关键词： eutheria   mammalian evolution   mitochondrial genomes   molecular phylogeny   phylogeny   evolutionary trees   paralogy   immunological distances  

摘要： DNA sequences provide a direct record of the genealogy of extant species. This tremendous reservoir of phylogenetic information is only beginning to be exploited. If progress in molecular phylogeny is being made, it should be most conspicuous amongst mammals, as their evolution is probably the best-studied. Indeed, surprising changes have recently been proposed for the tree of mammalian orders. These range from grouping whales with hippos, to placing African golden moles closer to elephants than to their fellow insectivores.

关键词： production   copepoda   flooded savanna  

摘要： This is the first contribution to the copepod production in Venezuelan tropical savannas. Total abundance, biomass, production and mean P/B ratio of nauplii, copepodids and adults were determined in interdaily samples from a flooded, embanked savanna during February and March (end of dry season), and May (beginning of rainy season). Highest values of biomass and total production were recorded during dry season (61.5 mg m(-3), 153.8 mg m(-3) day(-1), respectively), compared to the rainy season (5.6 mg m(-3), 45.9 mg m(-3) day(-1)). The last values are related to a low population density found during rainy season. Highest production were observed in copepodids at the end of the dry season. Significant differences of production between nauplii and copepodid stages, as well as between nauplii and adults, were also found. (C) 1998 Elsevier Science B.V. All rights reserved.

关键词： grasses   forbs   dwarf shrubs   offtake   herbivores   desertification   nomads  

摘要： A three-year study was done to determine herbaceous biomass production and its utilization by herbivores in an arid zone inhabited by a largely nomadic population in northern Kenya. The indicator selected for study was aboveground live standing biomass of grasses, forbs, dwarf shrubs and the total sum biomass of these vegetation categories (total herbaceous layer). Sampling was done along grazing gradients in order to estimate the utilization levels in the arid zone 7 and the semi-arid zone 6. Mean peak standing biomass for grasses, forbs, dwarf shrubs and total herbaceous layer under non-grazed conditions was 184.4, 374.2, 1094.4 and 1504.0 kg/ha in eco-zone 7, respectively. in the zone 6, mean peak standing biomass was 55.3, 98.8, 4259.1 and 4320.1 kg/ha under non-grazed conditions, respectively. Results indicated graminoid removal to be respectively 57.1-99.8% and 24.2-87.2% of mean peak standing aboveground live biomass in the zone 7 and 6. Forb utilization in zone 7 was estimated to be 51.5-99.3%. Mean peak forb standing biomass however, showed a general increase on grazed sites compared to the non-grazed plot in zone 6. The corresponding utilization of the dwarf shrub layer was estimated to be in the range 40.5-80.0% and 76.6-92.3% in zone 7 and 6 respectively. Total herbaceous layer consumption of 39.3-85.3% and 74.0-90% was estimated for zone 7 and 6 respectively. These findings suggest that herbivores exert considerable control over biomass dynamics of the herbaceous layer in this zone and contribute to degradation in heavily utilised areas. Efforts to understand and tackle the desertification problem in this area must therefore take this fact into account. (C) 1998 Elsevier Science B.V. All rights reserved.

关键词： Cleora scriptaria   Geometridae   Macropiper excelsum   Piperaceae   kawakawa   herbivory   induced defences   abscission   insect-plant interaction  

摘要： The pattern of herbivore damage on the New Zealand pepper tree (kawakawa;Macropiper excelsum) caused by its main insect herbivore (Cleora scriptaria) was investigated in the field and laboratory. In the field, only a small proportion of kawakawa leaves had no herbivore damage and C. scriptaria typically produced a number of small holes in each leaf. Leaves were shed at a rapid rate but leaf shedding was not increased by higher levels of herbivore damage. Some alder leaves had less damage than would be expected for their age, possibly suggesting some variation in leaf susceptibility to herbivory. The effect of previous leaf damage on subsequent herbivory was examined in the field and laboratory. On no occasion did artificial leaf damage, or herbivory by larvae of the same or another species, affect the edibility of leaves to C. scriptaria. The results suggest that leaf shedding by kawakawa was not primarily a response to herbivore damage and induced defences against herbivores could not be demonstrated in this system.

关键词： climax community   diversity   grazing   herbivory   scale   stability   succession   vegetation model  

