monogenea): susceptibility of Oncovhynchus mykiss (Walbaum) under experimental conditions

An experimental epidemiological approach was chosen to study the survival and infection dynamics of Gyrodactylus salaris on juvenile rainbow trout, Oncorhynchus mykiss , in the laboratory. A marked heterogeneity in the host stock was apparent. The rainbow trout could be divided into three groups on the basis of parasite survival and infection pattern on individually isolated fish: (1) hosts receptive to initial parasite attachment, but unreceptive to parasite establishment and reproduction; (2) hosts moderately susceptible to parasite establishment and reproduction, but which, after a period of restricted parasite population growth, responded, recovered and eliminated the parasites; and (3) hosts very susceptible to parasite infection and reproduction, but which, after a period of significant parasite population growth, responded, recovered and eliminated the parasites. These different patterns are considered to reflect genetic differences between host individuals. Parasite aggregation was also shown to be an important factor in the outcome of the host-parasite association. The parasites were finally eliminated on the individually isolated hosts, but not on hosts maintained in batches and so host population size and immigration of fresh. previously unexposed, hosts appeared to be important for growth and maintenance of the parasite population. The parasite was not found to cause host mortality. Rainbow trout was a suitable host for G. salaris , capable of transmitting the parasite to new localities as a consequence of stocking programmes or migratory behaviour.     

Competition as an amplifier of short-term vegetation responses to climate: an experimental test

1. Responses to a spring warming treatment were measured on five common herbaceous species grown in outdoor microcosms in Northern England. Although elevated temperature had a beneficial effect on canopy height and plant cover in monospecific cultures of all species, strongly divergent responses to warming occurred in mixtures.
2. We show that the effect of interspecific competition was to modify and amplify the vegetation response to the warming treatment through effects on phenology and morphological development.
3. The observed responses between the species to warming are consistent with predicted differential responses linked to genome size.
4. We conclude that the mechanism of competitive interaction proposed by Boysen-Jensen (1929 ) coupled with the insights related to genome size provide a basis for explaining and predicting the role of interannual variation in temperature in determining year to year fluctuations in the relative abundance of species in productive perennial herbaceous vegetation.     

食物、捕食及种间竞争对东方田鼠(Microtus fortis)种群攻击行为的作用

在野外围栏条件下,采用重复的2×2×2析因实验设计,测定食物、捕食和竞争物种黑线姬鼠对东方田鼠攻击行为作用的格局。东方田鼠各处理种群攻击水平与其种群密度的相关甚为复杂,与Chitty多态行为假设的预测不一致。东方田鼠双冲突个体间的攻击水平与其体重的相关不显著,而与冲突个体的体重差异则呈显著的负相关,体重差异越大,冲突个体间的攻击水平越低。雄体攻击水平与其繁殖特征无显著的相关关系,而雌体间攻击水平则受繁殖状态的影响,动情雌体间的攻击水平显著地高于非动情雌体。体重差异和食物对雄体的攻击行为具有极显著的独立作用,捕食和种间竞争对雄体攻击行为的独立作用不显著,而捕食与种间竞争交互作用对雄体攻击行为的效应则达到显著水平。体重差异、食物、捕食和种间竞争对动情雌体的攻击水平均有极显著的独立作用,3类外部因子交互作用的效应达到极显著水平;而3类外部因子交互作用对非动情雌体攻击行为的效应则不显著。结果检验了外部因子食物、捕食、种间竞争对田鼠类动物种群攻击行为具有独立和累加的整合效应的假设。     

Effects of exogenous serotonin on a motor behavior and shelter competition in juvenile lobsters (Homarus americanus)

