关键词： JANZEN-CONNELL MODEL   TROPICAL FORESTS   SPECIES-DIVERSITY   SPATIAL-PATTERNS   SOIL PATHOGENS   DENSITY   COEXISTENCE   HERBIVORY   SLUG   REPLACEMENT  

摘要： There is growing evidence that distance- and density-dependent recruitment are important factors in structuring tree communities in temperate forests. Although pathogens have generally been assumed to be driving these patterns, few studies have attempted to experimentally identify the agent responsible. Here we investigate patterns of recruitment in sycamore Acer pseudoplatanus, a temperate tree species, in its exotic range in Britain. Seedling mortality, the limiting stage in sycamore regeneration, was monitored with respect to the proximity and density of adult sycamores and seedlings in a grid of sampling units. We found that mortality was significantly greater beneath adult sycamores compared with heterospecifics, and was further elevated in areas of high conspecific basal area, a proxy for biomass. An experimental approach that manipulated the access of predators to seedlings demonstrated that herbivorous invertebrates were responsible for driving these distance- and biomass-dependent trends in mortality. Evidence of stem damage on dead seedlings suggests that slugs are the principal cause of mortality, although there was only partial evidence of higher slug activity beneath sycamores. The implications of these findings for sycamore invasion, species co-existence and management are discussed.

关键词： TREE-GRASS COEXISTENCE   AFRICAN SAVANNA   SEMIARID SAVANNAS   SIMULATION-MODEL   ARID ECOSYSTEMS   GRAZING SYSTEMS   DYNAMICS   FIRE   HETEROGENEITY   STABILITY  

摘要： In this paper, we address the question whether and through which mechanisms herbivores can induce spatial patterning in savanna vegetation, and how the role of herbivory as a determinant of vegetation patterning changes with herbivore density and the pre-existing pattern of vegetation. We thereto developed a spatially explicit simulation model, including growth of grasses and trees, vertical zonation of browseable biomass, and spatially explicit foraging by grazers and browsers. We show that herbivores can induce vegetation patterning when two key assumptions are fulfilled. First, herbivores have to increase the attractiveness of a site while foraging so that they will revisit this site, e.g. through an increased availability or quality of forage. Second, foraging should be spatially explicit, e.g. when foraging at a site influences vegetation at larger spatial scales or when vegetation at larger spatial scales influences the selection and utilisation of a site. The interaction between these two assumptions proved to be crucial for herbivores to produce spatial vegetation patterns, but then only at low to intermediate herbivore densities. High herbivore densities result in homogenisation of vegetation. Furthermore, our model shows that the pre-existing spatial pattern in vegetation influences the process of vegetation patterning through herbivory. However, this influence decreases when the heterogeneity and dominant scale of the initial vegetation decreases. Hence, the level of adherence of the herbivores to forage in pre-existing patches increases when these pre-existing patches increase in size and when the level of vegetation heterogeneity increases. The findings presented in this paper, and critical experimentation of their ecological validity, will increase our understanding of vegetation patterning in savanna ecosystems, and the role of plant-herbivore interactions therein.

关键词： tree spanner   stretch factor   fundamental cycle bases   complexity   combinatorial optimization  

摘要： Tree spanner problems have important applications in network design, e.g. in the telecommunications industry. Mathematically, there have been considered quite a number of max-stretch tree spanner problems and of average stretch tree spanner problems. We propose a unified notation for 20 tree spanner problems, which we investigate for graphs with general positive weights, with metric weights, and with unit weights. This covers several prominent problems of combinatorial optimization. Having this notation at hand, we can clearly identify which problems coincide. In the case of unweighted graphs, the formally 20 problems collapse to only five different problems. Moreover, our systematic notation for tree spanner problems enables us to identify a tree spanner problem whose complexity status has not been solved so far. We are able to provide an NP-hardness proof. Furthermore, due to our new notation of tree spanner problems, we are able to detect that an inapproximability result that is due to Galbiati [On min-max cycle bases, in: P. Eades, T. Takaoka (Eds.), ISAAC, Lecture Notes in Computer Science, vol. 2223, Springer, Berlin, 2001, pp. 116-123;On finding cycle bases and fundamental cycle bases with a shortest maximal cycle, Inform. Process. Lett. 88(4) (2003) 155-159] in fact applies to the classical max-stretch tree spanner problem. We conclude that the inapproximability factor for this problem thus is 2 - epsilon, instead, of of only 1 + root 5/2 approximate to 1.618 according to Peleg and Reshef [A variant of the arrow distributed directory with low average complexity, in: J. Wiedermann, P. van Emde Boas, M. Nielsen (Eds.), ICALP, Lecture Notes in Computer Science, vol. 1644, Springer, Berlin, 1999,pp.615-6241. (c) 2007 Elsevier B.V. All rights reserved.

