Age-specific differences in the winter foraging strategies of rooks Corvus frugilegus

Summary Foraging efficiency and intraspecific competition were compared between wild adult and immature rooks Corvus frugilegus with respect to flock size. Behavioural time budgets, and observations of prey selection and prey energetic values revealed that adult rooks in large flocks (> 50 individuals) consumed smaller, less profitable prey, but allocated more time to feeding and fed at a faster rate and with greater success than adults in small flocks. By contrast, immature rooks in flocks of more than 30 individuals allocated proportionally less time to feeding, fed at a lower rate and fed with no increase in success rate than when foraging in smaller flocks. Agonistic encounters and the avoidance of adults by immature rooks appeared responsible for such inefficient foraging. Hence immature rooks showed a preference for smaller flocks (< 50 individuals) with low adult: immature ratios while adults preferred larger flocks (> 50 individuals). We discuss the possible influence of competitive disadvantages on immature rook distribution, flock composition and post-natal dispersal.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Limited DNA elimination from the irradiated potato parent in fusion products of albino Lycopersicon esculentum and Solanum tuberosum

Summary This paper describes the analysis of the elimination of potato DNA from potato-tomato somatic cell hybrids. The hybrids were obtained by fusion of protoplasts of a cytoplasmic albino tomato genotype with leaf mesophyll protoplasts of a potato genotype carrying the -glucuronidase (GUS) gene of Escherichia coli. The potato protoplasts were either isolated from unirradiated plants or from plants irradiated with 50 or 500 Gy of -rays. Green calli were selected as putative fusion products. The hybridity of these calli was confirmed by isoenzyme analysis. All of the green calli tested contained a potato-specific chloroplast DNA restriction fragment, and most of the calli analysed were positive for -glucuronidase activity. In 72 of the hybrid calli we determined the percentage of potato nuclear DNA using species-specific probes. All of the tested green calli contained a considerable amount of potato genomic DNA, irrespective of the dose of irradiation of the potato parent. The limited degree of potato DNA elimination and the absence of true cybrids are discussed.     

Competition between two primary parasitoids,Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes,attacking the mealybug Rastrococcus invadens Williams and the influence of a hyperparasitoid Chartocerus hyalipennis Hayat

Two primary parasitoids of the mealybug Rastrococcus invadens Williams, (Hemiptera: Pseudococcidae), Gyranusoidea tebygi Noyes and Anagyrus mangicola Noyes (Hymenoptera: Encyrtidae), were studied. Both primary parasitoids were capable of eliminating the mealybug host but on occasions the parasitoids went extinct before the mealybugs. Three of four parasitoids known to attack G. tebygi also attacked A. mangicola. The latter species was more heavily parasitized than the former, especially by the gregarious species Chartocerus hyalipennis Hayat. In competition experiments the presence of hyperparasitoids slightly slowed the speed of extinction of either the mealybug or the primary parasitoid. A. mangicola was heavily parasitized by C. hyalipennis and the primary went extinct while many suitable mealybug hosts were still available. There were two situations where the primary parasitoids were in direct competition; in the first G. tebygi was more successful than A. mangicola while the reverse was true for the second. It is likely that the superiority of either parasitoid would depend on particular conditions but the introduction of A. mangicola is unlikely to lessen the control being exerted in West Africa by G. tebygi.     

The influence of habitat use and foraging on the replacement of one introduced wasp species by another in New Zealand

Abstract.

• 1 New Zealand was colonized by the German wasp, Vespula germanica (F.), in the 1940s and it subsequently became established throughout the country. The common wasp, V.vulgaris (L), colonized in the late 1970s and is still spreading.
• 2 The common wasp has replaced the German wasp in some habitats in New Zealand. Samples from a nationwide postal survey indicate that the common wasp is now the more abundant species in honeydew beech forests (Nothofagus spp.), and to a lesser extent in other native forests. The German wasp is still the more abundant wasp in rural areas (excluding forest). The two species are at present co-dominant in urban areas, although this may be a transient phase.
• 3 In honeydew beech forest the two species show different foraging patterns that provide the potential for local coexistence. Although both species are generalist feeders, the German wasp is more commonly found foraging for protein amongst the forest litter, whereas the common wasp forages more on shrubs and tree saplings. Despite this difference, the common wasp can still replace the German wasp in honeydew beech forest within a few years of invasion.
• 4 In honeydew beech forests in which the German wasp is the more abundant species it dominates honeydew trunks (sugar resource), whereas the common wasp dominates honeydew trunks in areas where it is the more abundant species. The change from German to common wasp domination of honeydew trunks is more rapid than the change in dominance in other microhabitats. Aggressive interactions may be taking place on this high quality, potentially defensible sugar resource.

     

Ejaculate size varies with socio-sexual situation in an insect

Abstract.

• 1 Males operate within a finite energy budget and cannot produce limitless supplies of sperm. On the other hand, when a female mates with a second male while still containing fertile sperm from a rival male, selection should favour the male that inseminates more sperm. Optimal strategy should thus be for males to exercise discretion in the allocation of sperm to individual females. Assuming the outcome of sperm competition to be based on either the raffle or kamikaze principles, the sperm competition hypothesis predicts a positive association between the probability that the sperm will find themselves in competition with sperm from rival males and the number of sperm inseminated.
• 2 The beetle, Tenebrio molitor L., behaves in accordance with this hypothesis. Males accompanied by a rival male before and during mating inseminate more sperm per ejaculate than unaccompanied males. Accompanied males are also faster to initiate mating and more likely to show post-copulatory guarding. Adjustment of number of sperm inseminated was shown by males subjected to both long-term (5 days) and short-term (5 min) exposure to potential intrasexual competitors. Individual males exposed to both levels also demonstrated the ability to adjust their ejaculate according to socio-sexual situation.
• 3 We conclude that male T.molitor adjust the number of sperm they inseminate according to some perception of the risk of sperm competition.