Three experiments were conducted to determine (1) the pharmacodynamics of 5-hydroxytryptamine in juvenile lobsters; (2) the effects of 5-hydroxytryptamine, using a range of dosages, on a motor behavior used to escape an aversive situation; and (3) the effect of doses that did and did not inhibit this motor behavior on measures of dominance and shelter competition. The fate of 5-hydroxytryptamine in hemolymph over a 60-min post-injection period showed that the concentration fell rapidly to a low plateau that was maintained for at least 1 h. Low doses of 5-hydroxytryptamine did not affect locomotor behavior, but higher doses inhibited it. Dominance and subsequent possession of a shelter were unaffected by a low dose of 5-hydroxytryptamine but a higher dose that inhibited locomotion resulted in lobsters that lost fights and did not secure or retain possession of the shelter. In the context of dominance and shelter competition, we were unable to demonstrate any advantage of the low dose of exogenous 5-hydroxytryptamine and a severe disadvantage with the higher dose. Previous reports of transient increases in aggression in 5-hydroxytryptamine-treated subordinate lobsters did not take into account motor inhibition as a possible critical variable in aggression. Accepted: 14 April 2000     

Two-species asymmetric competition: effects of age structure on intra- and interspecific interactions

1. The patterns of density-dependent resource competition and the mechanisms leading to competitive exclusion in an experimental two-species insect age-structured interaction were investigated. 2. The modes of competition (scramble or contest) and strength of competition (under- to overcompensatory) operating within and between the stages of the two species was found to be influenced by total competitor density, the age structure of the competitor community and whether competition is between stages of single or two species. 3. The effect of imposed resource limitation on survival was found to be asymmetric between stages and species. Environments supporting both dominant and subordinate competitors were found to increase survival of subordinate competitors at lower total competitor densities. Competitive environments during development within individual stage cohorts (i.e. small or large larvae), differed from the competitive environment in lumped age classes (i.e. development from egg-->pupae). 4. Competition within mixed-age, stage or species cohorts, when compared with uniform-aged or species cohorts, altered the position of a competitive environment on the scramble-contest spectrum. In some cases the competitive environment switched from undercompensatory contest to overcompensatory scramble competition. 5. Such switching modes of competition suggest that the relative importance of the mechanisms regulating single-species population dynamics (i.e. resource competition) may change when organisms are embedded within a wider community.     

Patch exploitation strategies of parasitic wasps under intraspecific competition

The optimal residence time of a forager exploiting a resourcepatch in the absence of competitors has been much studied sincethe development of the marginal value theorem. However, severalforagers are frequently observed exploiting the same patch simultaneously,and patch residence time has been surprisingly little studiedin such competitive situations. The few theoretical models developedon this topic predict that foragers should engage in a war ofattrition and stay in the patch longer than when foraging alone.We tested this prediction in Pachycrepoideus vindemmiae (Hymenoptera:Pteromalidae), a solitary parasitoid species in which femalesare known to defend the hosts they are exploiting via intraspecificfighting. By measuring the effect of direct (i.e., presenceof conspecifics) and indirect (i.e., presence of already-parasitizedhosts) competition on patch exploitation strategies, we revealedan apparent polymorphism of strategies. Indeed, in competitivesituations, some members of the population tended to retreatalmost immediately from the patch whereas other members tendedto remain, further exploit the patch, and be more involved ininteractions with conspecifics. The proportion of "retreaters"strongly depended on competition intensity and prior experienceof competition. The forager's physiological state (age) alsoaffects patch exploitation strategy and the intensity of interactionsbetween competing females. Our results highlight the necessityfor further theoretical studies that consider cases where contestsbetween foragers are costly and take into account prior experienceof competition and the forager's physiological state.     

The energetic equivalence of cover to juvenile coho salmon (Oncorhynchus kisutch): ideal free distribution theroy applied

Cover is often thought to be an important habitat characteristicfor juvenile stream salmonida. In addition to providing protectionfrom predators, cover may be associated with reduced food availability.Thus, an individual's use of cover is likely to reflect a trade-offbetween the conflicting demands of growth and survival. We measuredthe influence of cover on foraging-site selection in groupsof eight juvenile coho salmon (Oncorhynchus kisutch) by examiningtheir distribution across two stream channel patches, one providingaccess to cover but little food (the "poor" patch), the otherproviding more food but no cover (the "good" patch). Becausefish distributions in the absence of cover conformed to an idealfree distribution (IFD) for unequal competitors (i.e., the distributionof competitive abilities matched the distribution of food),we used IFD theory to quantify the energetic equivalence ofcover to the fish. In the presence of cover and a model avianpredator, use of the poor patch increased relative to the predictionsof the IFD model. Using this observed deviation from an IFD,we calculated how much extra food must be added to the goodpatch to return the distribution of fish to the previously observedIFD of unequal competitors. As predicted, adding this amountof food caused the fish to return to their previous distribution,demonstrating that IFD theory can be used to relate energy intakeand risk of predation in a common currency     

