Population consequences of huge nesting aggregations ofRopalidia plebeiana (Hymenoptera: Vespidae)

Nest survival rates and reproductive rates of females of the Australian paper wasp, Ropalidia plebeiana, in nest aggregations under a concrete bridge were studied. The annual colony cycle commenced in August with the reutilization of old nests and by the founding of new nests, mainly by associations of foundresses. Distribution patterns of the number of foundresses per nest was nearly random on new nests while contagious on old nests. About one-third of the increase in number of nest was achieved by dividing large, old nests and two-thirds by founding new nests. Nest survival rates for old nests (August to April) and new nests (November to April) were 89.6% and 88.8% respectively, far higher rates than those of other polistine wasps so far reported. The numbers of nests and reproductive females increased during a colony cycle by 2.30 and 10.98 times, respectively. Thus, huge aggregations of nests are probably beneficial for this species by enabling the realization of high reproductive rates. We could find no density-dependent effect in nest growth rate or production of female reproductives. Ratio of cells parasitized by ichneumonid wasps was low. Nests in the central part of an aggregation were prasitized at significantly lower rates than nests on the periphery or scattered nests outside a dense aggregation, suggesting a selfish herd effect.     

Experiments on clutch size and nest size in passerine birds

Summary Results of experiments on three passerine species suggest that brood size may be constrained by nest size, since the breeding success of pairs provided with large nestcups was greater than that of those provided with small artificial nestcups. These results may have important implications, e.g. to the design of experiments involving manipulation of clutch and brood size. A small nestcup is requisite for successful hatching during the incubation period, but a large one for successful rearing during the nestling period. In nature this difference may select for types of nesting materials that are elastic, such as mosses and lichens. However, experiments showed that such materials rapidly absorb rainwater but only slowly dry out. In addition, because large nests dry out more slowly than small nests, selection will favour small nests among those open-nesting species that have exposed nests. A further possible nest size constraint on open-nesters is nest predation. However, no difference in the predation rate was found in experiments with small and large artificial nests.     

Can we predict the effects of alien species? A case-history of the invasion of South Africa by Mytilus galloprovincialis (Lamarck)

The Mediterranean mussel Mytilus galloprovincialis invaded the shores of South Africa in about the mid-1970s. It was first detected at a harbour in Saldanha on the west coast and apparently arrived accidentally. From there, it spread at a rate of about 115 km·year⁻¹ and now occupies the whole of the west coast of South Africa and at least the southern half of Namibia. It was deliberately introduced from the west coast to the south coast for mariculture. In this case study, we record its effects on intertidal rocky shores, cast in terms of predictions based on (a) the history of its invasions elsewhere, (b) its mode of dispersal, (c) its physiological performance relative to indigenous mussels, (d) the role of wave action as a moderator of competition, (e) the influence of relative body sizes, (f) the projected effects of the mussel on infauna, (g) consumption by higher trophic levels, and (h) rates of parasitism.

Several properties of M. galloprovincialis itself, and of the recipient community, conspired to favour the spread and establishment of this alien mussel, including high productivity on the west coast of South Africa, prevalently strong wave action, a sparsity of predators, an absence of parasites, the mussel's fast growth and high reproductive output, and its possession of a planktotrophic larva. It competitively displaces several species because of its physiological performance. Some of the species gain a substitute substratum on the mussels themselves, but only if they are small enough to live and reproduce on the mussels. M. galloprovincialis has had little effect on infauna, but has provided an additional source of food for higher predators, including the rare and endangered African Black Oystercatcher (Haematopus moquini).

Nearly all these effects and conditions were forecasted (or, at the least, explainable with hindsight), but despite these successes in predicting the impacts of M. galloprovincialis, its spread was not only unavoidable but was encouraged by its transfer to the south coast for mariculture. Moreover, there was one completely unpredictable effect of M. galloprovincialis—which led to mass mortalities of a swimming crab. Given the failure of even quite detailed and accurate predictions to allow control of M. galloprovincialis once it arrived, prevention rather than cure must be the prime means of avoiding future unwanted introductions of invasive species.     