     

Population structure,spite, and the iterated Prisoner's Dilemma

Evolutionary stability (sensu Maynard Smith: Evolution and the Theory of Games, Cambridge: Cambridge University Press, 1982) of TIT FOR TAT (TFT) under the social ecology of the iterated Prisoner's Dilemma is a function of the number of pure TFT groups (dyads) in the population, relative to the social position of a focal invading defector. Defecting against TFT always raises the defector's relative intragroup fitness; when Axelrod's (Am. Polit. Sci. Rev. 75:306–318, 1981; The Evolution of Cooperation. New York: Basic Books, 1984) Evolutionary stable strategy (ESS) conditions are met, defection also lowers the absolute fitness of the defector. Here the retaliatory (punishing) character of TFT converts defection into spite, permitting pure TFT groups to sufficiently outproduce the defector for the latter's evolutionary suppression. Increasing the relative impact of spiteful defection on a population lowers the range of evolutionary stability for TFT. When individuals participate in multiple dyads, those participating in the greatest number of dyads are most likely to provide a vehicle for the successful invasion of defection. Within social networks, ESS conditions for TFT are thus individual specific. This logic is generalized to the context of an interated n-person Prisoner's Dilemma, providing a cooperative solution conceptually identical with TFT in the two-person game.     

Negative interactions between Willamette mites and Pacific mites: possible management strategies for grapes

Willamette mites and Pacific mites are often negatively associated on grape vines. In particular, vineyards with early season infestations of the less damaging Willamette mite rarely develop high populations of the economically important Pacific mite. We report that Willamette mites had a negative effect on Pacific mite populations in both the greenhouse and field. In some experiments, the negative effect was more pronounced when vines had been damaged by previous feeding of Willamette mites and in other experiments concurrent feeding by both mite species was necessary to demonstrate a negative effect. Therefore, we cannot conclude if the mechanism of the response involves induced resistance against Pacific mites, more conventional interspecific competition, or both. Since predators were uncommon in our experiments, predator build up on Willamette mites did not cause the low Pacific mite population that we observed, although this mechanism may be important in many vineyards. Further, larger scale experiments are necesary to determine if growers can introduce Willamette mites to help control Pacific mites.

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Résumé E. willametti et T. pacificus sont souvent antagonistes dans les vignobles. En particulier, dans les vignobles contaminés tôt dans la saison par E. willametti,-dont les dégâts sont moins conséquents-, on observe rarement des populations élevées de T. pacificus, dont les dégâts sont économiquement importants. Nous avons constaté que E. Willametti a réduit les populations de T. pacificus, tant en serres qu'à l'extérieur. Dans quelques cas, l'effet antagoniste était plus marqué quand les vignes avaient été préalablement endommagées par l'alimentation de E. willametti. Dans d'autres expériences, la compétition alimentaire entre les deux espèces d'acariens était nécessaire pour produire un effet antagoniste. Cependant, nous ne pouvons pas en conclure que la réponse implique une résistance induite contre T. pacificus, ou une compétition interspécifique plus classique, ou les deux à la fois. Puisque les prédateurs étaient rares dans nos expériences, une explosion des prédateurs sur E. willametti n'a pu provoquer le faible niveau de population observé avec T. pacificus, bien qu'un tel méchanisme puisse être important dans certains vignobles. Quoi qu'il en soit, des expériences à plus grande échelle sont nécessaires pour déterminer si les vignerons peuvent introduire Eotetranychus willametti afin de limiter les populations de Tetranychus pacificus.

     

Experimental measurement of resource competition between planktonic microalgae and macroalgae (seaweeds) in mesocosms simulating the San Francisco Bay-Estuary,California

Planktonic algae are not abundant in the brackish waters of San Francisco Bay-estuary (mean chlorophyll a 5 µg 1^–1), despite the high level of nutrients usually present due to the input of treated sewage from 3 million people. Macroalgae (seaweeds) are sometimes locally abundant in the Bay. Phytoplankton are abundant (chlorophyll a > 50 µg 1^–1) and seaweeds uncommon in the almost freshwater Delta and upper estuary despite lower nutrient levels. Direct competition between these algal groups could explain the observed distributions.Given the size of the algae, large containers were needed for the determination of possible resource competition. Experiments were carried out in flow-through mesocosms (analog tanks) of 3 m³ volume. The macroalgae Ulva lactuca or Gigartina exasperata and a diatom-dominated phytoplankton, all from San Francisco Bay, were grown separately and together and with and without treated sewage effluent or other artificial nutrient additions. When grown alone phytoplankton and macroalgae were greatly stimulated by wastewater addition to unmodified baywater. The phytoplankton grew much more slowly in the presence of natural densities of Ulva. Allelochemical effects were tested for but not demonstrated.Resource competition for inorganic nitrogen was determined to be the probable cause of the depression of phytoplankton by Ulva. At its rapid growth rates in the flow-through mesocosms (up to 14% day^–1) this macroalga can reduce inorganic nitrogen to low levels. Ulva has a greater affinity (lower KS) for nitrogen than do some of the plankton of the Bay. Ulva may outcompete phytoplankton by reducing nitrogen to levels below those capable of supporting phytoplankton growth. Other macroalgae such as Gigartina and Enteromorpha need to be studied to determine if they also can depress phytoplankton growth by resource competition.