Habitat-specific estimates of competition in stream salmonids: A field test of the isodar model of habitat selection

Summary The population densities of sympatric Atlantic salmon,Salmo salar and brook charr,Salvelinus fontinalis, were measured in riffle and pool stream habitats to test whether non-linear isodars, a multispecific model of habitat selection based on ideal distribution assumptions, could (1) predict the distribution of densities between habitats and (2) reproduce the processes postulated to underlie spatial segregation and species interactions in previous laboratory and field studies. The model provided a good fit to observed density patterns and indicated that habitat suitability declined non-linearly with increased heterospecific competitor densities. Competitive effects in riffles appeared to be due to exploitative resource use, with salmon always emerging as the superior competitor. No evidence was found for interference competition in riffles. In contrast, interspecific competition in pools seemed to occur through exploitation and interference. The specific identity of the superior competitor in pools depended on the density of both species; pools provided the charr with refuge from competition with the salmon, presumably through the adoption by the charr of density-dependent behaviours, such as schooling and group foraging, that mitigated the negative impact of the salmon. Charr were displaced from the riffles toward the pools as the total salmon density increased. The isodar analysis, based on limited density data, successfully reproduced the processes suggested to underlie spatial segregation in previous field and laboratory studies and provided new insights into how changes in competitor densities modify habitat suitability in this system.     

The foraging behavior of granivorous rodents and short-term apparent competition among seeds

The foraging behavior of a predator species is thought to bethe cause of short-term apparent competition among those preyspecies that share the predator. Short-term apparent competitionis the negative indirect effect that one prey species has onanother prey species via its effects on predator foraging behavior.In theory, the density-dependent foraging behavior of granivorousrodents and their preference for certain seeds are capable ofinducing short-term apparent competition among seed species.In this study, I examined the foraging behavior of two heteromyidrodent species (family Heteromyidae), Merriam's kangaroo rats(Dipodomys merriami) and little pocket mice (Perognathus longimembris).In one experiment I tested the preferences of both rodent speciesfor the seeds of eight plant species. Both rodent species exhibiteddistinct but variable preferences for some seeds and avoidanceof others. However, the differences in preference appearedto have only an occasional effect on the strength of the short-termapparent competition detected in a field experiment. In anotherexperiment, I found that captive individuals of both rodentspecies had approximately equal foraging effort (i.e., timespent foraging) in patches that contained a highly preferredseed type (Oryzopsis hymenoides) regardless of seed densityand the presence of a less preferred seed type (Astragalus cicer)in the patches. The rodents also harvested a large proportionof O. hymenoides seeds regardless of initial seed density;this precluded a negative indirect effect of A. cicer on O.hymenoides. But there was a negative indirect effect of O.hymenoides on A. cicer caused by rodents having a lower foragingeffort in patches that only contained A. cicer seeds than inpatches that contained A. cicer and O. hymenoides seeds. Theindirect interaction between O. hymenoides and A. cicer thusrepresented a case of short-term apparent competition thatwas non-reciprocal. Most importantly, it was caused by theforaging behavior of the rodents.     

Effects of interspecific competition on development and reproduction in two giant water bugs,Diplonychus japonicus vuillefroy andDiplonychus major esaki (Hemiptera: Belostomatidae)

We studied the interpecific competition between 2 species of predatory aquatic bugs, Diplonychus japonicus and D. major by conducting a field experiment. We set up 3 types of experimental plots in the paddy fields where D. major predominated. The two plots contained single species of either D. japonicus or D. major, respectively, and one plot had both species in equal number. We compared the development and the reproductive performance between plots in each species. In D. japonicus, the number of eggs and early instar nymphs were significantly smaller in the plots containing both species than in the monospecific plots. However, the numbers of late instar nymphs and newly emerged adults were not significantly different between plots. The proportions of starved nymphs in both plots were larger than those in the D. japonicus's natural habitats. The final densities of adults in both plots were lower than those in the natural habitats. These results suggest that lower density of D. japonicus in these paddy fields is due to the lack of available food for nymphs rather than the effects of interspecific competition with D. major. In D. major, significant differences were not found in the number of eggs, each instar nymphs and adults. These results suggest that the effects of interspecific competition did not affect the reproductive performance of D. major.     