摘要： Long-term (45-yr) basal area dynamics of dominant graminoid species were analyzed across three grazing intensity treatments (heavily grazed, moderately grazed and ungrazed) at the Texas A&M University Agricultural Research Station on the Edwards Plateau, Texas. Grazing intensity was identified as the primary influence on long-term variations in species composition. Periodic weather events, including a severe drought (1951-1956), had little direct influence on composition dynamics. However, the drought interacted with grazing intensity in the heavily grazed treatment to exacerbate directional changes caused by grazing intensity. Species response to grazing was individualistic and noisy. Three response groups were identified. Taller, more productive mid-grasses were most abundant under moderate or no grazing. Short grasses were most abundant under heavy grazing. Intermediate species were most abundant under moderate grazing and opportunistic to weather patterns. Graminoid diversity increased with the removal or reduction of grazing intensity. The moderately and ungrazed treatments appeared most resistant to short-term weather fluctuations, while the heavily grazed treatment demonstrated significance resilience when grazing intensity was reduced after over 110 yr of overgrazing. Identification of a 'climax' state is difficult. Significant directional change, which took nearly 20 yr, appears to continue in the ungrazed treatment after 45 yr of succession. The observed, relatively linear patterns of perennial glass composition within the herbaceous patches of this savanna were generally explained by traditional Clementsian succession. However, when dynamics of the herbaceous community are combined with the woody component of this savanna, the frequency and intensity of fire becomes more important. Across the landscape, successional changes follow several pathways. When vegetation change is influenced by several factors, a multi-scale model is necessary to demonstrate int

摘要： Aggregations in tadpoles of the West African savanna frog Phrynomantis microps were often observed in their breeding ponds in our study area, situated in Comoe National Park, Ivory Coast. Experiments under seminatural conditions demonstrated that this behaviour was only shown while predators were present. The tadpoles reacted differently to different predators. Factors inducing swarm behaviour were optical (the predator) and/or chemical (liquid from injured tadpoles). Alarm substances are not species-specific. Kinship seems not to play a role in swarm formation.

关键词： aucubin   catalpol   iridoid glycosides   chemical defense   sequestration   tritrophic interactions   diet breadth   host range   predation  

摘要： Recent studies demonstrate that generalist insect herbivores may be more subject to predation than specialists and suggest that this pattern is related to the availability and utilization of plant-derived natural products in diets of the latter. This intuitively attractive hypothesis has seldom been tested under natural conditions, however. A well-described plant-insect system, the buckeye butterfly. Junonia coenia, and its iridoid glycoside-containing host plants, was used to test the hypotheses: (1) that larval sequestration of iridoids reduces predation in the field, and (2) that this protection varies among populations. Fifth-instars reared in the laboratory on leaves of two host-plant species. Kickxia elatine (Scrophulariaceae) or Plantago lanceolata (Plantaginaceae). or on artificial diets were attached to monofilament tethers and set out into four natural populations. Only larvae id on the locally available host plant and artificial diet-fed caterpillars were used at each site. Despite high overall predation, significantly more leaf-fed caterpillars survived than did artificial diet-fed caterpillars in Plantago sites, but the trend was not significant in Kickxia sites. The results reflect the differences in iridoid glycoside availability in these two host-plant species. They indicate that predators exert a strong selective force on this insect, that sequestered iridoid glycosides can be effective deterrents to predation, and that host-plant choices by ovipositing females and feeding larvae have consequences for mortality from natural enemies. This protection. however, is not absolute, and depends on local predator abundance and/or selectivity. The ecological interactions between buckeye caterpillars, their host plants, and their natural enemies are locally variable in nature.

关键词： rumen   protozoa   defamation  

摘要： Samples and extracts of foliage from African multipurpose trees were screened for their effects on rumen protozoa and bacteria with a view to predicting their safety as feed supplements and for identifying species with potential antiprotozoal activity. The species tested were Acacia aneura, Chamaecytisus palmensis, Brachychiton populneum, Flindersia maculosa, Sesbania sesban, Leucaena leucocephala and Vernonia amyedalina. Antimicrobial effects were mild except for S. sesban, which was highly toxic to rumen protozoa in vitro, and A. aneura, which was toxic to rumen bacteria. The antiprotozoal factor in S. sesban was apparently associated with the fraction of the plant containing saponins. When S. sesban was fed to sheep, protozoal numbers fell by 60 % after 4 d, but the population recovered after a further 10 d, in vitro experiments demonstrated that washed protozoa from later times were no more resistant to S. sesban than on initial exposure, suggesting that other micro-organisms, probably the bacteria, adapted to detoxify the antiprotozoal agent. Thus S. sesban may be useful in suppressing protozoa and thereby improving protein how from the rumen, but only if the bacterial metabolism of the antiprotozoal factor can be avoided.

摘要： Several major alliances of metazoan phyla have been identified by small subunit rRNA sequence comparisons, It is possible to arrange the phyla to produce a parsimonious distribution of cleavage types, requiring only one change from a radial ancestral condition to spiral cleavage and one other to ''idiosyncratic'' cleavage;this arrangement is consistent with most of the recent molecular phylogenies. The cleavage shifts are correlated with changes in many of the features that once were used to distinguish Protostomia and Deuterostomia. It is hypothesized that changes in cleavage direction are causally associated with changes in blastomere fates and thus that cleavage type correlates with such features as the identity of mesoderm founder cells, which in turn can constrain the mode of origination of the eucelom. Cleavage changes may also affect the timing of cell fate specification. In a tree that emphasizes cleavage parsimony, radial cleavage, regulative development, and enterocely are ancestral within the Bilateria, and spiral or idiosyncratic cleavages, mosaic development, and schizocely are associated with a change in cleavage direction, Deuterostomy is presumably ancestral and is correlated with radial cleavage for this reason, rather than mechanistically.