Polydnavirus hidden face: The genes producing virus particles of parasitic wasps

Very few obligatory relationships involve viruses to the remarkable exception of polydnaviruses (PDVs) associated with tens of thousands species of parasitic wasps that develop within the body of lepidopteran larvae. PDV particles, injected along with parasite eggs into the host body, act by manipulating host immune defences, development and physiology, thereby enabling wasp larvae to survive in a potentially harmful environment. Particle production does not occur in infected tissues of parasitized caterpillars, but is restricted to specialized cells of the wasp ovaries. Moreover, the genome enclosed in the particles encodes almost no viral structural protein, but mostly factors used to manipulate the physiology of the parasitized host. We recently unravelled the viral nature of PDVs associated with braconid wasps by characterizing a large set of nudivirus genes residing permanently in the wasp chromosome(s). Many of these genes encode structural components of the bracovirus particles and their expression pattern correlates with particle production. They constitute a viral machinery comprising a large number of core genes shared by nudiviruses and baculoviruses. Thus bracoviruses do not appear to be nudiviruses remnants, but instead complex nudiviral devices carrying DNA for the delivery of virulence genes into lepidopteran hosts. This highlights the fact that viruses should no longer be exclusively considered obligatory parasites, and that in certain cases they are obligatory symbionts.     

Role of predation and parasitism in the extinction of the inoceramid bivalves: an evaluation

The inoceramid bivalves were dominant constituents of marine, epifaunal communities throughout the Late Mesozoic. They experienced a rapid decline in the Early Maastrichtian and virtually all taxa disappeared 1.5 Myr prior to the Cretaceous-Tertiary (K-T) boundary. The ultimate cause for their demise is still controversial. This study evaluates the role predation, parasitism and/or disease played in the evolution and extinction of Early Maastrichtian inoceramids from the Western Interior Seaway of North America (WIS). Escalation - the 'evolutionary arms race' between predators and prey - is said to be one of the most influential selective agents in evolution. Evidence of predation, parasitism, and disease in inoceramids is virtually undocumented prior to the Turonian. However, populations of inoceramids from the Late Cretaceous Pierre Shale show a marked increase in the number of individuals in which evidence for attempted predation and/or parasitism is preserved. The percentage of predation and/or parasitism steadily increases between the Baculites baculus and the B. grandis ammonite biozones (uppermost Campanian through Lower Maastrichtian) from 2.6% to values as high as 44.6%. The dramatic increase in shell deformities among inoceramids corresponds to a rapid radiation of shell-crushing brachyuran crabs and may be related to their activity. The introduction of new, efficient predators, such as brachyuran crabs, combined with parasitism and disease could have stressed inoceramid populations. Thus, they may have been more susceptible to environmental perturbations than under normal 'background' conditions. The disappearance of the inoceramids, at least from the WIS, may be one of the few cases where virtually an entire family lost the 'evolutionary arms race'.     

Sociobiology of slave-making ants

Social parasitism is the coexistence in the same nest of two species of social insects, one of which is parasitically dependent on the other. Though parasitism in general is known to be of crucial importance in the evolution of host species, social parasites, though intriguing, are often considered as a phenomenon of marginal interest and are typically not taken into account in reviews on parasitism. Nevertheless, social parasites are rather common in social bees, wasps, and ants and therefore may offer unique model systems to study a number of fundamental problems in evolutionary biology. Here we review several aspects of the peculiar life history of slave-making ants, which is characterized by socially parasitic founding of colonies and the pillage of broods from neighboring host colonies during slave raids. In particular we focus on the evolution of slave-making habits (dulosis), communication mechanisms between slave makers and their hosts, sex-allocation ratios and reproductive conflict, and the effect of slave makers on host populations. Received: 2 February 2000 / Received in revised form: 21 December 2000 / Accepted: 8 January 2001