The influence of competition between foragers on clutch size decisions in an insect parasitoid with scramble larval competition

The effect of competition between ovipositing females on theirclutch size decisions is studied in animals that lay their eggsin discrete units of larval food (hosts). In such species theeffect of competition depends on the form of the larval competitionwithin such units. In insect parasitoids, there might eitherbe contest (solitary parasitoids) or scramble competition (gregariousparasitoids) between larvae within a host For gregarious parasitoids,a decreasing clutch size with increasing competition betweenforagers is predicted. This prediction is tested in experimentsusing the parasitoid Aphaertta minuta. Parasitoids were eitherkept alone or in groups of four before the experiment, in whichthey were introduced singly in a patch containing unparasitizedhosts. Animals kept together laid on average clutches of 0.74eggs smaller than females kept alone (average clutch is 5.3),thereby confirming the prediction. Clutch size decreased withencounter number, which might be due to the adjustment of thefemale's estimate of the encounter rate with hosts. Finally,the results are compared with those reported for solitary parasitoids(that have scramble larval competition), for which it is predictedthat the clutch size will increase with increasing levels ofcompetition between females.     

A test of the sequential assessment game: the effect of increased cost of sampling

The sequential assessment game describes a fight between twoconspecifics as a statistical sampling process, allowing forspecific predictions about fight duration and number of repetitionsof a behavioral element depending on relative fighting ability.Fight duration and number of repetitions of a behavioral elementcorrelate moderately to relative fighting ability. Variationin real contests depends on differences between the contestantsin the ability to incur cost and the value of the contestedresource. I hypothesized that an additional source of variationis differences between individuals in their perception of howdangerous it is to fight. To investigate this, I used the riskof predation to study the effect on the use of different agonisticbehaviors in fights between males of the cichlid fish Nannacaraanomala. In the group subjected to predation risk, the time untilthe most escalated behavior (mouth wrestling) was more variableand increased significantly on average. In addition, the durationof fights was significantly longer. In the predator treatmentthe use of visual assessment and tail beating varied more thanin the control, giving a significant positive relationship betweenthe use of low-intensity behaviors and time to mouth wrestlingacross the group. These relationships were less pronounced in thecontrol group. The effect of predation risk on optimal information transferis discussed based on the behavioral mechanism suggested bythe sequential assessment game.     

The kinds of traits involved in male--male competition: a comparison of plumage, behavior, and body size in quail

I compared the role of ornate plumage, behavior, and body sizeduring male—male competition in two species of New Worldquail. Gambel's quail (Callipepla gambelii) is a highly ornateand dichromatic species, whereas scaled quail (C. squamata)is unornamented and monochromatic. During paired contests betweenunfamiliar males, high rates of testosterone-mediated behaviors(tidbitting, calling) and large body size (mass, tarsus, andtail length) corresponded to winners. In the highly ornate Gambel's quail, male head plumes also influenced the outcomeof contests. Plume enhancement made Gambel's quail more likelyto win contests, whereas plume removal made males more likelyto lose. Plume position also reflected male status. Winningmales erected plumes, whereas losers frequently flattened them.Some plumage ornaments, such as belly patches, did not playa primary role during male contests. Unlike static ornaments,head plumes are highly modifiable and likely signal immediateinformation regarding a male's intent, similar to a coverablebadge. Combined, intrasexual selection favored dynamic traits(fast display rates, modifiable ornaments) and static traits(body size) as indicators of male condition or motivation.In scaled quail only, male size was favored both by male—malecompetition and female choice. Accordingly, the degree of sizedimorphism (tarsus length) is greater in scaled than in Gambel'squail. The frequency of overt aggression (chases, pecks, displacement)also differed between species. Gambel's quail were very aggressive,and subordinates often challenged their opponents. In contrast, scaled quail were less aggressive, and subordinates rarely disputedrank. Interspecific comparison indicated differences in themaintenance of male status and possibly in the honesty of signaling.Both appear to be related to differences in social system.     