关键词： Acacia tortilis   nutrients   plant-animal interactions   tree-grass interactions   wildebeest  

摘要： Recently, cover of large trees in African savannas has rapidly declined due to elephant pressure, frequent fires and charcoal production. The reduction in large trees could have consequences for large herbivores through a change in forage quality. In Tarangire National Park, in Northern Tanzania, we studied the impact of large savanna trees on forage quality for wildebeest by collecting samples of dominant grass species in open grassland and under and around large Acacia tortilis trees. Grasses growing under trees had a much higher forage quality than grasses from the open field indicated by a more favourable leaf/stem ratio and higher protein and lower fibre concentrations. Analysing the grass leaf data with a linear programming model indicated that large savanna trees could be essential for the survival of wildebeest, the dominant herbivore in Tarangire. Due to the high fibre content and low nutrient and protein concentrations of grasses from the open field, maximum fibre intake is reached before nutrient requirements are satisfied. All requirements can only be satisfied by combining forage from open grassland with either forage from under or around tree canopies. Forage quality was also higher around dead trees than in the open field. So forage quality does not reduce immediately after trees die which explains why negative effects of reduced tree numbers probably go initially unnoticed. In conclusion our results suggest that continued destruction of large trees could affect future numbers of large herbivores in African savannas and better protection of large trees is probably necessary to sustain high animal densities in these ecosystems.

关键词： 树胸   模型   动物   肝炎病毒  

摘要： 肝炎的的大范围流行已成为国内外关注的重要公共卫生问题之一。甲肝、乙肝虽已有疫苗,但不能忽视未来由于病毒变异所带来的威胁。丙肝目前尚未发现非常有效的疫苗,且丙肝的病理学机制也尚未完全清晰,主要原因是缺乏理想的动物模型。树属于低等灵长类动物。研究发现,他对很多人类疾病易感,所以建立肝炎树动物模型成为现在肝炎研究的一个热点。本文介绍了各型肝炎主要是甲、乙、丙型肝炎研究中建立树动物模型的国内外进展情况及存在的问题。

关键词： MONTASTRAEA-ANNULARIS COMPLEX   CYTOCHROME-OXIDASE SUBUNIT   MOLECULAR EVIDENCE   OCTOCORALLIA CNIDARIA   PHYLOGENETIC ANALYSIS   ANIMAL PHYLOGENY   RIBOSOMAL-RNA   ANTHOZOA   SCLERACTINIA   GENOME   Anthozoa   cladistics   Ribosomal RNA   Scyphozoa   Genome   Hydrozoa   MT-CO1 gene   Scleractinia   DNA barcoding   Porifera  

摘要： The evolution rates of mtDNA in early metazoans hold important implications for DNA barcoding. Here, we present a comprehensive analysis of intra- and interspecific COI variabilities in Porifera and Cnidaria (separately as Anthozoa, Hydrozoa, and Scyphozoa) using a data set of 619 sequences from 224 species. We found variation within and between species to be much lower in Porifera and Anthozoa compared to Medusozoa (Hydrozoa and Scyphozoa), which has divergences similar to typical metazoans. Given that recent evidence has shown that fungi also exhibit limited COI divergence, slow-evolving mtDNA is likely to be plesiomorphic for the Metazoa. Higher rates of evolution could have originated independently in Medusozoa and Bilateria or been acquired in the Cnidaria + Bilateria clade and lost in the Anthozoa. Low identification success and substantial overlap between intra- and interspecific COI distances render the Anthozoa unsuitable for DNA barcoding. Caution is also advised for Porifera and Hydrozoa because of relatively low identification success rates as even threshold divergence that maximizes the "barcoding gap" does not improve identification success.

