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Host Abundance (host + abundance)
Selected AbstractsEffects of landscape structure and habitat type on a plant-herbivore-parasitoid communityECOGRAPHY, Issue 3 2003Andreas Kruess The effects of local habitat and large-scale landscape factors on species diversity and species interactions were studied using the insect community in stems of the creeping thistle Cirsium arvense. Thistle abundance was higher in fallows than in crop fields and field margins, with fallows providing 67% of thistle abundance within 15 study areas on a landscape scale. Species richness of the herbivores was positively related with thistle abundance, parasitoid species richness was influenced by habitat type and was positively correlated with herbivore species richness. The abundance of herbivores and parasitoids was affected by local factors such as habitat type and host abundance, but also by landscape factors such as the percentage of non-crop area and the isolation of habitats. The infestation rate caused by the agromyzid Melanagromyza aeneoventris was positively related to percent non-crop area, whereas the parasitism rate of this fly increased with increasing habitat diversity on the landscape scale. For these two interactions and for total herbivore abundance, a scale-dependency of the landscape effects was found. The results emphasize that biological diversity and ecological functions within a plant-insect community are not only affected by local habitat factors but also by large-scale landscape characteristics. Hence, to improve future agri-environmental schemes for biodiversity conservation and biological control large-scale landscape effects and their scale-dependency should be considered. [source] Contrasting frequencies of parasitism and host mortality among phorid and conopid parasitoids of bumble-beesECOLOGICAL ENTOMOLOGY, Issue 2 2002Michael C. Otterstatter Abstract 1. Phorid (Diptera, Phoridae) and conopid (Diptera, Conopidae) parasitism among four North American bumble-bee (Hymenoptera, Apidae) species was investigated. Male bumble-bees experienced a significantly higher incidence of parasitism by the phorid, Apocephalus borealis Brues, and a significantly lower incidence of parasitism by the conopid, Physocephala texana Williston, than did workers. 2. The incidence of parasitism by A. borealis and P. texana varied between bumble-bee sexes and species in patterns that did not reflect differences in relative host abundance. Differences in foraging behaviour between bumble-bee workers and males, as well as between species, may explain these results. 3. Bumble-bee workers and males parasitised by A. borealis had significantly shorter lifespans than unparasitised bees. Based on previous estimates of bumble-bee mortality, A. borealis parasitism may reduce worker lifespans by up to 70%. In contrast, the mortality rate of bees parasitised by P. texana was not significantly different from that of unparasitised bees. 4. These results contrast with previous work highlighting the importance of conopid parasitism to bumble-bee populations, and suggest that phorid parasitism may impose greater costs to bumble-bees than does conopid parasitism in local populations. [source] Effects of parasitoids on a mycophagous drosophilid community in northern Japan and an evaluation of the disproportionate parasitism hypothesisENTOMOLOGICAL SCIENCE, Issue 1 2006Hiroshi YOROZUYA Abstract In a host,parasitoid system comprising mycophagous drosophilids and their parasitoids, the drosophilid and parasitoid species assemblages, host use, and the prevalence of parasitism were assessed, and the "disproportionate parasitism hypothesis" was examined with consideration given to yearly variations. The mycophagous drosophilids, their fungal food resources and parasitoids were studied by carrying out an intensive census throughout the activity seasons of 4 years (2000,2003) in Hokkaido, northern Japan. Five hymenopterous parasitoid species, four braconids and one eucoilid, were found. Parasitoids of mycophagous drosophilids are reported for the first time from Asia. Most parasitism (99.2%) was by braconids, in contrast to the dominance of eucoilids in Europe. Parasitism was restricted to the summer, and the rate was high from early July to early August every year. There was considerable yearly variation in the composition of abundant fungus, drosophilid and parasitoid species, especially between 2000 and 2001. The alternation of dominant host species was coupled with the alternation of dominant parasitoid species that differed in host use. Despite the yearly variation in the system, the most dominant host species suffered disproportionately heavy parasitism by the correspondingly dominant parasitoid species every year. The parasitism rate was positively correlated with the relative host abundance. This thus indicates that the disproportionate parasitism mechanism may operate, via which species coexistence is promoted by a higher rate of parasitism of the dominant species. [source] Does Holling's disc equation explain the functional response of a kleptoparasite?JOURNAL OF ANIMAL ECOLOGY, Issue 4 2001R. W. G. Caldow Summary 1Type II functional responses, which can be described by Holling's disc equation, have been found in many studies of predator/prey and host/parasite interactions. However, an increasing number of studies have shown that the assumptions on which the disc equation is based do not necessarily hold. We examine the functional response of kleptoparasitically feeding Arctic skuas (Stercorarius parasiticus L.) to the abundance of fish-carrying auks and, by examination of the assumptions of the disc equation, test whether it can explain the function. 