Predation and the evolution of prey behavior: an experiment with tree hole mosquitoes

We tested for facultative changes in behavior of an aquaticinsect in response to cues from predation and for evolutionof prey behavior in response to experimental predation regimes.Larvae of the tree hole mosquito Aedes triseriatus reducedfiltering, browsing, and time below the surface in responseto water that had held a feeding larva of the predator Toxorhynchitesrutilus. We subjected experimental A. triseriatus populationsto culling of 50% of the larval population, either by T. rutiluspredation or by random removal. After two generations of laboratoryculling, behavior of the two treatment groups diverged. Aedestriseriatus in control-culled lines retained their facultativeshift from filtering to resting, but tended to lose the response of reduced browsing below the surface in water that had helda feeding predator. Predator-culled lines lost their facultativeresponse of reduced filtering in water that had held a feedingpredator and evolved toward more time resting and less timefiltering in both water that had held a feeding predator andwater that had held only A. triseriatus. Predator-culled linesretained their facultative response of reduced browsing belowthe surface in water that had held a feeding predator. Twofield populations and their reciprocal hybrids responded similarlyto cues from predation and did not differ in their evolutionaryresponse to experimental culling. We conclude that consistentpresence or absence of predation can select rapidly for divergencein prey behavior, including facultative behavioral responsesto predators.     

Temperature affects operational sex ratio and intensity of male-male competition: an experimental study of sand gobies, Pomatoschistus minutus

The operational sex ratio is intimately related to the intensityof sexual selection, but factors governing variation in theoperational sex ratio and their effects on mating competitionare still poorly understood. In this study, temperature wasfound to affect both the operational sex ratio and the intensityof male-male competitive interactions in the sand goby [Pomatoschistusminutus (Pallas)]. In an experiment with two different temperaturetreatments, the operational sex ratio became male biased inthe warm treatment (15°C) and males in that treatment interactedmore frequendy than in the cold treatment (8.5°C). Theseresults were as predicted since the potential reproductive rateof males increases faster with temperature than does the potentialreproductive rate of females. Thus, an environmental factor,water temperature, affects not only the reproductive rates ofthe sexes, but also the operational sex ratio and mating competition,and thereby the intensity of sexual selection. Operational sexratio was not found to be correlated with male behavior. Thismay suggest a direct effect of temperature or potential reproductiverates on mate competition. The mechanism behind the evolutionof such a direct relationship would, however, probably be theimpact of potential reproductive rates on operational sex ratio,which in turn direcdy affects sexual selection.     

Costs of coexistence along a gradient of competitor densities: an experiment with arvicoline rodents

1. Costs of coexistence for species with indirect resource competition usually increase monotonically with competitor numbers. Very little is known though about the shape of the cost function for species with direct interference competition. 2. Here we report the results of an experiment with two vole species in artificial coexistence in large enclosures, where density of the dominant competitor species (Microtus agrestis) was manipulated. Experimental populations of the subordinate vole species (Clethrionomys glareolus) were composed of same aged individuals to study distribution of costs of coexistence with a dominant species within an age-cohort. 3. Survival and space use decreased gradually with increasing field vole numbers. Thus, responses to interference competition in our system appeared to be similar as expected from resource competition. The total number of breeders was stable. Reproductive characteristics such as the timing of breeding, and the litter size were not affected. In the single species enclosures a proportion of surviving individuals were not able to establish a breeding territory against stronger conspecifics. Under competition with heterospecifics such nonbreeders suffered high mortality, whereas the breeders survived. 4. Combined interference of dominant conspecifics and heterospecifics probably increased the frequency of aggressive interactions, social stress and mortality for the weaker individuals within a homogeneous age cohort of the subordinate competitor population. 5. Our results suggest, that in open systems where bank voles are outcompeted over the breeding season by faster reproducing field voles, animals able to establish a territory may be able to withstand competitor pressure, while nonbreeding bank vole individuals are forced to emigrate to suboptimal forest habitats.     