关键词： coexistence   dietary partitioning   food preferences   folivory   granivory   gut morphology  

摘要： Using a comparative approach, we examined the gut morphology of 6 ecologically similar rodent species coexisting in an African savanna. Although all species are primarily granivores, we aimed to establish whether modifications of the gut morphology exist that would promote the potential for exploiting different food types, particularly foliage. The species examined exhibited a gradation in gut structure, with some showing modifications more toward granivory and others toward folivory. These findings are supported by studies of food preferences in captivity. Modifications of the gut, enabling the potential for exploitation of a greater folivorous diet in some species compared to others, may be I way in which dietary partitioning as a method of coexistence may be promoted. Nonetheless, other mechanisms, such as spatial and temporal segregation, social tolerance, and behavioral differences, also may determine coexistence in this community.

关键词： caching   deceptive behaviour   eastern grey squirrel   hoarding   pilfering   scatter hoarding   Sciurus carolinensis  

摘要： Behavioural deception has been studied experimentally primarily in captive-raised primates and corvids, and only in a laboratory setting. Here we show that free-living eastern grey squirrels, Sciurus carolinensis, caching food in the presence of conspecifics perform behavioural deception by covering additional empty sites where nothing has been cached. Such deceptive caching (1) occurred in two distinct populations, (2) occurred more often in the close presence of conspecifics and (3) reduced the probability of cache pilferage by surrogate (human) cache pilferers. In an additional experiment, in which we attempted to elicit deceptive behaviour by pilfering caches, deceptive caching appeared as one behaviour in a suite of pilferage-avoidance responses. To our knowledge, this is the first study to show evidence of behavioural deception by a rodent, and the first to use an experimental approach to studying deceptive behaviour in the wild. (C) 2007 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.

关键词： MUTUALISM   ANTS   BIODIVERSITY   BIOTIC communities   ECOLOGY   SAVANNAS   SYMBIOSIS   INSECT societies   HERBIVORES  

摘要： Mutualisms are key components of biodiversity and ecosystem function, yet the forces maintaining them are poorly understood. We investigated the effects of removing large mammals on an ant-Acacia mutualism in an African savanna. Ten years of large-herbivore exclusion reduced the nectar and housing provided by plants to ants, increasing antagonistic behavior by a mutualistic ant associate and shifting competitive dominance within the plant-ant community from this nectar-dependent mutualist to an antagonistic species that does not depend on plant rewards. Trees occupied by this antagonist suffered increased attack by stem-boring beetles, grew more slowly, and experienced doubled mortality relative to trees occupied by the mutualistic ant. These results show that large mammals maintain cooperation within a widespread symbiosis and suggest complex cascading effects of megafaunal extinction.

关键词： Piper methysticum   kava   kava pyrones   kavalactones   mutagenicity   mouse lymphoma  

摘要： Kava (Piper methysticum) is a member of the pepper family and has been cultivated by South Pacific islanders for centuries and used as a social and ceremonial drink. Traditionally, kava extracts are prepared by grinding or chewing the rhizome and mixing with water and coconut milk. The active constituents of kava are a group of approximately 18 compounds collectively referred to as kavalactones or kava pyrones. Kawain, dihydrokawain, methysticin, dihydromethysticin, yangonin, and desmethoxyyangonin are the six major kavalactones. Kava beverages and other preparations are known to be anxiolytic and are used for anxiety disorders. Dietary supplements containing the root of the kava shrub have been implicated in several cases of liver toxicity in humans, including several who required liver transplants after using kava supplements. In order to study the toxicity and mutagenicity, two commercial samples of kava, Kaviar and KavaPure, and the six pure kavalactones including both D-kawain and DL-kawain, were evaluated in L5178Y mouse lymphoma cells. Neither the kava samples nor the kavalactones induced a mutagenic response in the L5178Y mouse lymphoma mutation assay with the addition of human liver S9 activation. (c) 2007 Elsevier Ltd. All rights reserved.