2The rate at which individual skuas make successful chases is a decelerating function of the abundance of auks. However, it would appear that this is not determined by factors that influence their probability of success, but by the rate at which they initiate chases. This too is a decelerating function of the abundance of auks. Arctic skuas have a Type II functional response. 3Although Arctic skuas exhibited a direct numerical response there was no evidence that components of predation connected to the density of predators (direct prey stealing, or increased host avoidance) had any effect on the rate at which individual skuas made chases or were successful in their chases. We conclude that the observed functional response is free from any effects of interference. 4We suggest that abnormally high levels of foraging effort expended by breeding skuas and their poor breeding success in the years of observation argue against the limit to the observed functional response being determined by skuas' energetic requirements. 5Several of the assumptions underlying the disc equation do not hold. The duration of chases (handling time) was not a constant; it decreased with increasing host abundance. Moreover, the chase duration predicted by the disc equation, if handling time limited the functional response, was far in excess of that observed. Furthermore, the observed rate of decline in the searching time per victim with increasing host abundance suggested that skuas' instantaneous rate of discovery was also not constant. Possible reasons for these observations are discussed. The basic disc equation may describe Arctic skuas' functional response, but it cannot explain it. [source] Masting and trophic cascades: interplay between rowan trees, apple fruit moth, and their parasitoid in southern NorwayOIKOS, Issue 3 2004Akiko Satake We analyzed berry production in rowan, Sorbus aucuparia L., in southern Norway and examined the ramifying effects of rowan masting on the dynamics of the dominant seed predator and its parasitoid. The apple fruit moth, Argyresthia conjugella Zeller, is a pre-dispersal seed predator of rowan. The larva of the apple fruit moth rely on rowan berries, which in turn is attacked by the parasitoid wasp, Microgaster politus Marsh. We found classic masting in rowan: berry production varied across years (the mean coefficient of variation=1.02) and was spatially synchronized at large scale (the averaged correlation coefficient=0.67). Berry production represented a two-year cycle in western but a three-year cycle in eastern Norway. The abundance of the moth and the parasitoid also varied across years and were spatially synchronized. The degree of spatial synchrony decreased and cyclicity became obscure with increasing trophic level. We attempted to assess two different components to the predator satiation, functional and numerical satiations, based on a simple population dynamics model. The observed pattern of seed predation testified that both of functional and numerical satiations were at work in this system. In a comparison at different locations, rowan trees with more variable berry production were more effective in reducing losses to the seed predator. The parasitoids also seemed to experience satiation through the fluctuation in their host abundance. These results show that rowan masting has an adaptive foundation, which impacts the dynamics of higher trophic levels. [source] Synergism between plant viruses: a mathematical analysis of the epidemiological implicationsPLANT PATHOLOGY, Issue 6 2001X.-S. Zhang Many virus diseases of plants are caused by a synergistic interaction between viruses within the host plant. Such synergism can induce symptoms more severe than would be caused by additive effects. In a synergistic interaction, the virus titre of both, one, or neither virus may be enhanced and, as a consequence, the rate of disease spread may be affected. An epidemiological model was developed in which transmission and loss rates were attributed to the different virus infection possibilities. Sharing the same host population implies competition, and this imposes an increased constraint on the survival of both viruses. It was shown that, in order to ensure virus survival in a mixed infection, the basic reproductive number should exceed a critical value which is larger than unity (R0 > Rc > 1). Here R0 is used in the same sense as in the absence of superinfection. Increased virulence (equivalent to disease severity) in dually infected plants decreases the opportunities for both viruses to coexist, while increased virus transmission from dually infected plants increases such opportunities. The net effect of increased virulence and increased virus transmission on virus persistence was neutral if synergism caused the same proportional effect on both. Total host abundance was, however, reduced. The opportunity for virus persistence was increased if the enhancement of transmission exceeded that of virulence. Indeed, by this mechanism a virus which was nonviable alone could invade and persist in a chronic epidemic of another virus. Where the effect on virulence is greater than that on transmission, the viruses are likely to exclude each other, especially when the transmission rates of both viruses have intermediate values. In such cases, the final outcome is determined by both the parameter values and the initial state. [source] | |||||||||||||||||||