Evolutionary rescue and the coexistence of generalist and specialist competitors: an experimental test

Competition for resources is thought to play a critical role in both the origins and maintenance of biodiversity. Although numerous laboratory evolution experiments have confirmed that competition can be a key driver of adaptive diversification, few have demonstrated its role in the maintenance of the resulting diversity. We investigate the conditions that favour the origin and maintenance of alternative generalist and specialist resource-use phenotypes within the same population. Previously, we confirmed that competition for hosts among φ6 bacteriophage in a mixed novel (non-permissive) and ancestral (permissive) host microcosm triggered the evolution of a generalist phenotype capable of infecting both hosts. However, because the newly evolved generalists tended to competitively exclude the ancestral specialists, coexistence between the two phenotypes was rare. Here, we show that reducing the relative abundance of the novel host slowed the increase in frequency of the generalist phenotype, allowing sufficient time for the specialist to further adapt to the ancestral host. This adaptation resulted in ‘evolutionary rescue’ of the specialists, preventing their competitive exclusion by the generalists. Thus, our results suggest that competition promotes both the origin and maintenance of biodiversity when it is strong enough to favour a novel resource-use phenotype, but weak enough to allow adaptation of both the novel and ancestral phenotypes to their respective niches.     

Separating the effects of number of individuals sampled and competition on species diversity: an experimental and analytic approach

1 Species richness typically increases with the number of individuals sampled, although many ecological processes that influence species richness are also well known to depend on density of individuals. We separated the effects of density on species richness that are due to sampling, from those due to density-dependent ecological processes such as competition or predation, by manipulating the density of an entire community.
2 A seed bank from a community of desert annual plants that occur on semi-stabilized sand dunes in Israel was collected from the field and sown in an experimental garden at a range of densities from 1/16 to eight times the natural density. The species pool observed in the lowest density plots was used as the null community, which was repeatedly sampled to calculate the species richness (and other diversity indices) in higher density plots that would be expected from sampling considerations alone. The significance of deviations of observed diversity from this expected diversity was then evaluated.
3 Both observed and expected number of species increased substantially with the experimental increase in density. However, observed species richness, the Shannon–Wiener diversity index and Simpson's diversity index were often significantly lower than that expected based on sampling considerations. The magnitude of the deviation from expected increased significantly with increasing density for richness and the Shannon–Wiener index. This provides some of the first direct experimental evidence from diverse natural assemblages that increasing competition among all the individuals in a community can lead to competitive exclusion.     

Effects of physical and social experiences and octopamine receptor agonist on fighting behavior of male crickets Velarifictorus aspersus (Orthoptera: Gryllidae)

Fighting commonly occurs among animals and is very important for resolving conflicts between conspecific individuals over limited resources. The plasticity of fighting strategies and neurobiological mechanisms underlying fighting behavior of insects are not fully understood. In the present study, we examined whether physical and social experiences affected the aggressiveness of males of the cricket Velarifictorus aspersus Walker, and whether an octopamine (OA) receptor agonist could affected the aggressiveness of males exposed to different experiences. We found that flight and winning a fight significantly enhanced male aggressiveness, while losing a fight significantly suppressed male aggressiveness, consistent with the findings of existing studies on other cricket species. We also found that female presence had a stronger enhancing effect on male aggressiveness than flight or winning a fight. These findings demonstrated that physical and social experiences can affect the fighting behavior of male V. aspersus. Topical application of a 0.15?M solution of an OA receptor agonist (chlordimeform, CDM) significantly increased male aggression level, suggesting that OA may play an important role as a neuromodulator in controlling fighting behavior of males of this species. Despite displaying a significantly higher aggression level (level 5 or 6), CDM-treated losers did not escalate to physical combat, while fights between courting males usually resulted in physical escalation. It is likely that fighting behavior is only partly regulated by OA, and additional regulatory pathways may be involved in achieving physical combat.